/ 


UNIVERSITY  OF  CALIFORNIA 
AT   LOS  ANGELES 


MINERAL  DEPOSITS 


BY 

WALDEMAR  LINDGREN 

PROFESSOR  OF  ECONOMIC  GEOLOGY,  MASSACHUSETTS  INSTITUTE 

OF  TECHNOLOGY;  FORMERLY  GEOLOGIST,  UNITED 

STATES  GEOLOGICAL  SURVEY 


SECOND  EDITION 

REVISED,  ENLARGED  AND  ENTIRELY  RESET 
SECOND  IMPRESSION 


McGRAW-HILL  BOOK  COMPANY,  INC. 

NEW  YORK:    239   WEST  39TH  STREET 

LONDON:    6  &  8  BOUVERIE  ST.,  E.  C.  4 

1919 

6114      1  - 


COPYRIGHT,  1913,  1919,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY,  INC. 


PREFACE  TO  THE  SECOND  EDITION 

The  last  few  years  of  scientific  progress  frequently  loom  up  in 
magnified  proportions.  This  is  a  view  that  may  be  corrected 
by  a  proper  perspective  but  to  the  author  of  a  text-book  anxious 
to  carry  his  readers  up  to  date  it  is  certainly  disconcerting  to 
measure  the  rapid  accumulation  of  new  facts  and  theories. 

In  spite  of  the  disturbed  condition  of  the  world  during  the 
last  six  years,  students  of  minerals  and  mineral  deposits  have 
made  many  important  contributions  to  science.  Among  these 
may  be  counted  the  investigations  bearing  on  magmatic  and  con- 
tact-metamorphic  ore  deposits,  on  problems  of  oxidation  and 
supergene  sulphides,  and  particularly  the  application  of  metallo- 
graphic  methods  to  ores  by  which  the  complexity  of  metallic 
replacements  has  been  revealed. 

In  this  second  edition  all  of  the  chapters  have  been  revised 
and  those  on  deposition  of  minerals,  contact-metamorphism, 
oxidation  and  sulphide  enrichment,  have  been  largely  rewritten. 
A  discussion  of  metallogenetic  epochs  has  been  added,  as  well 
as  an  index  by  elements  which  will  enable  the  student  to  coordi- 
nate rapidly  the  deposits  of  any  given  metal.  Many  new  illus- 
trations have  been  introduced.  Owing  to  war  conditions  it  has 
not  always  proved  easy  to  obtain  late  statistical  data.  In  case 
of  some  European  countries  the  latest  figures  available  date 
from  1913. 

The  progress  of  science  emphasizes  the  difficult  task  of  con- 
densation. In  tha  present  volume  some  less  essential  descrip- 
tions have  been^omitted  so  that  its  bulk  has  not  been  greatly 
increased.  No  one  knows  better  than  the  author  that  errors 
may  easily  creep  in  and  corrections  and  suggestions  will  be  grate- 
fully received. 

The  genetic  arrangement  has  been  preserved  throughout. 
While  it  is  realized  that  this  may  make  the  study  less  easy  for 
beginners,  it  is  believed  that  any  merit  that  the  book  may  have 
is  due  to  this  mode  of  treatment. 

CAMBKIDGE, 
April,  1919. 


PREFACE  TO  FIRST  EDITION 

Mineral  deposits  are  usually  classified  and  described  by  the 
metals  or  the  substances  which  they  contain;  for  instance,  de- 
posits of  copper  are  described  together,  with  little  or  no  effort  to 
separate  them  into  genetic  groups.  Where  a  genetic  treatment 
has  been  attempted  it  appears  to  me  to  have  failed  in  not  giving 
due  weight  to  the  physical  conditions  attending  the  genesis. 
Furthermore,  it  is  the  custom  to  divide  the  mineral  deposits 
into  two  groups — the  metallic  and  the  nonmetallic — a  line  of 
division  which  can  hardly  be  defended  except  on  the  ground 
of  long-established  habit. 

This  book  is  the  outcome  of  a  desire  to  place  the  knowledge  of 
mineral  deposits  on  the  broader  and  more  comprehensive  basis 
of  a  consistent  genetic  classification  and  thus  bring  it  into  a  more 
worthy  position  as  an  important  branch  of  geology.  Opinions 
may  differ  as  to  whether  our  present  knowledge  is  sufficient  for 
such  an  undertaking.  Believing  that  the  time  has  come  for  a 
first  attempt,  I  present  this  volume,  in  the  hope  that  its  short- 
comings may  be  judged  leniently. 

The  impetus  to  the  work  came  during  the  preparation  of  a 
series  of  lectures  a  few  years  ago,  and  a  course  along  the  general 
lines  followed  in  this  volume  has  since  then  been  presented  an- 
nually at  the  Massachusetts  Institute  of  Technology. 

The  general  plan  has  been  to  select  a  few  suitable  examples  to 
illustrate  each  genetic  group  of  deposits.  These  examples  have 
been  chosen  regardless  of  their  geographic  location,  and  it  was  of 
course  necessary  to  give  up  any  attempt  to  describe  deposits  in 
detail  or  to  present  all  known  examples  of  any  particular  type. 
As  the  larger  part  of  my  experience  has  been  within  the  United 
States  of  America,  a  considerable  number  of  examples  were 
gathered  from  this  country.  This  experience  I  owe  to  the  United 
States  Geological  Survey,  in  which  I  have  had  the  honor  to  serve 
for  many  years.  My  indebtedness  to  my  friends  and  associates 
in  that  organization  is  greater  than  can  be  expressed  in  words. 

BOSTON,  August,  1913. 


VII 


CONTENTS 

PAGE 

PKEFACE v 

CHAPTER  I 

INTRODUCTION      1 

Economic  geology — Mineral  deposits — Definitions — Technical 
utility — Ore  and  gangue — Distribution  of  the  elements — Composi- 
tion of  the  earth's  crust — Traces  of  metals  in  rocks — General  state- 
ment— Copper— Lead  and  zinc — Gold  and  silver — Tenor  of  ores — 
Iron — Copper — Lead — Zinc — Silver — Gold — Tin,  etc. — Price  of 
metals — Production  of  ore  and  metal — Weights  and  measures — 
Conversion  tables. 

CHAPTER  II 

THE  FORMATION  OF  MINERALS 22 

Solution  and  precipitation — General  features — Influence  of  pres- 
sure— Influence  of  temperature — Precipitation  by  evaporation  of 
the  solvent — Precipitation  by  reaction  between  solutions — Precipi- 
tation by  reactions  between  aqueous  solutions  and  solids — Precipi- 
tation by  reactions  between  gases  or  between  gases,  and  solutions — 
Crystalline  minerals — Colloids. 

CHAPTER  III 

THE  FLOW  OF  UNDERGROUND  WATER 29 

General  statement — Pores  and  openings  in  rocks — Water  in  sands 
and  gravels — Water  in  rocks  of  uniform  texture — Water  in  sedi- 
mentary rocks — Influence  of  fractures — Influence  of  volcanism — 
Conclusions — Examples  of  movement  of  water — Depth  of  water 
level — Total  amount  of  free  water  in  earth's  crust. 

CHAPTER  IV 

THE  COMPOSITION  OF  UNDERGROUND  WATERS 42 

Introduction — Calcium  carbonate  waters  in  igneous  rocks — 
Calcium  carbonate  waters  in  sedimentary  rocks — Chloride  waters  in 
sedimentary  rocks — Chloride  waters  in  igneous  rocks — Sulphate 
waters  in  sedimentary  rocks — Acid  sulphate  waters  in  igneous 
rocks — Mine  waters  of  sulphate  type — Sodium  carbonate  waters  in 
sedimentary  rocks — Sodium  carbonate  waters  in  igneous  rocks — 
Sodium  sulphide  waters — Summary — Interpretation  of  water 
analyses. 


x  CONTENTS 

CHAPTER  V 

PAGE 

THE  CHEMICAL  WORK  OF  UNDERGROUND  WATER 66 

Metamorphism  and  mineral  deposits— Stability  of  minerals  and 
rocks — Metamorphism — Metasomatism  or  replacement — The  law 
of  equal  volume — General  definition  of  the  metamorphic  zones — 
Zone  of  weathering — The  intermediate  zone — The  deeper  zones — 
Relation  of  mineral  deposits  to  the  metamorphic  zones — Deposits 
related  to  igneous  activity — Derivation  of  minerals — Concentra- 
tion— Underground  temperatures. 

CHAPTER  VI 

THE   ORIGIN   OF   UNDERGROUND   WATER   AND   ITS   DISSOLVED   SUB- 
STANCES      86 

Origin  of  the  water — Meteoric  waters — Magmatic  or  juvenile 
waters — Examples  of  springs  in  volcanic  regions — Salts  from  sedi- 
mentary rocks — Salts  from  igneous  rocks — Salts  of  volcanic 
springs — Origin  of  the  dissolved  gases — Rarer  elements  contained 
in  waters — The  igneous  emanations. 

CHAPTER  VII 

THE  SPRING  DEPOSITS  AT  THE  SURFACE 99 

Deposits  of  limonite  and  calcium  carbonate — Deposits  of  silica — 
Deposits  of  other  gangue  minerals — Summary. 

CHAPTER  VIII 
RELATIONS  OF  MINERAL  DEPOSITS  TO  MINERAL  SPRINGS 109 

CHAPTER  IX 

FOLDING  AND  FAULTING 115 

Folds— Faults— General  terms— General  classification  of  faults- 
Faults  of  parallel  displacement — Faults  in  stratified  rocks — Slip — 
Shift — Throw — Offset — Faults  classified  according  to  the  direction 
of  the  movement — Classes  of  strike  faults — Extension  of  the  words 
normal  and  reserve  to  diagonal  and  dip  faults — Special  classes  of 
faults— Rotatory  faults— Mineralization  of  faults— Complexity 
of  faulting. 

CHAPTER  X 

OPENINGS  IN  ROCKS 137 

Origin  of  openings — By  the  original  mode  of  formation  of  the 
rocks— By  solution— By  fractures  of  various  modes  of  origin— Force 
of  crystallization. 


CONTENTS  xi 

CHAPTER  XI 

PAGE 

THE  FORM  AND  STRUCTURE  OF  MINERAL  DEPOSITS 147 

Syngenetic  deposits — Epigenetic  deposits — Spacial  relations  of 
veins — Veins  in  relation  to  the  country  rock — Vein  walls — Out- 
crops— Length  and  depth  of  veins. 

CHAPTER  XII 

THE  TEXTURE  OF  MINERAL  DEPOSITS 161 

Filling  and  replacement — Introduction — Texture  of  deposits  of 
igneous  origin — Texture  of  pegmatite  dikes — Texture  of  sedimen- 
tary deposits — Concretions — Texture  of  residual  and  oxidized 
deposits — Texture  of  epigenetic  deposits — Primary  texture  of 
filled  deposits — Secondary  texture  and  structure  of  filled  deposits — 
Metasomatism  in  mineral  deposits — Metasomatic  processes — 
Mode  of  replacement — Texture  of  metasomatic  rocks — Replace- 
ments at  high  temperature — Replacements  at  intermediate  temper- 
ature— Replacement  at  low  temperature — Criteria  of  replacement — 
Role  of  colloids  in  filling  and  replacement. 

CHAPTER  XIII 

ORE-SHOOTS      182 

Form  of  primary  ore-shoots — Shoots  of  successive  mineraliza- 
tions— Superficial  or  secondary  shoots — Causes  of  primary  ore- 
shoots — Decrease  of  pressure  and  temperature — Character  of  wall 
rock — Impermeable  barriers — Intersections. 

CHAPTER  XIV 

THE  CLASSIFICATION  OF  MINERAL  DEPOSITS 195 

Classification  by  form  and  substance — Genetic  classifications — 
Outline  of  proposed  classification — Detrital  and  sedimentary  de- 
posits— Concentration  of  substances  contained  in  the  rocks — 
Residual  weathering — Deep  circulating  waters — Regional  meta- 
morphism — Zeolitization — Introduced  ores  not  connected  with 
igneous  rocks — Deposits  genetically  connected  with  igneous  rocks — 
Products  of  magmatic  differentiation — Metamorphism  and  surface 
enrichment  of  deposits.  A  classification  of  mineral  deposits. 

CHAPTER  XV 

DEPOSITS  FORMED  BY  MECHANICAL  PROCESSES  OF  TRANSPORTATION 

AND  CONCENTRATION;  DETRITAL  DEPOSITS 207 

Introduction — Detrital  quartz  deposits — Detrital  clay  deposits — 
Fuller's  earth — Placer  deposits — Origin  and  distribution — Gold 
placers — Origin  of  placer  gold — Eluvial  deposits — Processes  of 


CONTENTS 

PAGI 

concentration — Eolian  deposits — Stream  deposits — Classification  of 
fluviatile  and  marine  placers — Marine  placers — Buried  placers — 
Size  and  mineral  association  of  placer  gold — Fineness  and  relation 
to  vein  gold — Gold  in  relation  to  bed-rock — Grade  of  auriferous 
watercourses — The  pay  streak  or  "run  of  gold" — Solution  and 
precipitation  of  gold — Relation  to  primary  deposits — Economic 
notes — The  gold-bearing  conglomerates  of  South  Africa — Platinum 
placers — Cassiterite  placers — Monazite  placers — Other  placers. 


CHAPTER  XVI 


DEPOSITS  PRODUCED  BY  CHEMICAL  PROCESSES  OF  CONCENTRATION  IN 

BODIES  OF  SURFACE  WATER  BY  REACTIONS  BETWEEN  SOLUTIONS  .  247 
Limestone — Definition  and  origin— Chalk — Lithographic  stone — 
Hydraulic  limestone — Lime — Uses — Dolomite — Importance  of  car- 
bonate rocks  as  related  to  ore  deposits — Cherts  and  diatomaceous 
earth — Sedimentary  sulphide  deposits — Sedimentary  iron  ores — 
Limonites  in  swamps  and  lakes  (bog  iron  ores) — Occurrence — 
Composition — Origin — Examples — The  siderites  of  marine  and 
brackish-water  strata — Occurrence — Examples — The  Jurassic 
siderites  of  England — The  oolitic  marine  limonites  and  hematites — 
The  oolitic  limonites — Occurrence — Examples — Origin — The  ma- 
rine oolitic  silicate  ores — The  marine  oolitic  hematite  ores — Occur- 
rence— The  Clinton  ores — The  brazilian  hematites — The  oolitic 
hematite-chamosite-siderite  ores — Review  of  the  sedimentary 
iron  ores — Sedimentary  manganese  ores — Bog  manganese  ore — 
Manganese  in  lacustrine  and  marine  beds — Sedimentary  phosphate 
beds — Composition  of  the  calcium  phosphates — Other  phos- 
phates— Phosphate  deposits — Use — Production — Origin  of  the 
phosphate  rocks — Occurrences  of  phosphate  rocks. 


CHAPTER  XVII 

DEPOSITS  FORMED  BY  EVAPORATION  OF  BODIES  OF  SURFACE  WATERS  .  .  287 
The  saline  residues — Introduction — Types  of  water — Normal  suc- 
cession of  salts — Structural  features — Gypsum  and  anhydrite — 
Occurrence — Uses — Stability  and  solubility — Sodium  sulphate  and 
sodium  carbonate — Occurrence — Sodium  nitrate — Borates — Gen- 
eral occurrence — Marine  borate  deposits — Borax  marshes — Terti- 
ary lake  beds — Production  and  uses — Origin — Soxlium  chloride — 
Occurrence— Examples— The  salt  deposits  of  the  gulf  coast- 
Composition,  production  and  use — The  German  potassium  salts — 
Other  sources  of  potassium  salts — Potassium  in  rocks  and  min- 
erals— Potassium  in  brines — Bromine  and  calcium  chloride. 


CONTENTS  xiii 

CHAPTER  XVIII 

PAGE 

MINERAL  DEPOSITS  RESULTING  FROM  PROCESSES  OP  ROCK  DECAY  AND 

WEATHERING 319 

General  conditions — Decomposition  of  minerals — Total  chemical 
changes  by  weathering — Residual  clay — Occurrence — Uses  and 
properties — Origin — Residual  iron  ores  (limonite  and  hematite) — 
Origin — Classification — Brown  hematites  of  the  Appalachian 
region — Iron  ores  of  Bilbao,  Spain — Residual  ores  of  Cuba — 
Distribution  and  stability  of  residual  iron  ore — Residual  manga- 
nese ores — Primary  sources — Manganese  deposits  in  the  United 
States — Brazil — India — Origin — Residual  barite — Residual  zinc 
ore — Residual  ochers — Residual  phosphates — Deposits  of  hy- 
drated  silicates  of  nickel — Bauxite — Introduction — Origin — Occur- 
rences— Uses  and  production. 

CHAPTER  XIX 

THE  HEMATITE  DEPOSITS  OP  THE  LAKE  SUPERIOR  REGION 357 

General  character  and  distribution — Geology — The  "iron  forma- 
tions"— The  iron  ores — Form  of  ore  bodies — Marquette  range — 
Menominee  range  —  Penokee-Gogebic  range  —  Cuyuna  range — 
Mesabi  range — Vermilion  range — Origin  of  Lake  Superior  iron 
ores — Re'sume'. 

CHAPTER  XX 

DEPOSITS  FORMED  BY  CONCENTRATION  OF  SUBSTANCES,  CONTAINED 

IN  THE  SURROUNDING  ROCKS,  BY  MEANS  OF  CIRCULATING  WATERS  .  375 
General  statement — Barite — Modes  of  occurrence  and  origin — 
Deposits  in  the  United  States — Foreign  deposits — Uses  and 
production — Celestite  and  strontianite — Sulphur — Modes  of  occur- 
rence— Origin  of  sulphur  deposits  in  gypsum — Examples — Produc- 
tion— Uses — Sulphuric  acid — The  magnesian  deposits — Serpen- 
tine— Magnesite — Origin — Occurrence — Production  and  use — 
Meerschaum — Talc  and  soapstone — General  occurrence  and  ori- 
gin— Occurrences — Production  and  uses — Pyrophyllite — Asbes- 
tos— Amphibole  asbestos — Serpentine  asbestos  (chrysotile) — 
Uses — Ores  of  copper,  lead,  vanadium,  and  uranium  in  sandstone 
and  shale — General  features — Origin — Copper  and  lead  deposits 
in  sandstone — European  occurrences — American  occurrences — 
South  America — Africa — Genesis  of  sedimentary  copper  ores — 
Vanadium  and  uranium  ores  in  sandstones — Composition — Occur- 
rence— Genesis — Production  and  use — The  copper-bearing  shales 
of  Mansfield — Copper  sulphide  veins  in  basic  lavas — General 
features — The  Nikolai  greenstone — Copper  sulphide  veins  in  intru- 
sive basic  rocks — Other  veins  deposited  by  waters  of  the  upper 
circulation. 


xiv  CONTENTS 

CHAPTER  XXI 

PAGE 

DEPOSITS  RESULTING  FROM  REGIONAL  METAMORPHISM 421 

CHAPTER  XXII 


DEPOSITS  OF  NATIVE  COPPER  WITH  ZEOLITES  IN  BASIC  LAVAS  ....  425 
General  statement — Origin  of  the  zeolitic  copper  ores — Probable 
source  of  copper — The  occurrence  of  zeolites  and  the  process  of 
zeolitization — The  Lake  Superior  copper  deposits — General  occur- 
rence— The  Calumet  conglomerate — The  amygdaloids — The  veins 
— Mineral  association — Origin — Mine  waters — Rock  alteration — 
Mining  and  smelting  operations — The  copper  deposit  of  Monte 
Catini — Native  copper  with  epidote  in  basic  lavas  (Catoctin 
type). 


CHAPTER  XXIII 

LEAD  AND  ZINC  DEPOSITS  IN  SEDIMENTARY  ROCKS;  ORIGIN  INDEPEND- 
ENT OF  IGNEOUS  ACTIVITY 444 

Characteristic  features — Origin— Moresnet — Silesia — Alpine  Trias 
— Other  European  localities — The  lead-zinc  ores  of  the  Mississippi 
Valley. 


CHAPTER  XXIV 


METALLIFEROUS  DEPOSITS  FORMED  NEAR  THE  SURFACE  BY  ASCENDING 
THERMAL  WATERS  AND  IN  GENETIC  CONNECTION  WITH  IGNEOUS 

ROCKS 465 

Character  and  origin — General  features — Successive  phases  of 
mineralization — Zeolitic  replacement — Primary  ore  shoots,  oxida- 
tion, and  sulphide  enrichment — Types  of  deposits— Older  repre- 
sentatives of  this  class — Genesis — Proof  of  depth  below  surface — 
Proof  of  temperature — Relation  to  other  veins — Metasomatic 
processes — Extent  of  alteration — Types  of  alteration — Metaso- 
matic processes  at  Thames  and  Waihi — Metasomatic  processes  at 
Tonopah — The  development  of  kaolin — Metasomatic  processes 
at  Silverton,  Colorado — Summary — Quicksilver  deposits — The 
ores  and  their  general  occurrence — Distribution,  production 
and  use — Geological  features — Mineralogy  of  quicksilver  ores — 
Structure — Genesis — Relation  to  other  ore  deposits — Stibnite 
deposits — Mineralogy,  production  and  uses — Occurrence — Gold- 


CONTENTS  xv 

PAGE 

quartz  veins  in  andesite — Transylvania — Hauraki  Peninsula, 
New  Zealand — El  Oro,  Mexico — Gold-quartz  veins  in  rhyolite — 
Argentite-gold-quartz  veins — Tonopah,  Nevada — The  Comstoick 
lode — Argentite  veins — Gold  telluride  veins — Cripple  Creek — Gold 
selenide  veins — Occurrence  of  selenides — Republic,  Washington — 
Sumatra — The  base-metal  veins — The  San  Juan  region,  Colo- 
rado— General  features — Telluride  district — Silverton  district — 
Ouray  district — Rico  district — La  Plata,  Durango,  and  Needle 
Mountains  quadrangles — Lake  City  district — Creede  district — 
Summary — Gold-alunite  deposits — General  features — Goldfield, 
Nevada. 


CHAPTER  XXV 

METALLIFEROUS   DEPOSITS   FORMED   AT   INTERMEDIATE    DEPTHS  BY 
ASCENDING  THERMAL  WATERS  AND  IN  GENETIC  CONNECTION  WITH 

INTRUSIVE  ROCKS 546 

General  features — Metasomatic  processes — General  character — Al- 
teration of  wall  rocks  adjoining  gold-quartz  veins — Interior  types 
— Paragenesis — Gold-quartz  veins  of  the  California  and  Victoria 
type — Principal  characteristics — Gold-quartz  veins  of  the  Sierra 
Nevada — The  gold-quartz  veins  of  the  interior  Cordilleran  region — 
Victoria,  Australia — New  South  Wales  and  Queensland — Nova 
Scotia — Gold-arsenopyrite  deposits — Gold-bearing  replacement 
deposits  in  limestone — Gold-bearing  replacement  deposits  in 
quartzite — Gold-bearing  replacement  deposits  in  porphyry — The 
Silver-lead  veins — General  features — Quartz-tetrahedrite-galena 
veins — Tetrahedrite-galena-siderite  veins  (Wood  river  type) — 
Galena- siderite  veins — Lead-silver  veins  with  calcite,  siderite,  and 
barite — Pyritic  galena-quartz  veins — The  silver-lead  replacement 
deposits  in  limestone — General  features — Park  City,  Utah — 
Tintic,  Utah — Aspen,  Colorado — Leadville,  Colorado — The  Lead- 
ville-Boulder  County  belt — The  tungsten  deposits  of  Boulder 
County — Summary — Deposits  with  native  silver — The  zeolitic  en- 
richments— The  silver-bearing  cobalt-nickel  veins  of  Saxony — The 
silver-bearing  cobalt-nickel  veins  of  Ontario,  Canada — Quartz-adu- 
laria-zeolite  veins  (.Alpine  type) — Occurrence  and  mineral  associa- 
tion— Origin — The  copper  veins — Chalcopyrite-quartz  veins — 
Bornite-quartz  veins — Pyrite-enargite  veins — The  pyritic  replace- 
ment deposits — Copper  deposits  of  Shasta  County,  California — 
The  pyritic  deposit  of  Mount  Lyell,  Tasmania — The  pyritic 
deposits  of  Rio  Tinto,  Spain — General  features — Geological  for- 
mations— The  ores— Genesis — The  pyritic  deposit  of  Rammelsberg, 
Germany — Geology  and  structural  features — The  ores — Origin — 
Cadmium  ores — Arsenic  deposits — Fluorite  deposits — Siderite 
deposits. 


xvi  CONTENTS 

CHAPTER  XXVI 

PAGE 

VEINS  AND  REPLACEMENT  DEPOSITS  FORMED  AT  HIGH  TEMPERATURE 
AND  PRESSURE  AND  IN  GENETIC  CONNECTION  WITH  INTRUSIVE 

ROCKS •  .    .    .    .  651 

General  features — High-temperature  minerals — Metasomatic  proc- 
esses— Temperature  and  pressure — Classes  of  deposits — Mode  of 
fissuring  and  filling — The  cassiterite  veins — Mineral  association — 
Metasomatic  processes — General  features — Metasomatic  proc- 
esses in  the  deposits  of  Cornwall — Development  of  greisen — 
Alteration  of  sedimentary  rocks — Origin  of  tin-bearing  veins — 
The  cassiterite  veins  of  Cornwall,  England — Literature — Cassiter- 
ite veins  of  Saxony — Tin  deposits  in  other  countries — Wolframite 
veins — Gold-quartz  veins — The  veins  of  the  southern  Appala- 
chians— The  quartz  veins  of  Ontario — The  pre-Cambrian  gold 
veins  of  the  Cordilleran  region — The  gold-bearing  veins  of  Brazil — 
The  gold-quartz  deposits  of  Silver  Peak,  Nevada — The  gold-quartz 
veins  of  southeastern  Alaska — Metasomatic  processes  in  veins  of 
southeastern  Alaska — The  gold-telluride  veins  of  western  Aus- 
tralia— The  gold-copper  deposits — Copper  deposits — The  copper- 
tourmaline  deposits — Chile — United  States — The  copper-bearing 
veins  allied  to  contact-metamorphic  deposits  and  pegmatites — 
Copper-titanium  veins — Copper  molybdenum  veins — The  lead- 
silver-zinc  deposits — Veins  with  tourmaline — Veins  with  garnet — 
The  cobalt-tourmaline  veins. 

CHAPTER  XXVII 

DEPOSITS  FORMED  BY  PROCESSES  OF  IGNEOUS  METAMORPHISM  ....  704 
Introduction — General  features — History — Contact-metamorphism 
— General  features — Form  and  texture — Mineralogy — Intensity  of 
metamorphism — Influence  of  composition  of  igneous  rock — 
Alteration  of  the  intrusive  rock — Succession  of  events — Suc- 
cession of  minerals — Volume  relations — Mode  of  Transfer — Phys- 
ical conditions  at  the  contact — Depth  of  formation — Piezo-meta- 
morphism — Principal  types  of  contact-metamorphic  deposits — 
Magnetite  deposits — General  character — Foreign  occurrences — 
Fierro,  New  Mexico — Heroult,  California — Iron  Springs,  Utah — 
Cornwall,  Pennsylvania — Chalcopyrite  deposits — General  charac- 
ter— New  Mexico — Clifton,  Arizona — Bisbee,  Arizona — Silver 
Bell,  Arizona — Cananea,  Mexico— Bingham,  Utah — Ketchikan, 
Alaska — Zinc  and  lead  deposits — Gold  deposits — Gold-arseno- 
pyrite  type— Telluride  type— Cassiterite  deposits— Titanium 
deposits — Scheelite  deposits — Graphite — Properties — General  oc- 
currence and  origin — Occurrences — Production  and  uses — Garnet 
— Deposits  due  to  igneous  metasomatism  not  distinctly  related 
to  contacts— General  features— Boundary  district— Ducktown, 
Tennessee— Franklin  Furnace,  New  Jersey— Metasomatic  mag- 
netite deposits  of  Sweden— Magnetite  deposits  in  the  United  States. 


CONTENTS  xvii 

CHAPTER  XXVIII 

PAGE 

MINERAL  DEPOSITS  OP  THE  PEGMATITE  DIKES      760 

Introduction — Mineralizers  and  the  nature  of  their  action — 
Temperature  of  consolidation — Occurrence  and  general  character — 
Types  of  pegmatites — Acidic  pegmatites — Basic  pegmatites — Eco- 
nomic features  of  pegmatite  dikes — Feldspar  and  quartz — Mica- 
Oxide  ores — Wolframite — Columbite  and  tantalite — Yttrium, 
thorium,  cerium  minerals — Monazite  and  zircon — Apatite — 
Lithium  minerals — Cryolite — Precious  stones— Native  metals, 
sulphides  and  arsenides — Molybdenite. 

CHAPTER  XXIX 

'  I  INERAL  DEPOSITS  FORMED  BY  CONCENTRATION  IN  MOLTEN  MAGMAS  780 
Constitution  of  magmas  and  their  differentiation  and  consolida- 
tion— General  features — Constitution  of  magmas — Crystallization 
of  magmas — Differentiation  in  magmas — Principal  types  of  de- 
posits— Diamonds — Other  precious  stones — Platinum  and  palla- 
dium— Production  and  use — Iron  and  nickel — Chromite — Ilmenite 
or  titanic  iron  ore — General  features — Microstructure  of  ilmenite — 
Irregular  bodies — Dikes — Occurrences — Influence  of  pressure — 
Magnetite — The  iron  ores  of  northern  Sweden — The  magnetites 
of  the  Ural  mountains — The  magnetites  of  the  Adirondacks — 
Corundum — General  mode  of  occurrence — Corundum  in  igneous 
magnesian  rocks — Corundum  in  syenite — Other  occurrences — 
Production  in  the  United  States — Uses — Sulphide  ores  of  igneous 
origin — General  principles — Types  of  deposits — Sulphides  in  peri- 
dotites  and  gabbros — Sudbury,  Ontario — Cape  Colony — Bornite 
deposits — Injected  pyritic  deposits — General  features — Bavaria — 
Sweden — Norway. 

CHAPTER  XXX 

METAMORPHOSED  DEPOSITS      822 

Processes  involved — Deformed  pyritic  deposits — Regionally  meta- 
morphosed iron  ores — General  features — Swedish  "dry  ores" — 
Norwegian  ores — United  States — The  zinc  ores  of  Ammeberg, 
Sweden. 

CHAPTER  XXXI 

OXIDATION  OP  METALLIC  ORES 829 

General  conditions — Depth  of  oxidation — Outcrops — Nomen- 
clature— Principles  of  oxidation — Textures  and  criteria  of  the 
oxidized  zone — Textures  of  the  supergene  sulphide  zone — Solu- 
tion— Precipitation — Supergene  sulphides — Criteria  of  super- 
gene  sulphide  enrichment — Iron — Copper — Minerals — Solution 
and  precipitations — Supergene  copper  sulphides — Theory  of  super- 


xviii  CONTENTS 

PAGE 

gene  copper  sulphides — The  relation  of  chalcocite,  covellite  and 
bornite — Oxidation  of  chalcocite  zones— Examples  of  oxidation  of 
copper  deposits — General  features — Rio  Tinto — Mount  Morgan — 
Butte — Ely — Bingham — The  southwestern  chalcocite  deposits — 
Globe  —  Ray  —  Chuquicamata — Zinc — Minerals — Solubility  and 
mineral  development — Supergene  shoots  of  zinc  ore — Supergene  zinc 
sulphides — Lead — Minerals — Reactions  in  the  oxidized  zone — 
Supergene  sulphides— Oxidation  in  the  Coeur  d'Alene  district — 
Oxidation  in  the  Mississippi  valley  district — Gold — Examples  of 
oxidation  of  gold  deposits — Silver — Minerals — Solubility  and 
mineral  development — Precipitation — Supergene  sulphide  enrich- 
ment— Zones  of  supergene  deposition — Enrichment  at  Granite- 
Bimetallic  mine — Enrichment  at  Georgetown — Enrichment  at 
Tonopah — Enrichment  at  Chanarcillo — Other  metals — Platinum 
and  palladium — Mercury — Secondary  sulphides  of  quicksilver — 
Cadmium — Nickel  and  cobalt — Chromium — Manganese — Tin — 
Tungsten — Vanadium— Molybdenum — Bismuth — Arsenic  —  Anti- 
•*  mony  —  Mine  waters  —  Chloride  waters  —  Carbonate  waters — 
Sulphate  waters — Oxidation  of  pyrite — Examples. 

CHAPTER  XXXII 

METALLOGENETIC  EPOCHS 909 

Introduction — Main  epochs — Europe — Pre-Cambrian  epochs — 
Paleozoic  epochs — Hercynian  epochs — Permo-Triassic  epochs — 
Jurassic  and  Cretaceous  epochs — Tertiary  epochs — Asia — Africa — 
Australasia — South  America — Central  America — The  Antilles — • 
North  America — The  pre-Cambrian  epochs — Paleozoic  sedimentary 
epochs — Paleozoic  intrusives — Paleozoic  epochs  of  saline  deposits — 
Epochs  of  Triassic  copper  deposits — Cretaceous  and  later  periods 
of  lead  and  zinc  concentration — Tertiary  and  recent  periods  of 
rock  decay — The  pre-Cambrian  epochs — The  early  Mesozoic  epoch 
— The  late  Mesozoic  epochs — The  early  Tertiary  epoch — 
The  late  Tertiary  epoch — The  post- Pliocene  epoch — Cretaceous  or 
later  epochs  of  copper  concentration  in  sedimentary  rocks — 
Index  to  mineral  deposits  by  elements. 

INDEX .   929 


MINERAL  DEPOSITS 

CHAPTER  I 
INTRODUCTION 

ECONOMIC    GEOLOGY 

The  application  of  geology  to  the  practical  problems  of  the 
industries  and  the  arts  constitutes  economic  geology.  TTiis 
branch  of  the  science  includes  as  its  most  important  division  the 
study  of  deposits  of  useful  minerals,  but  it  also  teache?  the  oc- 
currence of  underground  waters,  explains  the  derivation  and 
constitution  of  soils  in  relation  to  agriculture,  and  applies  geo- 
logic principles  to  the  planning  of  important  engineering  works. 

Only  a  part  of  the  whole  field  of  economic  geology  will  be 
covered  in  these  chapters.  They  will  be  confined  to  a  descrip- 
tion, by  classes  and  type  examples,  of  the  occurrence,  structure, 
and  origin  of  the  principal  deposits  of  metallic  and  non-metallic 
minerals  of  economic  importance.  The  subjects  of  coals, 
mineral  oils,  and  structural  materials  could  not  be  included 
without  unduly  increasing  the  bulk  of  the  volume.  Little  space 
has  been  given  to  statistics,  while  the  problems  of  correlation 
and  origin  have  been  treated  rather  fully.  A  general  part  de- 
scribing principles  of  universal  application  precedes  the  detailed 
characterization  of  the  various  classes. 

A  complete  treatment  of  the  subject  should  include  discussions 
of  distribution,  occurrence,  structure,  origin,  production,  and 
valuation  of  deposits,  as  well  as  statements  of  the  uses  of  the 
materials  mined,  processes  of  mining  and  reduction,  and  criteria 
for  judging  the  value  of  the  products.  Such  a  complete  presen- 
tation will  not  be  found  in  this  volume,  for  it  is  believed  that  by 
examining  the  subject  from  a  scientific  rather  than  from  a  utilita- 
rian viewpoint,  the  student  will  obtain  a  clearer  insight  into 
the  geologic  relationship  of  the  various  deposits. 

1 


2  MINERAL  DEPOSITS 

Throughout  its  broad  domain  economic  geology  stands  on  the 
fundamental  sciences  of  chemistry  and  physics.  It  is  related 
on  one  side  to  theoretical  geology,  paleontology,  mineralogy, 
and  petrography;  on  the  other  side  to  mining,  metallurgy,  and 
many  other  technologic  arts.  A  student  who  tries  to  approach 
the  subject  without  the  necessary  knowledge  of  the  allied 
sciences  and  arts  is 'building  on  poor  foundations.  Even  with 
this  aid  the  study  offers  peculiar  difficulties.  The  alteration  of 
rocks  close  to  many  mineral  deposits  is  intense  and,  as  a  result, 
the  student  who  is  familiar  with  only  the  fresh,  unaltered  speci- 
mens finds  himself  in  the  midst  of  puzzling  and  strange  types  that 
he  is  unable  to  classify  with  certainty.  Altered  andesites  may 
assume  the  aspect  of  quartzites;  a  question  may  arise  as  to 
whether  a  silicified  rock  was  once  a  limestone  or  a  porphyry; 
diabases  may  at  some  places  be  converted  into  white  fine-grained 
calcite-sericite-quartz  rocks  and  at  other  places  appear  as  ag- 
gregates consisting  mainly  of  epidote  and  chlorite.  These 
examples  suffice  to  show  that  rock  alteration  is  a  subject  of 
prime  importance  for  the  mining  geologist. 


MINERAL  DEPOSITS 

Definitions. — The  outer  shell  of  the  globe  is  commonly  called 
the  earth's  crust.  Of  this  shell  only  the  most  superficial 
part  is  accessible.  The  radius  of  the  earth  is  about  4,000  miles. 
The  deepest  shaft  attains  only  about  6,000  feet,1  the  deepest 
bore-hole  7, 350  feet.  This  crust  consists  of  manifold  mineral 
aggregates  formed  at  different  times  and  in  various  ways.  Each 
individualized  mass  of  mineral  aggregates— such  as  a  stratum, 
a  lava  flow,  an  intrusive  mass  of  igneous  rock,  a  dike,  a  vein,' 
or  a  lenticular  mass— is  called  a  "formation,"  a  "member,"  or  in 
general  a  "geologic  body."  Geologic  bodies  which  consist 
mainly  of  a  single  useful  mineral— for  instance,  beds  of  pure 
gypsum  or  coal— or  which  contain,  throughout  or  in  places, 
valuable  minerals  that  can  be  profitably  extracted— for  in- 
stance, veins  containing  disseminated  gold— are  called  "mineral 
deposits."  Geologic  bodies  that  are  not  worked  for  any  particu- 
lar mineral  or  minerals,  but  for  the  aggregate  of  minerals— the 
rock  itself— are  usually  designated  as  deposits  of  the  particular 

1  The  gold  mine  of  Morro  Velho,  Brazil,  p.  190. 


INTRODUCTION  3 

rock.  Thus  a  mass  of  roofing  slate  is  not  spoken  of  as  a  mineral 
deposit,  but  as  a  slate  deposit.  Economic  geology  treats  of  the 
occurrence,  composition,  structure,  and  origin  of  those  geologic 
bodies  which  can  be  technically  utilized;  it  shows  where  they 
may  be  searched  for  and  how  their  value  may  be  ascertained.1 
Technical  Utility. — The  limitation  of  technical  utility  must  of 
course  not  be  taken  too  literally,  especially  where  questions  of 
origin  are  concerned,  for  here,  as  in  many  other  phases  of  the 
subject,  applied  geology  merges  into  theoretic  geology.  More- 
over, it  is  no  uncommon  occurrence  that  the  useless  of  yesterday 
becomes  the  useful  of  to-day.  Examples  are  easily  cited.  About 
1900  the  cupriferous  monzonite  of  Bingham,  Utah,  which 
yields  an  average  of  30  cents  in  gold  and  14  cents  in  silver  to 
the  ton  and  1.5  per  cent,  of  copper,  would  probably  not  have 
been  classed  as  an  ore,  but  with  modern  methods  of  treatment  it 
is  an  important  ore  of  copper.  The  zinc  minerals  of  the  western 
States,  valueless  and  even  causing  loss  in  the  marketing  of  ores, 
can  now  be  profitably  sold.  The  tungsten  ores  of  Colorado, 
thrown  over  the  dump  not  long  ago,  have  attained  a  value 
of  $200  a  ton.  Low-grade  gold  ores — for  instance,  those  of 
Mercur,  Utah — considered  as  hopelessly  refractory  before  1890, 
became  rich  assets  with  the  introduction  of  the  cyanide  process. 
Many  iron  ores  rich  in  phosphorus  were  neglected  until  the 
Thomas  process  provided  means  for  their  profitable  reduction. 
Monazite  containing  thorium  acquired  importance  with  the 
invention  of  the  incandescent  mantle,  for  gas  burners.  New 
processes  of  reduction,  the  rising  price  of  some  commodity, 
inventions  calling  for  rare  and  unused  metals — any  of  these  may 
suddenly  cause  a  geologic  body  that  has  previously  been^  valueless 
to  become  of  great  importance.  Titanic  iron  ores  form  vast  de- 
posits which  are  now  useless  because  of  metallurgical  difficulties 
but  which  some  day  will,  no  doubt,  be  utilized.  This  principle 
also  works  the  other  way.  Decreasing  prices  may  make  a 
particular  deposit  unprofitable;  that  is  what  happened  to  many 
silver  mines  during  the  great  decline  in  the  price  of  silver  which 
began  in  1880.  Great  changes,  mainly  in  the  direction  of  rising 
prices  have  been  brought  about  by  the  great  war  beginning  in 
1914.  A  large  number  of  metals  have  doubled  in  price:  They 
include  silver,  platinum,  copper,  lead,  zinc,  tin,  antimony  and 

1  Stelzner  and  Bergeat,  Die  Erzlagerstatten,  vol.  1,  1904,  p.  1. 


4  MINERAL  DEPOSITS 

aluminum.     Gold  alone,   being  the  standard  by  which  other 
values  are  measured,  remains  stable. 

Ore  and  Gangue. — These  considerations  bring  us  to  the  terms 
ore  and  gangue.  "Ore"  is  a  word  which  has  been  used  in 
several  meanings.  An  "ore  mineral"  is  a  mineral  which  may  be 
used  for  the  extraction  of  one  or  more  metals.  An  "ore,  "as 
the  term  is  used  here,  is  that  part  of  a  geologic  body  from  which 
the  metal  or  metals  that  it  contains  may  be  extracted  profita- 
bly. Thus  galena  and  malachite  are  ore  minerals.  An  ore 
is  practically  always  a  mixture  of  minerals.  Local  usage  has 
adopted  several  terms  as  substitutes  for  "ore."  In  the  lead- 
zinc  district  of  Missouri  crude  ore  is  called  "dirt,"  while  con- 
centrates are  called  "ore."  In  Michigan  the  ore  is  called 
"rock"  and  the  concentrates  are  termed  "mineral."  Gold-bear- 
ing gravels  are  not  usually  referred  to  as  ore.  The  use  of  the 
term  "  ore  "  is  not  quite  consistent.  Ordinarily  it  implies  a  metal, 
but  the  expression  "sulphur  ore,"  meaning  py rite,  is  sometimes 
seen, 'and  occasionally  such  terms  as  "sapphire  ore"  are  found. 
The  useless  minerals  occurring  in  the  ore  are  termed  "gangue." 
Thus,  a  gold  ore  may  consist  of  quartz,  calcite,  siderite,  native 
gold,  auriferous  pyrite,  and  galena.  Here  the  first  three  are 
called  "gangue  minerals."  The  terms  are  not  inflexible;  for 
example,  siderite  may  under  some  circumstances  be  utilized  as 
an  iron  ore.  Moreover,  as  stated  above,  what  to-day  is  useless 
gangue  may  prove  valuable  ore  to-morrow.  It  is  therefore 
safe  to  make  the  definition  of  an  ore  rather  wider  than  the 
present  technical  limits.1 

It  is  hardly  necessary  to  call  attention  to  the  differences  in 
prices  of  metals  which  cause  wide  disparity  in  the  amounts  of 
different  metals  necessary  to  constitute  ores.  An  iron  ore  must 
ordinarily  contain  at  least  30  per  cent,  of  iron — usually  much 
more.  A  volcanic  rock  containing  15  per  cent,  of  iron  is  far 
from  being  an  iron  ore,  but  quartz  containing  0.05  per  cent,  of 
gold  is  a  rich  gold  ore,  worth  $330  a  metric  ton;  in  fact,  is  little 
as  0.0001  per  cent,  of  gold,  equivalent  to  1  gram  to  the  metric 
ton,  or  a  value  of  66  cents  a  ton,  if  occurring  in  an  ore  with  other 
useful  substances,  is  ordinarily  paid  for  by  smelting  works. 

1  For  a  full  discussion  of  the  subject  see  J.  F.  Kemp,  "What  is  an  Ore?" 
Jour.  Canadian  Min.  Inst.,  vol.  12,  1910,  pp.  356-367.  Also,  Min.  and  Sci. 
Press,  March  20,  1909. 


INTRODUCTION  5 

DISTRIBUTION   OF  THE  ELEMENTS 

To  obtain  data  regarding  the  relative  distribution  of  the  ele- 
ments, several  calculations  have  been  undertaken  on  the  basis 
of  a  great  number  of  reliable  rock  analyses.  Especially  notable 
are  the  papers  of  Clarke,1  Vogt,2  and  Washington.3  Clarke  used 
1,200  analyses  of  American  rocks;  Washington  1,800  from 
various  parts  of  the  world.  We  are  here  chiefly  concerned  with 
the  solid  crust  of  the  earth,  although  in  passing^it  is  deserving 
of  notice  that  enormous  quantities  of  salts  are  dissolved  in  the 
sea  water,  among  them  sodium  chloride,  sulphates  of  calcium, 
magnesium,  and  potassium,  and  carbonates  of  calcium  and  mag- 
nesium. The  volume  of  salts  in  the  sea  water,  according  to 
Clarke,  would  be  enough  to  cover  the  entire  area  of  the  United 
States  (exclusive  of  Alaska)  to  a  depth  of  1.6  miles,  or  the 
whole  globe  with  a  stratum  of  sodium  chloride  112  ft.  deep.  Ac- 
cording to  the  same  authority,  the  crust  of  the  globe  10  miles 
thick,  with  an  assumed  average  specific  gravity  of  2.5,  contains 
about  93  per  cent,  of  solid  matter  and  7  per  cent,  of  sea  water. 

Composition  of  the  Earth's  Crust. — In  calculating  the  average 
composition  of  the  accessible  portion  of  the  solid  crust  it  is  nec- 
essary to  consider  the  sedimentary  and  the  igneous  rocks.  The 
sedimentary  rocks  form  but  a  thin  veneer  compared  with  the 
igneous  rocks.  The  average  composition  of  the  latter  closely 
approximates  that  of  the  crust.  Clarke  calculates  that  the  crust 
to  a  depth  of  10  miles  is  composed  of  95  per  cent,  of  igneous 
rocks,  4  per  cent,  of  shales,  0.75  per  cent,  of  sandstones,  and 
0.25  per  cent,  of  limestones.  Van  Hise  and  others  arrive  at 
somewhat  different  figures.  The  sediments  average  poorer  in 
calcium,  magnesium,  and  especially  in  sodium  than  the  igneous 
rocks  and  thus  show  the  effect  of  leaching.  They  also  contain 
more  potash  and  carbon  dioxide,  but  on  the  whole  they  are 
similar  in  composition  to  the  igneous  rocks.  According  to 
Clarke  the  average  of  analyses  of  igneous  rocks  made  in  the 
laboratories  of  the  United  States  Geological  Survey  is  as  follows : 

1  F.  W.  Clarke,  Geochemistry:  Bull.  616,  U.  S.  Geol.  Survey,  1916,  pp. 
22-35.     Many  partial  analyses  are  also  included. 

2  J.  H.  L.  Vogt,  Ueber  die  relative  Verbreitung  der  Elemente,  etc. :  Zeit- 
schr.  prakt.  Geol,  1898,  pp.  225-238;  314-325. 

3H.  S.  Washington,  The  distribution  of  the  elements  in  igneous  rocks: 
Trans.  Am.  Inst.  Min.  Eng.,  vol.  39,  1908,  pp.  809-838. 


MINERAL  DEPOSITS 

AVERAGE  ANALYSIS  OF  IGNEOUS  ROCKS 


o  

47.29 

S  

0.103 

Si  

28.02 

Cl  

0.063 

Al  

7.96 

F  

0.10 

Fe  

4.56 

Ba  

0.092 

Mg  

2.29 

Sr  

0.033 

Ca  

3.47 

Mn  

0.078 

Na  

2.50 

Ni  

0.020 

K  

2.47 

Cr  

0.033 

H  

0.16 

V  

0.017 

Ti  

0.46 

Li  

0.004 

Zr  

0.017 



C  

0.13 

Total  

100.000 

P  

0.13 

The  eight  elements  first  named  above  make  up  98.56  per  cent, 
of  the  igneous  rocks. 

Among  the  six  principal  metals  shown  in  the  average  composi- 
tion only  iron,  magnesium  and  aluminum  are  of  economic  import- 
ance. The  lighter  elements  predominate,  the  atomic  weight  of 
each  falling  below  56  (Fe  55.9).  In  the  average  composition,  the 
rarer  metals  titanium,  zirconium,  barium,  strontium,  manganese, 
nickel,  chromium,  and  vanadium  are  represented,  but  except 
titanium,  which  amounts  to  0.45  per  cent.,  all  these  metals 
average  below  0.1  per  cent.  Platinum,  gold,  silver,  copper,  lead, 
zinc,  antimony,  arsenic,  tin,  quicksilver,  molybdenum,  tungsten, 
and  others  are  present  in  amounts  less  than  0.01  per  cent. 

For  some  of  these  more  definite  estimates  have  been  made  by 
Clarke  and  Steiger1  from  careful  analyses  of  large,  composite 
samples  of  rocks  and  clays.  The  average  percentages  are  as 
follows:  CuO,  0.0130;  ZnO,  0.0049;  PbO,  0.0022;  As205,  0.0005. 
These  figures  considered  as  orders  of  magnitude  have  a  high 
degree  of  probability;  possibly  they  are  a  trifle  too  high.  Silver 
may  constitute  0.00001  and  gold  perhaps  0.0000005  per  cent, 
of  the  crust. 

The  percentages  of  the  useful  metals  given  above  do  not  by 
any  means  indicate  the  amount  available  for  industrial  use. 
That  amount  indeed  is  so  infinitesimal  in  relation  to  the  volume 
of  the  crust  that  it  can  not  be  expressed  on  the  basis  of  per- 
centages. The  metals  in  the  deposits  of  useful  minerals  then 
comprise  only  a  minute  fraction  of  the  quantity  of  metals  in  the 

1  Jour.,  Washington  Acad.  of  Sci.,  vol.  4,  1914,  p.  57. 


INTRODUCTION  7 

crust — a  fraction  which  has  been  locally  accumulated  by  this  or 
that  process  of  concentration. 

In  general  igneous  rocks  contain  more  of  the  heavy  metals 
than  do  the  sedimentary  rocks.  We  are  well  justified  in  regard- 
ing the  former  as  the  original  source  of  these  metals.  Dissipa- 
tion by  solution  accompanies  sedimentation  and  the  many 
metals  found  in  traces  in  the  sea  water  furnish  evidence  of  this. 
On  the  other  hand,  it  is  true  that  certain  kinds  of  sedimentation 
will  result  in  a  concentration  of  metals,  such  as  iron,  zinc,  cobalt, 
nickel  and  vanadium. 

Vogt  and  Washington  have  also  formulated  some  rules  con- 
cerning the  relationship  of  certain  metals  with  certain  rocks. 
It  is  obvious,  however,  that  a  distinction  should  be  made  as  to 
whether  the  metal  is  an  integral  part  of  the  rock  or  whether 
simply  deposits  of  the  metal  occur  in  the  rock.  Thus,  for 
instance,  lead  deposits  are  characteristic  of  many  limestones, 
but  it  may  be  doubted  whether  lead  is  a  primary  constituent  of 
limestone;  it  is  present  because  the  rock  had  the  power  of  pre- 
cipitating the  metal  from  its  solution. 

In  highly  siliceous  rocks,  especially  in  granites  and  in  the 
pegmatite  dikes  accompanying  them,  we  find  minerals  contain- 
ing fluorine,  boron,  lithium,  zirconium,  tin,  tungsten,  tantalum, 
molybdenum,  thorium,  and  beryllium.  Highly  sodic  magmas 
are  also  accompanied  by  a  great  number  of  rare  metals. 

On  the  other  hand,  basic  rocks  in  which  the  darker  con- 
stituents predominate  contain  phosphorus,  sulphur,  chlorine, 
copper,  chromium,  nickel,  cobalt,  titanium  and  vanadium,  and 
some  of  them,  chiefly  peridotites,  contain  platinum  and  diamonds. 
Gold  and  silver  exist  in  minute  quantities  in  many  rocks,  particu- 
larly in  those  of  acidic  types,  like  granite  and  rhyolite. 

These  rarer  metals  are  not  everywhere  present  in  similar 
rocks.  Platinum,  for  instance,  is  contained  in  the  peridotites  of 
the  Ural  Mountains,  but  the  peridotites  of  the  Sierra  Nevada 
are  poor  in  that  metal  and  the  similar  rocks  in  the  Coast  Ranges 
of  California  contain  little  platinum,  as  do  the  serpentines  of  Asia 
Minor  and  of  Italy.  Similar  conditions  characterize  the  occur- 
rence of  rarer  metals  in  granitic  rocks. 

TRACES  OF  METALS  IN  ROCKS 

General  Statement. — In  order  to  formulate  a  theory  or  a 
hypothesis  of  the  origin  of  mineral  deposits  it  is  most  desirable 


8  MINERAL  DEPOSITS 

to  ascertain  to  what  extent  the  different  rocks  contain  the  rarer 
metals. 

J.  G.  Forchhammer  and  L.  Dieulafait  began  examinations 
for  this  purpose  about  1860  and  found  traces  of  silver,  copper, 
lead,  bismuth,  nickel,  cobalt,  zinc,  arsenic,  antimony,  and  tin  in 
many  rocks.  Somewhat  later  F.  von  Sandberger  followed  up 
this  line  of  investigation  and  ascertained  that  the  dark  silicates 
of  many  rocks  contained  lead,  copper,  tin,  antimony,  arsenic, 
nickel,  cobalt,  bismuth,  and  silver.  Some  doubt  has  been 
expressed  as  to  a  few  of  these  results  and  it  is  believed  that  in 
some  of  Sandberger's  specimens  the  metals  were  derived  from 
adjacent  veins  or  from  the  reagents  or  the  vessels  used  in  the 
analyses.  However,  in  spite  of  analytical  difficulties,  the  pres- 
ence of  many  of  those  metals  in  various  igneous  and  metamorphic 
rocks  is  clearly  proved.  Many  ordinary  analyses  show  the  pres- 
ence of  chromium,  cobalt,  and  nickel  in  basic  rocks  like  perido- 
tites,  serpentines,  and  pyroxenites.  Some  of  these  rocks  con- 
tain as  much  as  0.76  per  cent,  of  Cr203  and  up  to  0.3  per  cent, 
of  (Ni,Co)0.  Traces  of  nickel  and  cobalt  are  often  found  in 
diabases,  gabbros,  and  basalts;  occasionally  in  diorites.  A 
little  vanadium  is  common  in  all  rocks — usually  only  0.01  to 
0.05  per  cent  of  V2O3. 

In  the  following  paragraphs  some  of  the  most  reliable  data 
regarding  traces  of  rarer  metals  are  compiled.  More  extensive 
references  will  be  found  in  Clarke's  "Data  of  geochemistry." 

Copper. — A.  C.  Lane1  states  that  the  Keweenawan  ''traps" 
average  0.02  per  cent,  of  copper.  F.  F.  Grout2  found  0.029  and 
0.02  per  cent,  in  fresh  specimens  of  the  same  series  from  Minne- 
sota. J.  Volney  Lewis3  says  that  some  of  the  New  Jersey 
diabases  or  "traps"  contain  chalcopyrite  and  that  copper  is  also 
present  in  the  pyroxene  of  these  rocks.  The  average,  according 
to  numerous  analyses,  is  0.025  per  cent,  of  CuO.  R.  C.  Wells 
found  0.03  per  cent,  of  copper  in  a  perfectly  fresh  basaltic  lava 
from  The  Dalles,  Oregon. 

Analyses  made  for  W.  H.  Weed  in  the  laboratory  of  the  United 
States  Geological  Survey  show  that  Butte  granite  or  quartz 
monzonite  from  a  quarry  near  Walkerville,  Montana,  contains 
0.006  per  cent,  of  copper.  The  quartz  and  feldspar,  forming 

1  Mine  waters,  Proc.,  Lake  Superior  Min.  Inst.,  June,  1908,  p.  86 

2  Econ.  Geol,  vol.  5,  1910,  p.  471. 

1  Econ.  Geol,  vol.  2,  1907,  pp.  242-257. 


INTRODUCTION  9 

91  per  cent,  of  the  rock,  contain  little  or  no  copper;  the  mica 
and  hornblende,  which  constitute  7  per  cent,  of  the  rock,  yield 
0.047  per  cent,  of  copper;  there  is  thus  nearly  eight  times  as 
much  copper  in  the  ferromagnesian  minerals  as  in  the  rock. 
The  altered  rock  surrounding  the  veins  carries  more  copper  than 
the  fresh  rock. 

E.  T.  Allen  examined  18  samples  of  fresh  gabbros  and  diorites 
from  the  Encampment  district,  Wyoming,  and  found  copper 
in  all,  the  largest  quantity  noted  being  0.02  per  cent,  of  CuO.1 

A  composite  sample  of  seventy-one  Hawaiian  lavas  yielded 
Geo.  Steiger  0.0155  per  cent,  of  copper.  H.  I.  Jenssen  found 
0.034  per  cent,  of  copper  in  an  andesite  from  Fiji.2  E.  Coman- 
ducci  reported  0.0854  per  cent,  of  CuO  and  0.0038  per  cent,  of 
CoO  in  volcanic  ash  from  Vesuvius.3 

J.  B.  Harrison4  examined  36  igneous  and  metamorphic  rocks 
from  British  Guiana  and  found  that  6  contained  no  copper  and 
12  contained  copper  in  traces  only;  those  carrying  most  copper 
were  diabases  and  porphyrites;  a  feldspathic  tuff  yielded  0.13 
per  cent.  The  average  for  the  series  was  0.025  per  cent,  of  copper. 
In  some  of  these  rocks  the  copper  may  have  been  contained  in 
secondary  disseminated  sulphides. 

In  a  fresh  granodiorite  from  Steamboat  Springs,  Nevada, 
W.  H.  Melville8  detected  copper,  lead,  arsenic,  and  antimony. 
J.  D.  Robertson6  found  from  0.0024  to  0.0104  per  cent,  of  copper 
in  granite,  porphyry,  and  diabase  from  the  Archean  of  St. 
Francis  Mountain,  in  Missouri.  The  average  was  0.006  per  cent. 
Lead  and  zinc  were  also  recognized.  The  adjacent  Silurian 
and  Carboniferous  limestones  also  contained  these  metals,  but 
in  smaller  quantities. 

Native  copper  in  minute  scales  is  rather  common  in  shales  and 
copper  sulphides,  contemporaneous  with  the  metamorphism, 
occur  in  many  amphibolites.  Copper  has  been  repeatedly  de- 
tected in  sea  water  and  is  contained  in  the  red  and  blue  mud 
dredged  from  the  deep  seas. 

1  A.  C.  Spencer,  Prof.  Paper  25,  U.  S.  Geol.  Survey,  1904,  p.  49. 

2  Chem.  News,  vol.  96,  1907,  p.  245. 

3  Gazz.  chim.  ital.,  vol.  36,  pt.  2,  1906,  p.  797. 

4  Report   on  the   petrography  of  the   Cuyuni  and   Mazaruni  districts. 
Georgetown,  Demerara,  1905. 

5  Mon.  13,  U.  S.  Geol.  Survey,  1888,  p.  350. 

6  Missouri  Geol.  Survey,  vol.  7,  1894,  pp.  479-481. 


10  MINERAL  DEPOSITS 

From  this  evidence  the  conclusion  may  be  drawn  that  prob- 
ably all  igneous  rocks  contain  appreciable  amounts  of  copper 
and]that  acidic  rocks  contain  less  than  basic  rocks.  The  copper 
is  largely  associated  with  the  ferromagnesian  silicates,  and  in 
the  lavas  at  least  it  appears  to  be  present  as  a  silicate. 

Lead  and  Zinc. —  In  analyzing  the  quartzose  porphyries  of 
Leadville,  Colorado,  supposed  to  be  free  from  sulphides,  W.  F. 
Hillebrand1  found  that  of  18  carefully  selected  samples  15  con- 
tained lead,  the  richest  carrying  0.0064  per  cent,  of  PbO;  the 
average  was  0.002  per  cent.  The  same  analyst  found  0.008 
and  0.0043  per  cent,  of  ZnO  in  similar  rocks. 

J.  D.  Robertson2  determined  an  average  of  0.004  per  cent,  of 
lead  and  0.009  per  cent,  of  zinc  in  the  Archean  rocks  from  Missouri 
mentioned  above.  J.  B.  Weems3  determined  lead  and  zinc  in  the 
limestones  and  dolomites  of  the  Dubuque  region,  Iowa.  The 
average  of  9  samples  gave  0.00326  per  cent,  of  lead  and  0.00029 
per  cent,  of  zinc.  J.  B.  Harrison  looked  for  lead  in  23  samples  of 
rocks  from  British  Guiana  and  was  able  to  determine  the  metal 
in  five;  the  maximum  obtained  was  0.02  per  cent.  L.  Dieulafait 
detected  zinc  in  hundreds  of  samples  of  Jurassic  limestone  from 
central  France. 

On  the  other  hand,  W.  F.  Hillebrand4  was  unable  to  detect 
lead  or  zinc  in  samples  of  limestone  from  Mexico,  near  important 
lead  deposits.  Henry  W.  Nichols5  found  no  lead,  copper,  or 
zinc  in  calcareous  concretionary  deposits  of  the  Challenger  Banks, 
near  Bermuda.  Zinc  is  reported  by  Dieulafait  in  sea  water  and  in 
ashes  of  sea  weeds.  Lead  apparently  does  not  exist  in  the  water 
of  the  ocean. 

Gold  and  Silver. — A  large  number  of  experiments  have  been 
undertaken  to  decide  the  question  whether  fresh  igneous  or  sedi- 
mentary rocks  contain  gold  and  silver.  In  mining  districts  where 
the  solution  of  this  problem  has  been  frequently  attempted  it  is 
difficult  to  obtain  perfectly  satisfactory  samples,  free  from  con- 
tamination by  circulating  water.  Furthermore,  contamination 
is  possible  from  fluxes,  from  the  dust  of  assay  rooms,  from  mortars, 
or  from  bucking  boards.  The  mere  statement  that  the  assay 

1  Mon.  12,  U.  S.  Geol.  Survey,  1886,  pp.  591-594. 

2  Missouri  Geol.  Survey,  vol.  7,  1894,  pp.  479-481. 

3  Iowa  Geol.  Survey,  vol.  10,  1900,  p.  566. 

4  Oral  information. 

6  Econ.  Geol.,  vol.  2,  1907,  p.  309. 


INTRODUCTION  11 

indicates  gold  and  silver  in  a  rock  is  not  sufficient.  It  must  be 
corroborated  by  a  statement  of  the  methods  used  and  accom- 
panied by  the  evidence  of  microscopic  examination  as  to  the 
freshness  of  the  rock. 

It  is  satisfactorily  proved  that  many  fresh,  massive  igneous 
rocks  contain  gold.  The  best  evidence  thus  far  brought  forward 
is  probably  that  afforded  by  the  granite  from  the  Altar  district, 
Sonora,  Mexico,  described  by  G.  P.  Merrill.1  The  gold  occurs 
embedded  in  fresh  quartz  and  feldspar.  W.  Moricke2  found  gold 
in  a  pitchstone  from  Chile  and  believed  the  metal  to  be  primary. 
Gold  was  found  by  W.  F.  Ferrier  in  a  fresh  syenite  from 
Kamloops,  British  Columbia.  According  to  R.  W.  Brock3 
probably  primary  gold  was  found  in  a  porphyry  dike  on  North 
Fork  of  Salmon  River,  West  Kootenai,  British  Columbia.  Of 
twelve  dikes  of  porphyry  at  different  points  in  West  Kootenai, 
six  contained  gold,  most  of  them  being  wholly  unaltered.  Brock 
also  states  that  a  sample  of  alkali  syenite  porphyry  in  the  Valkyr 
Mountains,  east  of  Lower  Arrow  Lake,  British  Columbia,  con- 
tained gold  that  was  visible  to  the  naked  eye. 

Many  of  the  statements  of  this  sort  in  the  literature  must  be 
critically  scanned,  for  it  is  not  uncommon  to  find  gold  deposited 
by  mineralizing  solutions  in  massive  rocks,  especially  in  schists, 
under  circumstances  closely  simulating  original  deposition. 
Statements  regarding  primary  gold  in  the  chloritic  schists  of 
the  Sierra  Nevada  refer  to  occurrences  of  this  class. 

It  is  frequently  said  that  gold  occurs  in  pegmatite,  but  few  of 
the  assertions  have  the  requisite  backing  of  complete  evidence. 
The  probability  is  strong,  however,  that  gold  is  present  in  such 
dikes.  One  of  the  most  definite  descriptions  of  this  mode  of 
occurrence  is  furnished  by  J.  Catharinet.4  Sperrylite,  an  ar- 
senide of  platinum,  is  stated  by  Catharinet  to  occur  with  the 
gold.  Another  case  is  reported  by  C.  De  Kalb5  from  Mohave, 
California.  References  in  the  literature  to  primary  gold  in  rocks 
from  the  Ural  Mountains  and  from  Australia  do  not  appear  to 
be  sufficiently  substantiated. 

Having  procured  samples  of  various  rocks,  most  of  them  far 

1  Am.  Jour.  Sci.,  4th  ser.,  vol.  1,  1896,  p.  309. 

*  Min.  pet.  Mitt.,  vol.  12,  1891,  p.  195. 

3  Eng.  and  Min.  Jour.,  vol.  77,  March  31,  1904. 

*  Eng.  and  Min.  Jour.,  vol.  79,  1905,  p.  127. 

*  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  38,  1908,  p.  312. 


12 


MINERAL  DEPOSITS 


from  mining  districts,  Luther  Wagoner,1  of  San  Francisco, 
assayed  them  with  the  results  as  given  in  the  accompanying 
table.  His  method  consisted  in  cyanide  treatment  of  40  or  50 
grams  of  material  followed  by  blowpipe  cupellation.  All  par- 
ticulars of  the  operations  are  detailed. 

Wagoner  found  that  the  purest  obtainable  reagents,  such  as 
soda,  borax,  and  cyanide  of  potassium,  contain  gold  and  silver. 
A  sample  of  Merck's  "C.  P."  carbonate  of  soda  contained  3 
grams  of  silver  to  the  ton.  A  sample  of  cyanide  of  potassium 
yielded  147  milligrams  of  gold  and  26.05  milligrams  of  silver  to 
the  metric  ton.  Wagoner's  results  merit  attention,  but  it 
would  be  desirable  to  have  them  checked.  His  figures  for 
silver  seem  high.  It  will  be  noted  that  gold  values  obtained 
by  him  are  as  low  as  5  milligrams,  or  3<i3  of  a  grain,  to  the 
ton.  J.  R.  Don,  using  the  fire  assay,  was  able  to  detect  amounts 
as  low  as  6.4  milligrams  per  metric  ton. 


GOLD  AND  SILVER  CONTENTS  OF  VARIOUS  ROCKS 
[Milligrams  per  metric  ton] 


Rock  and  locality 

Gold 

Silver 

Granite,  Lake  Tenaya,  California  

104 

7,660 

Granite,  Lake  Tenaya,  California  

137 

1,220 

Granite,  head  of  American  River,  California  

115 

940 

Syenite,  Candelaria,  Nevada  

720 

15,430 

Granite,  Candelaria,  Nevada  

1,130 

5,590 

Sandstone,  Colusa  County,  California  

39 

540 

Sandstone,  Angel  Island,  California 

24 

450 

Sandstone,  Russian  Hill,  San  Francisco,  California  . 

21 

320 

Basalt,  Petaluma,  California  

26 

547 

Diabase,  Mariposa  County,  California  

76 

7,440 

Marble,  Tuolumne,  California. 

5 

212 

Marble,  Carrara,  Italy  

8.63 

201 

Sea  water  is  known  to  contain  small  quantities  of  gold  and 
silver.  To  illustrate  the  difficulty  and  uncertainty  attending 
the  measurement  of  minute  traces  of  metals  the  following  results, 
reached  by  several  chemists,  are  given: 

1  Trans.,  Am.  Inat.  Min.  Eng.,  vol.  31,  1901,  pp.  798-810. 


INTRODUCTION  13 

GOLD  AND  SILVER  IN  SEA  WATER 
[Milligrams  per  metric  ton] 


Chemist                                        Gold 

Silver 

Miinster  (Norway)     5-6 

19-20 

65-130 

Don  (New  Zealand)  4.5 
Wagoner  (San  Francisco)  12-16 
Malaguti  and  Durocher  None. 

None. 
1500-1900 
9 

Wagoner  found  457  milligrams  of  gold  and  54.4  grams  of 
silver  to  the  metric  ton  of  salt  evaporated  from  sea  water,  and 
later  reported  both  metals  in  appreciable  quantities  in  deep-sea 
dredgings.1 

J.  R.  Don2  assayed  a  large  number  of  rocks  both  close  to  and 
at  a  distance  from  ore  deposits,  to  ascertain  their  content  of 
gold  and  silver.  His  work  indicated  in  general  that  gold  and 
silver  are  contained  only  in  rocks  that  have  been  impregnated 
with  py rite  in  the  vicinity  of  ore  deposits.  These  results,  how- 
ever, do  not  agree  with  those  of  other  analysts  and  are  especially 
contradicted  by  Wagoner's  experiments.  Don's  conclusions, 
broadly  speaking,  are  probably  true,  but  his  methods  largely 
excluded  detection  of  the  possible  occurrence  of  gold  in  the 
silicates  of  the  rocks. 

A  few  evidently  authentic  occurrences  of  primary  gold  in 
crystalline  schists  are  known.  Spurr3  describes  an  auriferous 
quartz  diorite  gneiss  from  the  Ayrshire  mine,  Lomagundi, 
Mashonaland,  Africa.  The  gneiss  forms  a  dike  20  feet  in  width, 
of  which  15  feet  is  mined;  the  average  gold  content  is  said  to  be 
about  $15  a  ton.  The  gold  is  intergrown  with  orthoclase, 
plagioclase,  quartz,  epidote,  and  hornblende;  it  is  especially 
associated  with  the  bands  of  hornblende  and  is,  according  to 
Spurr,  unquestionably  of  primary  origin — that  is,  it  crystallized 
during  the  metamorphism  with  the  other  constituents  of  the 
gneiss.  This  does  not,  however,  absolutely  prove  that  it  was  a 

1  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  .38,  1907,  p.  704. 

2  The  genesis  of  certain  auriferous  lodes:  Trans.,  Am.  Inst.  Min.  Eng., 
vol.  27,  1898,  p.  564. 

3  J.  E.  Spurr,  Native  gold  original  in  metamorphic  gneisses:  Eng.  and 
Min.  Jour.,  vol.  76,  1903,  p.  500. 


14  MINERAL  DEPOSITS 

constituent  of  the  igneous  rock  from  which  the  gneiss  was 
derived. 

Lacroix1  reports  gold  in  extremely  abundant  and  minute 
crystals  in  a  biotite  gneiss  from  Madagascar;  also  in  a  magnetite- 
bearing  quartzite  from  the  same  island.  F.  C.  Lincoln  has 
recently  given  a  comprehensive  review  of  the  data  regarding 
gold  in  rocks.2 

Neither  primary  native  silver  nor  visible  silver  compounds 
are  reported  to  occur  in  igneous  or  metamorphic  rocks.  W.  F. 
Hillebrand  found  silver  in  a  number  of  porphyries  from  Lead- 
ville,  Colorado,  that  were  collected  and  assayed  with  the  greatest 
care;  the  average  amount  was  0.0265  ounce  to  the  ton  (884 
milligrams  to  the  metric  ton).3  Traces  of  gold  were  rarely 
found. 

J.  W.  Mallet4  found  silver  in  two  samples  of  volcanic  ash 
from  the  Andes,  to  the  amount  of  about  10  grams  to  the  metric 
ton. 

TENOR  OF  ORES 

While  it  is  not  possible  to  give  exact  data  as  to  the  minimum 
values  which  ores  of  the  different  metals  should  have  for  profit- 
able extraction,  some  approximate  statements  may  be  useful.5 
Local  conditions,  price  of  metals,  the  nature  of  the  ores,  and  the 
association  of  the  metals  must  of  course  be  considered. 

Iron. — Iron  ores  from  the  Lake  Superior  region  usually  con- 
tain 50  to  60  per  cent,  of  iron;  but  iron  ores  which  contain  less 
than  this  may  be  utilized,  especially  where  other  conditions 
are  favorable.  The  Clinton  ores  of  Alabama  contain  as  little  as 
30  per  cent,  of  metallic  iron;  some  types  of  easily  concentrated 
magnetites  may  contain  as  low  as  25  per  cent,  and  still  yield  a 
profit. 

Copper. — Copper  ores  of  the  Lake  Superior  region  may  be 
treated  with  profit,  under  favorable  circumstances,  with  as  little 

1  Lacroix,  A.,  Sur  1'origine  de  Tor  de  Madagascar:  Compt.  Rend.,  vol.  132, 
January  21,  1901,  pp.  180-182. 

2  Econ.  Geol,  vol.  6,  1911,  pp.  247-302. 

3  Mon.  12,  U.  S.  Geol.  Survey,  1886,  pp.  591-594. 

4  Chem.  Neivs,  vol.  55,  1887,  p.  17. 

BJ.  F.  Kemp,  Problem  of  the  metalliferous  veins:  Econ.  Geol.,  vol.  1, 
1906,  pp.  207-232. 


INTRODUCTION  15 

as  0.5  per  cent,  of  metallic  copper,  though  they  ordinarily  average 
somewhat  higher.  Sulphide  copper  ore  of  the  usual  type  can 
rarely  be  utilized  if  it  contains  below  1.5  per  cent,  of  copper  unless 
gold  and  silver  are  present  also,  and  in  many  districts  the  ores 
must  average  considerably  higher  than  this.  The  ores  treated 
at  Ely,  Nevada,  contain  about  2  per  cent,  copper,  besides  gold 
and  silver  to  the  value  of  40  cents  a  ton. 

Lead. — In  northern  Idaho  lead  ores  which  contain  5  to  6  per 
cent,  of  lead  and  3  ounces  in  silver  to  the  ton  are  profitably 
mined.  Non-argentiferous  ores  which  assay  from  5  to  7  per 
cent,  of  lead  are  utilized  in  southeastern  Missouri. 

Zinc. — Zinc  ores  vary  considerably  according  to  locality. 
At  Joplin,  Missouri,  much  of  the  crude  material  hoisted  yields 
less  than  3  per  cent,  of  zinc  sulphide  and  a  little  lead.  This 
is  concentrated  to  about  60  per  cent,  of  zinc.  In  localities 
more  remote  from  markets,  as  in  Colorado,  Utah,  and 
Idaho,  only  high-grade  zinc  ores  can  be  profitably  treated  or 
shipped. 

Silver. — With  rising  metal  prices  pure  silver  ores  again  attract 
attention.  Quartzose  ores  should  contain  not  less  than  15 
ounces  to  the  ton.  The  usual  ores  contain  silver  in  associa- 
tion with  lead,  copper,  or  gold  or  with  all  three.  In  complex 
ores  smelters  rarely  pay  for  less  than  2  ounces  of  silver  and 
0.01  ounce  of  gold  to  the  ton.  Gold  and  silver  are  separated 
from  the  lead  or  copper  bullion  by  zinc  desilverization  or  elec- 
trolytic refining  and  the  cost  of  that  process,  of  course,  imposes 
the  necessity  of  a  certain  minimum  tenor  of  gold  and  silver 
for  profitable  extraction,  but  at  many  plants  gold  and  silver, 
although  present  in  less  than  these  small  quantities,  are  ob- 
tained as  by-products  through  the  necessity  of  eliminating 
some  objectionable  constituent,  like  arsenic,  from  the  bullion. 

Gold. — Gold  has  been  profitably  extracted  from  ores  yielding 
less  than  one  dollar  to  the  ton,  but  the  ordinary  gold  quartz 
ores — for  instance,  those  of  California — yield  about  $5  to  the 
ton;  those  of  Nevada,  Colorado,  and  some  other  States  usually 
contain  more.  On  a  large  scale  gold  ores  containing  from  $2.50 
to  $3  a  ton,  or  even  less,  may  be  worked,  as  at  the  Treadwell 
mines,  in  Alaska.  In  gold  gravels  worked  by  the  hydraulic 
process  as  little  as  4  or  5  cents  to  the  cubic  yard  may  be  profitable. 
By  dredging,  gravels  containing  8  to  15  cents  a  cubic  yard  may  be 
utilized  in  California;  in  Alaska  they  should  contain  from  50 


16  MINERAL  DEPOSITS 

cents  to  $1  a  cubic  yard.  In  the  last  few  years  the  costs  of 
gold  dredging  have  been  brought  down  to  about  4  cents  a  cubic 
yard. 

Tin,  Etc. — Tin  ores  range  from  1.5  to  3  per  cent,  in  tin,  but  in 
tin-bearing  gravels  a  much  smaller  tenor  is  sufficient  to  yield 
a  profit.  Ores  of  quicksilver  contain  at  least  0.3  per  cent,  of 
that  metal;  aluminum  ores  at  least  30  per  cent,  of  aluminum. 
Nickel  should  be  present  to  the  amount  of  2  per  cent,  or 
more  to  constitute  a  workable  nickel  ore.  Manganese  ore 
should  contain  50  per  cent,  of  that  metal,  but  less  is  required  if 
iron  is  also  present.  Chromium  ore  must  contain  about  50  per 
cent,  of  chromic  oxide.  Poorer  ores  may  be  used  if  amenable 
to  concentration. 

PRICE  OF  METALS 


The  prices  which  the  various  metals  bring  express  the  result 
of  their  abundance,  of  the  demand  for  them,  and  of  the  cost  of 
reduction  of  their  ores.  The  value  of  gold  is  fixed  by  inter- 
national agreement,  hence  it  constitutes  the  standard  by  which 
the  prices  of  all  other  commodities  are  measured.  Aluminum,  the 
most  common  of  all  metals,  brings  a  high  price  because  it  can  be 
produced  from  only  a  few  of  the  minerals  containing  it. 

In  the  following  table  the  first  column  represents  what  may 
be  called  the  "normal"  prices  for  metals.  Even  in  normal  times 
there  is,  of  course,  constant  fluctuations  and  some  metals  like 
copper,  iron  and  tin  are  especially  susceptible  to  economic 
influences.  The  great  World  War  beginning  in  1914  proved  to 
have  a  potent  influence  on  prices  and  by  consulting  the  second 
column  it  will  be  found  that  the  prices  of  most  metals  had  been 
doubled  in  1918.  With  the  re-establishing  of  normal  conditions 
the  prices,  except  for  gold  and  silver,  are  likely  to  decline. 

Many  other  substances  used  for  war  materials  have  reached 
an  abnormal  price  in  1917:  High-grade  manganese  ore  sells  at 
$1  per  unit  and  about  the  same  price  is  obtained  for  chromite. 
Tungsten  ore  brings  $20  to  $26  per  unit  of  WO3,  and  molybdenite 
over  $2  per  pound. 

The  values  of  the  generally  used  metals  in  1914  and  1918 
compare  as  follows: 


INTRODUCTION  17 

COMPARATIVE  VALUES  OF  METALS 


March,  1914  February,  1918 


.  i 

Platinum  j     $44.00  per  ounce 

$106  .  00  per  ounce 

Gold  1       20.67     " 

20.67     " 

Silver  !         0.57     " 

0.87     " 

Quicksilver  •         0  .  52  per  pound 

1.71  per  pound 

Nickel  0.45     " 

0.50     " 

Tin  0.37     " 

0.85     " 

Aluminum  0.19     "         " 

0.37     " 

Copper  i         0.14     "         " 

0.23     " 

Antimony  '         0.07     "         " 

0.14     " 

Zinc  0.05    " 

0.08     " 

Lead  0.04     " 

0.07     " 

Pig  iron  0.006  " 

0.015  " 

Many  interesting  data  on  the  total  quantities  of  metals  pro- 
duced in  the  world  and  on  the  largest  amounts  mined  in  any  one 
deposit  are  given  by  J.  H.  L.  Vogt.1 

PRODUCTION   OF   ORE   AND   METAL 

An  interesting  table  reducing  metal  production  in  the  United 
States  to  a  uniform  basis  of  short  tons  is  published  annually  in 
Mineral  Resources.2  From  this  it  is  seen  that  in  1915,  for 
instance,  were  produced  nearly  33,000,000  tons  of  pig  iron, 
nearly  700,000  tons  of  copper,  507,000  tons  of  lead,  458,000  tons 
of  zinc,  2,570  tons  of  silver  and  167  tons  of  gold. 

The  ore  •  production  in  short  tons  of  the  same  year  was  as 
follows:  Iron  ores,  62,150,000;  copper  ore,  43,500,000;  lead  ore, 
7,500,000;  zinc  ore,  18,200,000;  silver  ore,  1,400,000;  gold  ore, 
11,300,000,  all  in  round  figures. 

WEIGHTS  AND  MEASURES 

Before  leaving  this  part  of  the  subject  a  few  words  on  weights 
and  measures  may  be  added.  The  contents  of  base-metal  ores, 
such  as  iron,  lead,  zinc,  and  copper,  are  measured  by  percentage. 

1  Beyschlag,  Krusch  and  Vogt,  Die  Lagerstatten,  etc.,  vol.  1,  1909,  pp. 
187-200. 

2  J.  P.  Dunlop,  Metals  and  Ores  in  1914  and  1915;  Mineral  Resources, 
U.  S.  Geol.  Survey,  pt.  1,  1915. 


18  MINERAL  DEPOSITS 

For  lead  and  copper  the  figures  given  often  do  not  mean  the  exact 
content  by  wet  analysis,  but  by  the  dry  assay,  which  is  1^ 
per  cent,  or  more  lower  than  the  exact  content.  In  some  cases 
the  lead  is  determined  by  wet  assay  of  the  button  from  a  cru- 
cible assay,  which  places  the  percentage  obtained  still  farther 
below  the  actual  content.  The  smelter  pays  for  the  metals  by 
the  "unit,"  which  means  1  per  cent.,  or  20  pounds  to  the  ton, 
or  else  by  a  "basis  price"  for  a  given  percentage  of  metal,  say  55 
per  cent,  for  bessemer  iron  ores  or  65  per  cent,  for  Joplin  zinc 
concentrates.  Tungsten  ore  is  sold  per  unit  of  tungsten  trioxide 
for  ore  carrying  60  per  cent,  or  more  of  this  compound.  Deduc- 
tions and  allowances  based  on  the  presence  or  absence  of  certain 
elements  and  certain  other  rules  complicate  the  smelter  schedules.1 

Precious  metals  in  ores  are  measured  in  England  and  its 
colonies  by  troy  fine  ounces  and  pennyweights,  per  long  or 
short  ton.  In  the  United  States  decimal  fractions  are  substi- 
tuted for  pennyweights;  gold  is  often  reported  in  dollars  and 
cents,  $1  corresponding  closely  to  1  pennyweight.  Silver  is 
measured  in  fine  ounces,  the  pennyweights  always  being  omitted. 
The  short  ton  is  always  used.  Practically  all  other  nations 
measure  these  metals  in  grams  per  metric  ton,  a  far  more 
sensible  way.  For  comparison  the  following  data,  computed  and 
arranged  by  W.  J.  Sharwood,2  are  given: 

Conversion  Tables/ — The  gram  is  taken  as  15.4320  grains. 
The  value  of  a  troy  ounce  of  fine  gold  is  assumed  as  being  exactly 
$20.67,  mstead  of  $20.6718346+, 3  resulting  in  an  error  of  less 
than  1  in  10,000.  Values  in  English  coin  are  based  on  the 
assumption  that  an  ounce  of  fine  gold  is  worth  4.25  pounds 
sterling,  or  85  shillings,  or  1,020  pence;  this  is  too  high  by 
about  1  part  in  2,000,  the  true  value  being  1,019.45  pence. 
It  is  useless  to  attempt  a  closer  approximation  in  practical  work, 
for  the  simple  reason  that  gold  bullion  assays  are  rarely  reported 
closer  than  the  nearest  half  millieme,  or  to  within  1  part  in 
2,000.  At  the  values  adopted  one  dollar  is  equivalent  to  4.11224 
shillings,  and  one  pound  sterling  to  $4.86353. 

1  C.  H.  Fulton,  The  buying  and  selling  of  ores  and  metallurgical  products, 
Tech.  Paper  83,  U.  S.  Bureau  of  Mines,  1915. 

2  Conversion  tables  for  assay  valuations,  Mines  and  Minerals,  January, 
1909,  p.  250. 

3  The  United  States  Mint  Bureau  and  the  United   States  Geological 
Survey  use  tables  compiled  on  the  basis  of  $20.671834625323. 


INTRODUCTION  19 

VOLUME  AND  WEIGHT  OF  FINE  GOLD  AND  SILVER 


One  cubic 
centimeter 

One  cubic 
inch 

One  cubic 
foot 

Fine  silver: 

Weight:  grams  

10.57 

173.21 

299307.00 

Weight:  troy  ounces  

.  339825 

5.5687 

9622.72 

Fine  gold: 

Weight  :  grams  ,....'. 

19.3 

316.269 

546,513 

Weight:  troy  ounces  

.6205 

10.1680 

17,570.39 

Value:  United  States  dollars  

$12.82" 

$210.17 

$363,180 

fl  647 

£43  214 

/74  674 

20 


MINERAL  DEPOSITS 


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CHAPTER  II 
THE  FORMATION  OF  MINERALS 

SOLUTION  AND  PRECIPITATION 

General  Features. — A  mineral  is  an  inorganic  body  of  definite 
chemical  composition  found  in  the  crust  of  the  earth.  Solid 
minerals1  are  formed  by  changes  in  chemical  energy  in  systems 
which  contain  one  fluid  or  vapor  phase.  Their  development, 
therefore,  generally  indicates  a  transition  from  a  mobile  to  a 
less  mobile  form  of  matter.  In  the  great  majority  of  cases  the 
minerals  in  nature  are  formed  by  precipitation  from  solutions,2 
and  this,  therefore,  becomes  a  process  of  the  highest  importance 
for  mineral  genesis. 

Supersaturation  and  consequently  precipitation  are  con- 
trolled by  the  thermodynamic  environment  of  the  system. 
Temperature  and  pressure  are  the  most  important  agencies 
though  at  times  electric  energy  and  light  may  also  be  active. 

In  most  cases  the  formation  of  minerals  involves  a  chemical 
reaction  subject  to  pressure  and  temperature,  and  this  may  be 
brought  about  in  several  ways: 

1.  By  reactions  between  liquids  or  liquid  solutions. 

2.  By  reactions  between  gases  or  gaseous  solutions. 

3.  By    reactions    between    liquids    or    liquid    solutions    and 


4.  By  reactions  between  solids  and  liquids  or  liquid  solutions 
or  gases. 

Few  minerals  are  formed  below  the  freezing  point  of  water. 

1  The  liquid  minerals  are  practically  confined  to  water,  mercury  and  to 
certain  hydrocarbons.  The  latter,  as  well  as  certain  undercooled  liquids 
like  obsidian,  are  not,  however,  considered  as  minerals  as  they  have  no 
definite  composition  expressible  in  a  chemical  formula. 

1  Certain  chemical  reactions  involving  the  formation  of  new  minerals 
may  apparently  take  place  by  the  interaction  of  solids  under  the  influence 
of  heat  and  pressure,  but  it  is  perhaps  probable  that  here  too  a  vapor  phase 
or  a  fluid  phase  is  present  in  the  system. 

22 


THE  FORMATION  OF  MINERALS  23 

Their  upper  limit  of  development  is  marked  by  the  temperature 
at  which  they  become  unstable  or  melt. 

After  the  inception  of  mineral  formation  concentration  to 
larger  masses  or  deposits  may  be  effected  by  the  continuation  of 
similar  processes  or  reactions.  Quite  frequently,  however, 
there  are  other  causes  which  have  contributed:  Gravity  may 
cause  the  sinking  of  heavy  minerals  in  lighter  media  as  when 
anhydrite  crystals  sink  to  the  bottom  in  sea  water,  or  when 
magnetite  sinks  in  a  residual  rock  melt.  Solution  and  disinte- 
gration may  do  its  share,  as  when  native  gold  is  set  free  from  a 
quartz  matrix,  and  is  separated  by  gravity  into  workable  gold 
deposits;  or,  as  when,  in  a  rock  composed  of  calcite  and  calcium 
phosphate,  the  former  is  dissolved,  leaving  a  phosphate  deposit 
of  economic  value. 

Influence  of  Pressure. — The  effect  of  change  of  pressure, 
according  to  LeChatelier's  law,  is  as  follows:  When  the  pressure 
in.  a  system  in  equilibrium  is  increased  that  reaction  takes  place 
which  is  accompanied  by  a  diminution  in  volume;  and  when  the 
pressure  is  diminished  a  reaction  ensues  which  is  accompanied 
by  an  increase  in  volume.  "The  direction  in  which  change  of 
concentration  will  occur  with  change  of  pressure  can  be  pre- 
dicted, if  it  is  known  whether  solution  is  accompanied  by  increase 
or  diminution  of  the  total  volume.  If  diminution  of  the  total 
volume  of  the  system  occurs  on  solution,  as  is  the  usual  case, 
increase  of  pressure  will  increase  the  solubility;  in  the  reverse 
case  increase  of  pressure  will  diminish  the  solubility."1  In 
general,  a  decrease  of  pressure,  which  results  when  solutions 
ascend  in  the  earth's  crust,  will  be  favorable  to  precipitation. 
The  influence  of  pressure  is,  however,  in  most  cases  slight.  For 
instance,  the  solubility  of  sodium  chloride  (in  grams  of  salt  in 
1  gram  of  solution)  at  the  pressure  of  one  atmosphere  is  expressed 
by  0.264  and  at  500  atmospheres  by  0.270.  In  systems  in  which 
one  or  more  of  the  components  are  volatile  the  effect  of  pressure 
may  be  very  great;  carbon  dioxide,  for  instance,  held  in  water 
by  pressure,  may  increase  the  solubility  of  calcium  carbonate 
owing  to  the  formation  of  bicarbonate. 

The  effects  of  uniform,  or  hydrostatic,  pressure  are  much  less 
marked  than  are  the  effects  of  stress,  or  unequal  pressure. 
Under  conditions  of  stress  a  given  pressure  will  lower  the  melting 

1  Alexander  Findlay:  The  Phase  Rule,  1908,  p.  107. 


24  MINERAL  DEPOSITS 

temperature  far  more  rapidly  than  when  the  pressure  is  equal 
from  all  sides.1 

Influence  of  Temperature.  —  In  a  solution  of  various  salts  in 
water  or  in  a  silicate  melt  changes  in  temperature  are  far  more 
effective  in  producing  precipitation  than  changes  in  pressure. 
Van't  Hoff's  law  states:  When  the  temperature  of  a  system  in 
equilibrium  is  raised  that  reaction  takes  place  which  is  accom- 
panied by  absorption  of  heat;  and,  conversely,  when  the  tem- 
perature is  lowered  that  reaction  occurs  which  is  accompanied 
by  an  evolution  of  heat.2  In  the  great  majority  of  cases  increase 
of  temperature  promotes  the  solubility  of  salts,  and  decreasing 
temperature  —  say  by  the  cooling  of  ascending  thermal  waters  or 
of  magmas  —  promotes  precipitation.  The  common  rule  for 
salts  —  to  which  a  number  of  exceptions  may  be  noted  —  is  that 
the  solubility  increases  to  temperatures  of  75°  C.  or  150°  C. 
beyond  which  a  lessening  of  the  quantity  dissolved  may  often 
be  noted.  Breaks  in  the  solubility  curve  usually  indicate  the 
limit  of  stability  for  a  particular  salt,  less  hydrated  forms,  for 
instance,  coming  in  at  higher  temperatures. 

In  any  hot,  complex  solution,  occurring  in  nature,  decreasing 
temperature  will,  in  general,  cause  precipitation  of  some  min- 
eral; with  continued  cooling  a  series  of  other  minerals  may  be 
precipitated,  as  the  solubility  limit  of  each  is  reached. 

As  no  compounds  are  absolutely  stable  under  the  varying 
conditions  obtaining  in  the  crust,  it  follows  that  minerals  once 
formed  may  subsequently  be  brought  into  solution,  transported, 
redeposited,  or  indeed  wholly  decomposed  so  that  its  elements 
may  enter  into  new  combinations. 

Precipitation  by  Evaporation  of  the  Solvent.—  The  salts 
contained  in  a  solution  are  naturally  precipitated  when  evapora- 
tion at  the  surface  so  reduces  the  amount  of  the  solvent  that 
supersaturation  ensues.  The  deposits  of  gypsum  and  salt  in 
various  formations  are  familiar  results  of  this  process.  In  some 
cases  carbon  dioxide  or  other  gases  may  be  the  solvent;  the  pre- 
cipitation of  calcium  carbonate  follows,  for  instance,  in  springs  at 


Johnston  and  Paul  Niggli,  The  general  principles  underlying 
metamorphic  processes.  Jour.  Geology,  vol.  21,  1913,  pp.  481-516  and 
588-624. 

John  Johnston,  Pressure  as  a  factor  in  the  formation  of  rocks  and 
minerals.  Jour.  Geology,  vol.  23,  1915,  pp.  730-747. 

2  Alexander  Findlay,  The  phase  rule,  1908,  p.  68. 


THE  FORMATION  OF  MINERALS  25 

their  point  of  issue  when  the  carbon  dioxide  escapes  which  holds 
the  salt  in  solution  as  a  bicarbonate. 

Precipitation  by  Reaction  between  Solutions. — Mingling  of 
different  solutions  is  one  of  the  most  common  occurrences  in 
nature,  as  when  rivers  discharge  their  waters  into  the  sea  or  as 
when  ascending  hot  waters  meet  surface  waters  of  different 
composition.  Precipitation  of  chemical  compounds  results  when 
any.  combination  of  the  various  ions  in  the  solution  can  form 
to  a  sufficient  extent  to  be  insoluble  in  the  liquid  present. 
Solutions  in  nature  are  usually  complex  and  the  various  reactions 
are  more  or  less  interfered  with.  In  general,  according  to 
Nernst's  law,  the  solubility  of  a  given  salt  is  reduced  by  the 
presence  in  the  solution  of  another  salt  which  has  a  common  ion 
but  is  increased  by  the  presence  of  another  salt  with  no  common 
ion.  For  instance,  the  solubility  of  lead  chloride  is  decreased 
by  the  presence  of  the  chloride  of  calcium  or  magnesium.  The 
presence  of  alkaline  carbonates  decreases  the  solubility  of  FeCOa; 
the  solubility  of  NaCl  is  decreased  by  CaCl2;  while  the  solubility 
of  CaS04  (gypsum)  is  increased  in  a  NaCl  solution.  If  calcite 
is  treated  with  a  saturated  solution  of  FeCOs,  ZnCOs,  or  MgCOs, 
a  part  of  the  calcite  will  be  dissolved  while  a  corresponding  part 
of  the  other  carbonates  is  precipitated;  the  solubility  of  CaCOa 
in  water  is  increased  by  Na2SO4  or  NaCl  but  decreased  by  MgC03. 

In  mixed  solutions  precipitation  is  often  delayed,  as  shown, 
for  instance,  by  the  slow  precipitation  of  barite  (BaSCh)  due  to 
the  presence  of  sodium  and  magnesium  chlorides  in  certain  mine 
waters  consisting  of  salt  brines.  Nernst's  law  offers  an  explana- 
tion of  these  anomalies. 

Slow  precipitation  in  dilute  solutions  generally  results  in  large 
crystals  being  formed,  while  rapid  precipitation  results  in 
colloids  or  fine  powders. 

Precipitation  by  Reactions  between  Aqueous  Solutions  and 
Solids. — In  nature,  solutions  act  constantly  upon  solid  minerals. 
At  the  surface  all  rocks  and  mineral  deposits  are  moistened  by 
rain  water  which  also  may  descend  to  great  depths  in  porous, 
fissured  or  broken  material.  Rising  hot  waters  soak  into  the 
rocks  from  the  fractures  upon  which  they  ascend.  Minerals 
are  attacked  to  a  greater  or  lesser  degree  by  these  various  kinds 
of  waters;  they  are  decomposed  and  partly  or  wholly  go  into 
solutions.  From  these  solutions  new  minerals  are  deposited  in 
open  spaces  and  this  is  a  very  common  mode  of  mineral  forma- 


26  MINERAL  DEPOSITS 

tion.  But  the  changes  also  proceed  in  the  solid  rocks  themselves 
and  such  processes  by  which  new  minerals  may  take  the  place  of 
old  ones  are  called  metasomatism  or  replacement.  The  water 
penetrates  the  rocks  in  capillary  openings.  By  the  phenomenon 
known  as  adsorption,  the  film  of  liquid  on  the  solid  contains 
more  than  an  average  amount  of  material  in  solution  and  these 
films  are  likely  to  become  supersaturated  in  advance  of  the 
remainder  of  the  solution,  so  that  chemical  reactions  will  be 
facilitated.  In  this  manner  the  mineralogical  and  structural 
character  of  rocks  may  be  changed:  Chlorite  may  replace 
augite,  and  sericite  and  quartz  may  replace  feldspars.  Metallic 
ores  are  often  formed  by  replacement.  Limestone  may,  for 
instance,  be  permeated  by  a  solution  of  zinc  sulphate  with  the 
result  that  the  calcium  carbonate  is  replaced  by  zinc  carbonate 
with  faithful  preservation  of  the  limestone  structure,  while 
calcium  sulphate  is  carried  off  in  solution.  It  is  not  even  neces- 
sary that  the  replacing  mineral  should  have  an  element  in 
common  with  the  older  mineral.  Pyrite  or  galena  may  replace 
feldspars  or  calcite  grains.  The  replacing  mineral  may  even 
develop  as  perfect  crystals  in  the  older  mineral.  The  phenomena 
of  replacement  are  of  the  utmost  importance  for  the  genesis  of 
mineral  deposits. 

Precipitation  by  Reactions  between  Gases  or  between  Gases 
and  Solutions. — Gases  may  produce  precipitation  in  solutions. 
Hydrogen  sulphide  in  some  mine  waters  precipitates  cuprous 
sulphide  from .  cupric  sulphate.  Less  important  is  the  action 
between  gases:  Native  sulphur  may  be  precipitated  in  volcanic 
regions  by  a  mixture  of  hydrogen  sulphide  and  sulphur  dioxide. 

Crystalline  Minerals.- — The  minerals  may  be  precipitated  as 
crystalloids  or  as  colloids.  In  mineral  deposits  formed  in  depth 
and  at  temperatures  higher  than  those  prevailing  at  the  surface 
crystalloids  are  almost  exclusively  present,  as  they  also  are  in 
igneous  and  metamorphic  rocks.  The  molecules  have  been 
allowed  to  arrange  themselves  in  the  symmetry  of  one  of  the  six 
crystal  systems  and  the  minerals  form  homogeneous  grains  or 
crystals.  Crystalline  minerals  develop  best  by  slow  precipita- 
tion in  solutions  contained  in  open  spaces;  under  such  conditions 
free  crystals  may  form  in  silicate  melts  or  magmas  and  in 
aqueous  solutions.  Crusts  of  minerals  may  develop  where 
crystals  adhere  to  walls  of  water  filled  fissures  and  this  is  a 
common  feature  in  mineral  veins.  The  first  impulse  to  crystal- 


THE  FORMATION  OF  MINERALS  27 

lization  may  be  given  by  adsorption  and  supersaturatioii  along 
the  walls.  Once  started  the  larger  crystals  become  further 
enlarged  because  smaller  crystals  dissolve  more  rapidly  than 
large  ones  and  the  liquid  remains  supersaturated  with  reference 
to  the  larger  growths.1 

When  crystallization  is  progressing  from  a  great  number  of 
points  in  the  solution,  a  granular  texture  is  developed  by  the 
mutual  interference  of  the  crystals.  In  mineral  deposits  the 
resulting  textures  are  usually  rather  coarse;  only  very  rarely 
do  we  find  fine-grained  aggregates  of  order  of  magnitude  of  the 
dense  groundmass  of  igneous  porphyritic  rocks.  In  mineral 
deposits  a  considerable  variety  of  textures  result  by  replacement 
in  solid  rock;  such  textures  are  in  general  analogous  to  those 
found  in  metamorphic  rocks. 

Colloids.2 — A  number  of  minerals  are  formed  both  as  crystal- 
loids and  as  colloids.  The  usage  requires  a  different  name  for 
the  two  forms.  The  opinion  is  gaining  ground  that  the  colloid 
state  is  not  a  separate  kind  of  matter  but  simply  crystalloids 
in  a  state  of  dispersion  ranging  from  comparatively  coarse  sus- 
pension down  to  almost  molecular  subdivisions.  A  colloid 
mixture  is  thus  a  two-phase,  heterogeneous  system  in  which 
the  solid,  divided  into  small  separate  volumes  is  known  as  the 
disperse  phase  and  the  liquid  as  the  dispersion  medium.  The 
disperse  phase  may  separate  from  the  mixture  in  a  gelatinous 
or  flocculent  form,  still  retaining  some  of  the  solvent.  In  one 
class  of  these  mixtures,  called  colloidal  solutions,  a  gelatinous 
mass  is  obtained  by  cooling  or  evaporation;  this  is  termed  a 
"gel."  Silica  in  aqueous  solution,  separating  as  opal  is  one  of 
the  more  common  of  these  gels  occurring  in  nature.  Gelatinized 
colloids  are  permeable  to  salts,  the  rate  of  diffusion  being  almost 
the  same  as  in  water,  but  they  diffuse  themselves  through  other 
colloids  or  through  porous  walls  with  the  utmost  difficulty.  The 
colloidal  solutions  are  not  coagulated  by  salts. 

Another  class  of  colloidal  mixtures,  known  as  "  colloidal  sus- 
pensions" is  more  abundantly  represented  in  nature;  it  is  be- 
lieved that  in  these  the  suspended  particles  are  of  much  larger 

1  W.  Ostwald,  The  scientific  foundations  of  analytical  chemistry,  1900, 
p.  22. 

2  Arthur  A.  Noyes,  The  preparation  and  properties  of  colloidal  mixtures. 
Jour.  Am.  Chem.  Soc.,  vol.  27,  1905. 

W.  Ostwald,  Handbook  of  colloid  chemistry,  1915. 


28  MINERAL  DEPOSITS 

size  than  in  the  colloidal  solutions.  These  colloidal  suspensions 
are  not  viscous  and  do  not  gelatinize  but  coagulate  readily  when 
an  electrolyte  is  added.  They  result  from  reactions  between 
two  chemical  compounds  in  the  absence  of  electrolytes.  When, 
for  instance,  hydrogen  sulphide  is  added  to  a  solution  of  arsenious 
oxide,  a  turbid  yellow  liquid  results  which  is  a  colloidal  suspen- 
sion of  arsenic  sulphide.  An  important  fact  is  that  a  small 
quantity  of  a  gelatinizing  colloid  prevents  the  coagulation  of 
colloidal  suspensions  by  electrolytes.  Thus  a  small  amount  of 
gelatine  may  keep  silver  chloride  indefinitely  in  a  state  of  colloidal 
suspension. 

The  importance  of  these  facts  as  related  to  reactions  in  natural 
solutions  is  obvious.  Many  waters  are  rich  in  colloidal  silica 
and  its  influence  may  prevent  a  precipitation  which  otherwise 
would  take  place.  Another  effect  is  the  retention  of  silica  jelly 
within  the  walls  of  fissures  while  gases  and  electrolytes  pass 
through  them. 

The  existence  of  colloid  minerals  was  recognized  by  the 
German  mineralogist  Breithaupt,  and  Berzelius  already  knew  of 
the  properties  of  many  colloids.  Grahams  studies  about  1864 
laid  the  real  foundation  for  colloidal  chemistry,  a  branch  which, 
however,  has  only  recently  begun  to  attract  the  attention  it 
merits.  Colloidal  minerals  form  in  great  abundance  at  ordinary 
temperatures,  near  the  surface,  within  the  oxidized  zone.  Some 
colloids  like  opal  may  be  deposited  at  higher  temperature  of 
100°  C.  to  200°  C.  perhaps  even  higher,  but  this  is  exceptional. 
The  solidified  colloids  have  a  great  tendency  to  acquire  crys- 
tallinity  and  in  time — rapidly  with  heat — become  transformed 
into  fibrous  or  cryptocrystalline  aggregates.  Such  crystallized 
colloids  may  be  called  metacolloids.1  Almost  all  metals,  oxides, 
hydroxides  and  sulphides  may  yield  colloidal  solutions  or  sus- 
pensions. Among  minerals  of  colloidal  origin  common  in  nature 
may  be  mentioned  opal,  limonite,  psilomelane,  calcium  phos- 
phate, arsenic  and  perhaps  some  secondary  varieties  of  chalcocite.2 

1  E.  Wherry,  Jour.  Washington  Acad.  Sci.,  vol.  4,  1914,  p.  112. 

2  F.    Cornu,  Many  papers  in.  Zeit.  f.  Chem.  u.  Ind.  d.  Colloide,  vol.  4, 
1909. 

E.  Wherry,  Op.  cit. 

Austin  F.  Rogers,  A  review  of  the  amorphous  minerals,  Jour.  Geology, 
vol.  25,  1917,  pp.  515-541. 


CHAPTER  III 
THE  FLOW  OF  UNDERGROUND  WATER 

General  Statement.1 — Much  of  the  rain  water  which  descends 
upon  the  land  runs  off  in  surface  drainage,  and  a  smaller  part 
evaporates,  but  a  certain  quantity  sinks  down  into  the  soil  and 
into  the  porous  and  fractured  rocks.  This  part  of  the  pre- 
cipitation adds  to  the  ground  water  and,  if  there  is  a  sufficiency 
of  rainfall,  it  saturates  the  material  at  a  certain  varying  depth 
below  the  surface.  This  upper  limit  of  the  saturated  zone 
indicated  by  the  depth  at  which  water  stands  in  wells  or  shafts  is 
called  the  water  level,  ground-water  level,  watertable,  or  hy- 
drostatic level.  It  is  as  a  matter  of  fact  a  warped  surface  which 
feebly  reflects  the  topographic  features  (Fig.  1).  The  water 
may  penetrate  to  considerable  depths,  particularly  along  fissures; 
gravity  and  heat  often  establish  a  circulation  of  the  ground  water 

1  C.  R.  Van  Hise,  A  treatise  on  metamorphism,  Mon.,  47,  U.  S.  Geol. 
Survey,  1894,  pp.  123-191,  arid  657-670. 

C.  R.  Van  Hise,  Some  principles  controlling  the  deposition  of  ores, 
Trans.,  Am.  Inst.  Min.  Eng.,  vol.  30,  1901,  pp.  27-176. 

L.  M.  Hoskins,  Flow  and  fracture  of  rocks  as  related  to  structure,  Six- 
teenth Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  1,  1896,  pp.  845-875. 

C.  S.  Slichter,  The  motion  of  ground  waters,  Nineteenth  Ann.  Rept., 
U.  S.  Geol.  Survey,  pt.  2,  1898,  pp.  295-384.  Also  Water-Supply  Paper 
67,  U.  S.  Geol.  Survey,  1902,  106  pp. 

F.  H.  King,  Principles  and  conditions  of  the  movements  of  ground- 
water,  Nineteenth  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  2,  1898,  pp.  59-294. 

J.  F.  Kemp,  R6le  of  the  igneous  rocks  in  the  formation  of  veins,  Trans., 
Am.  Inst.  Min.  Eng.,  vol.  31,  1901,  pp.  169-198. 

J.  F.  Kemp,  Igneous  rocks  and  circulating  waters,  etc.,  Idem.,  vol.  33, 
pp.  707-711. 

J.  F.  Kemp,  The  problem  of  metalliferous  veins,  Econ.  Geol.,  vol.  1, 
1906,  pp.  207-232. 

J.  W.  Finch,  The  circulation  of  underground  aqueous  solutions  and  the 
deposition  of  lode  ores,  Proc.,  Colo.  Sci.  Soc.,  vol.  7,  1904,  pp.  193-252. 

M.  L.  Fuller,  Total  amount  of  free  water  in  the  earth's  crust,  Water-Supply 
Paper  160,  U.  S.  Geol.  Survey,  1906,  pp.  59-72. 

T.  A.  Rickard,  Eng.  and  Min.  Jour.,  March  14,  1903;  Min.  and  Sci.  Press, 
June  27,  1908. 

29 


30  MINERAL  DEPOSITS 

so  that  it  may  again  reach  the  surface  after  a  long  underground 
journey.  In  all  its  aspects  the  ground  water  performs  im- 
portant geologic  work  by  solution  and  precipitation,  which  may 
result  in  the  concentration  of  useful  minerals. 

There  are  other  waters  which  may  be  found  in  the  crust: 
The  sea  water  may  at  times  invade  the  rocks,  or  may  have  been 
occluded  in  old  sediments.  There  is  a  small  class  of  hot  waters, 
ascending  from  great  depths  which  are  believed  by  many  geolo- 
gists to  be  of  magmatic  origin,  that  is  given  off  by  cooling 
magmas.  However  important  such  waters  may  be  in  the 
formation  of  certain  kinds  of  mineral  deposits,  they  are  insig- 
nificant in  quantity  compared  to  the  great  mass  of  water  of 
atmospheric  origin  which  is  contained  in  the  rocks. 

Pores  and  Openings  in  Rocks. — All  rocks  are  porous  and  are 
capable  of  absorbing  water.  By  porosity  is  understood  the 
percentage  of  pore  space  referred  to  the  total  volume  of  the  rock. 
The  ratio  of  absorption  is  the  ratio  between  the  weight  of  the 
water  absorbed  and  the  weight  of  the  rock  tested.  When  the 
pores  are  completely  filled  the  rock  is  said  to  be  saturated,  but  a 
saturated  rock  after  being  drained  always  retains  a  certain 
amount  of  water  which  adheres  to  the  walls  of  the  pores.  The 
pore  space*  varies  from  a  fraction  of  1  per  cent,  to  40  per  cent. 
In  fresh  granites  and  similar  compact  rocks  the  porosity  is  from 
0.2  to  0.5,  in  limestones  from  0.53  to  13.36,  in  sandstones  from 
5  to  28  per  cent. 

Under  the  assumption  that  a  sandstone  consists  of  spherical 
grains  packed  in  the  most  compact  arrangement  possible,  the 
space  between  the  spheres  would  amount  to  25.95  per  cent. 
Other  things  being  equal  the  porosity  increases  with  the  size  of 
the  grains. 

.In  loose  sand  and  gravel  the  porosity  is  highest,  ranging  from 
32  to  40.  The  absorbed  water  may  be  called  "free  water" 
in  contrast  to  that  existing  in  chemical  combination  in  the 
minerals  of  the  rock.  All  of  the  free  water  is  not  "available," 
for  instance  in  wells,  because  some  rocks,  like  clays,  have  the 
peculiarity  of  holding  in  their  pores  great  quantities  of  water 
which  is  released  only  at  an  extremely  slow  rate. 

On  the  basis  of  size,  openings  in  rocks  may  be  divided  into  (1) 
openings  which  are  larger  than  those  of  capillary  size,  or  super- 
capillary  openings;  (2)  capillary  openings;  and  (3)  sub-capillary 
openings.  For  water,  openings  larger  than  capillary  openings 


THE  FLOW  OF  UNDERGROUND  WATER       .    31 

may  be  considered  as  circular  tubes  which  exceed  0.508  mm.  in 
diameter,  or  sheet  openings,  such  as  those  furnished  by  faults, 
joints,  etc.,  whose  width  exceeds  0.254  mm.  Capillary  tubes 
or  sheet  spaces  are  those  smaller  than  the  dimensions  indi- 
cated but  larger  than  the  openings  in  which  the  molecular 
attraction  of  the  solid  material  extends  across  the  space,  and  to 
such  openings  the  laws  of  capillary  flow  apply.  In  sub-capillary 
openings  the  attraction  of  the  molecules  extends  from  wall 
to  wall,  and  this  class  includes  tubes  smaller  than  0.0002  mm. 
in  diameter  and  sheet  openings  smaller  than  0.0001  mm.  in 
width.  According  to  G.  Bakker1  the  spheres  of  molecular  at- 
traction are  only  six  to  seven  times  the  molecular  diameter  and 
consequently,  if  this  be  true,  capillary  movement  can  take  place 
in  tubes  very  much  smaller  than  0.0002  mm. 

The  flowage  of  water  through  super-capillary  openings  nearly 
follows  the  ordinary  laws  of  hydrostatics,  but  is  subject  to  a 
certain  retardation  on  account  of  friction.  The  super-capillary 
openings  include  the  greater  number  ofJaults,  joints,  partings, 
and  the  openings  in  coarser  sedimentsC  In  capillary  openings 
the  movement  is  very  slow  indeed,  so  that  many  rocks  in  which 
they  occur,  as  shales  and  clays,  are  spoken  of  as  impermeable.  In 
sub-capillary  openings  the  water  is  held  firmly  as  a  film  glued  to 
the  walls  by  adhesion;  there  is  no  free  water  and  the  flow  is 
practically  nil. 

The  capillary  forces  play  a  considerable  part  in  the  move- 
ment of  underground  waters,  but,  in  general,  they  cannot  pro- 
duce a  continuous  flow,  and  they  are  of  secondary  importance 
in  comparison  with  the  hydrostatic  pressure.  At  higher  temper- 
atures the  capillary  action  decreases  and  becomes  zero  at  the 
critical  points. 

Openings  in  rocks  do  not  persist  indefinitely  in  depth,  though 
very  hard  rocks  like  granite  will  sustain  far  greater  loads  than  at 
the  surface.  The  experiments  of  F.  D.  Adams2  indicate  that 
cavities  may  exist  in  granite  to  a  depth  of  at  least  11  miles,  or 
17,600  meters.  Most  rocks  tend  to  become  plastic  at  far  lesser 
depths  and  will  then  become  deformed  and  flow  without  fracture. 
Under  such  conditions  an  active  circulation  of  water  is  difficult. 

lZeitschr.    /.   phys.  chem.,  vol.  80,  1912,  p.  129.    See  also  J.  Johnston 
and  L.  H.  Adams,  Jour.  Geology,  vol.  22,  1914,  p.  13. 
2  Jour.  Geology,  vol.  20,  1912,  pp.  97-118. 


32    .  MINERAL  DEPOSITS 

Water  in  Sands  and  Gravels. — We  may  first  consider  the 
simpler  case,  of  loose  material  such  as  sands  and  gravels  which  are 
so  abundant  in  the  uppermost  part  of  the  crust,  and  in  comparison 
with  which  the  underlying  compact  rocks  may  be  regarded  as 
impermeable.  Under  the  influence  of  gravity  the  water  de- 
scends until  a  depth  is  reached  where  the  material  is  saturated, 
this  being  the  water  level.  In  the  valleys  the  water  level  will  lie 
close  to  the  ground  while  it  rises  slightly  under  the  ridges  so 
that  it  may  be  necessary  to  sink  deeply  for  wells  with  permanent 
water.  The  ground  water  is  not  stationary  but  moves  slowly 
from  the  higher  ground  toward  the  valleys;  and  underneath  the 
valleys  its  perceptible  movement  continues  down  stream  until 
ultimately,  with  lack  of  grade,  for  instance,  where  a  river  valley 
opens  toward  the  sea,  the  movement  will  become  slow  and  almost 
imperceptible.  That  such  a  movement  actually  takes  place 
has  been  proved  by  the  use  of  fluorescein  and  other  indicators  in 
wells  and  bore  holes. 

Water  in  Rocks  of  Uniform  Texture. — In  uniform  rocks,  like 
granite,  water  is  not  only  contained  in  the  pores  but  moves  more 
easily  on  the  ever  present  seams  and  joints.  Considerable 
water  may  be  stored,  though  the  quantity  per  unit  volume  of 
rock  will  be  much  smaller  than  in  sands  and  gravels.  The 
water  table  is  here  also  a  curved  surface  which  follows  approxi- 
mately the  topographic  relief  but  is  less  accentuated.  The 
elevation  of  the  water  table  or  water  level  may  fluctuate  consid- 
erably dependent  upon  seasonal  rainfall. 

In  his  paper,  cited  above,  J.  W.  Finch  distinguishes  the  space 
above  the  water  level  as  the  gathering  zone  or  zone  of  percolation, 
in  which  water  and  air  are  both  present,  and  in  which  the  water 
is  conducted  to  the  saturated  belt  (Fig.  1).  Even  in  very  arid 
regions  there  is  usually  a  deep  zone  of  saturation,  though  the 
quantity  of  water  stored  may  be  small.  It  would  also  be  pos- 
sible, however,  to  have  no  belt  of  saturation  and  the  water  would 
then  simply  percolate  feebly  downward  until  the  quantity  is 
diminished  to  zero. 

The  zone  of  discharge  "  embraces  that  part  of  the  belt  of  satura- 
tion which  has  a  means  of  horizontal  escape"  by  continuous 
gravitative  flow.1  The  movement  of  the  water  is  usually  more 
rapid  in  the  upper  part  of  this  zone  than  in  the  lower  part,  where, 
in  spite  of  greater  pressure,  the  obstructions  and  the  increasing 

1  Proc.,  Colorado  Sci.  Soc.,  vol.  7,  1904,  p.  202. 


THE  FLOW  OF  UNDERGROUND  WATER 


33 


compactness  of  the  rock  retard  the 
flow.  The  air  or  gas  filling  the  pore 
space  must  also  be  driven  out  before 
the  water  can  enter,  and  the  evapora- 
tion of  water  in  the  underground  at- 
mosphere may,  under  some  conditions, 
also  become  an  important  factor  in 
reducing  the  supply  of  water.  Still 
another  portion  of  the  water  enters  into 
chemical  combination  by  hydration. 

The  static  zone  is  the  third  and  deep- 
est of  the  divisions  proposed  by  Finch. 
It  extends  below  the  level  of  the 
lowest  point  of  discharge  and  the  water 
in  it  is  stagnant  or  moves  extremely 
slowly.  It  depends  upon  the  zone  of 
discharge  for  its  water,  as  it  is  simply 
the  bottom  part,  with  gradually  dimin- 
ishing quantity  of  water,  of  a  belt  of 
saturation  of  which  the  zone  of  dis- 
charge is  the  upper  and  flowing  part. 
The  lower  limit  of  the  third  zone, 
where  the  quantity  of  water  becomes 
exceedingly  small,  is  not  entirely  a 
matter  of  speculation,  for  many  defi- 
nite data  are  supplied  by  mining  oper- 
ations and  in  many  places  it  is  not 
more  than  1,500  feet  below  the  sur- 
face. Large  quantities  of  water  may 
be  stored  in  the  third  zone,  as,  for 
instance,  in  the  deep  artesian  basins 
where  impermeable  beds  prevent 
escape.  A  certain  amount  of  "rock 
moisture"  undoubtedly  persists  to 
great  depths. 

Water  in  Sedimentary  Rocks. — In 
a  series  of  sedimentary  beds  it  is 
common  to  find  impermeable  rocks 
like  clay  and  shale  alternate  with 
more  porous  beds  like  sandstone  and 
limestone.  Under  such  conditions  the 


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34 


MINERAL  DEPOSITS 


scending  surface  waters  is  likely  to  be  irregular.  Near  the  sur- 
face there  may  be  a  local  water  level  but  below  this  beds  heavily 
charged  with  water  may  alternate  with  almost  dry  strata.  Each 
bed  may  in  a  way  be  considered  as  a  unit  and  if  it  outcrops  it 

has  its  own  zone  of  gather- 
ing, zone  of  discharge  and 
its  static  zone.  The  Cre- 
taceous Dakota  sandstone 
presents  an  excellent  ex- 
ample of  a  porous  stratum' 
in  which  a  large  amount  of 
water  can  be  stored. 
Throughout  the  Great 
Plains  this  is  a  veritable 
reservoir  of  water,  which 
can  be  tapped  by  artesian 
wells  as  far  as  300  miles 
from  its  outcrop  and  at 
depths  of  a  few  hundred  to 
3,000  feet  (Fig.  2).  But 
this  stratum  at  present 
simply  contains  a  stagnant 
body  of  water,  and,  as  in 
most  other  artesian  basins, 
the  quantity  is  not  inex- 
haustible. This  very  case 
proves  how  impervious  the 
adjacent  sedimentary  beds 
are,  for  neither  upward 
nor  downward  is  an  avenue 
of  escape  afforded  in  spite 
of  the  strong  pressure. 
Should  profound  fissuring 
take  place  in  the  Great 
Plains  a  natural  avenue 
of  escape  would,  of  course, 
be  opened  and  a  deep  cir- 
culation established.  Kemp  and  Fuller  have  both  brought  out 
the  fact  that  the  deep  sedimentary  beds  are  often  remarkably 
dry.  The  well  4,262  feet  deep  at  Wheeling,  West  Virginia,  was 
in  absolutely  dry  rocks  for  the  lower  1,500  feet.  Wells  sunk  at 


THE  FLOW  OF  UNDERGROUND  WATER  35 

Northampton,  Massachusetts,  and  at  New  Haven,  Connecticut, 
to  depths  of  4,000  feet  have  failed  to  obtain  water.  A  number 
of  other  instances  are  mentioned,  and  in  many  cases  the  dry 
part  consists  of  sandstones  or  other  porous  rocks. 

Some  time  ago  it  was  suggested  by  A.  C.  Lane1  that  part  of 
the  salt  water  in  deeply  buried  beds  is  fossil  sea  water  or  "con- 
nate" water  occluded  in  the  sediments  at  the  time  of  deposition. 
There  can  be  little  doubt  that  dryness  as  well  as  salinity  increase 
with  depth.2 

Influence  of  Fractures. — The  simple  conditions  outlined 
above  are  seriously  disturbed  where  extensive  fracturing  has 
taken  place  and  paths  have  been  laid  out  on  which  the  water 
may  move  under  approximately  normal  hydrostatic  conditions. 
There  may  be  a  comparatively  slow  descent  of  the  water  along 
devious  joints  and  fractures  and  a  rapid  rise  under  hydrostatic 
head  where  the  descending  water  reaches  the  open  paths  of 


D   E 


FIG.  3. — Section  illustrating  flow  of  water  in  jointed  crystalline  rocks.  A, 
C,  flowing  wells  fed  by  joints;  B,  intermediate  well  of  greater  depth,  with  no 
water;  D,  deep  well  not  encountering  joints;  E,  pump  well  adjacent  to  D,  ob- 
taining water  at  shallow  depth;  S,  dry  hole  adjacent  to  spring.  After  M.  L. 
Fuller,  U.  S.  Geol.  Survey. 

important  faults  and  fissures.  Friction  during  the  descent 
undoubtedly  seriously  diminishes  the  theoretical  head,  but  the 
evidence  is  perfectly  clear  that  in  regions  of  dynamic  disturbance, 
such  as  the  Alps  and  the  Rocky  Mountains,  strong  ascending 
springs  may  result  from  these  conditions. 

At  the  point  of  issue  such  springs  may  be  warm  and  their 
temperature,  in  regions  where  no  recent  igneous  action  has 
taken  place,  may  be  a  good  indication  of  the  depth  attained  by 
the  water.  Such  springs  seldom  have  a  temperature  higher  than 
65°  C.,  and  the  composition  of  their  salts  corresponds  to  the 
character  of  the  beds  traversed.  On  the  supposition,  believed  to 
be  well  founded,  that  only  a  moderate  loss  in  heat  takes  place 

» Bull.  Geol.  Soc.  Am.,  vol.  19,  1908,  p.  502. 
2  See  Chapter  VI. 


36  MINERAL  DEPOSITS 

during  the  ascent,  a  water  of  the  temperature  named  would  be 
derived  from  a  depth  of  about  5,500  feet.  Large  regions  of  the 
earth,  such  as  the  Scandinavian  peninsula,  contain  no  warm 
springs,  and  the  eastern  part  of  the  American  Continent  yields 
very  few  of  them.  Fuller  says : 

The  results  of  drilling  in  sedimentary  and  crystalline  rocks,  as  well 
as  the  studies  of  deep  mines,  show  that  in  all  probability  water  does 
not  commonly  exist  in  the  rocks  under  great  pressure,  although  such 
may  be  exerted  in  an  occasional  crevice.  It  is  not  believed  that 
hydrostatic  waters  exist,  except  possibly  in  rare  instances,  at  depths 
of  over  10,000  feet,  and  that  in  reality  the  estimate  of  a  depth  of 
6,520  meters,  or  20,000  feet,  as  the  limit  of  the  zone  of  open  cavities 

is  closely  approximate  to  the  truth If  waters  were  freely 

circulating  at  great  depths,  within  the  zone  of  fracture,  hot  springs 
would  certainly  be  more  common  along  the  numerous  faults  of  the 
Piedmont,  Appalachian,  and  similar  regions.1 

Van  Hise  suggests  that  the  decreased  density  and  viscosity  of 
water  at  higher  temperatures  may  lessen  the  head  necessary  for 
ascending  springs,  but  it  may  be  doubted  whether  these  factors 
would  ever  offset  the  great  friction  encountered  during  the  down- 
ward passage.  Faulting  and  mountain-building  processes  de- 
velop heat  and  this  disturbance  of  the  conditions  of  temperature 
may  result  in  convection  currents  and  an  increased  circulation 
of  the  water  stored  in  the  rocks. 

Influence  of  Volcanism. — When  magmas  are  intruded  into 
the  zone  of  fracture  the  conditions  become  more  complicated. 
It  is  thought  by  some  that  atmospheric  waters  are  able  to  de- 
scend into  the  deep  regions  and  become  absorbed  by  the  magmas, 
but  this  view  appears  improbable.  Before  its  irruption  into  the 
zone  of  fracture  the  magma  is  assuredly  far  beyond  the  reach  of 
any  waters  percolating  from  the  surface.  Daubree's  well-known 
experiment  has  often  been  cited,  as  showing  how  water  may  pass 
through  a  disc  of  sandstone  against  a  certain  counter  pressure 
of  steam.  Recent  critical  examination2  has  shown  the  fallacies 
involved  in  the  experiment,  and  indicate  that  "the  probabilities 
are  all  against  the  notion  that  appreciable  amounts  of  meteoric 
waters  can  ever  penetrate  into  deep-seated  and  highly  heated 
rock  masses." 

1  Water-Supply  Paper  160,  U.  S.  Geol.  Survey,  1906,  p.  64. 
1  John  Johnston  and  L.  D.  Adams,  Observations  on  the  Daubree  experi- 
ment, etc.,  Jour.  Geology,  vol.  22,  1914,  pp.  1-15. 


THE  FLOW  OF  UNDERGROUND  WATER  37 

The  presence  of  a  heated  body  in  the  zone  of  fracture  would 
undoubtedly  quicken  the  circulation  of  water  by  inducing  strong 
convection  currents  and  expelling  the  stored  water  from  its 
reservoir.  Whether  this  action  is  sufficient  to  account  for  the 
remarkable  number  and  volume  of  hot  springs  rising  in  volcanic 
regions  may  well  be  doubted,  and  it  is  thought  that  the  magma 
itself  gives  up  most  of  its  constitutional  water,  partly  when 
moving  up  to  higher  levels,  partly  when  crystallizing  to  solid 
rocks. 

Conclusions. — In  conclusion  it  is  believed  that  water  in 
quantities  sufficient  to  supply  an  ascending  circulation  can  only 
exceptionally  attain  a  depth  of  10,000  feet  and  that,  except  in 
regions  of  great  dynamic  movements,  the  active  circulation  is 
confined  to  the  uppermost  few  thousand  feet.  More  commonly 
the  depth  of  active  circulation  is  measured  by  the  level  of  surface 
discharge  and  the  water  below  that  level  is  practically  stagnant; 
the  lower  limit  of  the  body  of  stagnant  water  then  forms  an 
irregular  surface  descending  to  greater  depths  along  the  fractures 
and  rising  higher  in  the  intervening  blocks  of  solid  ground. 

Examples  of  Movement  of  Water. — According  to  Fuller, 
water  supplies  in  wells  in  crystalline  rocks  are  usually  found 
within  200  or  300  feet  of  the  surface  and  it  is  ordinarily  useless 
to  go  below  a  depth  of  500  feet  (Fig.  3).  The  occurrence  of 
porous  strata  which  are  capable  of  holding  immense  quantities  of 
water  but  in  which  none  whatever  is  actually  found  is,  according 
to  Fuller,  a  common  experience  of  drillers  in  this  country, 
even  where  the  upper  strata  contain  a  well-defined  water  table. 

Investigations  of  joints  in  the  crystalline  rocks  of  Connecticut 
have  shown,  according  to  Fuller,  that  the  water  occurs  largely  in 
the  vertical  joints,  which  have  an  average  spacing  of  3  to  7 
feet  at  the  surface.  In  depth  these  joints  diminish  rapidly  or 
close  up  and  it  is  therefore  not  advisable  to  go  below  250  feet  in 
search  of  water.  It  is  estimated  by  E.  E.  Ellis  that  the  water 
present  in  the  upper  2,000  feet  of  the  crystalline  rocks  is  only 
16  per  cent,  of  their  capacity,  or  0.000007  of  the  rock  volume. 

The  evidence  from  many  mining  regions  is  of  considerable 
importance.  In  the  Sierra  Nevada  of  California  deep  canyons 
are  separated  by  broad-backed  ridges  capped  with  Tertiary 
gravels  and  andesitic  tuffs.  The  abundant  precipitation  per- 
colates into  the  porous  tuffs  and  gathers  in  the  gravel  basins, 
from  the  lower  parts  of  which  large  quantities  of  cold  springs 


38  MINERAL  DEPOSITS 

issue.  This  upper  zone  of  gathering  and  discharge  may  be  1,500 
feet  deep  and  may  lie  the  same  distance  above  the  bottom  of  the 
canyons.  In  spite  of  the  depth  of  the  percolating  zone,  the 
waters  are  potable,  pure,  and  cold.  A  part  of  the  water  sinks 
into  the  underlying  bedrock  of  slate  or  granite,  but  the  quantity 
is  far  less  than  in  the  more  porous  Tertiary  strata  and  it  finds 
its  lowest  level  of  discharge  along  the  beds  of  the  rivers.  For 
the  Sierra  as  a  whole  the  Great  Valley  of  California  forms  the 
ultimate  level  of  discharge.  In  the  whole  western  part  of  the 
range  there  are  no  thermal  springs  and  very  few  strong  ascend- 
ing springs,  in  spite  of  the  prevalent  fissility  and  jointing  in  the 
rocks.  Hot  springs  are  encountered  only  along  the  eastern  slope 
of  the  range,  a  region  which  in  the  late  Tertiary  and  Quaternary 
was  the  scene  of  great  dislocations  and  volcanic  activity.  In  the 
gold-quartz  veins  contained  in  the  old  rocks  of  the  western  slope 
much  water  is  found  in  fissures  to  a  depth  of  about  800  or  1,000 
feet.  Below  this  little  water  is  met  and  many  stopes  and  drifts 
are  entirely  dry,  and  this  applies  both  to  mines  high  up  on  the 
slopes,  as  at  Nevada  City  and  Grass  Valley,  and  to  the  Mother 
Lode  mines  of  the  foot-hill  region. 

Cripple  Creek,  Colorado,  is  another  interesting  example. 
Here  we  have  a  granitic  plateau  at  an  elevation  of  9,000  feet 
above  the  sea;  this  plateau  contains  a  volcanic  plug  about  2 
miles  in  diameter  which  is  largely  filled  with  porous  breccias  and 
tuffs.  The  water  fills  the  volcanic  rocks  as  in  a  sponge  inserted 
in  a  cup  and  the  mining  operations  to  a  depth  of  1,500  feet  have 
tapped  heavy  flows.  But  even  in  this  water-logged  mass  there 
are  solid  intrusive  bodies,  for  instance  at  the  Vindicator  mine, 
at  a  depth  of  1,000  feet,  which  are  so  dry  that  water  must  be 
sent  down  for  drilling.  The  data  thus  far  available  have  led 
Ransome  to  the  conclusion  that  even  at  Cripple  Creek  the  water 
is  slowly  diminishing  in  quantity  at  increasing  depth.  The  big 
drainage  tunnel  now  under  way,  which  will  tap  the  veins  at  a 
depth  of  800  feet  below  the  present  lowest  tunnels,  will  afford 
more  information  on  this  subject.  The  granite  which  surrounds 
this  water-soaked  plug  contains  very  little  water  and  at  most 
places  is  practically  dry,  in  spite  of  the  great  hydrostatic  pressure. 
The  ultimate  level  of  possible  discharge  would  be  in  the  valley 
of  the  Arkansas,  2,500  feet  lower  and  many  miles  distant,  but  it 
may  be  gravely  doubted  whether  any  water  from  the  Cripple 
Creek  mines  ever  finds  its  way  through  the  granite  mass  to  this 


THE  FLOW  OF  UNDERGROUND  WATER  39 

level.1  Van  Hise,  after  stating  (Metamorphism,  p.  1065)  that 
during  a'certain  time  the  Portland  mine,  at  Cripple  Creek,  yielded 
water  to  the  amount  of  between  300  and  900  gallons  per  minute, 
asks  whether  better  evidence  could  be  required  for  proving  the 
existence  of  an  extremely  active  circulation.  The  answer  to  this 
is  that  the  water  was  simply  stagnant,  stored  water  filling  an 
underground  reservoir. 

In  the  copper  mines  of  Butte,  Montana,  where  the  granitic 
rocks  are  greatly  faulted  by  movements  of  late  date,  much  water 
was  encountered,  extending  in  places  down  to  2,400  feet,  or  the 
bottom  of  the  mines.  No  ascending  springs  are  found  at  the 
surface,  nor  any  hot  springs,  although  a  high  range  adjoins  the 
mines  on  the  east  and  conditions  seem  to  be  favorable  for  deep 
circulation.  The  water  is  probably  almost  stagnant,  and  Weed 
mentions  the  existence  of  large  bodies  of  dry  rock.2  One  such 
body  on  the  1,600-foot  level,  1,200  feet  in  width,  is  absolutely 
dry. 

Leadville,  Colorado,  is  another  place  where  the  faulting  is 
extensive  and  of  comparatively  recent  date.  At  1,500  feet,  the 
greatest  depth  attained,  there  is  still  much  water,  mainly  along 
the  faults. 

At  Rossland,  British  Columbia,  according  to  Bernard  McDon- 
ald,3 the  mine  waters  increase  greatly  during  the  spring  months. 
The  water  level  is  at  40  feet  and  the  quantity  increases  to  a  depth 
of  200  to  350  feet.  Below  350  feet  a  decrease  begins,  slowly  at 
first  but  soon  more  rapid,  until  at  900  feet  there  is  only  a  slight 
seepage  and  below  1,000  feet  the  mine  is  dry.4  Weed  states 
that  in  the  copper  mine  at  Ely,  Vermont,  an  incline  shaft  was 
carried  down  for  a  length  of  3,600  feet,  attaining  a  vertical  depth 
of  1,700  feet.  There  is  no  water  here  below  a  vertical  depth  of 
600  feet.  At  Przibram,  Bohemia,  the  workings  are  dry  and 
dusty  at  a  depth  of  3,000  feet.  In  Cornwall,  and  in  New  Found- 
land  mines  have  been  worked  underneath  the  sea,  and  sometimes 
close  to  the  sea  bottom,  without  irruptions  of  salt  water.5 

'Lindgren  and  Ransome,  Prof.  Paper  54,  U.  S.  Geol.  Survey,  1906, 
pp.  233-251. 

Finch,  J.  W.,  op.  cit.,  p.  204. 

2  M.  L.  Fuller,  Water-Supply  Paper  160,  U.  S.  Geol.  Survey-,  1906,  p.  65. 

3  T.  A.  Rickard,  Min.  and  Set.  Press,  June  27,  1908. 

4  M.  L.  Fuller,  op.  cit.,  p.  65. 

6  For  other  examples  see  J.  F.  Kemp,  The  ground  waters,  Trans.,  Am. 
Last.  Min.  Eng.,  vol.  45,  1914,  pp.  3-25. 


40        .  MINERAL  DEPOSITS 

One  of  the  most  convincing  examples  is  that  furnished  by  the 
deep  copper  mines  of  Michigan  and  fully  set  forth  by  A.  C.  Lane.1 
He  shows  that  the  surface  waters  are  of  the  normal,  potable  type 
and  that  they  descend  in  diminishing  quantities  only  to  a  depth 
of  about  1,000  or  1,500  feet  below  the  surface.  Below  this  depth 
moisture  is  scant,  but  where  it  appears  it  consists  of  drippings  of 
strong  calcium  chloride  brine  which  cannot  in  any  way  be  ex- 
plained as  being  derived  from  the  surface  water.  Many  levels 
are  absolutely  dry  and  water  must  be  sent  down  for  drilling. 
This  case  is  particularly  convincing,  for  we  have  here  many  fea- 
tures in  favor  of  a  strong  circulation:  Moist  climate,  inclined 
position  of  beds,  and  great  permeability. 

No  certain  figure  can  be  assigned  to  the  depth  of  the  ground 
water;  it  may  be  shallow  or  the  water  may  descend  on  strong 
fractures  for  several  thousand  feet.  At  any  rate  the  quantity  is 
limited,  and  the  water  is  largely  stagnant  and  is  much  more 
likely  to  decrease  than  to  increase  at  depths  below  1,000  feet. 

Depth  of  Water  Level. — In  moist  climates  the  water  level  is 
usually  found  within  50  feet  of  the  surface,  but  in  regions  with 
less  rainfall  there  is  great  diversity  in  the  location  of  this  upper 
limit  of  the  zone  of  saturation.  In  the  more  arid  regions  the 
water  is  often  met  300  or  400  feet  below  the  surface.  In  the 
valley  of  Hachita,  New  Mexico,  no  water  is  found  in  the  sands 
and  gravels  until  a  depth  of  500  feet  is  reached;  at  the  Abe  Lin- 
coln gold-quartz  mine,  New  Mexico,  a  little  water  began  to 
come  in  1,300  feet  below  the  surface.  In  the  rich  deposits  of 
Tintic,  Utah,  the  water  level  in  limestone  lies  1,700  to  2,000  feet 
below  the  surface,  but  in  mines  in  andesite  and  porphyry  in  the 
same  district  water  may  be  found  at  much  less  depth. 

When  water  is  being  drained  or  pumped  from  a  mine  the  water 
level  is  artificially  depressed,  in  the  form  of  a  flat  funnel.  The 
pump  in  this  case  does  not  merely  drain  the  bottom  level,  but 
receives  water  from  higher  levels  farther  away  from  the  shaft. 
It  is  important  to  note  this,  for  the  water  thus  obtained  from 
the  bottom  of  a  wet  mine  may  not  have  the  same  composition  as 
that  originally  belonging  to  this  level. 

Total  Amount  of  Free  Water  in  Earth's  Crust. — Several  es- 
timates have  been  made  of  the  total  amount  of  uncombined  water 
contained  in  the  upper  crust.  The  older  estimates  by  Delesse, 

1  Mine  waters,  Trans.,  Lake  Superior  Min.  Inst.,  vol.  12,  1908,  pp.  154- 
163. 


THE  FLOW  OF  UNDERGROUND  WATER  41 

Dana,  and  Slichter  were  very  high.  Chamberlin  and  Salisbury1 
believed  that  the  water  in  the  earth  would  be  equivalent  to  a 
layer  800  feet  deep  over  its  entire  surface.  Van  Hise2  reduced 
the  estimate  materially  and  concluded  that  it  would  be  equivalent 
to  a  sheet  of  water  226  feet  (69  meters)  thick  over  the  continental 
areas. 

Fuller3  estimates,  after  a  careful  study  of  the  problem,  that 
the  total  water  would  be  equivalent  to  a  uniform  sheet  96  feet 
thick  over  the  entire  surface  of  the  earth.  This  estimate  is 
probably  more  nearly  correct  than  any  of  the  others. 

1  Geology,  1904,  vol.  1,  pp.  206-207. 

2  A  treatise  on  metamorphism,  Mon.  47,  U.  S.  Geol.  Survey,  1904,  pp. 
128-129,  570-571. 

1  Op.  tit.,  p.  72. 


CHAPTER  IV 
THE  COMPOSITION  OF  UNDERGROUND   WATERS 

INTRODUCTION 

Water  is  continually  evaporated  from  sea  and  land.  From 
the  gathered  clouds  it  is  precipitated  as  pure  rain  water,  which, 
by  the  aid  of  absorbed  oxygen  and  carbon  dioxide  immediately 
begins  the  attack  on  disintegrating  rocks.  The  rivers  finally 
carry  suspended  particles  and  dissolved  salts  to  the  sea.  Con- 
sidered more  closely,  the  rivers  are  the  products  of  the  weak 
solutions  of  the  immediate  run-off  and  the  stronger  ground 
water  solutions  from  the  zone  of  discharge  (Chapter  III)  which 
have  been  in  longer  contact  with  the  rocks  and  leached  them 
more  thoroughly.  This  inconspicuous  process  of  decomposition 
of  rocks  and  solution  of  resulting  salts  is  one  of  scarcely  realized 
geologic  importance.  F.  W.  Clarke1  states  that  the  Mississippi 
annually  carries  to  the  sea  about  108  metric  tons  of  salts  from 
each  square  mile  of  territory  drained;  the  Colorado  abstracts 
about  51  tons  from  the  same  unit  area. 

A  smaller  part  of  the  ground  water  sinks  to  enrich  the  static 
zone  of  stagnant  waters,  and  ultimately  becomes  highly  charged 
with  salts.  A  still  smaller  part  of  the  water  is  permanently 
withdrawn  by  entering  hydrated  compounds  like  kaolin. 

To  complete  the  picture  we  must  not  overlook  the  ascending 
hot  solutions  which  come  from  great  depths  and  which  in  part 
at  least  are  derived  from  rising  magmas. 

As  most  mineral  deposits  have  been  formed  by  aqueous 
solutions  the  composition  of  the  waters  of  rivers,  lakes  and  seas 
becomes  a  study  of  importance.  Even  more  important  is  the 
composition  of  the  underground  waters  of  wells  and  springs. 
In  considering  them  from  a  chemical  standpoint  it  will  be  best 

1  F.  W.  Clarke,  Geochemistry,  Bull.  616,  U.  S.  Geol.  Survey,  1916,  p. 
113. 

R.  B.  Dole  and  H.  Stabler,  Water-Supply  Paper  234,  U.  S.  Geol.  Survey, 
1909,  p.  78. 

42 


COMPOSITION  OF  UNDERGROUND  WATERS      43 

to  attempt  no  artificial  distinction  between  thermal  or  cold 
mineral  or  non-mineral  waters. 

The  substances  dissolved  in  the  ground  water  depend  upon 
the  formations  which  it  traverses.  At  the  immediate  surface 
organic  life  may  influence  the  composition.  In  general,  each 
formation  yields  its  characteristic  salts  to  the  precolating  waters 
Each  natural  water  is  a  chemical  system  of  balanced  constituents 
of  more  or  less  dissociated  electrolytes,  of  colloids,  and  of  gases. 

CALCIUM  CARBONATE  WATERS  IN  IGNEOUS  ROCKS 

Igneous  rocks,  of  deep-seated  origin,  as  well  as  crystalline 
schists  contain  only  small  amounts  of  soluble  salts.  The  surface 
waters  penetrating  them  are  charged  with  more  or  less  carbon 
dioxide,  which,  at  ordinary  temperatures,  gradually  decomposes 
the  silicates,  particularly  the  pyroxene,  amphibole,  biotite,  and 
the  calcium  feldspars;  the  alkali  feldspars  are  more  slowly 
attacked.  As  a  result  the  springs  in  such  terranes  will  have  a  low 
salinity,  rarely  above  1,000  parts  per  million,  and  will  contain 
principally  calcium  carbonate,  with  more  or  less  of  the  corre- 
sponding magnesium  salt;  a  smaller  amount  of  sodium  carbonat'e 
and  much  less  of  potassium  carbonate  are  present.  There  will 
be  little  of  the  chlorine  and  sulphuric  acid  radicles.  The  silica 
is  relatively  high.  Such  calcium  carbonate  waters  are  character- 
istic not  only  of  superficial  springs,  but  also  of  the  deeper  cir- 
culation in  crystalline  terranes;  in  the  latter  case  the  waters 
may  be  warm,  though  usually  they  are  cold.  The  spring-fed 
rivers  in  such  terranes  have  a  similar  composition. 

Where  magnesian  rocks  like  basalt  and  serpentine  abound,  the 
underground  waters  are  richer  in  magnesia  than  usual,  and  this 
substance  may  even  equal  the  calcium.  Waters  of  this  calcium 
carbonate  type  are  common  and,  when  encountered  as  ascending 
springs  or  elsewhere,  justify  the  presumption  of  surface  origin.1 

1  Some  years  ago,  in  a  report  on  the  gold-quartz  veins  of  Nevada  City 
and  Grass  Valley,  Cal.  (Seventeenth  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt. 
2,  1896,  p.  121),  I  presented  an  analysis  of  an  ascending  spring  found  in  the 
Federal  Loan  mine  which  carried  some  arsenic  and  hydrogen  sulphide.  At 
that  time  I  held  the  opinion  that  this  water  might  possibly  have  bad  some 
connection  with  the  genesis  of  the  vein,  but  it  is  now  apparent  that  it  is 
simply  water  of  the  general  surface  circulation  which  happened  to  find  its 
way  up  on  the  vein  and  which  dissolved  certain  constituents  from  it.  The 
analysis  is  quoted  on  page  44. 


44 


MINERAL  DEPOSITS 


It  often  happens  that  hot  springs  which  are  not  characterized 
by  an  abundance  of  calcium  carbonate  are  accompanied  by 
numerous  other  springs  of  somewhat  lower  temperature.  A 
comparison  of  analyses  will  usually  show  that  in  proportion  to  the 
lowering  of  the  temperature  the  quantity  of  calcium  carbonate 
increases;  this  indicates  a  cooling  admixture  of  surface  waters 
bearing  calcium. 

COMPOSITION  OF  SALTS  AND   TOTAL  SALINITY  OF  SURFACE  WATERS    IN 
CRYSTALLINE  ROCKS 


A. 

B 

C 

D 

CO3  

so,  

Cl  

31.91 
9.07 
4.03 

47.14 
6.67 

4  18 

57.80 
3.10 
1  30 

38.46 
15.35 
2  81 

s 

0  50 

Ca 

14  53 

22  67 

13  90 

13  24 

Mg  
Mn  
Na  
K  
(Al,Fe)203  
Si02  

2.93 

10.80 
2.72 
0.51 
23.50 

6.17 
I  5.32 
7.85 

2.30 
0.10 
5.50 
0.40 
1.70 
13.40 

4.33 

12.86 
3.76 

9.19 

100.00 


100.00 


100.00 


100.00 


Salinity, 

parts  per  million  .  .  i 

37 

280 

245 

282 

NOTES  RELATING  TO  ABOVE  ANALYSES 

A.  Cache  la  Poudre  River,  Colorado,  above  North  Fork.     In  schist  and 
granite.     Analysis  by  W.  P.  Headden.     See  F.  W.  Clarke,  Geochemistry, 
Bull.  616,  1916,  p.  65. 

B.  Aztec  Spring,  4  miles  east  of  Santa  Fe,  New  Mexico.     In  schist  and 
granite.     Cold.     Analysis  by  F.  W.  Clarke,  Geochemistry,  p.  64. 

C.  Cold  spring  in  Federal  Loan  mine,  in  granite  and  schist,   Nevada 
City,  California.     Approximate  analysis  by  W.  F.  Hillebrand;  contains  a 
little  manganese  and  trace  of  lead.     Deposits  calcium  carbonate,  limonite, 
and  some  arsenic.     Sulphur  probably  due  to  reduction  from  small  amount 
of  H2S  after  bottling.     Seventeenth  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  2, 
1896,  p.  121. 

D.  Cold  water  from  500-foot  level,  Geyser  mine,  Silver  Cliff ,  Colorado . 
Analysis  by  W.  F.  Hillebrand.    In  "The  mines  of  Custer  County,  Colorado," 
by  S.  F.  Emmons.     Seventeenth  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  2,  1896, 
p.  461.     Free  and,  semi-combined  CO,  38.8  parts  per  million. 


COMPOSITION  OF  UNDERGROUND  WATERS      45 

Waters  of  the  kind  described  above  are  generally  poor  in  the 
rarer  metals.  A  little  arsenic  is  found  in  some  cases  and  traces 
of  barium,  strontium,  lithium,  boron  and  phosphorus  are  some- 
times recorded.  Where  they  traverse  mineral  deposits, 
metals  contained  in  the  deposits  will  of  course  be  dissolved, 
as  in  the  water  from  the  Federal  Loan  mine,  Nevada  City, 
California.  The  springs  may,  under  favorable  conditions,  form 
crusts  of  calcium  carbonate  and  hydroxide  of  iron,  but  as  a 
rule  their  powers  of  solution  and  deposition  are  weak.  Where 
the  rocks  contain  much  pyrite,  as  often  is  the  case  in  mining 
districts,  the  sulphates,  especially  calcium  sulphate,  rapidly 
increase  in  the  waters. 


CALCIUM  CARBONATE  WATERS  IN  SEDIMENTARY  ROCKS 

Waters  of  the  type  described  above  are  not  confined  to  igneous 
rocks.  They  are  often  found  in  circulation  in  glacial  drift  and 
also  in  sedimentary  rocks — sandstones,  limestones,  and  dolo- 
mites. Such  waters  sometimes  contain  hydrogen  sulphide  and 
carbon  dioxide.  The  derivation  of  the  latter  is  not  always 
easily  explained.  In  some  cases  the  gas  may  emanate  from  a 
deep-seated  magma,  but  more  commonly  it  is  formed  by  decom- 
position of  carbonates.  An  example  of  such  water  is  furnished 
by  the  cold  Cresson  Spring  in  Pennsylvania,  which  issues  from  a 
shale  member  between  sandstones  in  a  3,000-foot  series  of  Coal 
Measures,  containing  practically  no  limestone.  This  water  is 
pure,  its  salinity  being  only  442  parts  per  million,  and  of  this 
272  may  be  calculated  as  calcium  carbonate,  76  as  sulphates 
of  sodium,  magnesium,  and  calcium,  and  11  as  sodium  chloride. 
According  to  a  careful  analysis  by  Genth  this  water  contains 
traces  of  nickel,  cobalt,  iron,  manganese,  copper,  strontium, 
barium,  and  fluorine,  0.17  part  per  million  of  the  last-named 
element  being  present.  Several  analyses  of  similar  well-known 
waters  are  quoted  on  page  46. 


46 


MINERAL  DEPOSITS 


COMPOSITION  OF   SALTS   AND  TOTAL  SALINITY  OF  CALCIUM  CARBONATE 
WATERS  IN  SEDIMENTARY   ROCKS 


A 

* 

C 

CO3                   .                   

40  02 

41  47 

'48  64 

SO,  :  :.... 
Cl  

21.73 
0.64 

3.93 

1  27 

6.30 
5  40 

PO 

0  03 

NO, 

0  23 

Ca             

23  35 

23  54 

16  56 

Me  .. 

5  82 

2  56 

7  64 

Na  

1.81 

2  38 

10  36 

K 

2  04 

0  80 

NH4 

0  03 

Mn                 ...              

0  17 

Fe,O, 

\ 

0  40 

A1203  
SKX  

0.58 
4.01 

\     0.10 
22  85 

0.20 

4  50 

BO2 

0  64 

100.00 

100.00 

100.00 

Salinity,  parts  per  million  

563 

199 

222 

HCO3. 


NOTES  RELATING  TO  ABOVE  ANALYSES 


A.  Virginia  Hot  Springs,  Virginia.     Analysis  by  F.  W.  Clarke.     "Geo- 
chemistry," p.  193.     Temperature  tepid.     Issues  from  Paleozoic  sediments. 
See  also  Bull.  32,  U.  S.  Geol.  Survey,  1886,  p.  61. 

B.  Hot  Springs,  Arkansas.     Spring  No.  16.     Temperature  62°  C.     Issues 
from  sharply  compressed  folds  of  Silurian  sandstone  and  shale.     CO2  from 
bicarbonates  28.34  cc.  per  liter;  nitrogen  8.39  cc.  per  liter;  oxygen  2.49 
cc.  per  liter;  H2S  none.     Arsenic  none;  trace  iodine  and  bromine.     Analysis 
by  J.  K.  Haywood.     The  Hot  Springs  of  Arkansas,  Senate  Doc.  282,  Fifty- 
seventh   Congress,   First   Session,   1902,   p.   94.     Recalculated  by  F.   W. 
Clarke,  "Geochemistry,"  1916,  p.  195. 

C.  Cold  water  from  well  of  Missouri  Lead  and  Zinc  Company,  Joplin, 
Missouri.     Depth  1,387  feet.     In  Paleozoic  limestone.     Analysis  by  Cleve- 
land and  Millar  Laboratory.     Water-Supply  Paper  195,  U.  S.  Geol.  Survey, 
1907,  p.  137.     Recalculated. 

These  waters  frequently  form  ascending  springs.  Bartlett 
Springs,  Lake  County,  California,  the  water  of  which  is  exten- 
sively used  in  that  State,  probably  belong  to  this  class.  The 


COMPOSITION  OF  UNDERGROUND  WATERS      47 

water  contains  782  parts  of  salts  per  million,  of  which  493  may 
be  calculated  as  calcium  carbonate.  It  is  rich  in  free  carbon 
dioxide  and  is  low  in  chlorine,  sulphuric  acid  radicle,  sodium, 
and  potassium,  but  contains  some  iron,  probably  as  carbonate, 
a  little  barium,  phosphoruSj  and  about  63  parts  of  silica  per 
million.1 

Carbonate  waters  are  undoubtedly  active  in  solution  and 
deposition  in  the  upper  part  of  the  crust,  and  especially  in 
the  formation  of  concentrations  from  weathering  rocks.  They 
may  deposit  calcareous  sinters  and  effect  concentrations  of  iron 
and  manganese.  Some  lead  and  zinc  deposits  in  limestone  may 
also  be  genetically  connected  with  them;  their  power  of  solution 
and  concentration  of  rarer  metals  appears  to  be  weak,  unless 
they  contain  carbon  dioxide  and  hydrogen  sulphide.  Such 
waters  in  Kansas,  Missouri,  and  Kentucky  have  been  found  to 
contain  zinc  and  probably  also  lead  and  copper. 

The  salts  are  surely  obtained  from  the  rocks  traversed. 

CHLORIDE  WATERS  IN  SEDIMENTARY  ROCKS 

Infiltration  from  Present  Oceans. — Wells  and  springs  along  the 
sea  coasts  usually  contain  a  higher  percentage  of  sodium  chloride 
than  farther  inland;  this  may  be  caused  either  by  infiltration  of 
sea  water  into  sediments  or  porous  igneous  rocks,  or  by  winds 
carrying  finely  divided  salt  from  the  spray  of  the  waves. 

Solution  of  Saline  Deposits. — Many  past  geologic  periods  in- 
cluded epochs  of  desiccation  and  desert  climate  when  salt  was 
precipitated  from  evaporating  waters  of  closed  basins.  Surface 
waters  encountering  such  sedimentary  deposits  easily  dissolve 
the  sodium  chloride,  and  wells  and  springs  rich  in  this  salt  are 
characteristic  of  many  regions.  Besides  sodium  these  waters 
contain  calcium  and  magnesium,  and  they  are  often  rich  in 
calcium  chloride.  They  are  poor  in  silica  and  potassium  and 
rarely  contain  much  calcium  which  can  be  combined  with 
carbon  dioxide.  The  presence  of  bromine  is  almost  character- 
istic; traces  of  iodine  and  boron  are  often  found.  Barium 
and  strontium  are  almost  always  present,  the  former  sometimes 
in  considerable  amount.  Free  carbon  dioxide  and  hydrogen 

1  Winslow  Anderson,   Mineral  springs,   etc.,  of  California,  1892,  p.  94. 
G.  A.  Waring,  Springs  of   California,  Water-Supply  Paper  338,   U.   S. 
Geol.    Survey,  1915. 


48  MINERAL  DEPOSITS 

sulphide  are  sometimes  found,  the  latter  especially  where  there 
is  an  abundance  of  calcium  sulphate.  Waters  of  this  general 
type  are  characteristic  of  certain  Paleozoic  beds  in  the  eastern 
United  Slates,  as.  for  instance,  the  Silurian  of  New  York  and 
Michigan  and  certain  parts  of  the  Carboniferous  in  Michigan. 
In  the  western  States  the  "Red  Beds,"  generally  of  Permian  or 
Triassic  age,  are  sometimes  rich  in  salt  and  gypsum,  and  this 
combination  appears  in  the  waters  of  these  terranes. 

There  are  many  similar  springs  and  wells  in  Pennsylvania,  and 
in  fact  all  through  the  interior  Paleozoic  basin,  from  Arkansas 
to  Canada.  The  Saratoga  Springs  of  New  York,  issuing  from 
Silurian  limestones,  probably  belong  to  this  class.  Their  tem- 
perature is  about  50°  F.;  the  total  solids  amount  to  about  11,000 
parts  per  million,  of  which  the  larger  part  is  sodium  chloride. 
Barium  is  conspicuously  present,  in  some  analyses  to  a  max- 
imum of  about  34  parts  per  million,  likewise  bromine  at  about 
1.20  parts  per  million.  Small  amounts  of  silica,  iron,  and  lithium, 
and  traces  of  boron,  iodine,  and  fluorine  are  recorded.  The 
origin  of  the  CO?  so  abundant  at  Saratoga  Springs  is  uncertain. 
J.  F.  Kemp  believes  it  to  be  of  magmatic  derivation.  Examples 
of  such  waters  are  given  in  the  table  of  analyses  on  page  50. 

Certain  of  these  waters  are  abnormally  rich  in  calcium  chlo- 
ride, that  most  easily  soluble  salt  which  remains  as  the  last 
liquid  residue  in  evaporating  brines.  Several  instances  of  such 
waters  have  been  interpreted  as  residual  or  connate  brines, 
remaining  in  early  isolated  Paleozoic  basins.1 

In  the  lower  peninsula  of  Michigan  brines  are  obtained  from 
deep  wells  in  the  Carboniferous  and  Silurian.  One  of  the  springs 
in  this  region  contains  12,000  parts  per  million  in  total  solids, 
with  6,000  calculated  as  NaCl,  1,600  as  MgCl2,  and  4,100  as  Ca 
C12.  The  researches  of  A.  C.  Lane  have  shown  that  the  scanty 
waters  in  the  deep  levels  of  the  copper  mines  near  Houghton 
have  a  similar  composition,  except  that  here  calcium  chloride 
prevails.  These  waters,  which  are  found  in  amygdaloid  lava 
flows  and  associated  sedimentary  rocks  of  the  Upper  Algonkian 
(Keweenawan),  are  perhaps  to  be  regarded  as  residual  oceanic 
waters,  which,  in  their  long  contact  with  the  rocks,  have  under- 
gone considerable  changes.  An  analysis  is  given  below  (p.  50) 
This  water  contains  no  barium. 

1  E.  M.  Shepard,  Underground  waters  of  Missouri,  Water-Supply  Paper 
195,  U,  S.  Geol.  Survey,  1907,  p.  81. 


COMPOSITION  OF  UNDERGROUND  WATERS      49 

In  oil  bearing  districts  salt  waters  are  of  very  frequent  occur- 
rence. They  are  rich  in  sodium  chloride  and  often  also  contain 
the  chlorides  of  calcium  and  magnesium,  as  well  as  more  or  less 
bicarbonates.  They  are  always  poor  in  sulphates,  and  this  is 
perhaps  due  to  their  reduction  by -the  hydrocarbons.  Such  salt 
solutions  have  been  variously  interpreted  as  connate  waters,  as 
solutions  of  saline  deposits  and  as  of  magmatic  origin.1 

In  the  western  States  many  similar  waters  occur  in  the  Red 
Beds,  but,  as  stated  they  are  usually  also  rich  in  calcium  sulphate. 
As  an  example  may  be  cited  the  tepid  Quelites  Spring  in  New 
Mexico,2  which  ascends  through  Red  Beds  and  contains  about 
2.6  per  cent,  of  solids;  one-half  is  calculated  as  sodium  chloride 
and  the  larger  part  of  the  remainder  as  calcium  sulphate.  Bro- 
mine, boron,  and  barium  are  present.  On  the  Pacific  coast 
such  waters  are  not  common.  Byron  Hot  Springs,  California, 
may  be  cited  as  an  example.  The  temperature  spring  is  76°  C. 
The  water  contains  about  13,000  parts  of  salts  per  million,  of 
which  over  10,000  parts  are  sodium  chloride.  A  large  portion  of 
the  remainder  consists  of  calcium  chloride.  Small  quantities  of 
bromine,  iodine,  and  barium  are  present.3 

The  Triassic  strata  of  the  French  Alps  and  the  Pyrenees  are 
rich  in  similar  waters,  many  of  which  are  warm.  The  mineral 
combination  is  a  characteristic  mingling  of  chlorides  and  sul- 
phates, and  undoubtedly  all  of  the  constituents  are  derived  from 
the  sedimentary  rocks  mentioned. 

The  Spring  of  Mey  in  Haute  Savoie,  with  a  temperature  of 
39.8°  C.,  may  be  taken  as  a  typical  example.  It  contains  both 
carbon  dioxide  and  hydrogen  sulphide  and  yields  a  total  of 
5,000  parts  per  million  of  dissolved  salts,  of  which  1,753  parts 
are  calculated  as  sodium  chloride,  1,773  as  sodium  sulphate,  and 
957  as  calcium  sulphate.  Some  bromine  and  traces  of  iodine, 
phosphorus,  and  arsenic  are  present.4 

A  celebrated  group  of  these  chloride  springs  are  found  in 
Germany  on  both  sides  of  the  Rhine.  Among  them  are  the 

1  C.  W.  Washburne,  Chlorides  in  oil  field  waters,  Trans.,  Am.  Inst.  Min. 
Eng.,  vol.  45,  1915,  pp.  687-693.     G.  S.  Rogers,  Chemical  relations  of  the 
oil  field  waters  in  the  San  Joaquin  Valley,  California,  Bull.  653,  U.  S.  Geol. 
Survey,  1917. 

2  F.  A.  Jones,  New  Mexico  mines  and  minerals,  1904,  p.  309. 

3  Winslow  Anderson,  Mineral  springs  and  health  resorts  of  California, 
1892,  p.  106. 

4  Jacquot  et  Willm,  Les  eaux  minerales  de  la  France,  Paris,  1894,  p.  243. 


50 


MINERAL  DEPOSITS 


waters  of  Soden,  Homburg,  Wiesbaden,  Kreutznach,  Kissingen, 
Nauheim.  Most  of  them  issue  from  or  ascend  through  salt- 
bearing  beds  of  Devonian,  Permian,  or  Triassic  age,  and  their 
composition  is  similar.  The  springs  of  Kreutznach  are  especially 
rich  in  calcium  chloride.  Some  of  the  springs  cited  are  hot, 
others  cold;  some  are  rich  in  carbon  dioxide.  In  regard  to 
Kreutznach  and  Wiesbaden  there  is  room  for  doubt,  for  the 
former  springs  stand  in  intimate  relation  to  eruptive  rocks, 
while  the  latter  issue  from  a  gneiss  and  are  by  some  authors  con- 
sidered of  juvenile  origin.  The  majority  of  them,  at  any  rate, 
have  certainly  derived  their  salts  from  sedimentary  beds. 

The  chloride  waters,  described  above,  are  capable  of  dissolving 
and  depositing  many  metallic  substances  and  have  strong 
dehydrating  power.  Their  relation  to  mineral  deposits  will  be 
mentioned  later. 

COMPOSITION  OF  SALTS  AND  TOTAL  SALINITY  OF  CHLORIDE  WATERS 
(Cited  from  Clarke's  Geochemistry,  1916,  pp.  182-186) 


A 

B 

c 

D 

E 

F 

Cl  55.83 
Br  0.04 
1  0.03 
SO4  3.12 
CO                               2  63 

58.79 
trace 

0.94 
0  61 

42.00 
1.13 
0.02 
0.08 
18  59 

62.31 
0.53 
0.01 
0.03 
0  27 

63.55 

0.01 
0  01 

56.58 
0.04 
trace 
0.78 
3  13 

B.O, 

0  01 

Na  33.09 
K  0.27 
Li  
NH«  

30.38 
3.76 

27.62 
0.78 
0.08 

18.35 
1.55 
0.04 
0.23 

5.63 

32.60 
1.16 
0.04 
0.07 

Ca  3.72 
Ba 

4.90 

6.03 
0  09 

13.86 

30.78 

4.05 
0  01 

Sr                                 .    . 

0  12 

Mg  1.13 
A12O3  

0.40 
0.02 

3.41 

2.53 
0.02 

0.01 

0.61 

Fe2O3                           0  06 

0  03 

Fe   . 

0  25 

0  04 

SiO2  !       0.08 

0.20 

0.14 

0.02 

0.01 

0.76 

100.00 

100.00 

100.00 

100.00 

100.00 

100.00 

Salinity,   parts         10,589 
per  million. 

23,309 

12,022 

309,175 

212,300 

8,241 

COMPOSITION  OF  UNDERGROUND  WATERS      51 

NOTES  RELATING  TO  ABOVE  ANALYSES 

A.  Cincinnati  artesian  well,  Cincinnati,  Ohio.     Analysis  by  E.  S.  Wayne, 
cited  by  A.  C.  Peale,  Bull.  32,  U.  S.  Geol.  Survey,  1886,   p.  133.     This 
water  contains  considerable  quantities  of  free  H2S  and  CO2. 

B.  Utah   Hot  Springs,  8  miles  north  of  Ogden,   Utah.     Temperature 
55°  C.     Analysis  by  F.  W.  Clarke,  Bull.  9,  U.  S.  Geol.  Survey,  1884,  p.  30. 

C.  Congress  Spring,  Saratoga,  New  York.     Analysis  by  C.  F.  Chandler, 
cited  by  A.  C.  Peale,  in  Bull.  32,  U.  S.  Geol.  Survey,  1886,  pp.   38,  39. 
Traces  of  F,  P,  B,  Sr,  and  Al.     Contains  much  free  C02. 

D.  Brine  from  well  2,667  feet  deep  at  Conneautsville,  Pennsylvania. 
Analysis  by  A.  E.  Robinson  and  C.  F.  Mabery,  Jour.,  Am.  Chem.  Soc.,  vol. 
18,  1896,  p.  915.     A  little  H2S  is  present. 

E.  Water  from  the  deep  levels  of  the  Quincy  mine,  Hancock,  Michigan. 
Analysis  by  George  Steiger. 

F.  The  Kochbrunnen,  Wiesbaden,  Germany.     Analysis  by  C.  R.  Fresen- 
ius.     This  water  also  contains  traces  of  I,  P,  and  As. 


CHLORIDE  WATERS  IN  IGNEOUS  ROCKS 

Waters  rich  in  chlorine  are  sometimes  found  as  ascending 
springs  in  igneous  rocks,  but  almost  always  close  to  regions  of 
comparatively  recent  volcanic  activity.  Their  composition  is 
somewhat  different  from  the  brines  resulting  from  the  dissolving 
of  salts  from  sedimentary  beds.  Bromine  is  seldom  present 
except  in  mere  traces,  wnile  boron  appears  in  considerable 
amounts.  Such  tepid  salt  waters  arise,  for  instance,  in  the 
volcanic  region  around  Clifton,  Arizona.  The  Paleozoic  rocks 
of  this  region  are  not  known  to  contain  either  salt  or  gypsum. 
Another  case  is  the  Glen  wood  Hot  Springs  in  western  Colorado; 
the  springs  at  this  place  issue  from  limestone,  but  the  structural 
relations  show  that  the  basal  granite  underlies  this  limestone  at 
slight  depths.  The  temperature  is  49.5°  C;  the  water  contains  a 
large  amount  of  sodium  chloride  and  relatively  small  amounts 
of  carbonates  and  sulphates.  Hydrogen  sulphide  and  free 
carbon  dioxide  are  present.  Still  another  case  is  Steamboat 
Springs,  Nevada,  which  issue  from  granodiorite  near  the  eastern 
base  of  the  Sierra  Nevada  in  a  region  of  Tertiary  volcanism. 

Many  of  these  springs  are  rich  in  carbon  dioxide  and  hydro- 
gen sulphide;  they  often  contain  many  of  the  rarer  elements,  as 
shown  in  the  analyses  quoted  below,  and  they  usually  appear  in 
regions  rich  in  ore  deposits.  Doubt  as  to  the  derivation  of  the 
salt  may  exist  in  many  cases,  as,  for  instance,  in  the  springs  of 
Kreutznach,  Germany,  which  issue  from  a  porphyry  said  by 


52 


MINERAL  DEPOSITS 


Laspeyres  to  contain  0.001  per  cent,  sodium  chloride.1  Delkes- 
kamp,2  on  the  other  hand,  holds  that  the  salt  is  derived  from 
sedimentary  deposits.  Another  notable  instance  of  chloride 
springs  of  this  class  is  mentioned  by  Daubree3  from  the  provinces 
of  Antioquia  and  Cauca  in  Columbia,  where  they  issue  in  great 
abundance  from  granite,  crystalline  schist,  and  late  volcanic 
rocks.  Great  difficulties  arise  in  attempting  to  trace  the  origin 
of  the  sodium  chloride  in  springs  of  this  class  to  surrounding 
rocks,  even  admitting  that  granite  and  other  crystalline  rocks 
may  contain  traces  of  this  salt.  Sinters  of  calcium  carbonate 
and  silica  are  often  deposited  at  the  orifices  of  these  springs. 

COMPOSITION  OF  SALTS  AND  TOTAL  SALINITY  OF  SODIUM  CHLORIDE  AND 

SILICA    WATERS 
(After  Clarke's  Geochemistry,  1916,  pp.   186  and  196) 


A 

B 

c 

D 

E 

Cl  

35.00 

36.61 

31.64 

13.52 

37.52 

Br 

0  25 

so,  
s  -  

CO 

4.58 
0.22 
5  08 

1.84 
0  15 

1.30 

8  78 

9.01 
0.32 
10  16 

4.96 

PO4  

0.03 

0.08 

AsO4 

0  24 

B4O7 

8  88 

2  24 

1   19 

Na  
K  
Li  
NH4  

30.35 
3.79 
0.27 

21.44 
4.45 
0.22 
0.02 

26.42 
1.93 
0.40 
trace 

19.71 

1.88 

0.28 

24.22 
0.36 

Ca 

0  25 

0  39 

0  11 

2  59 

Me 

0  01 

0  08 

0  04 

0  08 

0  19 

Fe            

trace 

trace 

trace 

As  
Sb 

0.10 
0  02 

A12O3  

0.01 

0.76 

0  12 

0.35 

SiO, 

11  41 

31  72 

27  58 

45  04 

29  81 

100.00 

100.00 

100.00 

100.00 

100.00 

Salinity,  parts  per  million  . 

2,850 

1,830 

1,388 

1,131 

2,735 

1Zeitschr.  Deutsch.  geol.  Gesell.,  vol.  19,  p.  854,  and  vol.  20,  p.  155. 

2  Verhandl'.  Polytech.  Gesell.  (Berlin),  1903,  II,  p.  161. 

3  Les  eaux  souterrainos.  etc.,  II,  2,  p.  106. 


COMPOSITION  OF  UNDERGROUND  WATERS      53 

A.  Steamboat  Springs,  Nev.     Analysis  by  W.  H.  Melville,  given  by  G.  F. 
Becker  in  Mon.  13,  U.  S.  Geol.  Survey,  1888,  p.  349.     Bicarbonate  reduced 
to  normal  salts.     Temperature  85°  C.     Contains  free  carbon  dioxide  and 
hydrogen  sulphide.     Traces  of  iron  and  quicksilver;  deposits  cinnabar  and 
stibnite. 

B.  Coral  Spring,  Norris  Basin,  Yellowstone  National  Park.     Analysis  by 
F.  A.  Gooch  and  J.  E.  Whitfield,  Bull.  47,  U.  S.  Geol.  Survey,  1888.     Tem- 
perature 73°  C.     H2S  none.     Free  CO2  42.5  parts  per  million. 

C.  Old   Faithful   Geyser.     Same   locality   and   analysts.     Temperature 
84°-88°  C.     H2S,  0.2  part  per  million. 

D.  Great  Geyser,  Iceland.     Analysis  by  F.  Sandberger. 

E.  Water  of  the  pink  terrace,  Roturoa  geyser.     Analysis  by  W.  Skey. 

Closely  related  to  this  group  are  the  predominant  springs  in 
the  great  geyser  regions  of  Yellowstone  National  Park,  New 
Zealand,  and  Iceland.  They  are  essentially  sodium  chloride 
waters  with  large  amounts  of  silica,  believed  to  exist  in  part  as 
sodium  silicate,  a  large  quantity  of  free  carbon  dioxide,  and  a 
little  hydrogen  sulphide.  Large  amounts  of  boron,  usually 
calculated  as  sodium  borate,  are  often  present,  and  also  fre- 
quently arsenic.  Bromine  is  rarely  recorded  in  quantities 
approaching  those  in  the  brines  from  sedimentary  formations. 
The  waters  are  always  hot  and  usually  ascend  through  volcanic 
rocks,  mostly  rhyolite;  from  these  the  silica  is  supposed  to  be 
derived,  but  no  such  explanation  seems  sufficient  to  account 
for  the  predominating  salt,  sodium  chloride,  or  for  the  boron. 
In  the  Yellowstone  Park  a  number  of  the  springs  issuing  near 
limestone  bear  evidence  of  their  passage  through  this  rock  in 
increased  quantities  of  calcium  and  magnesium.  Others  are  rich 
in  sulphate  of  sodium  and  other  sulphates,  but  these  springs  give 
an  acid  reaction  and  the  sulphates  are  in  all  probability  due  to 
the  oxidation  of  hydrogen  sulphide  and  the  replacement  of  silica 
in  sodium  silicate  by  sulphuric  acid. 


SULPHATE  WATERS  IN  SEDIMENTARY  ROCKS 

The  waters  which  traverse  sedimentary  rocks  are  often  rich 
in  salts,  particularly  in  sulphates.  The  gypsum  waters  have  been 
mentioned  and  are  connected  with  the  sodium  chloride  waters 
in  a  manner  corresponding  to  the  association  of  gypsum  and 
rock  salt.  By  interaction  of  calcium  sulphate  and  magnesium 
carbonate,  the  sulphate  of  magnesium  may  be  formed,  or  it  may 
be  derived  from  the  decomposition  of  a  pyritic  dolomite. 


54  MINERAL  DEPOSITS 

Sodium  sulphate  waters  are  almost  characteristic  of  certain  for- 
mations in  the  western  Cretaceous,  for  instance;  these  formations 
consist  mainly  of  sandstones  and  carbonaceous  shales,  the  lat- 
ter often  pyritiferous  and  the  whole  series  mainly  a  product 
of  near-shore  deposition.  The  oxidation  of  the  pyrite  furnishes 
solutions  containing  free  sulphuric  acid,  and  by  reaction  between 
this  and  various  other  substances  sulphates  of  calcium,  magne- 
sium, and  sodium  will  be  formed.  In  land  deposits  contained  in 
many  series  of  sedimentary  rocks  sodium  carbonate  and  sodium 
sulphate  are  formed  by  several  well-established  reactions,  and 
percolating  waters  will  easily  abstract  these  salts.  The  inter- 
action of  calcium  sulphate  and  sodium  carbonate  results  in 
sodium  sulphate  and  precipitation  of  calcium  carbonate.  So- 
dium sulphate  in  the  presence  of  free  carbon  dioxide  will  dissolve 
calcium  carbonate,  forming  sodium  bicarbonate  and  a'precipitate 
of  gypsum.1 

Reactions  in  soils  between  sodium  chloride  and  calcium 
sulphate  result,  according  to  Cameron,  in  calcium  chloride  and 
sodium  sulphate,  and  similar  reactions  take  place  between  sodium 
chloride  and  calcium  carbonate.  Sodium  sulphate  waters  are, 
as  stated,  common  in  the  western  Cretaceous,  especially  in 
the  shale  formations.  The  lowest  member  of  this  series,  the 
Dakota  sandstone,  is  particularly  noted  as  a  water-carrying  for- 
mation. The  water,  which  is  under  artesian  pressure,  penetrates 
this  formation  for  several  hundred  miles  underground  from  its 
outcrop  and  in  places  contains  so  much  sodium  sulphate  as 
to  be  unfit  for  irrigation  purposes.  There  is  no  evidence  that  this 
water  has  formed  mineral  deposits  in  the  sandstone. 

A  well  1,400  feet  deep,  in  Dakota  sandstone  at  Pueblo,  Colo- 
rado, contains,  according  to  Darton,2  1,337  parts  per  million  of 
total  solids,  of  which  about  one-half  is  calculated  as  sodium  sul- 
phate and  one-fourth  as  calcium  sulphate.  Very  little  silica  and 
chlorine,  but  a  little  iron  and  carbon  dioxide  are  present.  This 
analysis  appears  to  be  typical.  In  many  waters  in  sedimentary 
formations  chlorides  and  earthy  carbonates  appear  mixed  with 
sulphates.  Waters  from  artesian  wells  at  Roswell,  New  Mexico, 

1  F.  W.  Clarke,  Geochemistry,  1916,  p.  241. 

E.  W.  Hilgard,  Am.  Jour.  Sci.,  4th  ser.,  vol.  2,  1896,  p.  100. 
Cameron  and  Bell,  Bull.  33,  Bureau  of  Soils,  1906. 

F.  K.  Cameron,  Butt.  17,  Bureau  of  Soils. 

2  N.  H.  Darton,  Prof.  Paper  32,  U.  S.  Geol.  Survey,  1905,  p.  355. 


COMPOSITION  OF  UNDERGROUND  WATERS      55 


about  400  feet  deep,  derived  from  Permian  limestones,1  have  a 
temperature  of  64°-70°  F.  and  contain  from  600  to  1,200  parts 
per  million  of  solid  salts,  of  which  300  to  576  are  calcium  and 
magnesium  sulphates  and  the  remainder  carbonates  and  chloride 
of  sodium.  In  regions  of  dislocations  such  waters  may  be  hot 
and  then  the  ordinarily  low  percentage  of  silica  may  increase  con- 
siderably. The  Arrowhead  Spring2  of  San  Bernardino  Valley 
in  southern  California,  issuing  from  Tertiary  sediments,  has  a 
temperature  of  184°  F.  and  contains  1,086  parts  per  million  of 
solids,  of  which  735  are  calculated  as  sodium  sulphate,  69  as 
potassium  sulphate,  23  as  calcium  sulphate,  3  as  magnesium  sul- 
phate, 141  as  sodium  chloride,  and  23  as  calcium  carbonate;  85 
are  present  as  silica.  In  well  waters  of  the  same  valley  the  solids 
range  from  191  to  260  parts  per  million  and  the  relation  is 
CaC03>MgC03  =  NaSO4  >  NaC03>  >NaCl.  Silica  amounts 
to  24  to  32  parts  per  million. 

PERCENTAGE  COMPOSITION  AND  SALINITY  OF  SULPHATE  WATERS 
DERIVED  FROM  SEDIMENTARY  FORMATIONS 


A 

B 

cr 

D 

H  SO  (free) 

9  37 

Cl  
S04  
CO 

0.48 
66.28 
0  60 

11.10 
59.68 
1  67 

76.57 

0.32 

68.21 

Na  
K  

o&  

Fe" 

30.46 
1.08 
0.67 
0.41 

13.89 
0.49 
2.91 
10.19 

1.19 

5.82 
3.39 

4  28 

0.22 
0.11 
0.38 
1.11 
1.19 

Al 

7.36 

11.08 

SiO2  

0.02 

0.07 

1.39 

7.11 

100.00 

100.00 

100.00 

99.10 

Salinity,  parts  per  million  .  . 

74,733 

15,682 

3,303 

464 

A.  Abilena  Well,  Abilene,  Kansas,  130  feet  deep.  Analysis  by  E.  H.  S- 
Bailey,  Geol.  Survey,  Kansas,  vol.  7,  1902,  p.  166.  In  Permian  strata- 
From  Clarke's  Geochemistry,  1916,  p.  187. 

1  C.  A.  Fisher,  Report  on  the  Roswell  artesian  area,  Water-Supply  Paper 
158,  U.  S.  Geol.  Survey,  1906. 

2W.  C.^Mendenhall,  Hydrology  of  San  Bernardino  Valley,  Water-Supply 
Paper  142,  U.  S.  Geol.  Survey,  1905. 


56  MINERAL  DEPOSITS 

B.  King's  Mineral  Spring  near  Dallas,  Indiana.     Twenty-sixth  Annual 
Report,  Indiana  Dept!  Geol.,  1901,  p.  32.     Traces  of  Al,  Fe,  Ba,  Sr,  Li, 
Mn,  Ni,  Zn,  Br,  PC>4  and  B4O7.     Geological  horizon  Paleozoic  shale. 

C.  Alum  Well,  Versailles,  Missouri.     Analysis  by  P.  Schweitzer,  Geol. 
Surv.  Missouri,  vol.  3,  1892,  p.  131.     In  Pennsylvanian  shale. 

D.  Rockbridge  Alum  Springs,   Virginia.     Analysis  by   M.   B.   Hardin. 
Cited  in  Clarke's  Geochemistry,  p.  98.     From  pyritic  shale.     Contains  also 
0.69  Mn,  0.01  Li,  0.05  Co,  0.07  Ni,  0.08  Zn,  and  traces  of  Cu,  HNO3,  and 
P04. 

Waters  percolating  through  oxidizing  pyritic  shales  sometimes 
contain  large  amounts  of  the  sulphates  of  aluminum  and  ferrous 
iron;  evidently  this  happens  only  when  comparatively  large 
amounts  of  sulphuric  acid,  which  is  capable  of  attacking  alumin- 
ous silicates,  are  set  free. 

Such  waters  are  not  uncommon  in  the  eastern  and  central 
States  and  usually  contain  small  amounts  of  rarer  metals; 
traces  of  nickel,  zinc,  and  arsenic  are  common.  The  sulphate 
water,  especially  those  rich  in  iron  and  aluminum,  are  of  great 
importance  in  the  genesis  of  deposits  in  the  oxidizing  zone,  and 
the  latter  often  form,  at  their  orifices,  large  quantities  of  ocherous 
deposits.  Many  waters  of  this  kind  are  known  from  Virginia, 
issuing  from  pyritic  shales,  and  Peale1  quotes  some  interesting  and 
reliable  analyses.  A  water  from  Alleghany  Springs  in  Mont- 
gomery. County,  analyzed  by  Genth,  contained  3,129  parts  per 
million  of  solids,  of  which  the  principal  constituents  were  calcu- 
lated as  1,955  parts  CaSO4,  255  parts  MgSO4,  and  61  parts  CaC03. 
Small  quantities  of  strontium,  barium,  fluorine,  and  silica  and 
traces  of  zinc,  lead,  copper,  and  cobalt  are  noted.  Some  free 
carbon  dioxide  and  a  trace  of  hydrogen  sulphide  are  present. 

The  Jordan  Alum  Springs  in  Rockbridge  County,  Virginia,  of 
which  several  analyses  by  J.  W.  Mallet  are  recorded,  contain 
from  306  to  935  parts  per  million  of  solid  salts,  of  which  the 
larger  amount  consists  of  aluminum  sulphate,  35  to  85  parts  of 
ferric  sulphate,  and  from  8  to  17  parts  of  manganese  sulphate. 
Small  quantities  of  copper,  zinc,  cadmium,  nickel,  and  cobalt 
are  determined,  also  a  trace  of  fluorine.  One  of  the  waters 
contained  102  parts  of  copper  and  9  parts  of  zinc.  In  the 
Rockbridge  Alum  Springs,  in  the  same  State,  small  quantities 
of  copper,  nickel,  cobalt,  zinc,  and  a  trace  of  lead  were  determined. 

1  A.  C.  Peale,  Lists  and  analyses  of  the  mineral  springs  of  the  Unite  d 
States,  Bull.  32,  U.  S.  Geol.  Survey,  1886,  pp.  58-65. 


COMPOSITION  OF  UNDERGROUND  WATERS      57 

Free  sulphuric  acid  is  present  in  the  Bedford  Alum  Spring  to 
the  amount  of  70  parts  per  million,  according  to  M.  B.  Hardin. 
The  total  solids  are  1,207  parts,  practically  all  sulphates,  and 
about  one-third  consists  of  ferric  sulphate.  Small  quantities, 
about  0.8  part  per  million  of  each,  of  nickel,  cobalt,  copper,  and 
zinc  were  determined.  Springs  of  similar  composition  are  found 
in  Pennsylvania  and  other  eastern  States.  All  these  acid 
springs  are  poor  in  silica  and  contain  very  little  chlorine. 

ACID  SULPHATE  WATERS  IN  IGNEOUS  ROCKS 

Sulphate  springs  in  connection  with  igneous  rocks  and  vol- 
canism  appear  mainly  as  products  of  the  oxidation  of  ascending 
waters  of  alkaline  reaction,  containing  free  hydrogen  sulphide, 
but  there  is  evidence  that  in  regions  of  volcanic  activity  such 
oxidation  takes  place  on  a  large  scale  and  that  these  acid  waters 
are  of  high  importance  in  effecting  rock  alteration,  particularly 
by  attacking  aluminum  silicate  and  developing  alunite.  By 
similar  reactions  free  hydrochloric  acid  may  be  generated,  for 
instance  by  the  decomposition  of  chlorides  by  free  sulphuric  acid. 

As  a  consequence  it  is  common  to  find  such  waters  near  the 
orifices  of  hot  springs,  as  well  as  at  volcanoes.  The  develop- 
ment of  free  acid  of  course  displaces  the  equilibrium  and  the 
oxidized  water  may  differ  greatly  from  its  parent  liquid;  thus 
it  happens  that  a  single  ascending  hot  spring  may  yield  a  whole 
series  of  derivatives  of  varying  temperature  and  composition 
by  mingling  with  other  waters  and  by  oxidation. 

A  number  of  analyses  of  such  waters  are  quoted  in  Clarke's 
Geochemistry.  Some  of  them,  especially  from  pools  or  lakes  near 
volcanoes,  are  remarkably  rich  in  hydrochloric  acid.  The 
peculiar  water  from  the  Yellowstone  National  Park  known  as  the 
Devil's  Inkpot  contains,  besides  free  acids,  a  large  amount  of 
sulphate  of  ammonia.  The  water  from  Roturoa,  New  Zealand, 
the  analysis  of  which  is  quoted  below,  is  a  more  characteristic 
product  of  the  oxidation  of  normal  thermal  waters. 

The  geysers  of  Sonoma  County,  California,  of  which  there  is  a 
good  series  of  analyses  by  Dr.  Winslow  Anderson,1  form  a  most 
remarkable  illustration  of  the  oxidation  of  hot  waters.  There 
are  at  this  place  a  great  number  of  springs  of  varying  temperature 

1  Winslow  Anderson,  Mineral  springs,  etc.,  of  California,  1892,  pp.  136-154 
G.  A.  Waring,  Springs  of  California,  Water-Supply  Paper  338,  U.  S.  Geol. 
Survey,  1915,  p.  109. 


58 


MINERAL  DEPOSITS 


and  composition,  all  of  them  heavily  charged  with  hydrogen  sul- 
phide. The  primary  water  at  a  temperature  of  110°  F.  appears 
to  contain  chiefly  carbonate  of  magnesium  with  some  of  calcium. 
The  total  solids  amount  to  about  568  parts  per  million,  most  of 
which  consist  of  the  above-mentioned  carbonates;  there  are  92 
parts  of  silica  per  million.  This  water  is  probably  of  mixed 
origin;  the  carbonates  are  clearly  derived  from  the  serpentinoid 
rocks  of  the  vicinity,  but  the  hydrogen  sulphide  is  most  likely  of 
magmatic  origin.  Free  sulphuric  acid  is  generated  by  oxidation 
and  gives  rise  to  a  long  series  of  peculiar  sulphate  waters,  most 
of  them  rich  in  dissolved  solids  and  of  high  temperature.  An 
analysis  of  one  of  these  shows  3,262  parts  per  million  of  total 
salts  and  acids,  among  which  the  sulphates  of  magnesium,  sodium, 
and  aluminum  prevail.  There  are  544  parts  per  million  of  free 
sulphuric  acid  and  20  parts  of  free  hydrochloric  acid. 

Finally,  acid  water  may  result  directly  from  the  oxidation  of 
deposits  of  pyrite  or  of  sulphur.  A  water  of  the  latter  type  is 
described  by  W.  T.  Lee  from  Beaver  County,  Utah. 

PERCENTAGE  COMPOSITION  OF  SALTS  AND  TOTAL  SALINITY  OF 
ACID  WATERS 


A 

B 

C 

HC1  free                                          .        . 

0.18 

5  60 

H2SO4free  
HBO 

1.29 
2  73 

59.11 

,46.39 

Cl 

0  08 

SO 

67  66 

20  21 

32  63 

Na                                  

0  73 

8  35 

\ 

K                        

0  24 

0  32 

\      1.48 

Li                  

0  01 

NH 

22  85 

Ca 

1  18 

0  47 

1  63 

Mg                                      

0  36 

0  22 

2  50 

Fe"  '                        

trace 

5.76 

Fe'"         

0.33 

8.25 

Al 

0  10 

trace 

SiO 

2  67 

5  39 

1  28 

100.00 

100.00 

100.00 

Salts  and  acids,  parts  per  million  

3,365 

1,862 

9,716 

Includes  some  alumina. 


COMPOSITION  OF  UNDERGROUND  WATERS      59 

A.  Devil's  Inkpot,  Yellowstone  National  Park.     Analysis  by  F.  A.  Gooch 
and  J.  E.  Whitfield,  Bull.  47,  U.  S.  Geol.  Survey,  1888,  p.  80.     Contains 
also  65  parts  of  free  CO2  and  5  parts  of  H2S  per  million.     Cited  in  Clarke's 
Geochemistry,  1916,  p.  199. 

B.  Cameron's  Bath,  Roturoa  geyser  district,  New  Zealand.     Analysis  by 
W.  Skey,  Trans.,  New  Zealand  Institute,  vol.  10,  1877,  p.  423.     Contains 
6  parts  per  million  of  H2S.     Cited  in  Clarke's  Geochemistry,  p.  200. 

C.  Water  at  Sulphur  mine  of  Cove  Creek,  Beaver  Valley,  Utah.     Analy- 
sis by  W.  M.  Barr,  Water-Supply  Paper  217,  U.  S.  Geol.  Survey,  1908,  p.  20. 
Contains  also  much  free  H2S. 

MINE  WATERS   OF  SULPHATE  TYPE 

Mine  waters  consist  as  a  rule  of  the  normal  surface  waters  of 
the  rock  containing  the  ore  deposit,  modified  by  the  salts  re- 
sulting from  the  decomposition  of  the  minerals  of  the  deposit. 
In  deposits  free  from  sulphides,  such  as  the  copper  and  iron 
mines  of  Lake  Superior,  there  is  little  difference  between  the 
mine  waters  of  the  upper  levels  and  the  normal  surface  waters  of 
low  salinity;  both  are  comparatively  high  in  silica  and  calcium 
carbonate.  Where  much  pyrite  or  marcasite  is  present,  as  in 
coal  mines  and  in  most  metal  mines,  the  surface  waters  will  con- 
tain sulphates  of  ferrous  iron  and  aluminum  and  frequently  also 
of  the  rarer  metals.  When  these  waters  mingle  with  normal 
surface  waters  rich  in  calcium  carbonate  the  iron  and  alumina 
may  be  precipitated  as  hydroxides  and  calcium  sulphate 
remains  in  solution.  Calcium  sulphate  waters  often  spread 
over  a  considerable  area  surrounding  pyritic  deposits. 

The  mine  waters  will  be  discussed  in  the  chapter  on  oxidation 
and  secondary  sulphides. 

SODIUM  CARBONATE  WATERS  IN  SEDIMENTARY  ROCKS 

Waters  containing  sodium  carbonate  in  large  amounts  are  not 
common  in  sedimentary  rocks,  but  here  and  there  wells  or  springs 
of  this  character  are  encountered;  they  are  usually  cold  and  often 
contain  some  free  carbon  dioxide  and  hydrogen  sulphide.  The 
alkaline  carbonate  is  probably,  as  suggested  above,  derived  from 
a  reaction  between  sodium  sulphate  and  calcium  carbonate  or 
between  sodium  chloride  and  calcium  carbonate.  Waters  of 
this  kind  occur  at  a  few  places  in  the  eastern  and  central  States. 
G.  L.  Gumming  has  described  such  waters  from  some  artesian 
wells  in  Silurian  limestone  on  the  Island  of  Montreal.1  They 
contain  from  500  to  700  parts  per  million  of  solids,  chiefly  sodium 

1  Mem.  72,  Geol.  Survey  Canada,  1915. 


60  MINERAL  DEPOSITS 

carbonate  with  the  remainder  calculated  as  calcium  chloride  and 
sodium  sulphate.  Gumming  shows  that  the  waters  are  of  com- 
plicated origin  but  believes  that  the  sodium  carbonate  solutions 
are  derived  from  dikes  and  intrusions  in  the  limestone.  A  good 
instance  is  furnished  by  some  Missouri  waters  in  Carboniferous 
limestone,  one  of  which  is  quoted  under  E  in  the  following  table. 
Similar  waters  are  those  of  the  wells  at  La  Junta,  Denver,  and 
Greeley,  Colorado.  The  artesian  wells  at  Denver,  about  1,200 
feet  deep,  are  in  the  Arapahoe  Eocene,  while  the  Greeley  well, 
of  the  same  depth,  is  sunk  in  Laramie  sandstone.  The  maxi- 
mum of  total  solids  is  about  1,530,  divided  between  sodium 
carbonate  and  sodium  chloride.  Some  free  CO  2  is  present.  Many 
artesian  waters  in  New  South  Wales  are  rich  in  sodium  car- 
bonate. Sedimentary  beds  containing  volcanic  tuffs  often  yield 
sodium  carbonate  waters. 

SODIUM  CARBONATE  WATERS  IN  IGNEOUS  ROCKS 

Ascending  sodium  carbonate  waters  are  most  characteristic 
of  regions  of  subsiding  or  expiring  volcanism.  During  surface 
eruptions  alkaline  chlorides  and  carbonates  always  appear  as 
sublimates  and  waters  traversing  tuffs,  breccias  and  lava  flows 
may  dissolve  these  salts  together  with  other  volcanic  exhalations 
such  as  borates.  The  characteristic  sodium  carbonate  waters 
are,  however,  of  deep-seated  origin  and  usually  break  in  through 
the  older  igneous  or  metamorphic  rock  underlying  the  lavas  in 
regions  where  the  active  volcanism  has  ceased,  and  the  pre- 
vailing opinion  is  that  these  waters  with  their  charge  are  in  whole 
or  in  part  of  magmatic  origin.  They  rarely  contain  much  calcium 
and  they  are  poor  in  silica,  but  are  usually  heavily  charged  with 
carbon  dioxide  and  sometimes  hydrogen  sulphide.  They  almost 
always  contain  many  rarer  substances  such  as  boron,  fluorine, 
iodine,  arsenic  and  various  metals. 

Such  waters  would  attack  silicates  with  great  energy  and  it  is 
suggested  that  their  strong  percentage  of  sodium  may  have  been 
leached  from  walls  of  the  fissures,  during  the  conversion  of  sodium 
silicates  to  potassium  silicates  as  in  the  process  of  sericitization, 
so  common  in  mineral  veins.  It  is  certain  that  these  waters  are 
of  the  utmost  importance  in  the  genesis  of  orebearing  veins. 

An  excellent  instance  of  a  province  of  such  waters  is 
furnished  by  the  volcanic  district  of  central  France.1  An  analy- 

^acquot  and  Willm,  Les  eaux  minerales  de  la  France,  Paris,  1894. 


COMPOSITION  OF  UNDERGROUND  WATERS      61 


sis  of  the  celebrated  Vichy  Springs  is  given  in  the  table  on 
this  page.  Sodium  preponderates  as  bicarbonate,  but  smaller 
quantities  of  sodium  chloride  and  sulphate  are  also  present.  The 
whole  region  of  the  Central  Plateau  is  rich  in  carbon  dioxide, 
occurring  both  in  springs  and  as  exhalations  (for  instance,  at 
the  Pontgibaud  lead-silver  mines).  The  magmatic  source  of  the 
gas  is  rarely  questioned,  whatever  opinion  may  be  held  about 
the  origin  of  the  water  (Fig.  4). 

In  the- volcanic  regions  of  Taunus  and  Vogelsgebirge  on  the 
Rhine  in  Germany  are  the  springs  of  Ems  and  Fachingen.  The 
springs  of  Ems  issue  with  a  temperature  of  46°  C.  and  contain 
about  2,870  parts  per  million  of  solids,  of  which  about  one-half 
may  be  calculated  at  sodium  carbonate  and  a  large  part  of  the 
remainder  as  sodium  chloride. 

PERCENTAGE  COMPOSITION  OF  SALTS  AND  TOTAL  SALINITY  OF  SODIUM 

CARBONATE     WATERS 
(Cited  from  Clarke's  Geochemistry,  pp.  191,  193  and  197) 


Cl 

6  17 

8  85 

11  52 

13  57 

6  63 

4  01 

F  

0.19 

0.03 

so4  
s  

3.75 

5.77 

31.19 

0.32 

6.21 
0  06 

4.26 

CO, 

45  57 

41  91 

19  15 

22  38 

44  yg 

47  45 

PO4  
AsO4  

1.52 
0.04 

0.01 

0.01 

BA  

Na 

35  27 

0.16 

38  08 

32  49 

27.98 
33  97 

;;;;;;;; 

41  07 

40  09 

K  

2.88 

1.20 

1  35 

0  48 

0  38 

NH4  

0  05 

Li  

0.12 

Ca  

2.29 

0  87 

2  23 

0  41 

0  30 

0  27 

Sr.. 

0  04 

0  05 

0  01 

Mg  
Mn  

1.11 

0.41 

0.65 
0  01 

0.11 

0.12 

0.15 

Fe 

0  02 

0  14 

Fe,O,. 

0  04 

0  06 

A12O3          .    . 

0  02 

0  20 

SiO 

1  32 

2  30 

1  34 

0  73 

0  85 

3  05 

100.00 

100.00 

100.00 

100.00 

100.00 

100.00 

Salinity,   parts 
per  million 

5,249 

2,614 

5,431 

5,096 

2,069 

1,668 

62  MINERAL  DEPOSITS 

NOTES    RELATING    TO    ABOVE    ANALYSES 

A.  The  Grand-Grille  spring,  Vichy,  France.     Analysis  by  J.  Bouquet. 
Small  quantity  of  fluorine  present.     Temperature  44°  C.     Issues  from 
Tertiary  beds. 

B.  Ojo  Caliente  spring,  near  Taos,  New  Mexico.     Analysis  by  W.  F. 
Hillebrand.     Trace  of  barium  and  arsenic.     In  lake  beds  and  gneiss. 

C.  The  Sprudel,  Carlsbad,  Bohemia.     Analysis  by  F.  Ragzsky.     Contains 
0.76  gram  free  and  half-combined  CO2  per  kilogram.     Traces  of  Br,  I,  Li, 
B,  Rb,  and  Cs.     Temperature  72°  C.     In  granite. 

D.  Hot   water  from   the   Hermann   shaft,    Sulphur   Bank,    California. 
Analysis  by  W.  H.  Melville.     A  little  H2S  and  a  considerable  amount  of 
CO2  present.     Temperature  80°  C.     In  basalt  and  sandstone. 

E.  McClelland  well,  Cass  County,  Missouri,  45  feet  deep,  in  Carboniferous 
limestone.     Analysis  by  P.  Schweitzer.     Contains  H2S. 

F.  Artesian  water,  La  Junta,  Colorado.     Well  386  feet  deep.     Analysis 
by  W.  F.  Hillebrand.     In  Cretaceous  beds. 

At  the  .foot  of  the  Erzgebirge,  in  the  Tertiary  volcanic  region 
of  northern  Bohemia,  issue  a  series  of  hot  springs,  extending 
from  Teplitz  to  Carlsbad  and  Eger.  Most  of  these  belong  to  the 
class  of  sodium  carbonate  waters  with  free  carbon  dioxide. 
They  contain  an  abundance  of  salts,  and  in  the  Teplitz  and 
Bilin  springs  sodium  carbonate  predominates.  In  the  Carls- 
bad (C  in  table  of  analyses)  and  Marienbad  spring  the  sul- 
phuric acid  radicle  is  prominent  and  must  largely  exist  in  so- 
dium sulphate.  The  Carlsbad  springs  contain  fluorine  and 
barium  with  traces  of  many  rarer  metals  which  are  mentioned 
on  page  97. 

IntheCordilleran  Ranges  in  North  America  and  South  America 
sodium  carbonate  waters  are  abundant  and  always  closely  con- 
nected with  areas  of  Tertiary  volcanic  activity. 

In  New  Mexico  the  Ojo  Caliente  (B  in  table  of  analyses), 
Fay  wood,  and  Las  Vegas  springs  may  be  mentioned;  in  Colorado 
the  Idaho  Springs,  Middle  Park  Springs,  Poncha  Springs,  and 
the  water  in  the  Geyser  mine  at  Silver  Cliff;  in  Idaho  the  Boise 
Hot  Springs.  In  California  sodium  carbonate  waters  are 
especially  abundant  and  characteristic;  they  follow  the  Coast 
Range  from  San  Diego  to  Mendocino  County  and  appear  to 
stand  in  some  causal  connection  with  the  late  Teritary  or  Quat- 
ernary eruptions  of  basalt.  Some  of  the  waters  are  clearly 
admixed  with  magnesium  from  the  serpentinoid  rocks  which 
they  have  traversed,  but  in  general  the  type  is  perfectly  distinct. 
The  following  data  are  taken  from  the  U.  S.  Geological  Survey, 
Bulletin  32,  by  A.  C.  Peale. 


COMPOSITION  OF  UNDERGROUND  WATERS      63 


Source  of  water 

Salinity, 
parts  per 
million 

Composition  and  quantity  of 
principal  salts 

San  Juan  Capistrano  (T.  50°  C.) .  I 

Skaggs  Springs  (T.  54°  C.) | 

Paso  Robles  Springs  (T.  42°  C.) . ! 
New  Almaden  Vichy  (T.  17°  C.). 

NapaSoda  (T.  17°  C.) ! 

Pacific  Congress  (T.  10°  C.) 

Ukiah  Vichy  (T.  34°  C.) . . . 


290 

HNaCO3> 

NaCl 

>SiO2 

111 

105 

70 

2,556 

HNaCO3> 

BO2 

>SiO2 

2,083 

176 

151 

1,581        HNaCO3> 

NaCl 

>Na2SO4 

850 

469 

136 

7,361     !  HNaCO3> 

CaSO4 

>CaCO3 

3,400 

680 

544 

1,156 

HNaCO3>MgCO3 

>NaCl 

561 

187 

85 

5,678       HNaCO3> 

NaCl 

>CaCO3 

2,091 

1,923 

289 

4,624 

HNaCO3> 

NaCl 

>MgCO3 

3,369 

459 

374 

An  interesting  type  of  these  waters  is  represented  by  the  hot 
spring  of  Sulphur  Bank  (D  in  table  of  analyses),  which  contains 
boron  and  is  depositing  cinnabar.  On  the  whole  these  waters  are 
rich  in  unusual  constituents  and  have  great  solvent  powers. 

SODIUM  SULPHIDE  WATERS 

It  is  believed  that  in  some  of  the  springs  already  referred 
to — for  example,  Steamboat  Springs,  Nevada — sodium  sulphide 
or  other  sulphur  salts  of  sodium  are  present.  In  the  Pyrenees 
of  France  and  Spain  is  found  a  group  of  Springs  in  which  sodium 
sulphide  is  constantly  present.  These  springs  have  a  high  tem- 
perature and  a  low  salinity,  containing  from  250  to  350  parts 
per  million  of  salts;  they  usually  issue  in  crystalline  schists  or 
on  the  contact  of  the  schists  with  Paleozoic  strata.  A  charac- 
teristic spring  mentioned  among  others  by  Jacquot  and  Willm1 
contains  total  CO2  52,  S  (in  sulphides)  31,  Na  97,  CO3  26,  Cl  55, 
andJ3iC>2j)3  parts  per  million.  Some  organic  matter  is  present 
and  strong  traces  of  boron,  arsenic,  copper,  etc.,  are  mentioned. 
There  appears  to  be  considerable  difficulty  in  the  explanation  of 
this  combination  on  the  hypothesis  of  leaching  from  the  sur- 
rounding country  rock.  Where  contaminated  by  surface  water 

1  I*ea  eaux  minerales  de  la  France,  Paris,  1894. 


64  MINERAL  DEPOSITS 

or  where  locally  issuing  through  Triassic  strata  they  beocme 
calcic.     By  oxidation  they  acquire  hyposulphites. 

SUMMARY 

In  sedimentary  formations;  beyond  the  influences  of  igneous 
activity,  the  waters  are  of  many  differing  types.  Some  con- 
tain mainly  calcium  carbonate;  others  are  of  the  chloride  type, 
with  sodium  or  calcium  as  the  prevalent  base;  still  others, 
a  very  abundant  class,  are  rich  in  calcium  or  sodium  sulphates;  a 
rarer  type  is  that  of  the  sodium  carbonate  waters.  Naturally  many 
waters  show  a  mingling  of  these  types.  Most  of  these  waters  are 
cold;  many  are  tepid;  few  of  them  are  hot.  Whether  warm  or 
cold,  both  hydrogen  sulphide  and  carbon  dioxide  may  be  present. 

In  older  igneous  rocks  where  the  effects  of  volcanism  have  sub- 
sided the  types  vary  less  widely.  The  ordinary  surface  waters 
are  always — unless  some  disturbing  influence  interferes — of  the 
calcium  carbonate  type,  often  with  sodium  chloride,  ferrous 
and  magnesium  carbonate,  and  considerable  silica,  but  low 
salinity.  These  waters  sometimes,  but  not  often,  appear  as 
tepid  ascending  springs.  If  the  rocks  contain  iron  disulphide 
the  waters  may  locally  contain  free  sulphuric  acid  and  the 
sulphates  of  calcium,  aluminum  and  iron. 

The  remaining  classes  of  water  in  igneous  rocks  are  ascending 
and  confined  to  regions  of  recent  or  Tertiary  volcanic  activity. 
They  are  tepid  to  hot,  though  cold  waters  are  also  known.  They 
easily  fall  into  two  classes:  (1)  the  sodium  chloride  waters,  of 
which  the  siliceous  "geyser  waters"  form  a  sub-class;  (2)  the 
sodium  carbonate  waters,  which  are  generally  rich  in  free  carbon 
dioxide.  Transitions  between  the  two  classes  are  plentiful,  and 
the  latter  class  may  in  rarer  cases  also  contain  notable  amounts 
of  sodium  sulphate;  of  this  class  the  Carlsbad  Springs  form  a 
prominent  example. 

INTERPRETATION  OF  WATER  ANALYSES 

Analyses  of  waters  are  usually  stated  in  parts  per  million  of 
radicles  and  metals.  From  this  form  a  calculation  will  be  neces- 
sary to  ascertain  whether  the  water  is  alkaline,  neutral,  or  acid. 

Stabler1  has  suggested  that  for  this  purpose  the  quantities  deter- 

1  Herman  Stabler,  The  mineral  analysis  of  water  for  industrial  purposes 
and  its  interpretation  by  the  engineer:  Eng.  News,  vol.  60,  1908,  p.  356. 
Also,  chapter  on  the  industrial  application  of  water  analyses  in  Water- 
Supply  Paper  274,  U.  S.  Geol.  Survey,  1911,  pp.  165-181. 


COMPOSITION  OF  UNDERGROUND  WATERS      65 

mined  may  be  multiplied  by  the  reciprocals  of  the  equivalents. 
The  products  are  called  the  reacting  values.  If  the  water  is 
neutral  the  reacting  values  of  acids  and  basic  radicles  should 
balance.  Palmer,1  in  his  method  of  geological  interpretation 
of  water  analyses,  finds  it  convenient  to  express  the  reacting 
values  in  percentages,  thus  eliminating  the  factor  of  concentra- 
tion. Palmer's  classification  emphasizes  the  fact  that  a  solution 
in  which  strong  acids  are  exactly  balanced  with  strong  bases 
is  relatively  inert,  whereas  one  in  which  either  group  exceeds 
the  other  is  relatively  active.  It  is  of  special  use  in  showing  the 
relationship  and  the  nature  of  chemical  action  of  different  waters. 

Alkalinity  and  Salinity  are  the  fundamental  properties. 
Salinity  is  measured  by  the  strong  acid  radicles  (SO  4,  Cl).  If 
the  basic  radicles  are  partly  or  wholly  alkaline  metals  their  pro- 
portion of  the  salinity  is  said  to  be  primary.  The  remaining  salin- 
ity due  to  radicles  Ca,  Mg,  Fe  is  called  secondary.  If  the  acid 
radicles  are  in  excess,  tertiary  salinity  or  acidity  results.  The 
measure  of  primary  alkalinity  is  the  excess  of  alkaline  metal 
radicles  over  the  strong  acids;  the  weak-acid  radicles  C03  and 
HCOs  which  balance  any  excess  of  the  alkaline  earth  metals  over 
the  stronger  acids  produce  secondary  alkalinity.2 

In  spite  of  an  objectionable  terminology  Palmer's  method 
furnishes  a  convenient  basis  for  comparative  study  but  as  a 
classification  of  natural  waters  it  is  unwieldy  and  uncertain.  It 
is  not  always  a  safe  guide  to  the  geological  history  of  the  water. 

The  constants  used  in  converting  grains  per  gallon  to  parts  per  million 
and  vice  versa  are  as  follows: 

1  grain  per  U.  S.  gallon  =  17.138  parts  per  million 
1  grain  per  Imperial  gallon  =  14.285  parts  per  million 
1  part  per  million  =  0.0588  grain  per  U.  S.  gallon 
1  part  per  million  =  0.07  grain  per  Imperial  gallon 

1  Chase  Palmer,  Geochemical  interpretation  of  water  analyses,  Bull.  479, 
U.  S.  Geol.  Survey,  1911. 

2  Cfr.  F.  W.  Clarke,  Geochemistry,  1916,  p.  63. 

G.  S.  Rogers,  The  interpretation  of  water  analyses  by  the  geologist. 
Econ.  Geol,  vol.  12,  1917,  pp.  56-88. 


CHAPTER  V 
THE  CHEMICAL  WORK  OF  UNDERGROUND  WATER 

METAMORPHISM  AND  MINERAL  DEPOSITS 

Stability  of  Minerals  and  Rocks. — The  underground  water 
plays  a  very  important  part  in  the  changes  which  take  place  in 
rocks,  and  the  majority  of  mineral  deposits  are  formed  by  the 
aid  of  it.  Near  the  surface  it  may  completely  saturate  the  rocks 
or  move  in  large  volumes  on  fractures.  At  greater  depths  where 
there  is  no  active  circulation  it  may  be  sparingly  present  as  rock 
moisture.  The  great  mass  of  underground  water  is  of  atmos- 
pheric origin  but  as  all  magmas  contain  water  which  is  given  off 
upon  solidification  some  waters  in  the  rocks  may  be  of  magmatic 
origin.  Solution  and  precipitation  go  on  continuously;  one  or 
the  other  may  predominate  at  any  given  place.  The  reactions 
which  take  place  in  the  underground  solutions  extend  over  a 
wide  range  as  to  temperature,  pressure,  substances,  concentration 
and  time,  and  they  differ  markedly  under  the  varying  conditions. 
The  study  of  these  reactions  was  first  seriously  undertaken  by  G. 
Bischof  and  Justus  Roth1  and  these  pioneers  have  been  followed 
by  many  eminent  geologists  who  have  devoted  themselves  to  the 
study  of  chemical  geology. 

1  G.  Bischof,  Lehrbuch  der  chemischen  und  physikalischen  Geologic, 
1863-1866. 

Justus  Roth,  Allgemeine  and  chemische  Geologie,  vol.  1,  Berlin,  1879. 

C.  R.  Van  Hise, 'A  treatise  on  metamorphism,  Man.  47,  U.  S.  Geol. 
Survey,  1904. 

C.  R.  Van  Hise,  Metamorphism  of  rocks  and  rock  flowage,  Bull.,  Geol. 
Soc.  America,  vol.  9,  1898,  pp.  269-328. 

C.  R.  Van  Hise,  Some  principles  controlling  the  deposition  of  ores,  Trans., 
Am.  Inst.  Min.  Eng.,  vol.  30,  19QO,  pp.  27-177. 

U.  Grubenmann,  Die  krystalHnen  Schiefer,  Berlin,  1910. 

F.  Becke,  Ueber  Mineralbestand  urid  Struktur  der  krystallinen  Schiefer, 
Ninth  Session  Internat.  Geol.  Congress,  Vienna,  1903;  also  Sitz.  Ber., 
k.  k.  Akad.,  Vienna,  1903. 

John  Johnston  and  Paul  Niggli,  The  general  principles  underlying  meta- 
morphic  processes,  Jour.  Geology,  vol.  21,  1913,  pp.  481-516;  588-624. 

C.  K.  Leith  and  W.  J.  Mead,  Metamorphic  Geology,  New  York,  1915. 
66 


CHEMICAL  WORK  OF  UNDERGROUND  WATER    67 

One  of  the  most  fruitful  conceptions  developed  in  recent 
years  is  that  of  the  limits  of  stability  of  minerals  and  rocks. 
Conforming  to  increasing  heat  and  pressure,  zones  exist  in  the 
earth's  crust,  gradually  merging  into  one  another  but  each 
characterized  by  certain  groups  of  minerals  that  are  stable  only 
under  the  conditions  prevailing  in  that  particular  zone.  No 
mineral  is  absolutely  stable.  If  subjected  to  certain  conditions 
of  temperature  or  in  contact  with  certain  solutions  it  will  melt, 
decompose,  dissociate  or  dissolve.  At  the  surface  under  the 
influence  of  atmospheric  waters  with  oxygen  and  carbon  dioxide 
practically  no  minerals  are  stable  except  a  few  oxides,  hydroxides 
and  native  elements. 

In  consequence  of  the  reversible  nature  of  chemical  processes 
under  changing  conditions  each  mineral  thus  has  its  stability 
field  or  "critical  level"  which  it  can  not  leave  without  undergoing 
decomposition.  The  mineral  aggregates,  that  is,  the  rocks,  also 
follow  this  law  and  as  the  rock  minerals  have  usually  been  formed 
in  closely  analogous  ways  most  of  the  component  minerals  will 
become  unstable  more  or  less  simultaneously. 

Certain  minerals,  few  in  number,  are  less  sensitive  than  others 
to  such  changes  and  recur  under  the  most  different  conditions. 
They  are  designated  "persistent  minerals"  and  are  in  general 
of  simple  composition  and  do  not  contain  the  hydroxyl  mole- 
cule; among  them  are  quartz,  magnetite,  pyrite,  chalcopyrite, 
fluorite,  calcite  and  native  gold.  Orthoclase,  all  plagioclases, 
biotite,  augite,  olivine,  the  spinels,  cordierite,  and  garnets  develop 
and  are  fully  stable  only  at  high  temperatures.  Minerals  rich  in 
water,  like  chlorite,  serpentine,  and  talc,  are  characteristic  of 
lower  temperatures.  Other  minerals,  like  muscovite,  zoisite, 
epidote,  hornblende,  and  albite,  develop  by  preference  under 
strong  pressure. 

The  varied  composition  of  the  crust,  the  unequal  distribution 
of  the  underground  water,  the  changing  pressure,  and  the  great 
differences  in  temperatures  even  at  the  same  horizon  make  it 
difficult  to  establish  strict  rules  and  well-defined  zones.  One 
merges  into  another.  Besides,  stability  is  a  relative  term.  Some 
rocks,  like  granite,  are  really  stable  only  shortly  after  their  com- 
plete consolidation.  Under  the  influence  of  percolating  deep 
waters  the  minerals  of  the  granite  are  unstable,  as  they  are  in 
the  zone  of  weathering.  But  the  changes  take  place  so  slowly 
that  at  many  places  they  can  scarcely  be  perceived.  Other 


68  MINERAL  DEPOSITS 

rocks,  like  calcareous  shales,  are  stable  at  moderate  depths, 
but  easily  subject  to  recrystallization  under  pressure  and  rising 
temperature.  The  results  of  the  reactions  differ  widely  according 
to  the  composition  of  the  waters.  The  minerals  that  develop 
in  a  rock  charged  with  a  slight  amount  of  moisture  are  not  the 
same  as  those  that  appear  when  the  rock  is  penetrated  by  rapidly 
moving  solutions,  charged  with  salts  and  gases  of  foreign  origin. 

Metamorphism.1 — The  term  metamorphism  meaning  strictly 
"a  change  in  form,"  was  proposed  by  Lyell  in  1833  to  express 
the  changes  of  sedimentary  beds  to  slates,  quartzite,  crystalline 
limestone,  etc.  Later  it  was  extended  to  the  development  of 
schists  and  slates  from  igneous  rocks  by  pressure  and  recrystalliza- 
tion. Still  later,  for  instance,  by  C.  R.  Van  Hise  it  has  been 
employed  in  a  wide  sense  so  as  to  cover  any  change  in  the  com- 
position and  structure  of  any  rock,  through  whatever  agency  and 
with  or  without  gain  or  loss  of  substance.  This  would  include 
weathering  and  the  development  of  any  kind  of  epigenetic  deposit, 
such  as  mineral  veins,  in  a  rock.  Geologists  have  not  generally 
accepted  this  wide  definition.  Metamorphism  is  here  reserved 
for  the  processes  which  result  in  a  partial  or  complete  crystalliza- 
tion or  recrystallization  of  solid  masses  of  rocks,2  as  in  gneiss  from 
granite  or  mica  schist  from  clay  shale.  Though  the  mechanical 
effects  of  pressure  may  be  conspicuous,  metamorphism  is  always 
characterized  by  chemical  changes  in  the  component  minerals. 
The  composition  of  the  rock  as  a  whole  may  remain  fairly 
constant. 

For  practical  purposes  we  may  distinguish  between  static, 
dynamic,  igneous  and  hydrothermal  metamorphism.  Static 
metamorphism  proceeds  without  stress,  at  slight  depths  and 
under  influence  of  a  slight  amount  of  water.  At  great  depths 
and  high  temperatures  a  static  recrystallization  under  great  load 
may  be  recognized.3  Dynamic  metamorphism  is  effected  under 
stress  at  higher  or  lower  temperatures.  These  two  are  regional 
and  proceed  without  marked  changes  in  composition. 

Igneous  metamorphism  includes  the  effects  of  magmas  on 
adjacent  rocks  and  is  a  high  temperature  process.  It  is  about 

1  For  a  thorough  discussion  of  the  various  uses  of  this  term,  see:  R.  A. 
Daly,  Metamorphism  and  its  phases,  Bull.  Geol.  Soc.  Am.,  vol.  28,  1917, 
pp.  375-418. 

2  A.  Barker,  Geol.  Mag.,  vol.  6,  1889,  p.  15. 
3R.  A.  Daly,. op.  cit.,  p.  400. 


CHEMICAL  WORK  OF  UNDERGROUND  WATER    69 

equivalent  to  contact  metamorphism  but  includes  also  the  effect 
of  igneous  injection  and  pegmatitization. 

Hydrothermal  metamorphism  includes  the  changes  effected 
in  rocks  by  circulating  hot  ascending  waters.  Igneous  metamor- 
phism may  be  local  or  regional  and  in  part  involves  changes  of 
composition.  Hydrothermal  metamorphism  is  local  and  almost 
always  involves  changes  of  composition. 

Metasomatism  or  Replacement. — The  geological  importance 
of  metasomatism  or  replacement  has  already  been  pointed  out 
on  p.  26.  The  word  metasomatism,  meaning  a  change  of  body, 
first  used  by  C.  Naumann  to  designate  some  kinds  of  pseudo- 
morphism, is  now  applied  to  the  process  of  practically  simultane- 
ous capillary  solution  and  deposition  by  which  a  new  mineral 
of  partly  or  wholly  differing  chemical  composition  may  grow  in 
the  body  of  an  old  mineral  or  mineral  aggregate.  The  secondary 
minerals  of  any  metamorphic  rock  result  from  metasomatic 
action.  Rocks  are  termed  metasomatic  if  their  composition  has 
been  materially  changed  by  replacement  of  the  original  minerals. 
Pseudomorphs  and  petrifications  often  furnish  direct  and  in- 
controvertible evidence  of  processes  of  replacement. 

Metasomatism  is  met  everywhere  and  at  all  depths  in  sedi- 
mentary and  igneous  rocks  and  shows  that  the  rock  minerals  have 
been  subjected  to  conditions  under  which  they  were  unstable. 
The  development  of  chlorite  in  augite,  sericite  or  kaolin  or  calcite 
in  feldspars,  or  galena  in  limestone  is  due  to  metasomatism. 
The  typical  metasomatic  processes,  traced  with  the  highest 
magnifying  power,  show  no  space  between  the  parent  mineral 
and  the  metasome,  as  the  newly  developed  mineral  may  be 
designated.  The  fibers  and  blades  of  sericite  project  into  quartz 
without  the  slightest  break  in  the  contact.  Rhombohedrons  of 
siderite  develop  in  quartzite,  their  crystal  faces  cutting  across  the 
grains  without  any  interstices.  Perfect  prisms  of  tourmaline 
develop  in  feldspar  grains,  and  sharp  cubes  of  pyrite  in  primary 
feldspar  or  quartz. 

Metasomatic  rocks,  that  is  rocks  which  have  suffered  a  change 
in  composition,  are  very  common  in  mineral  deposits  and  are  often 
produced  by  strong  and  rapidly  moving  solutions  (usually  a.que- 
ous,  sometimes  gaseous)  which  penetrate  the  material  through 
veinlets  and  pores.  There  are  many  cases  of  complete  or  almost 
complete  metasomatism,  for  instance  of  limestone  by  sulphides 
and  quartz  in  which  the  chemical  composition  has  been  absolutely 


70  MINERAL  DEPOSITS 

changed.  In  contrast  to  this  the  ordinary  metamorphic  processes 
in  rocks  are  carried  on  by  the  scant  rock  moisture  and  while  there 
is  metasomatism  in  detail,  the  composition  as  a  whole  is  but  little 
changed.  For  description  of  metasomatic  processes  and  for 
criteria  of  metasomatism  see  Chapter  XI. 

Dissemination  refers  to  grains  or  crystals  distributed  in  a 
rock  and  is  without  genetic  significance. 

Impregnation  is  a  genetic  term  and  means  that  the  mineral 
introduced  is  later  than  the  rock;  it  may  have  developed  by 
metasomatic  processes  or  by  filling  of  pore  spaces  or  other 
cavities. 

Cementation  is  used  to  indicate  the  filling  of  interstices  in  porous 
or  shattered  rocks. 

The  Law  of  Equal  Volume.— It  is  necessary  to  distinguish 
between  (1)  metasomatic  changes  proceeding  in  free  crystals  or 
grains,  or  in  loose  aggregates  under  light  load,  where  the  force  of 
crystallization  can  easily  overcome  the  restraining  pressure,  and 
(2)  metasomatic  changes  proceeding  in  rigid  rocks  where  the  new 
mineral  is  forced  to  make  room  for  itself  by  solution  of  the  host 
mineral. 

In  the  first  case  the  volume  changes  proceed  according  to  the 
chemical  formula.  In  the  second  case,  the  replacing  mineral 
occupies  exactly  the  space  formerly  filled  by  the  primary  mineral; 
the  force  of  crystallization  is  of  little  or  no  direct  influence,  but 
as  the  pressure  differs  in  intensity  according  to  the  crystallo- 
graphic  directions  and  as  solution  proceeds  most  actively  at 
points  of  greatest  pressure  the  development  of  crystal  faces  is 
thereby  explained.1 

The  chemical  formulas  by  which  some  kinds  of  replacement  are 
usually  expressed  do  not  represent  the  actual  change  for  these 
formulas  are  based  on  equal  weights  and  will  indicate  definite 
changes  in  volume.  The  conversion  of  orthoclase  to  sericite  is 
usually  considered  to  take  place  according  to  the  following 
reaction, 

3KAlSi308  +  H20  +  C02  =  KH2Al3Si3Oi2  +  K2C03  +  6SiO2 

Orthoclase  Sericite 

which  involves  a  decrease  in  volume  of  15.5  per  cent,  even  if 
the  Si02  is  assumed  to  have  recrystallized  as  quartz.  If,  how- 

x  W.  Lindgren,  The  nature  of  replacement,  Econ.  Geol,  vol.  7,  1912,  pp. 
521-535.  Volume  changes  in  metamorphism,  Jour.  Geol,  vol.  26,  1918, 
pp.  542-555. 


CHEMICAL  WORK  OF  UNDERGROUND  WATER    71 

ever,  one  volume  of  orthoclase  has  been  replaced  by  an  equal 
volume  of  sericite  this  equation  is  not  correct,  and  by  a  calcu- 
lation of  the  quantities  of  silica,  alumina,  etc.,  contained  in  one 
cubic  centimeter  of  orthoclase  and  sericite,  respectively,  it  will 
be  found  that  a  considerable  addition  of  alumina  is  necessary. 
The  actual  formula  is  probably  very  complicated  and  could  be 
established  only  if  all  the  reactions  taking  place  in  the  solution 
during  the  conversion  of  one  mineral  to  the  other  were  known. 
Many  kinds  of  metasomatism,  for  instance,  galena  or  barite 
replacing  calcite  (Figs.  62  and  64)  can  not  be  expressed  by 
chemical  formulas.  One  crystal,  for  instance,  of  pyrite  may 
simultaneously  replace  parts  of  adjacent  grains  of  different 
minerals,  or  may  replace  an  aggregate  of  minerals  in  a  fine 
grained  rock.  These  well  known  facts  will  at  once  show  that 
replacement  is  not  the  expression  of  one  definite  chemical 
reaction. 

The  law  of  equal  volumes  has  been  repeatedly  verified  by  many 
independent  observers  and  there  is  little  doubt  that  it  holds  for 
most  metasomatic  processes,  both  on  a  large  and  a  small  scale, 
both  in  general  metamorphism  and  in  mineral  deposits.  The 
most  fundamental  changes  in  rocks  take  place  with  practical 
constancy  of  volume.  A  great  deal  has  been  written  on  changes 
of  volume  and  energy  liberated  or  absorbed,  that  is  absolutely 
valueless  as  a  measure  of  the  processes  that  have  been  going  on 
in  rocks.  The  time  will  soon  come  when  these  relations  are  more 
clearly  recognized. 

Metasomatism  in  solid  rocks  proceeds  independently  of  molec- 
ular weight,  molecular  volume  and  specific  gravity.  It  does  not 
take  place  "molecule  for  molecule,"  and  it  is  not  expressible  in 
simple  chemical  equations.  At  the  same  time  it  is  molecular 
or  at  least  sub-microscopic  in  the  sense  that  complex  processes 
of  solution  and  precipitation  constantly  take  place  in  the 
solution  films. 

Metasomatism  by  equal  volume  takes  place  in  most  perfect 
form  when  a  rock  is  permeated  by  stagnant  or  slowly  moving 
solutions.  When,  as  sometimes  happens,  the  solutions  move 
rapidly,  the  nice  equilibrium  is  disturbed.  Local  excess  of 
solution  over  deposition  is  then  expressed  in  drusy  or  cellular 
structure,  and  the  law  of  equal  volumes  may  'fail  to  hold. 

It  is  not  meant  that  the  processes  of  solution,  precipitation 
and  chemical  reactions  which  make  up  metasomatism  do  not 


72  MINERAL  DEPOSITS 

obey  the  law  of  mass  action  and  laws  of  LeChatelier  and  Van't 
Hoff.  They  undoubtedly  do.  With  increasing  pressure  a  denser 
mineral  will  tend  to  form  but  the  saturated  solutions  that  fill  the 
rock  will  at  once  proceed  to  fill  any  available  space  created  by  the 
deposition  of  the  denser  mineral  with  the  next  combination 
ready  to  be  precipitated. 

General  Definition  of  the  Metamorphic  Zones. — That  part 
of  the  earth's  crust  which  is  within  our  observation  is  called  the 
lithosphere.  It  may  be  observed  directly  by  borings  or  mining 
operations  or  indirectly  when  deformation  and  denudation  bring 
up  to  the  surface  rocks  which  we  know  were  once  deeply  buried. 
The  lower  limit  of  the  lithosphere  can,  of  course,  not  be  ac- 
curately fixed;  F.  W.  Clarke  has  suggested  that  it  may  be  defined 
as  extending  10  miles,  or  16  kilometers,  below  the  surface.  Below 
the  lithosphere  lies  the  centrosphere,  concerning  which  we  have 
little  definite  information. 

The  conceptions  of  Albert  Heim  developed  by  C.  R.  Van 
Hise  led  to  a  division  of  the  lithosphere  into  an  upper  zone  of 
fracture  and  a  lower  zone  of  rock  flowage,  in  which  only  sub- 
capillary  openings  exist  (p.  30)  and  deformation  is  effected 
by  granulation. and  recrystallization.  Between  them  intervenes 
a  middle  zone  of  combined  fracture  and  flowage.  The  limits  of 
these  zones  are  very  indefinite  owing  to  the  greatly  differing 
plasticity  of  rocks,  e.g.,  a  granite  and  a  calcareous  shale.  The 
experimental  proof  given  by  F.  D.  Adams1  that  in  supported 
rocks  in  depth  openings  in  granite  can  persist  to  depths  of  at  least 
11  miles,  or  about  58,000  feet,  at  a  uniform  pressure  of  70,000 
pounds  per  square  inch  and  at  temperatures  supposedly  corre- 
sponding, that  is,  550°  C.  shows  that  the  zones  overlap  widely 
and  have  only  value  as  relative  conceptions. 

Van  Hise  divided  the  zone  of  fracture  in  an  upper  zone  of 
weathering  and  a  lower  zone  of  cementation.  The  zone  of 
flowage  corresponds  to  the  deep  metamorphic  zones  in  which 
minerals  form  by  replacement  only  and  in  which  the  temperature 
is  high  and  the  pressure  largely  stress.2 

1  F.  D.  Adams,  Jour.  Geology,  vol.  20,  1912,  pp.  97-118. 

2  Van  Hise  called  the  upper  two  zones  the  realm  of  katamorphism  and 
the  lower  that  of  anamorphism.     In  the  zone  of  katamorphism  (kata,  down) 
complex  silicates  break  down  and  simpler,  less  dense  minerals  form.     In 
the  zone  of  anamorphism  (ana,  up)  silicates  are  supposed  to  be  built  up 
with  forming  of  denser  minerals  and  compact  texture.     Since  Leith  and 


CHEMICAL  WORK  OF  UNDERGROUND  WATER    73 

Later  investigations  have  shown  that  any  rock  may  be  de- 
formed under  stress.1  The  thrust  required  to  develop  deforma- 
tion in  marble  at  a  pressure  corresponding  to  4.2  miles  would  be 
66,400  pounds  per  square  inch;  in  case  of  granite,  138,500 
pounds  per  square  inch.  At  greater  depths  the  required  stress 
increases  markedly.  The  pressure  necessary  for  plastic  deforma- 
tion is  very  much  greater  than  the  crushing  strength  of  the  rock 
at  the  surface. 

Zone  of  Weathering. — The  best  defined  zone  is  that  of  weather- 
ing, the  depth  of  which  is  determined  by  the  level  of  the  ground- 
water,  or  by  the  depth  to  which  free  oxygen  can  penetrate  in 
large  quantities.  In  the  zone  of  weathering  the  water  percolates 
downward  more  freely  than  in  the  underlying  zone,  there  is  a 
tendency  to  the  destruction  of  the  rocks  as  units,  and  active 
transportation  and  concentration  are  characteristics. 

Chemical  work  progresses  by  means  of  water  solutions  and 
gases,  also  extensively  through  the  medium  of  organic  life;  me- 
chanical disintegration  is  also  important.  The  chemical  reac- 
tions are  oxidation,  carbonatization,  desilication,  and  hydration, 
the  two  first  named  mainly  through  decomposition  of  silicates 
by  water  containing  carbon  dioxide.  As  a  consequence  of  these 
reactions  the  volume  should  'increase,  but  so  much  is  carried 
away  by  solution  that  a  great  reduction  of  volume  ensues. 

Disintegration  works  hand  in  hand  with  decomposition  and  in 
advance  of  it;  calcium,  magnesium,  sodium,  and  potassium  are 
leached;  the  final  products  are  a  small  number  of  minerals, 
largely  hydrated  compounds  with  low  specific  gravity  and,  for 
the  most  part,  comparatively  simple  molecules.  Almost  all 
rock-forming  minerals  are  unstable,  as  are  the  sulphides.  These 
processes  give  rise  to  many  mineral  deposits  of  oxidized  ores, 
which  will  be  described  in  a  later  chapter. 

The  great  extent  of  weathering  and  the  intensity  of  the  changes 
are  justly  emphasized,  especially  in  regions  of  soluble  rocks  like 
limestone.  It  is  well  to  bear  in  mind  that  this  is  not  because  of 
rapid  attack  by  waters,  but  because  of  long-continued  action  by 
extremely  dilute  solutions.  This  is  shown  by  the  relative  purity 


Mead  have  changed  these  conceptions  (Metamorphic  Geology,  1915)  and 
now  confine  katamorphism  to  processes  of  weathering  a  confusion  has  been 
introduced  that  is  best  cured  by  the  dropping  of  both  terms. 

1  F.  D.  Adams  and  J.  A.  Bancroft,  Jour.  Geology,  vol.  25,  1917,  pp.  597- 
637. 


74  MINERAL  DEPOSITS 

of  the  surface  waters,  which  contain  calcium  and  magnesium 
carbonates  with  lesser  amounts  of  alkaline  salts.  The  soluble 
products  mainly  escape  into  the  rivers  through  the  zone  of  dis- 
charge, which  lies  below  the  zone  of  weathering,  and  finally  into 
the  oceans. 

The  Intermediate  Zone. — The  rocks  immediately  below  the 
zone  of  weathering  are  often  saturated  with  water  which  dimin- 
ishes in  quantity  with  increasing  depth.  The  small  pressure 
permits  fracturing  and  brecciation,  and  the  openings  created  by 
these  processes,  as  well  as  those  resulting  from  porosity,  are 
filled  with  minerals  deposited  by  circulating  solutions.  To  a 
small  extent  these  minerals  result  from  material  abstracted  from 
the  zone  of  weathering,  but  that  zone  is  shallow  in  comparison 
with  the  zone  of  cementation,  and  the  salts  available  from  the 
weathering  are,  to  a  large  extent,  carried  away  by  the  surface 
drainage.  The  larger  part  of  the  minerals  deposited  have  been 
derived  from  the  rocks  themselves;  to  a  considerable  extent  they 
are  derived  from  deep-seated  sources,  as,  for  instance,  in  the  ce- 
mentation by  quartz  veins  and  veinlets  near  igneous  intrusions. 
Hydration  and  carbonatization  are  the  principal  processes. 
Minerals  like  chlorite,  serpentine,  talc,  sericite,  epidote,  and  cal- 
cite  develop,  largely  by  metasomatic  processes.  Replacement 
and  filling  work  together. 

Where  stress  is  present  it  is  mainly  in  one  direction  and 
shearing  and  schistosity  may  develop;  in  metamorphic  schists 
some  of  the  minerals  formed  are  muscovite,  chlorite,  talc,  horn- 
blende, zoisite,  epidote,  and  albite;  also  quartz,  pyrite,  and  cal- 
cite,  probably  magnetite  and  specularite.  The  clay  slates  with 
muscovite  and  albite,  the  chloritic  schists,  and  the  talc  schists 
belong  to  this  zone.  According  to  the  views  of  Van  Hise  such 
schistose  rocks  can  only  develop  in  the  deeper  zones. 

The  Deeper  Zones. — In  the  deeper  belts  (included  by  Van 
Hise  under  the  name  of  the  anamorphic  zone)  the  pressure  and 
temperature  are  high;  the  latter  in  general  above  200°  C.  Very 
little  water  is  present.  Minerals  are  formed  mainly  by  replace- 
ment. In  the  upper  part  of  the  zone  temperature  and  pressure 
work  in  the  direction  of  diminished  molecular  volume.  The 
pressure  is  largely  stress— that  is,  acting  in  one  direction — but 
hydrostatic  pressure  (transmitted  in  all  directions)  is  becoming 
of  importance.  The  important  reactions  are  dehydration,  the 
development  of  silicates,  and  deoxidation.  Heavy  silicates,  like 


CHEMICAL  WORK  OF  UNDERGROUND  WATER    75 

wollastonite,  garnet,  tremolite,  and  diopside,  form  in  siliceous 
limestones  or  in  pure  limestones  where  the  silica  is  supplied  by 
plutonic  intrusions.  The  minerals  produced  are  numerous,  stable, 
heavy,  and  complex.  The  rocks  formed  are  compact  and  strong. 
However,  the  temperature  is  not  sufficiently  high  to  break  up 
the  molecules  in  which  hydroxyl  is  firmly  contained. 

The  recrystallization  takes  place  according  to  the  law  of 
Riecke,.1  so  that  the  solution  prevails  at  places  of  maximum 
pressure,  and  deposition  at  those  of  minimum  pressure.  The  re- 
crystallized  products  may  assume  lamellar  structure  extending 
perpendicularly  to  the  pressure;  this  results  in  a  "schistosity 
by  crystallization."  Among  the  minerals  of  this  zone  are  mus- 
covite,  microcline,  albite,  microperthite,  oligoclase,  biotite, 
zoisite,  epidote,  hornblende,  staurolite,  garnet,  cyanite,  titanite. 
magnetite,  and  ilmenite.  Most  of  the  micaceous  and  horn- 
blendic  gneisses  containing  garnet,  staurolite,  etc.,  belong  to  this 
zone;  also  the  mica  schists,  amphibolites,  and  glaucophane  rocks. 

Where  there  is  no  stress  in  this  zone,  many  igneous  rocks,  like 
granite,  basalt,  and  rhyolite,  are  stable. 

In  the  lower  part  of  the  deep-seated  zone  the  temperature  is 
high  and  the  tendency  is  toward  an  increase  of  volume.  The 
hydrostatic  pressure  is  enormous  and  stress  almost  non-existent, 
but  high  temperature  is  the  dominant  feature.  There  are  no 
minerals  containing  the  hydroxyl  molecule  except  biotite,  and 
the  characteristics  are,  therefore,  the  prevalence  of  anhydrous 
minerals  of  great  molecular  volume.  Characteristic  minerals  in 
the  crystalline  schists  of  this  zone  are  orthoclase,  all  plagioclases, 
biotite,  augite,  olivine,  garnet,  cordierite,  sillimanite,  magnetite, 
and  ilmenite.  Many  of  the  minerals  of  this  zone  also  appear  in 
the  massive  igneous  rocks  and  in  the  contact-metamorphic 
rocks.  The  rocks  are  mostly  gneisses,  gradually  approaching 
granites;  also  granulites,  eclogites,  and  augite  gneisses.  Most 
of  the  igneous  rocks  are  stable  in  this  zone. 

The  orthoclase  or  microcline  in  the  crystalline  schists  of  the 
deepest  zone  tends  to  microperthite  in  the  middle  depths  and  to 
sericite  in  the  upper  zone.  Plagioclases  of  the  deep  zone  may  be 
transformed  into  albite  and  anorthite  and  finally  to  albite  and 
zoisite  or  sericite.  The  augites  change  to  hornblende  and  finally 

1  E.  Riecke,  Ueber  das  Gleichgewicht  zwischen  einem  festen  homogen 
deformierten  Korper  und  einer  fliissigen  Phase,  etc.,  Nachr.,  Gesell.  d. 
Wissensch.,  Gottingen,  1894,  4,  pp.  278-284. 


76  MINERAL  DEPOSITS 

to  chlorite.  Olivine  of  the  deep  zone  is  transformed  to  horn- 
blende or  (with  feldspar)  to  garnet  and  becomes  serpentine  in 
the  upper  zone. 

Carbon  dioxide  and  water  doubtless  escape  from  the  deep 
zones  upward  wherever  calcareous  rocks  containing  free  water  or 
hydrated  compounds  become  submerged  in  it. 

Exceptional  supplies  of  heat  contributed  by  igneous  intru- 
sions may  carry  the  reactions  of  the  lower  zones  close  to  the 
surface. 

Relation  of  Mineral  Deposits  to  the  Metamorphic  Zones.— 
Though  certain  kinds  of  mineral  deposits  have  originated  at 
the  surface  or  in  the  zone  of  weathering,  the  largest  number  have 
undoubtedly  been  formed  in  the  zone  of  fracture,  where  circulation 
of  solutions  is  comparatively  easy.  It  is  safe  to  assert  that  the 
great  majority  of  ore  deposits  have  been  formed  within  15,000 
feet  of  the  surface. 

Ore  deposits  do  not,  as  a  rule,  form  in  the  zone  of  flowage  or 
anamorphism  where  the  passage  of  solutions  is  prevented.  An 
exception  to  this  is  where  hot  emanations  from  intrusive  bodies 
penetrate  and  impregnate  certain  rocks  like  limestone  without 
the  necessity  of  ducts  and  cavities. 

Ore  deposits  may  form  also  in  the  hottest  zone  where  the 
solutions  consist  of  magmas  in  which  the  free  rearrangement  of 
molecules  is  possible. 

During  the  ordinary  metamorphic  processes  under  static  or 
dynamic  conditions  extensive  changes  in  mineral  composition  and 
structure  may  be  effected  with  minimal  changes  in  the  chemical 
composition  of  the  rocks,  so  that  it  is  possible  to  trace  the  origin 
of  highly  metamorphosed  rocks  by  the  aid  of  analyses.  Meta- 
morphism  can  be,  and  usually  is,  effected  with  the  aid  of  minute 
quantities  of  rock  moisture  and  during  the  process  there  is  little 
opportunity  for  extensive  concentration  of  rarer  constituents. 
Mineral  deposits  due  to  simple  hydration  or  chemical  rearrange- 
ment within  the  mass  may  result.  Examples:  soapstone  by 
hydration  of  magnesian  minerals;  magnesite  from  carbonati- 
zation  of  serpentine;  sulphur  from  reduction  of  gypsum  by 
organic  compounds;  garnets  developed  in  crystalline  schists; 
concentration  of  hematite  from  lean  primary  ores;  and  many 
similar  instances. 

A  comparison  of  the  mineral  records  of  ore  deposits,  formed 
at  various  levels  in  the  earth's  crust,  with  the  results  obtained 


CHEMICAL  WORK  OF  UNDERGROUND  WATER    77 

by  a  study  of  general  metamorphism  soon  brings  out  the  fact 
that  the  same  laws  do  not  apply  to  both  cases,  although  there 
are  points  of  similarity.  Attention  was  called  to  this  important 
feature  in  a  paper  on  the  metasomatic  processes  in  fissure  veins, 
and  increasing  knowledge  emphasizes  the  distinction.1  Neither 
the  rules  of  Van  Hise  nor  the  three  zones  of  Grubenmann  will  fit 
closely  the  case  of  the  ore  deposits.  The  reason  for  this  is  not 
difficult  to  find.  In  metamorphism  one  deals  with  small  quan- 
tities of  solutions,  free  from  large  amounts  of  carbon  dioxide 
and  hydrogen  sulphide.  The  majority  of  ore  deposits,  on  the 
other  hand,  were  formed  by  large  quantities  of  waters  rich  in 
these  gases  and  heavily  charged  with  alkaline  salts.  A  large 
number  of  silicates  and  other  minerals,  fairly  stable  under  the  in- 
fluence of  ordinary  deep  ground  water,  are  incapable  of  existence 
in  many  vein-forming  solutions.  Biotite,  amphibole,  soda-lime 
feldspars,  often  also  chlorite,  serpentine,  and  magnetite  are 
included  among  these. 

Deposits  Related  to  Igneous  Activity. — Several  important 
groups  of  ore  deposits  must  therefore  be  considered  apart  from 
the  ordinary  processes  of  metamorphism.  This  especially  applies 
to  those  deposits  of  the  rarer  metals  which  stand  in  closest 
genetic  connection  with  intrusion  and  eruption  of  igneous  rocks. 
It  will  be  found  that  classified  according  to  gangue  minerals,  they 
form  three  groups:  (1)  Those  characterized  by  gangue  minerals 
such  as  garnet,  biotite,  hornblende,  pyroxene,  specularite,  mag- 
netite, tourmaline,  topaz,  apatite,  and  scapolite,  all  associated 
with  quartz;  (2)  those  in  which  quartz,  calcite,  dolomite,  siderite, 
barite,  sericite,  chlorite,  and  albite  occur;  (3)  those  in  which 
quartz,  chalcedony,  opal,  calcite,  dolomite,  barite,  fluorite,  seri- 
cite, chlorite,  and  adularia  form  the  more  abundant  gangue 
minerals.  Zeolites  occur  only  exceptionally  in  these  groups,  and 
kaolin  is  considered  almost  wholly  a  product  of  descending 
waters. 

The  first  group  may  be  called  the  high-temperature  deposits, 
though  the  highest  temperatures  during  their  genesis  probably 
did  not  exceed  500°  C.  Ordinarily,  but  not  necessarily,  they  were 

1  W.  Lindgren,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  30,  1900,  p.  601. 

W.  Lindgren,  The  relation  of  ore  deposition  to  physical  conditions, 
Econ.  Gcol.,  vol.  2,  1907,  pp.  105-127.  Also  Compte  Rendu  de  laaXeme 
session  du  Congrts  Geologique  international,  Mexico,  vol.  2,  1906,  pp. 
701-724. 


78  MINERAL  DEPOSITS 

formed  at  great  depths  and  are  sometimes  designated  as  deep- 
seated  deposits. 

The  second  group  is  formed  under  intermediate  conditions  of 
temperature  and  in  general  also  of  depth. 

The  third  group  is  formed  at  moderate  temperatures,  prob- 
ably rarely  exceeding  150°  C.,  within  a  few  thousand  feet  of  the 
surface;  many  of  them  were  developed  close  to  the  surface. 

These  groups  are  described  in  more  detail  in  a  later  chapter. 

On  the  whole  it  is  evident  that  the  great  majority  of  ore 
deposits  have  been  formed  relatively  near  the  surface  and  well 
within  the  zone  of  fracture,  probably  well  within  the  upper  15,000 
feet  of  the  crust,  and  most  of  them  within  10,000  feet  of  the 
surface;  this  applies  even  to  contact-metamorphic  deposits, 
many  of  which  have  developed  along  intrusive  masses  injected 
high  into  the  zone  of  fracture.  Some  instances  are  known  of  such 
deposits  having  been  formed  within  3,000  feet  of  the  surface. 
Only  one  class,  that  of  the  magmatic  segregations,  may  have  its 
origin  at  abyssal  depths;  but  it  is  thought  that  more  commonly 
the  differentiation  of  these  ores  was  effected  after  the  intrusion 
of  the  magmas  into  the  zone  of  fracture. 

Derivation  of  Minerals. — Regarding  the  derivation  of  the  val- 
uable substances  of  mineral  deposits  we  have  to  distinguish 
several  groups: 

1.  Those  which  require  little  or  no  concentration,  but  merely 
involve   chemical   readjustment,    as   the  formation   of  sulphur 
from  sulphate. 

2.  Those  which-  are  formed  by  precipitation  from  solutions, 
the  origin  of  which  is  beyond  doubt,  as,  for  instance,  the  salt 
beds  derived  from  evaporation  of  sea  water. 

3.  Those  which  are  concentrated  by  mechanical  means  from 
well-known  sources,  like  the  gold  placers. 

4.  Those  which  are  derived  by  the  solution  and  removal  of 
waste  material,  like  residual  manganese  or  phosphate  deposits. 

•  5.  Those  which  are  derived  from  concentration  in  magmatic 
solutions  by  processes  of  differentiation;  for  example,  certain 
titaniferous  magnetites. 

6.  Those  deposits,  mainly  of  rarer  metals,  which  require  great 
concentration  and  concerning  the  origin  of  which  more  or  less 
uncertainty  still  prevails. 

Concentration. — Regarding  the  last  group,  it  should  first  be 
stated  that  almost  all  rocks  and  inferentially  all  magmas  contain 


CHEMICAL  WORK  OF  UNDERGROUND  WATER    79 

small  quantities  of  these  rarer  metals.  There  are  two  ways  in 
which  concentration  of  the  rarer  elements  is  possible.  The 
first  is  by  solution,  by  means  of  descending  surface  waters,  of 
the  small  traces  contained  in  the  rocks  and  by  corresponding 
deposition  during  the  subsequent  ascent  of  the  waters.  This 
solution  of  minor  constituents  is  a  slow  and  imperfect  process, 
but  that  it  actually  occurs  is  shown  by  examination  of  natural 
waters  and  their  deposits.  The  smaller  the  metal  traces  the 
slower  and  less  complete  is  this  process  of  solution.  J.  F.  Kemp 
has  studied  this  problem  in  some  detail1  and  concludes  that  the 
amount  which  can  be  extracted  by  water  percolating  through 
the  cracks  of  a  rock  is  only  one-sixth  to  one  one-hundredth  of 
the  total  amount  of  the  particular  metal  contained.  The  leach- 
ing of  compact  masses  of  rocks  by  underground  water  is,  therefore, 
at  best  an  exceedingly  imperfect  process,  and  one  on  which  it  does 
not  seem  safe  to  rely  for  the  concentration  of  the  richer  ores 
of  the  rarer  metals  like  gold  and  silver.  Veins  and  other  deposits, 
conceded  to  have  been  laid  down  by  purely  meteoric  waters, 
contain  in  fact  little  or  no  gold,  silver,  molybdenum,  tungsten, 
and  other  rare  metals. 

Moreover,  the  descending  waters  are  cool  and  dilute,  and  thus 
their  chemical  action  is  slow.  To  obtain  effective  concentration 
of  such  metals  the  first  few  thousand  feet  of  percolated  rock 
should  probably  be  left  out  of  consideration. 

The  second  way  of  concentration  is  by  disturbing  the  equilib- 
rium of  a  molten  magmatic  solution.  Such  disturbances  would 
take  place  by  the  irruption  of  magmas  into  higher  levels  of  the 
crust  or  by  cooling  of  the  magma,  or,  in  other  words,  by  changes  in 
pressure  and  temperature.  From  the  study  of  volcanic  phe- 
nomena it  is  known  that  under  such  circumstances  certain 
substances  are  expelled  from  the  magma,  and  that  among  these 
are  water,  halogens,  alkaline  salts,  and  a  number  of  the  rarer 
metals.  From  the  study  of  plutonic  phenomena  we  infer  that 
a  still  more  thorough  expulsion  of  these  substances  was  effected 
during  the  intrusions  of  deep-seated  magmas.  Gold,  silver, 
lead,  zinc,  copper,  molybdenum,  tungsten,  tin,  and  many  other 
rarer  metals  certainly  have  a  place  in  the  list  of  magmatic 
emanations.  Associated  with  these  elements  are  the  ions  of  sul- 
phur, carbon,  chlorine,  fluorine,  boron,  and  other  elements. 

1  J.  F.  Kemp,  Problem  of  the  metalliferous  veins,  Econ,  GeoL,  vol.  1  , 
1905-1906,  pp.  207-232. 


80  MINERAL  DEPOSITS 

This  concentration  is  effected  automatically  and  with  ease, 
and  these  elements,  dissolved  in  water,  ascend,  propelled  by 
the  expansive  force  of  the  gases.  The  gaseous  solutions  will 
seek  the  fractures  and  fissures  on  their  upward  paths.  Their 
high  temperature  facilitates  the  solution  of  other  elements  in 
the  surrounding  rocks.  In  upper  and  cooler  levels  the  gases 
condense  to  liquid  solution;  precipitation  begins  by  reduction  of 
pressure  and  temperature  or  by  reaction  with  the  adjoining 
rock  minerals.  Finally,  meteoric  waters  mingle  with  the  mag- 
matic  and  this  again  causes  deposition  and  ultimately  the  still 
warm  waters  issue  as  ascending  springs  at  the  surface. 

Both  methods  are  used  in  the  work  of  deposition  by  waters 
in  the  crust.  The  first  is  applicable  to  the  more  common  gangue 
minerals  and  to  the  more  abundant  ore  minerals.  The  second, 
it  is  thought,  must  be  assigned  as  the  main  cause  of  the  rich 
deposits  of  gold,  silver,  and  other  rare  metals.  The  former 
class  of  minerals  is  found  in  all  parts  of  the  world ;  the  latter  is 
confined  to  districts  where  igneous  forces  have  been  active. 

This  conclusion  is  supported  by  an  impressive  array  of  ob- 
servations of  the  various  mineral  deposits  related  to  igneous 
rocks.  The  weight  of  the  cumulative  evidence  is  exceedingly 
strong,  but  is  perhaps  not  fully  appreciated,  except  by  those 
who  have  made  the  study  of  these  deposits  their  specialty. 

UNDERGROUND  TEMPERATURES1 

The  increment  in  temperature  in  the  upper  part  of  the  earth's 
crust  is  generally  assumed  to  average  1°  C.  for  30  meters  or 
nearly  100  feet.  Beginning  with  a  surface  temperature  of 
11°  C.  at  a  depth  of  100  feet,  corresponding  to  the  mean  annual 
temperature  of  a  place  in  the  temperate  zone,  we  would  have  at 
a  depth  of  1,000  feet,  20°  C.;  at  9,000  feet,  100°  C.;  at  20,000  feet, 
210°  C.;  and  at  35,000  feet,  360°  C.,  which  is  near  the  critical  tem- 
perature of  water  (364°  C.).  As  a  matter  of  fact  but  little  is 
known  about  the  increment  at  great  depths.  Actual  measure- 
ments within  the  accessible  zone  or  to  depths  of  about  6,000  feet 
show  considerable  divergences  from  the  average  figure  given  above. 
In  some  cases  the  increase  in  temperature  is  not  quite  uniform. 

1  J.  D.  Everett,  Evidence  before  the  Royal  Commission  on  coal  supplies, 
London,  1904.  Also  in  Reports  of  the  British  Association,  1882-1904. 
An  increment  of  1°  C.  in  100  feet  equals  1°  F.  in  55  feet.  The  Royal  Com- 
mission considered  that  the  average  would  be  1°  F.  in  64  feet. 


CHEMICAL  WORK  OF  UNDERGROUND  WATER    81 


The  best  summary  of  the  results  obtained  in  widely  separated 
parts  of  the  world  has  been  given  by  Koenigsberger,1  who  has 
also  given  important  data  regarding  the  influences  which  increase 
or  diminish  the  geothermal  gradient. 

The  results  best  available  for  general  statements  of  the  nor- 
mal increment  have,  as  a  rule,  been  obtained  from  deep  bore- 
holes in  regions  of  slight  relief,  far  from  large  bodies  of  water, 
and  in  little-altered  rocks,  with  no  Tertiary  or  post-Tertiary 
intrusions,  and  containing  no  large  deposits  of  coal  or  oil.  Obser- 
vations in  deep  mines  are  probably  somewhat  vitiated  by  the 
cooling  effect  of  ventilation;  in  new  drifts  and  stopes,  ventilation 
should  not  greatly  affect  the  results  obtained  from  bore-holes  in 
the  rocks. 

The  following  data  are  abstracted  from  the  tables  of  Koenigs- 
berger : 

GEOTHERMAL    GRADIENTS    IN    UNALTERED  ROCKS  (NOT  RECENT  ERUP- 
TIVES)  AND  REGIONS  OF  FLAT  RELIEF.     BORE-HOLES 


Locality 

Gradient 
in  meters 

Gradient 
infect 

Depth 
in  meters 

Depth 

inl"' 

Author 

Martincourt,1  France  .... 

31.0 

101 

1,200 

3,937 

Sperenberg,  Berlin  

32.5 

107 

1,268 

4,160 

Dunker. 

Sennewitz,  Halle  

36.6 

120 

1,048 

3,438 

Schladebach,  Merseburg.  . 

35.7 

117 

1,236 

4,055 

Dunker. 

Paruschowitz,*  Silesia  .... 

30.7 

101 

1,959 

6,428 

Czuchow,*  Silesia  

29.6 

97 

2,239 

7,346 

Michael  and  Quitzew. 

Bay  City,  Michigan  

36.8 

121 

1,050 

3,445 

Lane. 

Marietta,  West  Virginia  .  . 

37.9 

124 

1,360 

4,462 

Hallock. 

Homewood,  Pennsylvania 

36.7 

120 

1,309 

4,295 

Cummins. 

Wheeling,  West  Virginia 

40.7 

133 

1,360 

4,462 

1 J.  Koenigsberger  and  M.  Miihlberg,  Ueber  Messungen  der  geother- 
mischen  Tiefenstufe,  Neues  Jahrbuch,  Bail.  Ed.  31,  1911,  pp.  107-157. 
(Contains  also  list  of  literature  and  technique  of  measuring  temperatures.) 
Recent  investigations  relating  to  the  measurement  of  temperatures  in  deep 
drill  holes  by  maximum  thermometers  and  thermo-electric  methods  are 
found  in  John  Johnston  and  L.  H.  Adams,  Econ.  Geol.,  vol.  11,  1916,  pp. 
741-762. 

Note. — The  deepest  bore  hole  in  the  world  is  that  of  the  Goff  Farm  near 
Clarksburg,  West  Virginia.  On  January  24,  1918,  this  had  reached  7,350 
feet  and  the  temperature  gradient  is  1°  F.  in  51  feet.  The  boiling  point  of 
water  should  be  reached  at  about  10,000  feet.  U.  S.  Geol.  Survey,  Press 
Bulletin  357,  1918. 

2  At  Paruschowitz,  Czuchow,  and  Martincourt  some  coal  beds  are  present. 


82 


MINERAL  DEPOSITS 


The  influence  of  cool  bodies  of  water  in  lowering  the  earth 
temperature  is  shown  in  the  following  data: 


Locality 

Gradient 
in  meters 

Gradient 
in   feet 

Depth 
in  meters 

Depth 
infect 

Author 

Port  Jackson,  N.  S.  W., 

44.0 

144 

833 

2,733 

David. 

Australia, 

Tokio   Japan  

39.8 

130 

'    361 

1.184 

Tanakadate. 

Pas  de  Calais,  France  

56.6 

185 

1,400    . 

4,593 

Le  Prince-Rinquet. 

Copper    mines  of    Lake 

Superior: 

a.  Osceola,  8  km.   from 

42 

138 

303 

994' 

the  lake, 

b.  Atlantic,  3  to  5  km. 

52-55 

171-180 

276 

905 

Wheeler  and 

from  the  lake, 

Supan. 

c.  1.5  km.  from  the  lake, 

67 

220 

508 

1,667 

d.  Close  to  the  lake  

123 

404 

1,396 

4,580 

Underneath  high  ridges  and  mountains  the  increase  is  slow: 


Locality 

Gradient 
in  meters 

Gradient 
in  feet 

Depth 
in  meters 

i    Depth 
infect 

Author 

Mont  Cenis  (summit) 

50 

164 

Gotthard  (summit) 

44 

144 

Stapff 

43  5 

143 

Schardt 

In  or  near  recent  eruptive  rocks  the  increase  is  often  rapid. 
This  rapid  increase  is  even  noticeable  in  Tertiary  eruptions.  The 
following  data  are  from  borings: 


Locality 

Gradient 
in  meters 

Gradient 
in  feet 

Depth 
in  meters 

Depth 
in  feet 

Author 

Sulz  (Wurttemberg)  
Macholles,  France  
Buda-Pest  

24.1 
14.2 
15.0 

79 
46 
49 

710 
1,005 
903 

2,329 
3,329 
2  963 

Braun  and  Waitz. 
Michel-Levy. 
Szab6 

In  the  vicinity  of  heat-producing  waters,  or  where  chemical 
processes  of  decomposition  are  active,  the  increase  is  especially 
rapid. 


CHEMICAL  WORK  OF  UNDERGROUND  WATER    83 


Locality 

Gradient 
in  meters 

Gradient 
in  feet 

Depth 
in  meters 

Depth 
in  feet 

Author 

Idria,  Austria  

10 

33 

329 

1,079 

Scheinpflug     and 

Holler. 

Comstock,  Nevada  

17.1 

56 

(457 
\672 

l,499l 
2,205  / 

G.  F.  Becker. 

In  coal  mines  and  in  borings  in  coal-bearing  strata  the  increase 
is  more  rapid  than  the  normal,  owing  to  the  chemical  processes  in 
the  coal  beds. 


Locality 

Gradient 
in  meters 

Gradient 
in  feet 

Depth 
in  meters 

Depth 
in  feet 

Author 

Charmoy,  Creusot   (bore- 

26 

85 

1,168 

3,832 

Michel-Le"vy. 

hole). 

Paruschowitz,  Silesia 

(bore-hole): 

Above  coal  

26 

85 

1,122 

3,681 

Hendrick. 

Below  coal  

35 

115 

1,959 

6,427 

Hendrick. 

Gelsenkirchen,  Germany.  . 

23.5 

77 

705 

2,313 

Rosebridge,     England 

29.5 

97 

(cooled  by  ventilation). 

Similar  relations  appear  to  exist  in  regions  producing  petro- 
leum.1 


Locality 

Gradient 
in  meters 

Gradient 
infect 

Depth 
in  meters 

Depth 
infect 

Author 

Santa  Maria,  California.  . 

23.0 

76 

1097 

3,599 

Arnold  and  Ander- 

Berekei,  Caucasus  
Apsheron,  Russia  

23.0 
28.4 

76 
.       93 

1000 
300-700 

3,281 
984-2,297 

.    son. 
Kelickij. 
Solubjatnikow. 

In  mines  of  various  kinds  the  increase  may  be  more  rapid  or 
more  slow  than  normal.  The  cooling  by  ventilation  reduces  the 
temperature  to  some  extent. 

1  For  later  information  on  this  subject  see  H.  Hoefer,  Temperature  in  oil 
regions,  Econ.  GeoL,  vol.  7,  1912,  pp.  536-541  and  E.  De  Golyer,  The  sig- 
nificance of  certain  Mexican  oil  temperatures,  Econ.  Geol.,  vol.  13,  1918, 
pp.  275-301. 


84 


MINERAL  DEPOSITS 


Locality 

Gradient 
in  meters 

Gradient 
infect 

Depth 
in  meters 

Depth 
in  feet 

Author 

31.0 

102 

457 

ca.  1,500 

D'Aubuisson. 

33  0 

108 

610 

ca.  2,000 

Thomas. 

Bendigo,  Victoria  (New  Chum 

42.7 

140 

1,110 

3,645 

Jenkins. 

Railway). 

Ballarat,  Victoria  

44.2 

145 

634 

2,080 

Jenkins. 

Witwatersrand,  S.  A  

115 

377 

1,200 

3,900 

Marriott. 

Althoughjin  some  mines  the  increase  is  about  normal,  in 
other  mines  it  is  remarkably  slow.  At  Bendigo,1  where  gold- 
bearing  quartz  veins  occur  in  Ordovician  sandstone,  the  rock 
temperature  at  the  greatest  depth  attained,  4,600  feet,  is  only 
112°  F.  (44.5°  C.).  At  this  depth  the  water  is  salty  and  has  .a 
temperature  of  114°  F.  At  St.  John  del  Rey,2  a  gold  mine  in 
the  schists  of  southern  Brazil,  at  4,000  feet  below  the  adit  tunnel, 
the  temperature  is  only  95°  F.,  or  35°  C.  On  the  Witwatersrand, 
in  the  Transvaal,3  a  temperature  of  65°  F.  prevails  at  500  feet; 
the  increase  down  to  3,900  feet  is  regular  at  the  rate  of  1°  C.  per 
360  feet;  at  the  lowest  depth  the  temperature  is  only  84.4°  F.,  or 
about  29°  C.  According  to  Marriott  the  natural  ventilation 
reduces  the  temperature  near  the  workings  5°  to  6°  C. 

No  reason  is  known  for  the  slow  increase  of  temperature  in 
the  Transvaal  and  in  Victoria.  Koenigsberger  has  suggested 
that  the  decided  increase  in  the  earth  temperature  near  oil  pools 
and  beds  of  coal  (except  anthracites)  may  be  utilized  for  the 
prognostication  of  the  occurrence  of  these  substances  near  a  given 
bore-hole. 

In  temperature  measurements  a  maximum  instrument  con- 
structed on  the  same  principle  as  a  clinical  thermometer  is  most 
practical;  an  instrument  about  25  centimeters  long  is  recom- 
mended. An  ordinary  high-class  chemical  thermometer  reading 
to  0.2°  C.  may  also  be  used.  For  the  measurement  of  tempera- 

1  W.  J.  Rickard,  Deep  mining  at  Bendigo,  Mining  Magazine,  London, 
1910,  pp.  281-282. 

2  Eng.  and  Min.  Jour.,  July  3,  1909. 

>  H.  F.  Marriott,  An  investigation  of  earth  temperatures,  etc.,  Trans., 
Inst.  Min.  and  Met.,  1906. 

See  also  The  Mining  Journal  (London),  April,  1906,  p.  479. 


CHEMICAL  WORK  OF  UNDERGROUND  WATER    85 

ture  in  mine  workings  H.  C.  Jenkins1  suggests  some  rules  summar- 
ized below: 

1.  Temperatures  should  be  taken  in  new  workings  which  are  rapidly 
pushed.     Otherwise  cooling  may  affect  the  result. 

2.  When  possible  the  rock  should  be  free  from  easily  oxidized  sulphides. 
Considerable  heat  is  developed  when  pyrite  oxidizes,  as  is  well  illustrated 
in  many  mines. 

3.  Holes  should  be  bored  6  feet  and,  if  possible,  horizontal. 

4.  Wet  ground  should  be  avoided,  as  the  readings  will  generally  be  too  low. 

5.  One  or  two  days  should  be  allowed  to  permit  the  heat  of  drilling  to 
dissipate. 

6.  The  thermometer  should  be  inserted,  supported  upon  cork  mounts 
in  an  outer  closed  glass  tube,  and  the  bore-hole  closed. 

Note  1. — In  measuring  rock  temperatures  in  some  Alpine  tunnels2  a  hole 
1.5  meters  deep  was  bored  in  the  side  of  the  tunnel  about  a  meter  above 
the  floor  and  slightly  inclined  upward.  An  ordinary  thermometer  was 
used,  its  length  approximating  25  centimeters.  It  was  cemented  in  a  glass 
tube  and  inclosed  in  a  double  metal  cylinder  with  cork  rings.  The  mercury 
bulb  was  inclosed  in  a  mixture  of  wax  and  turpentine.  The  metal  cylinder 
with  the  thermometer  was  pushed  in  by  means  of  a  metal  wire.  The  hole 
was  then  closed  by  a  long  plug  of  wood  wrapped  with  woolen  cloth,  and 
stoppered  by  a  wooden  plug  covered  with  gypsum  plaster.  The  readings 
were  taken  24  hours  after  the  insertion.  The  errors  or  differences  from  the 
actual  rock  temperature  are  ±  0.5°  C. 

Note  2. — A.  C.  Lane3  doubts  Koenigsberger's  conclusion  that  the  vicinity 
of  Lake  Superior  affects  the  temperatures,  and  believes  rather  that  climate 
changes  may  be  responsible  for  the  present  slow  increase.  Lane  gives  the 
average  gradient  at  the  Calumet  &  Hecla  as  189  feet  for  1°  C. 

1  H.  C.  Jenkins,  Rock  temperatures  in  Victoria,  Proc.,  Aust.  Assoc.  Adv. 
Sci.,  vol.  9,  1902,  pp.  309-318. 

2  E.  Kiinzli,  Geologische  Beschreibung  des  Weissensteintunnels,  Beitrdge 
zur  Geologischen  Karte  der  Schweiz,  Neue  Folge,  21.  Lieferung,  Bern,  1908, 
p.  128. 

3  The  Keweenaw  series  of  Michigan,  Lansing,  1911,  p.  763. 


CHAPTER  VI 

THE  ORIGIN  OF  UNDERGROUND  WATER  AND  ITS 
DISSOLVED  SUBSTANCES 

Origin  of  the  Water. — There  is  no  physical  or  chemical  criterion 
by  which  the  origin  of  a  given  water  can  be  determined.  A  pure 
water  might  possibly  rise  from  interior  sources  and  acquire 
saline  constituents  during  the  ascent.  A  water  of  superficial 
derivation  might  be  conceived  to  have  become  charged  with 
magmatic  products.  If  it  is  possible  to  distinguish  between 
waters  derived  from  the  surface  and  those  brought  up  from  the 
interior  of  the  earth,  the  evidence  must  be  circumstantial  and 
depend  on  geologic  and  physiographic  testimony,  such  as  geo- 
logic structure,  igneous  history,  rainfall,  and  drainage  basins. 
There  are  then  two  modes  of  derivation : 

1.  Meteoric  Waters. — (a)  The  water  is  derived  from  the 
rain  that  falls  on  the  surface,  or  from  the  water  courses,  or  from 
the  lakes,  or  from  the  present  oceans  and  has  simply  descended 
into  the  earth  in  the  cavities,  fissures,  or  capillary  openings  to 
ascend  at  suitable  places  under  hydrostatic  conditions,  or  to 
remain  stored  in  the  rocks  and  almost  stagnant  (Chapter  III). 
The  term  meteoric  waters,1  or  surface  waters,  is  applied  to  this 
group. 

(6)  The  water  was  mechanically  included  in  the  sediments  of 
ancient  oceans  and  has  for  geologic  periods  been  a  constituent  of 
these  strata.  The  term  "connate"  has  been  proposed  by  A.  C. 

1  Reginald  A.  Daly,  Genetic  classification  of  underground  volatile  agents, 
Econ.  Geol,  vol.  12,  1917,  pp.  487-504.  Daly  shows  that  Posepny's 
term  "vadose"  (vadus  =  shallow)  was  applied  by  him  to  the  descending 
waters  above  the  water  level,  that  is  to  the  zone  of  gathering  (Chapter  III). 
Authors  have  used  it  since  with  different  meanings,  in  each  case  including 
a  certain  part  of  the  atmospheric  waters.  It  seems  as  if  the  science  could 
dispense  with  the  word  vadose.  Daly  following  Archibald  Geikie  has 
suggested  "epigene"  to  cover  the  underground  activities  of  both  fresh  and 
marine  waters.  The  term  phreatic,  applied  by  Daubre"e  to  a  somewhat 
indefinite  part  of  the  meteoric  waters,  may  likewise  be  dispensed  with. 


THE  ORIGIN  OF  UNDERGROUND  WATER         87 

Lane1  to  cover  the  origin  of  such  waters,  which  really  like  those 
of  the  preceding  class,  are  of  meteoric  origin. 

2.  Magmatic  Waters. — The  water  existed  in  the  solution  con- 
stituting an  igneous  magma.  Crystallization  of  the  magma  or 
its  irruption  into  higher  levels  of  the  earth's  crust  liberated  the 
water  as  one  of  the  most  volatile  constituents,  thus  permitting 
its  ascent  to  cooler  levels.  Such  water  may  be  called  magmatic 
or  juvenile.2 

Underground  waters  may  then  be  meteoric  or  magmatic  or  a 
mixture  of  both.  Large  quantities  of  magmatic  water  are  rarely 
found  except  in  regions. of  present  or  recent  igneous  activity. 

Smaller  parts  of  meteoric  or  magmatic  waters  may  permanently 
or  temporarily  be  withdrawn  from  the  circulation  by  being  held 
firmly  by  capillarity,  by  forming  inclusions  in  minerals  or  by 
entering  chemical  compounds.  Heat,  pressure  and  chemical  ac- 
tion may  release  part  of  these  imprisoned  waters  when  rocks  sink 
into  warmer  zones  or  are  engulfed  by  rising  magmas.  Thus 
while  no  one  may  doubt  that  the  magma  contains  primary  water 
a  certain  small  part  of  it  may  be  derived  by  the  melting  of  rocks 
immersed  in  magmas.3 

Magmatic  or  Juvenile  Waters. — Volcanic  phenomena  are 
almost  always  accompanied  by  the  emission  of  large  quantities 
of  steam  and  other  volatile  substances,  and  geologists  generally 
have  agreed  that  part  of  this  water  is  a  contribution  to  the 
atmosphere  and  hydrosphere  from  the  magmas.4 

More  recently  A.  Brun5  in  a  work  of  much  merit  on  the  volcanic 
exhalations  has  arrived  at  the  result  that  the  magmas  are  anhy- 
drous, a  view  which  is  difficult  to  accept,  though  undoubtedly 
some  classes  of  lavas  like  basalt,  when  arriving  at  the  surface, 
are  relatively  poor  in  water.  The  clouds  of  vapors  attending 
volcanic  eruptions  are,  according  to  Brun,  mainly  volatilized 
chlorides,  mixed  with  dust  from  explosions.  Day  and  Shepherd6 

1  Bull.  Geol.  Soc.  Am.,  vol.  19,  1908,  p.  502. 

2  E.  Suess,   Verb,  der  Gesell.  deut.  Naturf.  und  Aertze,  1902,  pp.  133- 
150;  Das  Antlitz  der  Erde,  Wien,  Bd.  3,  2te  Halfte,  1909,  pp.  630,  655. 

3  R.  A.  Daly,  Am.  Jour.  Sd.,  4th  ser.,  1908,  p.  48;  Igneous  rocks  and  their 
origin,  New  York,  1914,  p.  249.     Daly  terms  such  waters  re-surgent. 

4  T.  C.  Chamberlain,  Jour.  Geology,  vol.  7,  1899,  p.  559. 

*  Recherches  sur  1'exhalaison  volcanique,  Geneva,  1911.  See  also  F.  W. 
Clarke,  Geochemistry,  Butt.  616,  U.  S.  Geol.  Survey,  1916,  p.  282.  A.  N. 
Winchell,  Brun's  new  data  on  volcanism,  Econ.  Geol.,  vol.  7,  1912,  pp.  1-14. 

6  Arthur  L.  Day  and  E.  S.  Shepherd,  Water  and  volcanic  activity,  Bull., 
Geol.  Soc.  Am.,  vol.  24,  1913,  pp.  573-606. 


88  MINERAL  DEPOSITS 

recently  disproved  Brim's  thesis  by  subjecting  the  gases  of  the 
Kilauea  crater  on  the  island  of  Hawaii  to  a  very  careful  study, 
and  ascertained  that  when  free  from  contamination  of  air  they 
consist  of  nitrogen,  water  gas,  carbon  dioxide,  sulphur  dioxide  and 
hydrogen.  They  concluded  that  the  water  released  from  the 
liquid  lava  as  it  reaches  the  surface  is  entitled  to  be  considered 
an  original  component  of  the  lava  with  as  much  right  as  the  sul- 
phur or  the  carbon.  It  follows  logically  that  some  of  this  water 
from  cooling  lavas,  with  associated  gases  must  mingle  with  the 
waters  of  meteoric  origin. 

Regarding  plutonic  rocks  the  direct  evidence  is  lacking  but 
indirect  testimony  is  supplied  by  the  inclusions  of  aqueous  solu- 
tions so  commonly  found  in  granular  rocks  and  by  the  presence  of 
minerals  like  mica  and  amphibole  which  contain  the  hydroxyl 
molecule. 

The  best  general  evidence  of  the  existence  of  juvenile  waters  is 
furnished,  not  by  observation  of  the  present  springs,  but  by  the 
study  of  old  intrusive  regions.  Here  the  granites  merge  into 
pegmatite  dikes,  the  latter  change  into  pegmatite  quartz,  and  this 
into  veins  carrying  quartz  and  metallic  ores,  such  as  cassiterite 
and  wolframite.  Here  we  have  evidence  difficult  to  controvert 
that  dikes  consolidated  from  magmas  gradually  turn  into  de- 
posits the  structure  and  minerals  of  which  testify  to  purely  aque- 
ous deposition;  this  admitted,  it  is  difficult  to  see  what  would 
prevent  such  waters  from  reaching  the  surface  in  the  form  of 
ascending  springs. 

Elie  de  Beaumont1  was  the  first  to  give  full  expression  to  this 
view.  He  believed  that  there  were  two  classes  of  hot  springs: 
The  first  (the  more  common)  is  intimately  related  to  volcanism 
and  derives  its  waters  and  dissolved  solids  from  this  source;  the 
second,  and  more  exceptional,  derives  its  water  from  simple 
infiltration.  This  view  was  accepted  by  de  Lapparent,  but 
Daubree  arrived  at  the  contrary  conclusion,  that  both  volcanism 
and  thermal  springs  result  from  the  infiltration  of  water  from 
the  surface;  similar  views  were  held  by  Fouque  and  have  more 
recently  been  adopted  by  de  Launay.2  The  views  of  Daubree 
found  general  acceptance  in  other  countries;  in  the  United  States 
they  were  accepted  by  Le  Conte,  Van  Hise,  and  others.  All 

1  Bull.,  Soc.  Geol.  de  France,  Serie  2,  1847,  Tome  4,  p.  1272. 

2  L.  de  Launay,  Recherche,  captage  et  ame'nagement  des  sources  thermo- 
mine' rales,  Paris,  1892. 


THE  ORIGIN  OF  UNDERGROUND  WATER         89 

waters  appearing  at  the  surface  were  considered  of  atmospheric 
origin  and  their  salts  were  dissolved  from  the  rocks  percolated. 
About  the  year  1900  the  importance  of  magmatic  exhalations  for 
the  formation  of  mineral  deposits  began  to  be  reasserted  by 
various  mining  geologists — among  them  Vogt  in  Norway,  and 
Spurr,  Kemp,  Weed,  and  Lindgren  in  the  United  States.  In 
1902  Suess,1  the  eminent  Austrian  geologist,  announced  his  belief 
that  many  of  the  springs  in  volcanic  regions  were  of  "juvenile" 
origin — that  is,  that  they  now  reach  the  surface  for  the  first  time 
and  yield  a  permanent  addition  of  water  and  salts,  carried  up 
from  magmas  cooling  at  great  depth.  As  an  excellent  example 
of  this  the  Carlsbad  Springs  were  cited. 

The  question  now  arises  whether  it  be  possible  to  establish 
criteria  by  which  the  magmatic  waters  may  be  distinguished  from 
those  of  meteoric  origin.  Delkeskamp  in  Germany  has  attempted 
the  solution  of  this  problem  in  a  series  of  suggestive  papers.2 
He  rightly  considers  temperature  of  little  value  as  a  criterion  and 
points  out  that  many  springs  of  meteoric  origin  are  hot,  while 
some,  strongly  suspected  to  be  of  juvenile  origin,  are  cold. 
The  constant  admixture  with  vadose  waters  forms  another  diffi- 
culty, but  accounts  well  for  the  many  derivatives  of  varying 
characteristics  which  accompany  every  spring  of  deep-seated 
origin.  Seasonal  variations  of  temperature,  salinity,  and 
quantity  of  water  constitute  excellent  proofs  of  superficial  origin. 
A  practical  constancy  of  salinity,  temperature  and  quantity  is 
said  to  be  the  best  proof  of  a  juvenile  origin.  Among  the  juvenile 
springs  are  those  of  Carlsbad  in  Austria,  Ems  and  Wiesbaden  in 
Germany. 

It  is  doubtful  whether  these  criteria  can  be  accepted.  Much 
more  work  must  be  done  before  we  shall  be  able  to  establish  the 
magmatic  origin  of  any  given  spring. 

Examples  of  Springs  in  Volcanic  Regions. — As  pointed  out  on 
p.  63  there  are  two  types  of  ascending  hot  waters  which  may  be 

1  Verhandl.  Gesell.  deutscher  Nat.  u.  Aerzte,  Karlsbad,  1902. 

2  R.  Delkeskamp,  Juvenile  und  vadose  Quellen,  Balneologische  Zeitung, 
16,  No.  5,  Feb.  20,  1905,  p.  15. 

R.  Delkeskamp,  Die  Genesis  der  Thermalquellen  von  Ems,  Wiesbaden, 
und  Kreutznach  und  deren  Beziehungen  zu  den  Erz — und  Mineralgangen 
des  Taunus  und  der  Pfalz.  Verhandlungen  Gesell.  deutscher  Nat.  und 
Aerzte,  1903,  2,  First  Part. 

A.  Gautier,  Compt.  Rend.  vol.  150,  1910,  p.  436. 

See  also  reference  in  Econ.  Geol.  vol.  1,  1905,  pp.  602-612. 


90  MINERAL  DEPOSITS 

of  juvenile  origin.  They  are  the  sodium  carbonate  and  the 
sodium  chloride-silica  types,  both  common  in  regions  of  expiring 
volcanism.  The  former  appear,  for  instance,  in  central  Germany, 
in  central  France,  in  California  and  at  various  places  in  our 
Western  States.  The  latter  characterize  the  great  geyser  regions 
of  Yellowstone  Park,  Iceland  and  New  Zealand.  The  two  classes 
break  up  through  volcanic  rocks  and  through  the  underlying 
plutonic  rocks  or  crystalline  schists.  Whether  these  waters  are 
wholly  or  partly  of  magmatic  origin  is  a  doubtful  question. 
Arnold  Hague,  who  spent  many  years  in  the  study  of  the  Yellow- 
stone Park  has  expressed  the  decided  opinion  that  the  present  hot 
springs  at  this  locality  are  of  meteoric  origin.1  Such  an  origin 
is  probably  more  difficult  to  establish  for  the  geyser  district  of 
New  Zealand.  On  the  other  hand  many  geologists  are  of  the 
opinion  that  some  of  .the  dissolved  salts  and  gases  at  all  of  these 
places  are  of  magmatic  or  juvenile  origin. 

Salts  from  Sedimentary  Rocks. — There  is  little  difficulty  in 
the  large  sedimentary  areas,  where  volcanism  is  absent.  The 
great  majority  of  underground  waters  are  here  simply  of  atmos- 
pheric origin,  and,  in  spite  of  great  diversity,  the  saline  constitu- 
ents, as  well  as  the  gases,  are  readily  traced  to  the  sediments 
traversed.  It  is  evidently  possible  for  atmospheric  waters  to 
attain  sufficient  depth  to  acquire  a  high  temperature,  though 
this  rarely  exceeds  60°  C.  The  Hot  Springs  of  Virginia,  the  Ar- 
kansas Hot  Springs,  the  Arrowhead  Springs  of  southern  California, 
and  the  Utah  Springs  in  the  Salt  Lake  Basin  clearly  derived 
their  saline  constituents  from  the  surrounding  sedimentary  rocks. 
Examples  of  this  class  from  the  French  Alpine  region  are  plentiful. 

In  all  these  waters  the  principal  constituents  are  those  of  the 
surrounding  sediments — calcium-magnesium  carbonates  from  the 
limestones  and  dolomites,  brines  from  the  saline  formations, 
calcium  sulphate  from  gypsiferous  Triassic  formations,  sodium 
sulphate  from  the  Cretaceous  shales,  hydrogen  sulphide  from  the 
reduction  of  sulphates  by  oil  or  other  organic  matter  often  present 
in  the  strata,  carbon  dioxide  from  reactions  between  carbonate  of 
calcium  and  other  salts.  The  presence  of  connate  waters  is 
difficult  to  prove.  It  is  simply  inferred  from  the  occurrence  of 
strong  sodium  chloride  and  calcium  chloride  brines  in  certain 
sedimentary  rocks.  Any  marine  beds  must  necessarily  have  con- 

1  Origin  of  the  thermal  waters  in  the  Yellowstone  National  Park,  Bull., 
Geol.  Soc.  Am.,  vol.  22,  1911,  pp.  101-122. 


THE  ORIGIN  OF  UNDERGROUND  WATER         91 

tained  occluded  sea  water,  but  many  geologists  doubt  whether  it 
would  have  remained  undisturbed  during  long  ages. 

Salts  from  Igneous  Rocks. — The  various  types  of  waters  from 
sedimentary  areas  are  closely  paralleled  by  those  from  igneous 
rocks.  The  waters  of  the  upper  circulation  in  igneous  rocks 
are  characterized  by  their  consistent  content  of  calcium  car- 
bonate, to  which  large  amounts  of  magnesium  and  ferrous 
carbonates  are  sometimes  added.  The  attack  of  carbon  dioxide 
on  alkaline  silicates  gives  alkaline  carbonates  and  the  oxidation 
of  pyrite  affords  a  small  amount  of  sulphates.  Soluble  silica 
is  added  from  all  silicates,  suffering  partial  decomposition. 
Occasionally  these  waters  are  tepid  or  hot,  but  probably  only 
where  they  have  exceptional  opportunities  for  deep  descent 
in  regions  of  strong  dislocations  or  contact  with  hot  eruptive 
rocks. 

Salts  of  Volcanic  Springs.- — Some  of  the  hot  ascending  springs 
in  volcanic  regions  carry  much  sodium  carbonate  as  stated 
above.  The  long-continued  action  of  the  hot  water  saturated 
with  carbon  dioxide  on  the  feldspars  of  the  surrounding  rock 
undoubtedly  yields  this  salt  in  large  quantities,  and  the  scarcity 
of  calcium  and  magnesium  carbonates  is  explained  by  their  pre- 
cipitation with  increasing  percentage  of  alkaline  carbonates. 

Considerable  quantities  of  sodium  chloride  are,  however, 
always  associated  with  the  sodium  carbonate  and  sometimes 
indeed  predominate;  to  find  an  adequate  explanation  of  this  is 
more  difficult.  Igneous  rocks  average,  according  to  Clarke's 
calculation,  only  0.07  per  cent,  of  chlorine,  and  while  there  are 
some  exceptional  rocks  containing  sodalite,  the  sodium  chloride 
waters  are  by  no  means  particularly  associated  with  this  mineral. 
Considering  that  the  water  could  extract  only  a  small  part 
of  this  chlorine,  it  is  not  easy  to  estimate  the  amount  of 
rock  which  must  be  percolated  to  obtain  a  sustained  flow  of 
chloride  waters  of  the  concentration  often  found  in  hot  springs. 
The  sam.e  reasoning  applies  to  the  alkaline  sulphates  which  are 
abundant  in  some  waters.  It  might  be  imagined  that  surface 
waters  moving  downward  could  have  become  charged  with 
sodium  chloride  or  sulphate  while  traversing  saline  sedimentary 
rocks,  but  such  an  explanation  seems  somewhat  forced  in  the 
case  of  springs  which  issue  from  granite  in  a  region  where  no 
such  sedimentary  beds  are  known  to  occur.  Boron  is  a  common 
constituent  of  many  of  these  springs,  for  instance,  the  Steamboat 


92  MINERAL  DEPOSITS 

Springs,  Nevada,  and  Ojo  Caliente,  New  Mexico,  both  of  which 
issue  from  granitic  rocks.  It  is  still  more  difficult  to  find  a 
reasonable  explanation  for  the  presence  of  this  substance  on  any 
hypothesis  of  leaching.  Tourmaline  and  datolite  are  of  course 
present  in  some  rocks,  but  the  springs  carrying  boron  exhibit  no 
marked  relation  to  areas  where  such  boron  minerals  occur.  It 
is  true  that  boron  occurs  in  saline  sedimentary  beds  and  that 
traces  of  it  are  often  found  in  waters  traversing  them,  but  the 
quantities  do  not  compare  with  those  determined  in  many 
waters  of  volcanic  associations.  Similar  statements  can  be 
applied  to  fluorine,  though  it  is  less  abundant  than  boron. 

The  geyser  springs  of  Iceland,  the  Yellowstone  Park,  and  New 
Zealand  are  rich  in  silica,  and  as  most  of  them  ascend  through 
easily  decomposed  rhyolitic  rocks,  that  substance  may  well  be 
derived  from  leaching  of  the  country  rock.  And  yet  when  we 
note  how  veins  rich  in  quartz  are  at  places  directly  connected 
with  pegmatite  dikes,  and  how  strong  the  evidence  is  •  against 
their  deposition  by  leaching  from  surrounding  rocks,  we  may 
well  wonder  whether  this  silica  in  the  thermal  waters  is  neces- 
sarily derived  by  solution  of  rock  comparatively  near  the  surface. 
And  again,  when  we  observe  that  chlorides  form  part  of  magmas, 
as  indicated  by  the  presence  of  sodalite,  and  remember  that 
sodium  chloride  occurs  as  small  crystals  in  the  fluid  inclusions 
of  quartz  phenocrysts,  and  finally  note  the  abundance  of  chlo- 
rides at  volcanic  eruptions,  would  it  not  then  be  easier  to  account 
for  this  salt  in  the  springs  of  volcanic  regions  by  an  easily  effected 
concentration  of  volatile  substances  while  the  magma  was  still 
fluid,  than  by  a  laborious  search  for  traces  of  chlorides  in  the 
congealed  igneous  rocks? 

Origin  of  the  Dissolved  Gases. — Reduction  of  sulphates  by 
organic  matter  and  oil  accounts  satisfactorily  for  the  hydrogen 
sulphide  abundant  in  many  sulphate  and  chloride  waters  in  sedi- 
mentary rocks.  Carbon  dioxide  occurs  in  many  such  waters,  but 
the  reactions  by  which  this  gas  is  produced  are  less  well  known. 
The  rain  water  contains,  of  course,  little  carbon  dioxide  and  far 
more  is  taken  up  during  the  percolation  of  the  humus  of  the  soil. 
This  is  soon  absorbed  in  the  formation  of  bicarbonates,  but  from 
the  decomposition  of  these  compounds  gas  may  be  set  free. 

In  igneous  rocks  similar  processes  may  apply  on  a  small  scale, 
but  many  of  the  ascending  waters  in  volcanic  regions  contain 
such  enormous  amounts  of  carbon  dioxide  that  its  explanation 


THE  ORIGIN  OF  UNDERGROUND  WATER         93 

by  chemical  reactions  within  the  water  itself  appears  utterly 
insufficient.  From  the  earliest  times  of  geological  science  this 
difficulty  has  been  recognized  and  the  literature  on  the  deri- 
vation of  the  carbon  dioxide  is  extensive.  It  has  lately  been 
summarized  by  R.  Delkeskamp.1  In  addition  to  the  carbon 
dioxide  absorbed  by  the  water  from  the  atmosphere  and  the 
soil  humus,  a  possible  source  of  superficial  origin  may  be  found 
in  decomposing  organic  deposits  such  as  peat  and  lignite,  though 
the  coals  are  more  likely  to  give  off  hydrocarbons  than  carbon 
dioxide.  Still  another  way  in  which  the  latter  gas  may  be  formed 
is  by  the  reaction  between  acid  waters,  such  as  are  often  found  in 
mines,  and  limestone. 

A  more  deep-seated  source  lies  in  the  replacement  near  intru- 
sive contacts  of  limestone  or  dolomite  by  calcic  or  magnesic 
silicates.  This  process  has  taken  place,  on  a  larger  or  smaller 
scale,  at  the  contacts  of  all  intrusives  in  calcareous  rocks,  and 
there  is  no  good  reason  to  doubt  that  it  is  going  on,  in  some 
localities,  at  present.  The  quantity  of  carbon  dioxide  available 
from  this  reaction  is  large  and  it  is  likely,  indeed,  that  many 
thermal  springs  in  regions  of  intrusives  have  been  fed  from  such 
sources.  But,  on  the  other  hand,  coal  beds  and  limestones 
cannot  be  supposed  to  exist  underneath  every  volcanic  region, 
and  in  large  areas  of  granite  rocks  the  hypothesis  becomes 
decidedly  improbable. 

It  is  known  that  many  igneous  rocks,  particularly  granites, 
contain  liquid  carbon  dioxide  as  minute  fluid  inclusions,  though 
it  may  be  doubted  whether  they  are  as  abundant  as  is  assumed 
by  some  writers.  On  the  assumption  that  the  quartz  contains  a 
maximum  of.  5  per  cent,  by  volume  of  these  inclusions  Laspeyres 
has  calculated  the  content  of  CO  2  in  a  cubic  kilometer  of  granite 
as  sufficient  to  furnish  the  springs  of  Nauheim,  Germany,  with 
carbon  dioxide  for  273,000  years.  Such  calculations  carry 
little  conviction  to  those  who  realize  the  difficulty  involved  in  the 
absorption  of  any  but  a  minimal  quantity  of  this  gas  from  the 
quartz  grains  by  percolating  waters;  and  besides,  exhalations  of 
carbon  dioxide  are  not  characteristic  of  areas  of  granite,  but 
appear  in  regions  of  volcanic  rocks  without  reference  to  the 
character  of  the  basement  rock  traversed  (Fig.  4). 

The  interesting  experiments  of  A.  Gautier  and  others  on  the 

1  R.  Delkeskamp,  Vadose  und  juvenile  Kohlensaure,  Zeitschr.  prakl. 
Geol,  February,  1906. 


94 


MINERAL  DEPOSITS 


gases  included  or  occluded  (absorbed)  in  the  minerals  of  a  rock 
and  set  free  on  heating  have  been  summarized  by  several  writers, 
including  F.  W.  Clarke1  and  F.  L.  Ransome.2  A  great  number 
of  exact  analyses  of  these  gases  were  made  recently  by  R.  T. 
Chamberlin,3  who  found  in  general  that  the  various  pulverized 
rocks  yielded  a  total  amount  of  gases  (at  0°  C.  and  760  mm. 
pressure)  equal  to  from  a  fraction  up  to  as  much  as  30  times  the 
unit  volume  of  rock;  the  gases  determined  were  H2S,  CO2,  CO, 


••* 

0   10  20  30  4p  5jQ  60  Ki|flmeters 


FIG.  4. — Carbon  dioxide  and  sodium  carbonate  springs  of  central  France. 
Black  dots  are  springs.     Shaded  area  shows  extent  of  basaltic  eruptions. 


CH4,  H2,  and  N2,  among  which  CO2  and  H2  always  predominated. 
The  carbon  dioxide  is  believed  to  be  derived  from  the  decom- 
position of  small  quantities  of  secondary  carbonates,  while 
in  part  it  may  also  be  included  or  occluded.  The  hydrogen 
and  other  carbon  compounds  are  probably  due  to  reactions 
of  water  vapor  and  carbon  dioxide  with  some  of  the  substances 

1  Geochemistry.  Bull.  616,  U.  S.  Geol.  Survey,  1916,  pp.  276-281. 
J  Econ.  Geol,  vol.  1,  1906,  p.  688. 

1  R.  T.  Chamberlin,  The  gases  in  rocks,   Carnegie  Inst.   Washington, 
1908,  p.  80. 


THE  ORIGIN  OF  UNDERGROUND  WATER         95 

contained  in  the  rock,  notably  ferrous  compounds.  All  of 
of  these  results  are  highly  important  and  suggestive,  and  various 
hypotheses  have  been  advanced  showing  how,  by  a  sort  of  distil- 
lation, carbon  dioxide  and  other  gases  might  be  given  off  and 
absorbed  by  ascending  waters.  There  is  indeed  a  possibility  that 
some  of  the  carbon  dioxide  in  deep  waters  may  have  been  derived 
in  this  way.  But  it  is,  perhaps,  scarcely  recognized  that  there  is 
a  great  difference  between  heating  a  small  quantity  of  pulverized 
rock  in  the  air  and  obtaining  the  same  amount  of  gases  from  a 
solid  mass  at  great  depth.  It  seems  probable  that  pressure  would 
prevent  the  escape  of  these  gases,  and  if  the  mass  of  rock  were 
heated  to  the  melting-point  it  would  undoubtedly  acquire  a 
capacity  for  absorption  of  far  greater  amounts  of  gases  than 
those  expelled  by  heating  the  powder  to  redness. 

The  presence  of  liquid  carbon  dioxide  in  cavities  in  minerals 
of  igneous  rocks  is  proof  of  its  occurrence  in  the  molten  magma 
consolidated  in  depth.  Every  eruption  brings  new  evidence  of 
exhalations  from  magmas  congealing  near  the  surface;  and 
almost  every  volcanic  district  of  recently  closed  igneous  activity 
testifies  to  the  persistence  of  this  gas  in  escaping  from  the  cooling 
lavas  below.  The  Cripple  Creek  district,  where  gold-tellurium 
veins  cut  through  the  core  of  an  old  volcano,  presents  an  excel- 
lent illustration  of  this  condition.  Imperceptible  at  the  surface, 
exhalations  of  carbon  dioxide  become  more  marked  in  depth 
and  their  temperature,  higher  than  that  of  the  surrounding 
rocks,  indicates  that  they  came  from  below.  In  the  extinct 
volcanoes  of  the  Auvergne  in  France  and  of  the  Eifel  on  the 
Rhine,  waters  highly  charged  with  carbon  dioxide  and  exhala- 
tions of  the  same  gas  are  extremely  abundant.  It  seems  difficult 
to  escape  the  conclusion  that  the  enormous  quantities  of  this 
gas  contained  in  the  ascending  waters  of  volcanic  regions  are  of 
igneous  origin — a  volatile  constituent  of  the  magma  released 
when  the  magma  was  brought  up  to  higher  levels  of  less  pressure 
in  the  earth's  crust. 

These  considerations  apply  equally  well  to  the  hydrogen  sul- 
phide with  which  some  of  these  springs  are  so  abundantly  sup- 
plied. The  decomposition  of  sulphates  by  organic  matter  or  other 
reducing  agents  may  be  appealed  to  in  places,  but  in  igneous 
•rocks,  like  granite,  it  does  not  appear  to  be  quantitatively  suffi- 
cient, and  as  we  know  that  this  gas  plays  a  prominent  part  in 
volcanic  eruptions  we  may  well  feel  justified  in  believing  that  the 


96  MINERAL  DEPOSITS 

waters  ascending  in  regions  of  such  eruptions  may  absorb  this 
gas  or  alkaline  sulphides  and  carry  them  to  the  surface. 

Rarer  Elements  Contained  in  Waters. — In  the  meteoric  waters 
in  crystalline  rocks  the  salinity  is  low  and  rarer  metals  are 
generally  absent.  There  are  usually  some  iron  and  manga- 
nese, and  possibly  refined  methods  might  discover  other  heavy 
metals  in  extremely  small  amounts.  Where  these  waters  appear 
in  mines  they  naturally  take  up  certain  amounts  of  the  metals 
of  the  deposits.  Careful  examination  of  calcium  carbonate 
waters  in  sedimentary  rocks  has  disclosed  traces  of  nickel,  cobalt, 
copper,  lead,  zinc,  strontium,  and  barium,  much  more  rarely 
fluorine,  boron,  and  iodine.  Arsenic  is  often  present  and 
where  the  waters  precipitate  limonite  from  dissolved  ferrous  car- 
bonate, the  ocher  almost  always  contains  traces  of  that  metal. 
Where  sulphates  of  iron  and  aluminum  are  among  the  saline  con- 
stituents of  waters  in  sedimentary  rocks,  determinable  amounts 
of  copper,  zinc,  cadmium,  nickel,  and  cobalt  may  be  found,  with 
traces  of  lead,  barium,  and  strontium.  The  quantity  of  cobalt 
usually  exceeds  that  of  nickel.  Such  waters  issue  at  many 
places  from  beds  of  pyritic  shale  and  owe  their  strong  solvent 
power  to  the  sulphuric  acid  generated  by  oxidation  of  pyrite. 

Chloride  waters  from  sedimentary  rocks  are  always  com- 
paratively rich  in  bromine  and  barium,  with  traces  at  least  of 
iodine  and  traces  of  boron,  fluorine,  and  sometimes  arsenic. 
From  Wildbad,  in  Wiirttemberg,  in  a  strong  sodium  chloride 
water,  traces  of  tin  besides  the  substances  mentioned,  have  been 
reported.1  From  the  calcium  chloride  springs  of  Cannstatt,  the 
same  authorities  mention  nitric  acid,  boron,  iodine,  fluorine, 
barium,  arsenic,  and  manganese;  in  the  ochery  deposits  also 
copper,  lead,  and  antimony  or  tin.  From  the  cold  sodium  chlo- 
ride water  of  Homburg,  nickel,  copper,  arsenic,  antimony,  boron, 
and  fluorine  are  reported;  in  the  hot  waters  of  Wiesbaden, 
copper,  tin,  arsenic,  and  boron.  In  almost  all  of  the  various 
waters  mentioned  above,  traces  of  phosphoric  acid  are  found. 

In  hot  ascending  sodium  chloride  springs  which  issue  in  vol- 
canic regions  rarer  elements  have  often  been  determined;  such 
waters  are  often  rich  in  boron.  Steamboat  Springs,  Nevada, 
contain  notable  amounts  of  arsenic  and  antimony  with  traces  of 
quicksilver.  The  springs  of  the  Yellowstone  Park  carry  boron 
and  arsenic,  but  are  poor  in  other  rarer  constituents. 

1Stelzner  and  Bergeat,  Die  Erzlagerstatten,  2,  1905-06,  p.  1220. 


THE  ORIGIN  OF  UNDERGROUND  WATER         97 

The  ascending  sodium  carbonate  springs  in  volcanic  districts 
also  frequently  contain  boron  and  fluorine  in  notable  amounts. 
Arsenic  and  copper  have  been  found  in  the  springs  of  Ems,  and 
the  same  metals  with  lead  also  at  Vichy.  The  Carlsbad  Sprudel 
contains,  according  to  Gottl,1  traces  of  bromine,  iodine,  fluorine, 
selenium,  phosphorus,  boron,  barium,  strontium,  lithium,  titanium, 
tin,  arsenic,  antimony,  copper,  chromium,  zinc,  cobalt,  nickel, 
and  gold. 

The  statements  summarized  above  show  clearly  that  traces  of 
metals  are  by  no  means  confined  to  springs  of  supposedly  deep- 
seated,  "magmatic"  or  "juvenile"  origin,  but  that  they  occur  in 
many  different  kinds  of  water.  The  presence  of  silver  has  appar- 
ently not  been  recorded,  and  that  of  gold  only  from  the  Carlsbad 
Springs.  Quicksilver  and  large  quantities  of  antimony  seem  to 
occur  only  in  sodium  chloride  or  sodium  carbonate  waters  of 
the  volcanic  type,  of  which  also  higher  amounts  of  boron  and 
fluorine  are  characteristic.  Arsenic  is  probably  the  most  common 
of  the  rarer  metals  and  has  been  found  in  all  kinds  of  water. 
Copper,  zinc,  nickel,  and  cobalt  are  not  uncommon,  both  in  waters 
of  sedimentary  and  in  those  of  igneous  origin.  Lead  is  of  rare 
occurrence.  In  minute  quantities  it  is  contained,  together  with 
zinc,  in  some  calcium  carbonate  waters  issuing  from  Paleozoic 
rocks  of  the  Mississippi  basin.  Iron  is  characteristic  of  meteoric 
waters  and  occurs  only  in  minute  quantities  in  the  hot  ascending 
sodium  springs. 

The  Igneous  Emanations.2 — At  several  places  above  the 
igneous  emanations  have  been  mentioned  and  the  inference 
drawn  that  the  waters  in  the  crust  of  whatever  origin  must  at 
places  have  absorbed  such  volatile  substances.  It  may  be  well 
to  describe  briefly  the  character  of  these  emanations. 

The  active  volcanoes  constantly  emit  volatile  matter  from 
lava  flows,  craters,  and  fumaroles.  Some  of  the  less  volatile 
materials  crystallize  as  sublimates  near  the  gas  vent;  other  parts 
escape  into  the  atmosphere. 

Chlorides3  are  given  off  in  abundance  by  the  erupted  lavas 

1  J.  Roth,  Allgemeine  und  chemische  Geologie,  Berlin,  vol.  1,  1879,  p.  570. 

2  For  summary  on  this  subject  see  F.  C.  Lincoln,  Magmatic  emanations, 
Econ.  Geol,  vol.  2,  1907,  pp.  258-274. 

3  According  to  Brun  congealed  lavas,  upon  heating,  give  off  considerable 
quantities  of  chlorides,  chlorine,  hydrochloric  acid,  sulphur  dioxide,  and 
nitrogen.     This  statement  still  awaits  confirmation. 


98  MINERAL  DEPOSITS 

and  crystallize  near  the  fumarolic  vents;  they  comprise  the  salts 
of  sodium,  potassium,  aluminum,  ammonium,  iron,  copper, 
lead,  and  manganese.  Sulphates  come  next  in  abundance;  more 
rarely  are  fluorides  or  oxyfluorides  found.  Boric  anhydride  and 
sulphur  are  common  products  of  sublimation  at  many  places. 
Selenium  and  tellurium  have  been  recognized  in  sulphur  of 
volcanic  origin.  Arsenic  in  the  form  of  realgar  is  reported,  and 
the  presence  of  cobalt,  tin,  bismuth,  and  molybdenum  has  been 
established,  most  of  the  detailed  work  having  been  done  at  the 
Italian  volcanoes.  Among  the  sulphides,  pyrite,  pyrrhotite, 
and  galena1  are  mentioned.  Specularite  is  a  common  product 
of  the  reactions  effected  by  the  volcanic  gases.  Even  silicates, 
such  as  leucite,  augite,  hornblende,  and  sodalite,  may  be  formed 
by  sublimation. 

Among  the  volcanic  gases  nitrogen,  carbon  dioxide,  and 
hydrogen  sulphide2  are  the  most  important  and  their  emission, 
particularly  that  of  carbon  dioxide,  may  continue  long  after  the 
igneous  activity  has  subsided  and  even  after  the  volcanic  cone 
has  been  eroded.  Evidence  of  this  is  furnished  by  the  exhala- 
tions of  carbon  dioxide  and  nitrogen  in  the  mines  at  Cripple 
Creek;  of  nitrogen  at  Creede  and  Tonopah;  of  carbon  dioxide 
in  the  Tertiary  gold  deposits  of  New  Zealand. 

Some  fumaroles  in  volcanic  regions  give  off  superheated  steam 
with  many  associated  salts  and  gases.  Very  .interesting  are  the 
"soffioni"3  of  Toscana,  Italy.  These  are  vents  emitting  super- 
heated steam  of  a  temperature  of  up  to  190°  C.  and  a  pressure  of 
2  to  4  atmospheres.  They  contain  much  boric  acid  undoubtedly 
of  volcanic  origin. 

If  all  these  substances  are  given  off  at  the  surface  the  intruding 
masses  in  depth  must  part  with  still  more  of  their  volatile  con- 
stituents. All  of  these,  whether  from  lavas  or  intrusives,  must 
to  greater  or  lesser  degree  mingle  with  the  ground  water.  Beyond 
all  doubt,  ascending  waters  in  regions  of  igneous  activity  must  in 
places  carry  a  considerable  load  of  magmatic  eamnations. 

1  F.  Zambonini,  Mineralogia  Vesuviana,  Naples,  1910. 

A.  Bergeat,  Die  Aeolischen  Inseln,  Abh.  math-phys.  Klasse,  K.  bayer 
Akad.,  20,  1,  1899,  p.  193. 

2  Oxygen  is  usually  and  marsh  gas  (CH4)  sometimes  present. 

3  P.  Toso,  Bolletino  del  R.  Comitate  Geol.,  vol.  42,  Rome,  1913,  p.  122. 


CHAPTER  VII 
THE  SPRING  DEPOSITS  AT  THE  SURFACE 

Processes  of  solution  and  precipitation  are  in  continual  progress 
in  the  underground  waters.  By  a  comparison  between  the  prod- 
ucts of  the  laboratory  and  those  of  nature  we  have  arrived  at 
the  conclusion  that  a  majority  of  mineral  deposits  have  resulted 
from  reactions  in  the  underground  water  channels.  Only  at  the 
surface,  however,  is  it  possible  to  study  the  actual  progress  of 
these  chemical  changes,  and  great  interest,  therefore,  attaches 
to  the  deposits  formed  by  the  natural  waters  where  they  issue  as 
springs  from  their  underground  path.  The  precipitation  taking 
place  in  rivers,  lakes  and  seas  will  be  described  in  later  chapters. 

On  the  whole  the  composition  of  the  material  deposited  by 
springs  is  simple.  Three  main  divisions  are  recognized:  De- 
posits of  limonite  (iron  hydroxide),  calcium  carbonate  and  silica. 
Mixtures  of  two  or  all  of  these  substances  are  frequently  ob- 
served. The  deposits  are  known  as  1.  Ochers,  2.  Tufas,  traver- 
tines or  calcareous  sinters,  3.  Sinters  or  siliceous  sinter. 

The  precipitation  is  in  part  due  to  cooling  or  escape  of  carbon 
dioxide  but  algae  and  micro-organisms  frequently  aid  by  secreting 
silica  jelly,  calcium  carbonate,  colloidal  ferric  hydroxides  or 
manganese  oxide.1 

Deposits  of  Limonite  and  Calcium  Carbonate. — Limonite  is 
frequently  deposited  by  superficial  meteoric  waters  which  con- 
tain ferrous  carbonate  and  ferrous  sulphate.  Many  such  ochers 
contain  a  little  manganese  and  traces  of  arsenic,  nickel  and 
cobalt.  Other  waters  also  deposit  some  limonite  so  that  many 
sinters  and  tufas  are  stained  by  this  compound.  Analyses  of 
such  ochers  are  quoted  by  F.  W.  Clarke.2 

Calcium  carbonate  is  probably  the  most  common  spring 
deposit,  though  the  ordinary  dilute  surface  waters  rarely  are 

1  W.  H.  Weed,  Ninth  Ann.  Rept.,  U.  S.  Geol.  Survey,  1889,  pp.  613-676. 
H.  Molisch,  Die  Eisenbakterien,  Jena,  1910. 

Jour.  Chem.  Soc.,  vol.  92,  pt.  2,  1907,  p.  888,  abstract. 

2  Geochemistry,  Bull.  616,  U.  S.  Geol.  Survey,  1916,  p.  205. 


100  MINERAL  DEPOSITS 

able  to  form  important  precipitates.  Hot  carbonated  waters 
issuing  from  limestone  often  deposit  large  masses  of  such  tufa, 
covering  many  acres  with  thick  terraced  beds.  The  Mammoth 
Hot  Springs  in  the  Yellowstone  Park  offers  a  beautiful  example 
of  such  tufa.  The  precipitates  are  almost  pure  calcium  carbonate 
with  a  little  magnesium  carbonate.  In  many  of  these  springs 
the  calcium  carbonate  is  the  least  soluble  constituent  which 
remains  after  the  others  have  been  carried  away.  Thus,  the 
sodium  chloride  springs  of  Glenwood,  Colorado,  yield  a  con- 
siderable deposit,  and  the  sodium  carbonate  springs  of  Ojo 
Caliente,  New  Mexico,  which  are  very  poor  in  calcium,  deposited 
at  their  former  point  of  issue  a  porous  tufa  containing  over  90 
per  cent,  of  calcium  carbonate.  This  carbonate  is  no  doubt 
deposited  in  crystalline  form,  though  it  is  usually  fine  grained. 
Such  deposits  are  not  always  calcite,  for  the  presence  of 
aragqnite  has  been  proved  in  many  spring  deposits,  for  in- 
stance those  of  Hammam  Meskoutine,  in  Algeria,  and  of 
Carlsbad,  in  Bohemia.1 

Deposits  of  Silica. — At  hot  springs  containing  much  silica,  this 
substance  is  abundantly  precipitated  because  of  evaporation, 
through  mixture  with  other  waters,  or,  according  to  W.  H.  Weed, 
by  the  action  of  certain  hot-water  algae.  The  material  is  de- 
posited as  a  colloid  jelly  which  subsequently  hardens  to  opaline  or 
chalcedonic  silica.  Such  sinters  are  formed  by  the  hot  springs  of 
the  Yellowstone  Park  and  may  contain  up  to  95  per  cent,  of 
silica.  Sodium  is  often  present  as  chloride  or  carbonate.  The 
Steamboat  Springs  of  Nevada2  deposit  a  sinter  of  pure  silica  or 
mixtures  of  calcium  carbonate  and  silica,  the  latter  being  present 
as  chalcedony,  or  small  crystals  of  quartz.  (See  Fig.  5.)  This 
sinter  contains  weighable  quantities  of  sulphides  of  mercury, 
lead,  copper,  arsenic,  and  antimony;  the  presence  of  gold  and 
silver  was  also  determined,  and  traces  of  manganese,  zinc,  cobalt, 
and  nickel  were  found.  Antimony  sulphide,  Sb2S3,  is  deposited 
as  the  amorphous  "  metastibnite "  in  quantities  large  enough  to 
color  the  sinter  red  in  places.  In  a  shaft  sunk  into  the  gravel 
immediately  adjoining  the  granite  hill  from  which  the  springs 
issue,  Lindgren3  discovered  delicate  crystals  of  stibnite  covering 

1  H.  Vater,  Zeitschr.  Kryst.  u.  Min.,  vol.  35,  1902,  p.  149. 

2  G.  F.  Becker,  The  quicksilver  deposits  of  the  Pacific  coast,  Mon.  13, 
U.  S.  Geol.  Survey,  1888,  p.  341. 

3  W.  Lindgren,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  36,  1906,  pp.  27-36. 


THE  SPRING  DEPOSITS  AT  THE  SURFACE     101 

the  pebbles  and  associated  with  thin  crusts  of  black  opal  and 
grains  of  pyrite  or  marcasite. 

The  sinter  of  the  Yellowstone  Park  often  contains  arsenic, 
especially  in  the  form  of  scorodite  (FeAsO4.2H2O),  and  near 
one  of  the  springs  which  was  impregnated  with  pyrite  Weed1 
noted  rhyolite  that  contained  traces  of  gold  and  silver.  On 
the  whole,  however,  the  Yellowstone  spring  deposits  are  poor 
t  n  the  rarer  metals.  The  same  author,  associated  with  Pirsson,2 


FIG.  5. — Section  of  chalcedonic  spring  deposits  from  Steamboat  Springs, 
Nevada.  White  areas  microcrystalline  quartz.  Magnified  29  diameters. 
Crossed  nicols. 

reports  the  occurrence  of  orpiment  and  realgar  with  native 
sulphur  in  a  siliceous  sinter  from  the  Norris  geyser  basin.  De 
Launay  mentions  a  deposit  containing  orpiment  at  St.  Nectaire, 
Puy-de-D6me,  France. 

A  calcareous  sinter  deposited  by  an  ascending  sodium  car- 
bonate spring  in  the  Geyser  mine,  Silver  Cliff,  Colorado,  on  the 

1  Mineral  vein  formation  at  Boulder  Hot  Springs,   Montana,   Twenty- 
first  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  2,  1899-1900,  pp.  233-255. 

2  Occurrence   of   sulphur,    orpiment,    and   realgar   in   the   Yellowstone 
National  Park,  Am.  Jour.  Sci.,  3d  ser.,  vol.  42,  1891,  pp.  401-405. 


102 


MINERAL  DEPOSITS 


2000-foot  level,  yielded  traces  of  lead,  copper,  zinc,  nickel,  and 
cobalt.  At  Hammam  Meskoutine,  in  Algeria,  a  similar  spring, 
according  to  Daubree,  deposits  tufas  and  pisolitic  sinters  in 
which,  in  the  concretions,  shells  of  calcium  carbonate  alternate 
with  shells  of  pyrite;  strontianite  is  deposited  by  the  same  spring. 
Quicksilver,  gold,  and  silver  have  been  recognized  in  the  spring 
deposits  of  the  geyser  districts  in  New  Zealand.  From  the 
Whakarewarewa  hot  springs  at  Roturoa  (sodium  chloride-silica 


FIG.  6. — Section  of  chalcedonic  spring  deposits,  from  De  Lamar,  Idaho, 
showing  vegetable  remains.     Magnified  35  diameters.     Ordinary  light. 


type)  suiters  have  lately1  been  analyzed  which  yielded  nearly  5 
ounces  of  silver  and  about  $1  in  gold  per  ton. 

At  De  Lamar,  Idaho,  Lindgren  found  in  rhyolite  spring  de- 
posits of  flinty  chalcedony,  which  included  casts  of  vegetable 
remains  and  yielded  traces  of  gold  and  silver.2  (Fig.  6.) 

S.  Meunier3  reported  0.5  per  cent,  of  cassiterite  in  siliceous 

1  J.  M.  Bell,  First  Ann.  Rept.  N.  Z.  Geol.  Surv.,  1907,  p.  100. 
3  The  gold  and  silver  veins  of  Silver  City,  etc.,  Twentieth  Ann.    Rept... 
U.  S.  Geol.  Survey,  pt.  3,  1898,  p.  187. 
3  Compt.  Rend.,  vol.  110,  1890,  p.  1083. 


THE  SPRING  DEPOSITS  AT  THE  SURFACE      103 

sinter  deposited  by  a  hot  spring  at  Selangor,  in  the  Federated 
Malay  States,  but  this  statement  has  lately  been  challenged.1 

It  has  been  shown  that  springs,  hot  or  cold,  may  deposit  lim- 
onite  in  abundance,  with  arsenic,  manganese,  and  traces  of 
other  metals;  and  it  is  likewise  proved  that  the  carbonate  and 
silica  sinters  of  hot  springs,  particularly  those  of  the  NaCl  or 
Na2C03  type,  contain  small  quantities  of  the  rarer  metals, 
including  gold,  silver,  copper,  lead,  zinc,  antimony,  arsenic, 
tin,  and  quicksilver.  In  very  few  instances  has  commercial 
ore  been  obtained  from  spring  deposits  at  the  surface.  Quick- 
silver ores  have  been  mined  in  New  Zealand  and  ores  of  iron  and 
manganese  have  been  utilized  in  rare  instances.  The  evidence 
that  such  waters  have  formed  workable  ore  deposits  is  therefore 
strong  but  hardly  conclusive;  the  remarkable  poverty  in  metals 
of  the  deposits  of  the  springs  in  the  Yellowstone  National  Park, 
for  instance,  will  to  many  seem  an  argument  against  the  hydro- 
thermal  theory  of  the  genesis  of  ore  deposits. 

Deposits  of  Other  Gangue  Minerals.2 — Calcite,  quartz,  chal- 
cedony, and  opal  are  common  products  of  deposition  at  the  sur- 
face, but  besides  these  the  mineral  deposits  often  contain  such 
minerals  as  barite,  ankerite  and  siderite,  fluorite,  and  more 
rarely  gypsum,  strontianite,  celestite,  and  zeolites.  It  will  be 
necessary  to  examine  the  competency  of  the  various  waters  to 
form  these  gangue  minerals. 

Fluorine  is  present  in  traces  in  many  waters,  both  vadose  and 
deep,  but  appears  in  larger  quantities  in  waters  of  the  sodium 
carbonate  type.  Few  authenticated  instances  of  actual  deposi- 
tion of  fluorite  by  springs  are  recorded;  the  substance  rarely 
occurs  in  crystallized  form  and  the  chemists  have  probably 
often  neglected  to  test  the  sinters  for  fluorine.  The  Carlsbad 
Springs  deposit  a  pisolitic  sinter  of  aragonite  and  calcite.  Accord- 
ing to  Berzelius3  and  later  chemists  this  contains  a  notable  quan- 
tity of  calcium  fluoride.  A  limonitic  variety  of  the  spring  deposit 
yielded  0.272  per  cent,  arsenic.4  The  analyses  on  page  104  also 
demonstrate  that  various  phosphates  may  be  precipitated  as  well 
as  the  carbonates  of  iron  and  strontium. 

1  J.  B.  Scrivenor,  Origin  of  tin-deposits,  Perak,  Chamber  of  Mines,  p.  5. 

2  The  best  resume'  of  the  older  data  regarding  spring  deposits  are  found 
in  Roth's  Allgemeine  und  Chemische  Geologie,  vol.  1,  1879,  pp.  564-596. 

3  Pogg.  Ann.,  74,  1823,  p.  149. 

4  Blum  and  Leddin,  Am.  Chem.  Pharm.,  vol.  73,  1850,  p.  217. 


104  MINERAL  DEPOSITS 

COMPOSITION  OF  DEPOSITS  OF  CARLSBAD  SPRINGS 
Berzelius,  Analyst 


A 

B 

12.13 

Ferric  oxide  
Manganese  oxide  

19.35 
53  20 

0.43 
trace 
96  47 

0  30 

Basic  ferric  phosphate  
Aluminum  phosphate  .                

1.77 
0.60 

0.10 

0  06 

Calcium  fluoride  

0.99 

Silica 

3  95 

Water                                .                   ... 

9.00 

1  59 

,  At  Plombieres,  in  the  French  Vosges,  springs  with  a  tempera- 
ture of  70°  C.  issue  from  granite.  They  have  a  low  salinity  (360 
parts  per  million)  and  contain  mainly  sodium  sulphate  and  silica, 
believed  to  be  present  in  part  as  sodium  silicate,  also  traces  of 
arsenic  and  fluorine.  The  derivation  of  these  salts  is  doubtful 
and  the  springs  are  apparently  not  directly  related  to  volcanic 
rocks.  They  issue  from  well-defined  fissure  veins  containing 
quartz  and  fluorite,  and  DaubreV  found  that  the  waters  had 
actually  deposited  calcite,  aragonite,  and  fluorite.  The  bricks 
and  cements  used  by  the  Romans  2,000  years  ago  in  the  construc- 
tion of  the  baths  at  Plombieres  were  found  to  contain  zeolites, 
chiefly  apophyllite  (containing  fluorine)  and  chabazite,  with 
opal  and  chalcedony.  Chabazite  is  also  reported  by  Daubre"e  as 
deposited  from  springs  at  Luxeuil  and  at  Bourbonne-les-Bains, 
which  have  a  temperature  of  46°  and  68°  C.  respectively. 

In  some  of  the  mines  of  Cripple  Creek,  Colorado,  gypsum  is 
found  suggesting  deposition  by  ascending  springs.  Crystals  of 
gypsum  occur  commonly  near  springs  charged  with  calcium 
sulphate.  Weed2  has  described  how  the  Hunter  Hot  Springs, 
near  Livingston,  Montana,  deposit  this  mineral  in  fractures  in 
Tertiary  sandstone;  stilbite,  a  zeolite,  is  forming  with  the  gypsum. 
The  springs  have  a  temperature  of  64°  C.  and  are  weak  mineral 

1Les  eaux  souterraines,  3,  p.  31. 

2  Economic  value  of  hot  springs  and  hot  springs  deposits.  Bull.  260, 
U.  S.  Geol.  Survey,  1904,  pp.  298-604. 


THE  SPRING  DEPOSITS  AT  THE  SURFACE      105 

waters.  According  to  a  somewhat  doubtful  analysis  they  are 
rich  in  silica  and  alumina,  but  poor  in  calcium  sulphate,  so  that 
Weed  believes  that  at  present  they  deposit  more  stilbite  than 
gypsum.  The  presence  of  stilbite  has  also  been  noted  by  Weed1 
in  the  vein-like  deposits,  containing  gold  and  silver,  believed  to  be 
made  by  the  present  Hot  Springs  at  Boulder,  Montana;  the  stil- 
bite occurs  in  the  predominating  quartz,  chalcedony  and  calcite. 
Lindgren  noted  the  presence  of  a  little  adularia  in  the  material. 

According  to  Lindgren2  a  spring  deposit  in  New  Mexico 
contains  about  89.60  per  cent,  of  calcium  carbonate  and  0.9  per 
cent,  of  calcium  fluoride.  There  are  no  springs  now  at  this  place, 
but  it  is  probable  that  the  sodium  carbonate  of  Ojo  Caliente,  a 
short  distance  lower  down  in  the  valley,  formerly  issued  here. 
As  shown  by  an  analysis  on  page  61,  the  water  contains  a  notable 
amount  of  fluorine.  A  vein  of  white  crystalline  fluorite  is 
opened  by  a  shaft  close  by  the  calcareous  tufa  and  is  believed 
to  have  been  formed  by  the  same  waters.  Both  tufa  and  vein 
contain  traces  of  gold  and  silver,  and  a  few  crystals  of  barite 
were  observed  in  the  vein  material.  W.  H.  Emmons  and  E.  S. 
Larsen3  have  described  a  similar  case  from  Wagon  Wheel  Gap, 
Colorado. 

Veins  and  replacements  of  fluorite  in  quartz  porphyry  and  Cre- 
taceous sandstone  near  the  sodium  carbonate  springs  of  Teplitz, 
Bohemia,  have  been  described  by  J.  E.  Hibsch4  and  the  evidence 
is  convincing  that  fluorite  was  deposited  by  these  thermal  waters. 

Barite  is  deposited  far  more  abundantly  than  fluorite.  As 
shown  above,  many  carbonate  and  even  sulphate  waters  contain 
a  little  barium.  It  has  been  proved  that  alkaline  bicarbonates 
with  an  excess  of  carbon  dioxide  can  hold  barium  in  solution, 
notwithstanding  the  presence  of  sulphates;  sodium  chloride  and 
other  salts  also  seem  to  retard  the  formation  of  barium  sulphate. 
Haidinger  observed  that  barite  was  deposited  by  the  hot  waters 
at  Carlsbad,8  and  Becke  noted  the  same  at  Teplitz.6  At  Idaho 

1  Mineral  vein  formation  at  Boulder  Hot  Springs,  Montana.     Twenty' 
first  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  2,  1899-1900,  pp.  233,  255. 

2  Econ.  Geol,  vol.  5,  1910,  pp.  22-27. 

3  Econ.  Geol,  vol.  8,  1913,  pp.  235-246. 

4  Tsch.  M.  u.  P.  Mitt,  25,  1906,  pp.  482-488. 

5  Jahrb.  K.  k.  Reichsanstalt,  5,  1854,  p.  142. 

See  also  Delkeskamp,  R.,  Entstehung  und  Wegfiihrung  des  Baryts. 
Notizblatt  d.  Ver.f.  Erdkunde  (Darmstadt)  (4),  21,  1900,  pp.  55-83. 
•  Tsch.  M.  u.  P.  Mitt.,  5,  1883,  p.  115. 


106  MINERAL  DEPOSITS 

Springs,  in  Colorado,  a  hot  sodium  carbonate  water  issues  from 
granitic  rocks,  and  barite  crystals  are  found  in  abundance  in  a 
cellular  and  decomposed  dike  rock  at  the  mouth  of  the  spring. 
Spurr1  shows  that  the  barium  is  contained  in  this  dike  rock  and 
believes  that  the  barite  resulted  from  the  reaction  of  the  water  on 
the  rock. 

Barium  is,  however,  far  more  commonly  contained  in  sodium 
chloride  waters,  particularly  in  the  brines  of  sedimentary  strata. 
Many  writers  record  the  deposition  of  barite  from  such  waters, 
and  it  is  probable  that  wherever  this  mineral  appears  in  large 
quantities  in  mineral  deposits  waters  of  this  type  have  been 
active. 

An  excellent  example  is  reported  from  a  mine  near  Clausthal, 
Germany,2  where  a  spring  of  strong  brine,  which  undoubtedly 
derived  its  salts  from  sedimentary  strata,  was  encountered  at  a 
depth  of  1,200  feet;  this  brine  contained  in  grams  per  liter 
67.555  sodium  chloride,  10.509  calcium  chloride,  4.360  magnesium 
chloride,  0.350  potassium  chloride,  0.314  barium  chloride,  and 
0.854  strontium  chloride.  When  it  was  mingled  with  the  ordi- 
nary mine  waters,  which  carried  sulphates,  abundant  precipita- 
tion of  barium  and  strontium  sulphate  took  place  in  the  pumps 
and  elsewhere,  so  that  within  a  few  years  the  capacity  of  the  pipes 
became  much  reduced  by  this  deposit.  This  compact  material 
contained  92.44  per  cent,  barium  sulphate  and  4.32  per  cent, 
strontium  sulphate.  The  reaction  is  believed  to  be  retarded  by 
the  presence  of  sodium  and  magnesium  chlorides. 

According  to  P.  Krusch,3  barite  is  precipitated  in  the  pumps  at 
some  Westphalian  coal  mines  by  a  similar  reaction  between  strong 
salt  brine  from  the  Triassic  sandstones  and  potable  water  with 
sulphates,  each  ascending  on  separate  faults  and  each  deriving 
its  contents  of  salts  from  sedimentary  strata.  Veins  of  barite 
with  small  amounts  of  galena,  pyrite,  etc.,  appear  in  the  Carbon- 
iferous and  in  the  Devonian.  At  a  lower  horizon  quartz  veins 
contain  galena  and  zinc  blende,  both  kinds  of  deposits  having 
been  made,  according  to  Krusch,  by  these  saline  waters.  Simi- 

^J.  E.  Spurr,  Prof.  Paper  63,  U.  S.  Geol.  Survey,  1908,  p.  165. 

2  Lattermann,    Die  Lautenthaler  Soolquelle  und  ihre   Absatze.     Jahrb. 
Preuss.  geol.  Landesanstalt,  1888,  pp.  259-283.     Ref.  in  Stelzner  and  Ber- 
geat,  Die  Erzlagerstatten,  II,  1905-06,  p.  1218. 

3  Monatsberichte  Deutsch.  geol.   Qesell.,   1904,  p.   36;  Ref.  in   Zeitschr. 
prakt.  Geol,  12,  1904,  p.  252. 


THE  SPRING  DEPOSITS  AT  THE  SURFACE      107 

lar  deposits  of  barite  in  the  pipes  of  the  pumping  apparatus 
have  been  described  from  English  coal  mines.1 

An  account  by  Headden2  describes  an  interesting  group  of 
springs  on  the  North  Fork  of  the  Gunnison  River,  Delta  County, 
Colorado.  They  are  cold,  but  contain  free  carbon  dioxide  and  a 
little  hydrogen  sulphide  and  are  essentially  sodium  chloride 
waters.  At  least  one  spring  contains  barium  and  all  of  them 
yield  a  little  strontium.  The  Drinking  Spring  has  a  total 
salinity  of  about  1,656  parts  per  million.  Small  quantities  of 
calcium,  potassium,  magnesium,  barium  (0.0132  gram  per  liter), 
strontium  (0.0066  gram  per  liter),  lithium,  manganese,  ammonia, 
iron,  aluminum,  also  a  trace  of  zinc,  are  present  in  the  order 
stated;  also  sulphuric  acid  radicle  (0.6254  gram  per  liter),  silica, 
boron,  and  bromine,  the  latter  three  in  very  small  amounts. 
The  spring  deposits  a  calcium  carbonate  sinter,  which  was  found 
to  contain  5.42  per  cent,  barite,  but  no  gypsum  or  sulphur. 

Ferrous  carbonate,  or  siderite,  is  sometimes  observed,  as  in 
the  Carlsbad  "  Sprudelstein  "  and  in  deposits  of  limonite  formed 
in  bogs  and  peat.  Deposits  of  magnesium  minerals  are  rare. 
H.  Leitmeier3  describes  a  deposit  of  hydrous  carbonate  of  mag- 
nesia from  the  springs  of  Lohitsch  in  Styria;  many  springs, 
especially  those  whose  waters  have  traversed  sedimentary  beds, 
contain  organic  matter  and  are  probably  competent  to  deposit 
hydrocarbons. 

The  more  common  gangue  minerals  in  certain  classes  of  veins 
are  thus  deposited  by  spring  waters,  particularly  by  the  warm 
sodium  chloride  and  sodium  carbonate  springs.  There  are,  of 
course,  a  great  number  of  gangue  minerals  like  tourmaline, 
garnet,  feldspars,  and  similar  silicates  which  cannot  be  expected 
to  develop  in  water  under  the  conditions  of  temperature  and 
pressure  prevailing  at  the  surface. 

Summary. — The  deposits  of  ascending  springs  of  undoubted 
meteoric  origin  contain  opal,  chalcedony,  calcium  carbonate, 
limonite,  hydroxide  of  manganese,  barite,  siderite  and  pyrite. 
They  often  deposit  sulphur  by  the  oxidation  of  hydrogen  sulphide. 
The  ochery  deposits  very  frequently  yield  small  quantities  of 
arsenic,  copper,  lead,  zinc,  nickel  and  cobalt. 

The  springs  of  the  sodium  carbonate  and  sodium  chloride- 

1  J.  T.  Dunn,  Chem.  News,  vol.  35,  1877,  p.  140. 

2  The  Doughty  Springs,  etc.,  Proc.,  Colo.  Sci.  Soc.,  8,  1905,  pp.  1-30. 

3  Zeitschr.  KrystaU.  u.  Min.,  vol.  47,  1909,  p.  104. 


108  MINERAL  DEPOSITS 

silica  type  in  volcanic  regions  yield  abundant  deposits  of  opal, 
chalcedony,  quartz,  calcium  carbonate,  limonite,  barite,  siderite 
sometimes  also  pyrite.  They  also  deposit  fluorite  which  is 
rarely  if  ever  found  in  the  sinters  of  meteoric  waters  and  yield 
smaller  quantities  of  quicksilver,  antimony,  arsenic,  lead,  copper, 
zinc,  tin,  silver  and  gold.  The  rarer  metals  are  thus  more  promi- 
nent and  the  waters  are  particularly  characterized  by  a  relative 
abundance  of  borates,  arsenates  and  fluorides. 

The  list  of  recognizable  minerals  deposited  by  springs  at  the 
surface  is  as  follows :  Sulphur,  quartz,  opal,  chalcedony,  limonite, 
wad,  calcite,  aragonite,  siderite,  strontianite,  barite,  gypsum, 
celestite,  fluorite,  scorodite,  pyrite,  realgar,  orpiment,  cinnabar, 
stibnite,  chabazite,  apophyllite  and  stilbite. 


CHAPTER  VIII 

RELATIONS  OF  MINERAL  DEPOSITS  TO  MINERAL 
SPRINGS 

The  deposition  of  many  valuable  minerals  can  be  directly  ob- 
served in  nature:  limonite,  for  instance,  from  the  evaporation  of 
water  containing  iron,  or  from  precipitation  in  bogs  and  lakes; 
sulphur  by  the  decomposition  of  hydrogen  sulphide  dissolved 
in  water;  residual  deposits  of  limonite,  nickel  silicates,  and 
pyrolusite  by  the  decomposition  of  rocks  by  meteoric  waters;  com- 
mon salt  and  borax  by  the  evaporation  of  lake  waters.  A  large 
class  of  deposits,  such  as  the  deep-seated  veins  containing  metals 
and  ores  developing  near  intrusive  contacts,  we  can  never  hope 
to  observe  in  nature  in  the  process  of  formation. 

Ascending  mineral  springs  are  not  uncommonly  observed  in 
mineral  deposits,  particularly  in  those  which  follow  fissures,  but 
caution  must  be  used  in  attributing  a  genetic  role  to  these 
springs.  If  we  find  such  a  spring  in  a  contact-metamorphic 
deposit  or  in  a  vein  of  deep-seated  origin,  as  a  cassiterite  vein, 
it  would  be  unlikely  indeed  that  this  spring  had  anything  to  do 
with  the  formation  of  the  deposit,  for  it  could  scarcely  be  as- 
sumed that  the  circulation  of  underground  waters  could  be 
maintained  in  the  same  path  during  the  many  vicissitudes  of 
deep  erosion,  involving  the  laying  bare  of  rocks  once  many  thou- 
sand feet  below  the  surface.  The  formation  of  ore  deposits 
usually  occupies  comparatively  short  epochs,  and  the  agencies 
to  which  they  owe  their  origin  are  evanescent  phenomena. 

In  a  rather  large  class  of  veins,  however,  of  which  we  know 
that  they  were  formed  near  the  surface  and  in  recent  geological 
times,  we  may  look  with  more  confidence  for  a  maintenance  of 
the  originating  solutions,  but  even  here  it  is  well  to  investigate 
carefully;  the  spring  may  simply  be  a  water  of  the  upper  cir- 
culation which  selected  the  fissure  as  a  convenient  path. 

The  case  of  Plombieres  has  already  been  mentioned  (p.  104) 
and  there  seems  to  be  little  reason  to  doubt  that  the  quartz- 
fluorite  veins  at  this  place  have  been  deposited  by  the  same  hot 

109 


110  MINERAL  DEPOSITS 

waters  which,  still  issue  from  the  fissures.  The  Triassic  sand- 
stone, covering  the  granite  in  that  vicinity,  is  in  part  replaced 
by  silica  and  also  contains  fluorite  and  barite.  DaubreV  cites 
the  frequent  occurrence,  in  the  Triassic  beds  of  the  Central 
Plateau  and  the  Vosges,  of  veins  and  extensive  silicification 
similar  to  that  at  Plombieres.  Barite,  fluorite,  and  sometimes 
galena  accompany  the  quartz. 

According  to  Jacquot  and  Willm,2  the  sodium  chloride  springs 
of  Bourbon-l'Archambault,  at  the  north  end  of  the  Central 
Plateau  region,  issue  from  a  fracture  in  Triassic  strata,  which 
contains  quartz  with  galena,  barite,  and  fluorite.  Dikes  of  mica- 
ceous porphyry  (minette?)  follow  the  fissures.  The  waters  have 
a  temperature  of  53°  C.  and  the  total  solids  aggregate  3,186  parts 
per  million,  of  which  1,770  are  sodium  chloride.  Bromine, 
iodine,  fluorine,  arsenic,  and  copper  are  present,  and  the  saline 
constituents  are  attributed  to  the  Triassic  and  Permian  strata. 
The  spring  deposits  contain  earthy  carbonates  and  0.07  per  cent, 
copper  oxide.  The  springs  of  Contrexeville,  which  issue  from 
the  Triassic  and  carry  traces  of  fluorine  and  arsenic,  have  a 
temperature  of  11°  C.,  and  the  salts,  among  which  calcium  sul- 
phate and  calcium  carbonate  prevail,  also  bear  marks  of  deri- 
vation from  the  sediments. 

The  springs  of  Lamalou,  near  Montpelier,  southern  France, 
have  a  temperature  of  34°-47°  C.  and  1,500  parts  per  million 
of  total  solids;  the  alkaline  carbonates  prevail,  but  they  also 
contain  calcium  and  magnesium  carbonates,  suggesting  an 
admixture  of  the  meteoric  waters.  Traces  of  barium,  arsenic, 
copper,  lead,  nickel,  and  cobalt  were  determined.3  These 
springs  are  believed  to  be  genetically  connected  with  the  erup- 
tion of  a  neighboring  basalt  area  and  stand  in  close  relation- 
ship to  veins  containing  pyrite,  arsenopyrite,  and  chalcopyrite 
in  a  gangue  of  quartz  and  barite,  the  exploitation  of  which  had  to 
be  stopped  on  account  of  the  fear  of  tapping  large  volumes  of 
water.  Barite  is  believed  to  be  deposited  by  the  present  waters. 

The  sodium  carbonate  springs  at  Ems,4  according  to  Delkes- 
kamp,  issue  from  a  fissure  which  forms  the  extension  of  a  quartz 
vein  and  contains  chalcopyrite.  Basalt  occurs  in  the  same  vicinity. 

1  Les  eaux  souterraines  aux  epoques  anciennes,  p.  151. 
2Les  eaux  mine"rales  de  la  France,  Paris,  1894,  p.  107. 

3  L.  De  Launay,  Recherche,  etc.,  des  sources  thermomine'rales,  1892. 

4  Verhandl.  Gesell.  deutscher  Nat.  u.  Aerzte,  1903,42,^first  part. 


RELATIONS  TO  MINERAL  SPRINGS  111 

Sandberger  and  Delkeskamp  state  that  the  hot  sodium  chlo- 
ride springs  of  Wiesbaden  are  closely  connected  with  a  quartz 
vein  containing  tetrahedrite;  veins  of  barite  and  calcite  are 
common,  as  are  impregnations  of  barite ;  the  latter  are  attributed 
to  earlier  (Tertiary)  spring  waters. 

Close  connection  with  ore-bearing  veins  is  also,  according  to 
Delkeskamp,  indicated  by  the  sodium  chloride  springs  of  Kreutz- 
nach,  which  issue  close  to  a  number  of  veins  containing  cal- 
cite, barite,  and  fluorite  with  ores  of  copper  and  quicksilver. 
Here,  also,  Tertiary  strata  higher  than  the  springs  are  impregnated 
with  barite,  suggesting  a  considerable  age  and  a  formerly  higher 
point  of  issue  of  the  springs.  The  saline  constituents  of  the 
water  are  believed  to  be  derived  from  sedimentary  rocks. 

The  evidence  presented  by  Delkeskamp  does  not  suffice  to 
establish  direct  connection  of  the  springs  with  the  ore  deposition, 
but  the  widespread  occurrence  of  barite,  in  close  association 
with  strong  sodium  chloride  springs,  is  assuredly  suggestive. 

Mineral  springs  with  a  maximum  temperature  of  26°  C.  have 
been  opened  at  several  places  in  the  mines  of  Freiberg,  Saxony, 
and  are  described  in  some  detail  by  Stelzner  and  Bergeat,1  but 
there  is  little  reason  to  believe  that  they  are  genetically  connected 
with  the  deposits.  The  same  authors  describe  weak  sodium 
carbonate  springs,  which  were  encountered  in  the  veins  of 
Joachimsthal,  in  Bohemia;  their  highest  temperature  was  28°  C. 
This  is  only  about  10  miles  from  Carlsbad  and  at  a  lower  level. 
Posepny  has  suggested  that  the  waters  may  be  derived  from  the 
same  general  source  which  supplies  the  springs  at  Carlsbad. 
Finally  should  be  mentioned  the  hot  waters  which  broke  into  the 
copper  mine  of  Bocheggiano2  at  Massa  Maritima,  Tuscany,  at  a 
depth  of  over  1,000  feet,  and  which  had  a  temperature  of  40° 
C.  They  contained  from  769  to  2,053  parts  per  million  of  total 
solids,  mainly  sulphates  of  calcium  and  magnesium,  with  a  not- 
able amount  of  boric  acid.  The  waters,  except  for  the  boron, 
are  of  the  type  of  ordinary  mine  waters  and  may  be  of  meteoric 
origin  with  an  admixture  of  boron  from  volcanic  exhalations. 
Warm  springs  have  been  encountered  in  the  mines  of  Corn- 
wall ;  and  one  of  them  in  a  tin  vein  near  Redruth  is  said  to  have 
contained  much  lithium,  which  is  not  surprising  considering 
the  general  distribution  of  lithium-bearing  muscovite  in  that 

1  Die  Erzlagerstatten,  2,  1905-06,  p.  1227. 

2  B.  Lotti  and  K.  Ermisch,  Zeitschr.  prakt.  Geol,  1905,  pp.  206-247 


112  MINERAL  DEPOSITS 

region.  It  seems  difficult  to  believe  that  these  springs  are  the 
remains  of  the  waters  which  deposited  the  veins,  for  the  veins 
were  formed  at  a  great  depth  and  under  high  pressure  and  tem- 
perature at  a  remote  geological  time. 

In  the  Cordilleran  region  of  the  United  States  examples  of 
mineral  springs  in  mineral  veins  are  not  so  common  as  might  be 
expected  from  the  coexistence  of  a  late  mineralization  and  pres- 
ent abundance  of  thermal  waters.  One  reason  for  this  lack  lies 
probably  in  the  great  physiographic  changes  which  in  most  parts 
of  this  region  have  taken  place  in  relatively  late  times  and  which 
would  tend  to  lower  or  divert  the  discharges  of  the  springs.  At 
Silver  Cliff  S.  F.  Emmons1  found  issuing  from  the  2,000-foot  level 
of  the  Geyser  mine  a  strong  spring  of  sodium  carbonate  water 
with  free  carbon  dioxide,  yielding  small  quantities  of  copper,  lead, 
and  zinc;  the  temperature  was  26.5°  C.  The  shaft  was  sunk 
to  a  depth  of  1,850  feet  in  rhyolite  tuff;  at  this  depth,  at  the 
contact  between  the  tuff  and  pre-Cambrian  gneiss,  a  vein  was 
found  containing  galena,  zinc  blende,  tetrahedrite,  argentite,  etc., 
in  a  gangue  of  calcite,  barite,  and  quartz.  The  water  deposited  a 
calcium  carbonate  sinter  with  traces  of  lead,  zinc,  copper,  nickel, 
and  cobalt.  In  this  instance  it  is  possible  that  the  ascending 
water  may  have  had  a  genetic  connection  with  the  deposit. 

At  the  Comstock  lode2  hot  waters  were  encountered  at  an 
early  date  and  have  made  exploitation  difficult.  It  can  scarcely 
be  doubted  that  these  waters  stand  in  causal  relation  to  the 
vein  and  it  is  certain  that  they  now  dissolve  and  precipitate 
gold  and  silver,  as  well  as  pyrite.  The  heat  of  the  lode  has 
been  attributed  to  oxidation  of  pyrite  or  to  kaolinization  of 
the  feldspars  of  the  country  rock,  but  Becker  has  shown  that  it  is 
clearly  due  to  the  ascending  waters.  Reid3  has  examined  the 
evaporated  residue  from  water  collected  on  the  2,250-foot  level 
of  the  C.  and  C.  shaft.  He  found  2.92  milligrams  of  silver  and 
0.298  milligrams  of  gold  per  ton  of  solution.  This  water,  which 
has  a  temperature  of  46°  to  81°  C.,  contains  965  parts  per  million 
of  solids,  mostly  sulphates  of  calcium  and  sodium  but  including 

1  The  mines  of  Custer  County,  Colorado,  Seventeenth  Ann.  Rept.,  U.  S. 
Geol.  Survey,  pt.  2,  1896,  p.  461. 

2  G.  F.  Becker,   Geology  of  the  Comstock  lode,   Mon.  3,   U.  S.  Geol. 
Survey,  1882,  p.  230. 

3  John  A.  Reid,  The  structure  and  genesis  of  the  Comstock  lode,  Bull.  4, 
California  Univ.  Dept.  Geology,  1905,  pp.  177-191. 


RELATIONS  TO  MINERAL  SPRINGS  113 

133  parts  of  silica.  This  water  is  assuredly  not  one  of  the  pure 
types  of  ascending  waters;  its  composition  is  in  the  main  the 
same  as  that  of  the  ordinary  mine  waters  and  it  may  be  a  mix- 
ture of  meteoric  mine  waters  with  a  very  hot  ascending  water. 
Particularly  convincing  of  the  competency  of  the  ascending  "vol- 
canic" springs  to  deposits  gold  and  silver-bearing  veins  are  the 
data  given  on  p.  105  in  relation  to  the  Ojo  Caliente  springs  of 
New  Mexico  and  those  of  Wagon  Wheel  Gap  in  Colorado.  To 
this  is  added  the  evidence  of  the  gold  and  silver-bearing  sinters 
of  New  Zealand  (p.  102)  and  Steamboat  Springs,  Nevada  (p.  100). 
The  widely  cited  occurrence  at  Sulphur  Bank,  in  Lake  County, 
California,1  is  considered  to  furnish  good  proof  of  deposition 
of  cinnabar  by  hot  sodium  chloride  waters,  also  heavily  charged 
with  boron  (analysis  on  page  61).  The  springs  issue  through 
Quaternary  basalt  in  which  cinnabar  was  deposited  with  opal  as 
crusts  along  crevices,  sometimes  as  delicate  crystals  loosely 
attached  to  the  walls,  or  as  impregnations  of  the  porous  basalt  ; 
the  pyrite  or  marcasite  was  mostly  disseminated  in  the  rock,  but 
occurred  also  as  crusts  alternating  with  cinnabar  and  opal. 
Melville  found  traces  of  gold  and  copper  in  the  marcasite.  At 
the  surface  no  cinnabar  was  observed,  but  sulphur,  derived  from 
the  oxidation  of  H2S,  was  present.  A  few  feet  below  the  surface 
the  cinnabar  appeared  and  continued  down  to  about  300  feet, 
into  the  sandstones  on  which  the  basalt  rested.  No  quicksilver 
was  found  in  the  water,  but  no  one  who  has  studied  the  occur- 
rence has  doubted  that  cinnabar,  pyrite,  and  opal  have  been  pre- 
cipitated from  the  water  which  still  ascends  in  these  channels. 
The  gases  dissolved  in  the  water  consist  mostly  of  carbon  dioxide, 
with  hydrogen  sulphide,  hydrocarbon,  nitrogen,  and  some  am- 
monia. The  evidence  gains  in  importance  when  it  is  realized  that 
the  mineral  combination  and  general  mode  of  occurrence  cited 
are  characteristic  of  the  quicksilver  deposits  of  the  Coast  Ranges. 
A  number  of  other  instances  of  deposition  of  cinnabar  by  as- 
cending waters  are  given  in  Chapter  XXIV. 

1  G.  F.  Becker,  Geology  of  the  quicksilver  deposits  of  the  Pacific  slope, 
Man.  13,  U.  S.  Geol.  Survey,  1888,  pp.  251-268. 

Joseph  Le  Conte  and  W.  B.  Rising,  The  phenomena  of  metalliferous  vein 
formation  now  in  progress  at  Sulphur  Bank,  Am.  Jour.  Sci.,  3d  ser.,  vol. 
24,  1882,  pp.  23-33. 

F.  Posepny,  The  genesis  of  ore  deposits,  2d  ed.,  Pub.  by  the  Am.  Inst. 
Min.  Eng.,  1902,  pp.  32-36. 


114  '  MINERAL  DEPOSITS 

Summary. — There  is  then,  convincing  testimony  that  deposits 
of  quicksilver,  antimony,  arsenic,  gold  and  silver,  may  be  formed 
close  to  the  surface  by  hot  ascending  waters  of  the  kind  related 
to  volcanic  phenomena.  It  is  probable,  indeed,  that  the  majority 
of  fissure  veins  which  contain  notable  amounts  of  gold  and  silver 
together  with  sulphides  of  the  baser  metals  have  been  formed 
by  these  waters.  Of  this,  more  conclusive  evidence  is  yielded 
by  the  many  water  deposited  veins  which  so  frequently,  like  a 
metallic  aureole,  surround  the  areas  of  igneous  intrusive  rocks. 

On  the  other  hand  it  is  certain  that  warm  and  even  cold  waters 
of  the  meteoric  circulation  in  non-volcanic  regions  are  likewise 
competent  to  form  mineral  deposits  of  the  baser  metals  containing 
oxides  and  carbonates  of  iron  and  manganese,  and  sulphides  of 
copper,  lead  and  zinc  with  very  small  quantities  of  gold  and 
silver.  There  is  even  evidence  that  such  waters  may  develop 
deposits  of  minerals  of  vanadium,  and  uranium  with  radium 
(Chapter  XX,  p.  399).  The  waters  most  competent  to  per- 
form this  work  appear  to  be  the  calcium  carbonate  solutions  and 
the  chloride  brines  which  at  the  same  time  contain  carbon  dioxide 
and  hydrogen  sulphides.1 

1  C.  E.  Siebenthal,  Zinc  and  lead  deposits  of  the  Joplin  region,  Bull. 
606,  U.  S.  Geol.  Survey,  1915,  p.  154. 


CHAPTER  IX 

FOLDING  AND  FAULTING1 

FOLDS 

Sedimentary  beds  and  ore  deposits  contained  in  them  are 
often  bent,  corrugated,  and  folded  in  more  or  less  complex 
manner.  Extensive  folding  is  usually  effected  by  horizontal  or 
"tangential"  thrust,  but  minor  bends  and  monoclines  (Fig.  7) 
may  originate  by  thrust  in  any  direction.  In  extreme  cases  any 
fold  or  bend  may  pass  over  into  a  break  or  fault.  In  folding  on 
a  large  scale  it  is  necessary  that  the  sedimentary  complex  have 


FIG.  7. — Monocline  near  Gallup,  New  Mexico.     After  E.  Howdl. 

beds  of  sufficient  strength  (competent  beds)  to  transmit  the 
thrust  and  support  the  structures;  if  the  complex  is  plastic  it 
will  be  deformed  by  flowage  and  no  regular  folds  will  result. 
Folds  are  synclinal  (Figs.  8  and  9),  trough-like;  or  anticlinal 
(Fig.  10),  shaped  like  a  saddle.  A  plane  which  bisects  the 
average  angle  between  the  limbs  is  called  the  axial  plane  of  the 
fold.  By  complex  movements  the  axial  plane  may  become  a 
curved  surface.  If  this  axial  plane  is  vertical  the  limbs  dip 
at  like  angles;  if  the  axial  plane  is  inclined  the  limbs  have  unequal 
dips.  In  close  folding  the  limbs  dip  steeply  (Fig.  11).  When 
the  axial  plane  of  folds  inclines  strongly  in  one  direction  we  speak 

1  E.  de  Margerie  and  A.  Heim,  Les  dislocations  de  Pecorce  t^rrestre, 
Ztirich,  1888,  pp.  49-63. 

Bailey  Willis,  The  mechanics  of  Appalachian  structure,  Thirteenth  Ann. 
Rept.,  U.  S.  Geol.  Survey,  1894,  pp.  211-281. 

C.  R.  Van  Hise,  Principles  of  North  American  pre-Cambrian  geology, 
Sixteenth  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  1,  1896,  pp.  589-633. 

115 


116 


MINERAL  DEPOSITS 


FIG.  8. — Open  syncline  showing  Carboniferous  phosphate  beds  uncon- 
formably  covered  by  Eocene  beds.  Beaver  Creek,  Utah.  After  E.  Black- 
welder,  U.  S.  Geol  Survey. 


FIG.  9. — Eroded  syncline,  Georgetown  Canyon,  Idaho,  showing 
phosphate  bed.     After  H.  S.  Gale,  U.  S.  Geol.  Survey. 


FEET 
Above  sea-leve 

-8500 


-7500 
-6500 
-5500 
-4-500 


FIG.  10. — Eroded  anticline,  Montpelier,  Idaho,  showing  bending  of  copper- 
bearing  beds  of  Triassic  age.     After  H.  S.  Gale,  U.  S.  Geol.  Survey. 


FOLDING  AND  FAULTING 


117 


of  overturned  folds,  and  these  by  further  compression  may  easily 
pass  over  into  overthrust  faults  (Fig.  12),  causing  a  part  of  the 


1000  2000  3000  *ooo  5000  Feet 


Upper  diabase 


Lower  diabase 


FIG.  11.— Close  folding  with   overthrusts   and   thickening  of  strata   by 
duplication.     After  M.  Koch. 

folded  series  to  slide  over  the  other.     In  flat  overthrust  faults 
the  horizontal  movement  may  amount  to  many  miles. 


118  MINERAL  DEPOSITS 

Synclines  and  anticlines  extend  naturally  about  perpendicu- 
larly to  the  direction  of  compressive  stress.  Their  direction  is  in- 
dicated by  a  line  passing  through  all  the  highest  or  lowest'points 
of  a  given  stratum.  These  crest  lines  or  trough  lines  have 
usually  a  distinct  dip;  the  angle  of  this  line  with  the  horizontal 
is  called  the  pitch  of  the  fold.  Minor  plications  on  the  limbs 


FIG.  12. — Diagram  showing  development  of  an  overthrust  fault  from  a 
fold.     After  C.  R.  Van  Hise,  U.  S.  Geol.  Survey. 

often  indicate  the  pitch  of  the  fold.  Thrusts  in  two  directions 
result  in  cross-folding  with,  the  development  of  bending  in 
forms  known  as  canoes,  domes,  and  basins. 

When  beds  are  lifted  in  dome  shape  so  that  they  dip  away  from 
a  central  point  they  form  a  quaquaversal. 

In  sharp  folding  of  a  sedimentary  complex,  the  strata  become 
thicker  by  compression  at  the  points  of  greatest  bending  (Fig. 
13).  The  harder  strata  of  sandstone  or  limestone  will  yield  to 
tension  by  breaking  or  tearing;  the  softer,  shaly  strata  do  not 
break,  but  yield  to  deformation.  Strata  of  differing  hardness 
may  slide  over  one  another  at  such  points,  and  openings  may  be 
produced  which,  for  instance,  may  later  be  filled  with  quartz. 
In  slates  and  crystalline  schists  which  have  been  deformed  at 
great  depths  by  flowage  the  harder  or  "competent"  layers,  like 
quartz  veins,  may  be  corrugated  in  an  extraordinarily  complicated 
manner.  Some  quartz  veins  of  Nova  Scotia,  called  "barrel 
quartz,"  are  believed  to  owe  their  form  to  such  conditions 
(Fig.  14). 


FOLDING  AND  FAULTING 


119 


FIG.  13. — Overturned  anticline  of  crystalline  limestone,  Lenox,  Mass., 
showing  thickening  and  breaking  of  strata  at  points  of  bending.  After 
T.  Nelson  Dale,  U.  S.  Geol.  Survey. 


FIG.  14. — Gneiss  with  corrugated  veinlets  of  quartz.     After  C.  R.  Van  Hise, 
U.  S.  Geol.  Survey. 


120 


MINERAL  DEPOSITS 


FAULTS^ 

Sedimentary  beds  and  deposits,  as  well  as  deposits  of  later 
origin  which  persistently  follow  a  certain  horizon  in  a  sedimen- 
tary series,  are  sometimes  abruptly  cut  off  by  structural  planes. 
b 


FIG.  15. — Faulting  of  Mendota  vein,  Silver  Plume,  Colorado,     a,  granite; 
b,  quartz;  c,  galena  and  zinc  blende.     After  J.  E.  Spurr,  U.  S.  Geol.  Survey 

1  E.  de  Margerie  and  Heim,  Bailey  Willis,  C.  R.  Van  Hise,  op.  cit. 

J.  E.  Spurr,  Geology  applied  to  mining,  New  York,  1904,  pp.  149-163. 

J.  E.  Spurr,   Geology  of  the  Tonopah  mining  district,  Nevada,  Prof. 
Paper  42,  U.  S.  Geol.  Survey,  1905,  p.  144. 

J.  E.  Spurr,  Measurements  of  faults,  Jour.  Geology,  vol.  5,  1897,  p.  723. 

J.  E.  Spurr,  The  relation   of  ore   deposition   to  faulting.  Econ.  GeoL, 
vol.  11,  1916,  pp.  601-622. 

&F.  L.  Ransom e,  The  direction  of  movement  and  the  nomenclature  of 
faults,  Econ.  Geol.,  vol.  1,  1906,  p.  777. 

C.  F.  Tolman,  Jr.,  Graphic  solution  of  fault  problems,  Min.  and  Sci. 
Press,  June  17,  1911,  et  seq.     Reprinted,  San  Francisco  and  London,  1911. 

C.  F.  Tolman,  Jr.,  Econ.  Geol,  vol.  2,  1907,  pp.  506-511. 

H.  F.  Reid,  Geometry  of  faults,  Bull,  Geol.  Soc.  Am.,  vol.  20,  1909, 
pp.  171-196. 

•  H.  F.  Reid,  W.  M.  Davis,  A.  C.  Lawson,  and  F.  L.  Ransome,  Proposed 
nomenclature  of  faults,  Bull,  Geol.  Soc.  Am.,  vol.  24,  1913. 

C.  K.  Leith,  Structural  Geology,  New  York,  1913. 

F.  H.  Lahee,  Field  Geology,  New  York,  1916,  Chapters  VII  and  VIII. 


FOLDING  AND  FAULTING 


121 


When  such  an  occurrence  is  found  it  is  safe  to  conclude  that 
the  interruption  is  due  to  a  fault — -that  is,  to  a  fracture  along  which 
movement  has  taken  place — and  that  the  continuation  of  the  bed 
exists  somewhere  beyond  this  break  (Fig.  15). 


FIG.  16. — Sketch  showing  replacement  of  shale  by  pyrite.  Natural  size. 
The  small  fissures  are  older  than  the  pyrite  and  are  crossed  by  its  banded 
structures.  After  F.  L.  Ransome,  U.  S.  Geol.  Survey. 

In  the  case  of  epigenetic  deposits  not  closely  following  the 
original  lines  of  structure  in  the  rocks,  such  a  conclusion  is  often, 
but  not 'always  justified.  The  interruption  of  the  ore-body  may 


FIG.  17. — Plan  of  a  vein  in  the  Homer  mine,  Idaho  Springs,  Colorado, 
showing  deflection  of  the  vein  upon  meeting  a  dike.  After  J.  E.  Spurr, 
U.  S.  Geol.  Survey. 

be  due  to  an  actual  post-mineral  dislocation,  or  it  may  be 
caused  by  a  cessation  of  mineralization  on  account  of  structures 
existing  before  the  mineralization  began.  The  replacement  of 


122 


MINERAL  DEPOSITS 


limestone  by  galena  or  shale  by  pyrite  (Fig.  16)  may  stop  sud- 
denly at  a  clay-coated  seam,  which  offered  a  barrier  to  the  solu- 
tions. A  vein-filled  fissure  may  terminate  abruptly  or  split  up 
within  a  few  feet  upon  encountering  softer  and  more  plastic  rocks, 
such  as  clay  shales,  thick  gouge  seams,  or  soft  tuffs.  A  vein 
traversing  formations  of  varying  hardness  often  suffers  abrupt 
deflection  at  rock  contacts.  It  may  also  be  deflected  by  encoun- 
tering older  dikes  or  fissures,  either  barren  or  filled  with  vein 
material  (Fig.  17). 

The  distinction  between  faults  and  deviations  is  most  impor- 
tant.    The  appearance  of  detached  fragments  of  the  ore — usually 


Scale 


2  feet 


FIG.  18. — Sketch  of  faulted   quartz  vein  in  andesite,    showing   "drag." 
After  J.  E.  Spurr,  U.  S.  Geol.  Survey. 

termed  "drag"— on  the  faulting  plane  (Fig.  18),  the  direction 
of  the  striations,  and  the  interrelations  of  dip  and  strike  of  fault- 
ing, fissure,  and  ore-body  are  all  valuable  data  which  must  be 
interpreted  in  each  case. 

There  are  many  geometrical  rules  for  the  finding  of  a  faulted 
ore-body,  but  they  are  of  little  value  unless  the  character  of 
the  interruption  is  known.  Each  case  must  be  considered  and 
judged  by  itself. 


FOLDING  AND  FAULTING  123 

Too  often  faulting  is  considered  in  only  two  dimensions — that 
is,  as  either  normal  or  reverse  movement  in  a  vertical  plane. 
The  fact  is  that  most  faulting  movements  have  lateral  as  well  as 
vertical  components;  every  mining  engineer  knows  the  frequent 
occurrence  of  inclined  or  horizontal  striation  on  fault  planes. 

Several  proposals  have  been  made  looking  to  a  uniform  nomen- 
clature of  the  various  elements  involved  in  faulting  movements; 
the  best  of  these  are  advocated  by  J.  E.  Spurr,  C.  F.  Tolman,  Jr., 
and  H.  F.  Reid.  Lately  a  committee  of  the  Geological  Society  of 
America  has  been  instructed  to  examine  this  question  in  more 
detail,  and  their  conclusions,  in  large  part  based  on  the  work 
of  Reid,  will  probably  be  adopted  by  American  geologists.  An 
abstract  of  their  report1  will  be  found  in  the  following  pages. 

Measurements  of  fault  movement  are  made  in  the  fault  plane 
itself,  in  a  plane  normal  to  the  trace  of  the  faulted  body. on  the 
fault  plane,  in  any  normal  plane,  and  in  a  horizontal  plane. 

General  Terms 

A  fault  is  a  fracture  in  the  rock  of  the  earth's  crust,  accompa- 
nied by  a  displacement  of  one  side  with  respect  to  the  other  in  a 
direction  parallel  with  the  fracture.  The  fracture  is  usually  not 
an  open  crack,  and  an  open  crack  would  not  be  a  fault  unless  one 
of  the  sides  had  moved  parallel  with  the  crack  relatively  to  the 
other. 

As  we  pass  from  one  part  of  a  fault  to  another,  we  find  that 
certain  characteristics  vary.  Definitions  descriptive  of  charac- 
teristics must  therefore  be  considered  as  referring  to  the  parts 
of  the  fault  to  which  they  are  applied  and  not  necessarily  to  the 
fault  as  a  whole. 

A  closed  fault  is  one  in  which  the  two  walls  of  a  fault  are  in 
contact. 

An  open  fault  is  one  in  which  the  two  walls  of  a  fault  are 
separated.  The  same  fault  may  be  closed  in  one  part  and  open 
in  another. 

The  fault  space  is  the  space  between  the  walls  of  an  open  fault. 

A.  fault  surface  is  the  surface  along  which  dislocation  has 
taken  place;  this  may  be  called  a  fault  plane  if  it  is  without 
notable  curvature. 

A  fault  line  is  the  intersection  of  a  fault  surface  with  the 

1  Reid,  Davis,  Lawson,  and  Ransome,  op.  cit. 


124  MINERAL  DEPOSITS 

earth's  surface  or  with  any  artificial  surface  of  reference,  such  as 
a  level  of  a  mine. 

U  When  a  fault  is  made  up  of  a  number  of  slips  on  closely  spaced 
surfaces,  the  section  of  the  earth's  crust  containing  these  minor 
faults  is  called  the  shear  zone.  This  name  would  also  apply 
to  the  brecciated  zone  which  characterizes  some  faults. 

The  fault  breccia,  or  fault  rock,  is  the  breccia  which  is  frequently 
found  in  the  shear  zone,  more  especially  in  the  case  of  thrust 
faults. 

Gouge  is  a  fine-grained,  impervious  clay,  usually  a  mixture  of 
minerals  which  sometimes  occurs  between  the  walls  of  a  fault. 

A  horse  is  a  mass  of  rock  broken  from  one  wall  and  caught 
between  the  walls  of  the  fault. 

The  fault  strike  is  the  direction  of  the  intersection  of  the  fault 
surface,  or  the  shear  zone,  with  a  horizontal  plane.  It  is  measured 
from  the  astronomic  or  from  the  magnetic  meridian. 

The  fault  dip  is  the  vertical  inclination  of  the  fault  surface, 
or  shear  zone,  measured  from  horizontal  plane. 

The  hade  is  the  inclination  of  the  fault  surface,  or  shear  zone, 
measured  from  the  vertical;  it  is  the  complement  of  the  dip. 

The  hanging  watt  is  the  upper  wall  of  the  fault. 

The  foot  watt  is  the  lower  wall  of  the  fault. 

General  Classification  of  Faults 

Faults  of  parallel  displacement  are  those  in  which  all  straight 
lines  on  opposite  sides  of  the  fault  and  outside  of  the  dislocated 
zone,  which  were  parallel  before  the  displacement,  are  parallel 
afterward. 

Rotatory  faults  are  those  in  which  some  straight  lines  on  oppo- 
site sides  of  the  fault  and  outside  of  the  dislocated  zone,  parallel 
before  the  displacements,  are  no  longer  parallel  afterward — that 
is,  where  one  side  has  suffered  a  rotation  relative  to  the  other. 

Determinations  of  throw  are  almost  always  relative,  and  hence 
we  can  rarely  tell  which  side  of  the  fault  has  moved;  therefore 
the  terms  "upthrow"  and  " downthrow,"  which  are  used  accord- 
ing to  the  side  from  which  the  fault  is  viewed,  are  objectionable, 
as  they  suggest  that  a  particular  side  of  the  fault  has  actually 
been  moved.  They  are  in  very  general  use  and  should  be 
retained,  but  it  should  be  definitely  understood  that  they  refer 
merely  to  a  relative  and  not  to  an  absolute  displacement. 


FOLDING  AND  FAULTING  125 

Faults  of  Parallel  Displacement. — No  faults  of  any  magnitude 
consist  of  simple  parallel  displacements  over  their  whole  length. 
Faults  die  out  at  their  limits,  and  the  displacement  is  not  uni- 
form along  their  courses,  so  that  there  is  necessarily  some  slight 
rotation,  varying  in  amount  in  the  different  parts  of  the  fault's 
course.  Probably  the  greatest  number  of  faults,  certainly  of 
large  faults,  are  of  this  character.  The  variations  in  rotation 
and  displacement  are  permitted  by  slight  plastic  deformation. 
If,  however,  we  confine  our  attention  to  a  small  length  of  the 
fault,  we  may  describe  the  displacement  there  as  if  the  rock  were 
rigid;  and  if  the  rotation  is  very  small,  we  may  describe  it  as  if 
a  parallel  displacement  had  occurred.  It  sometimes  happens 
that  the  strikes  on  the  opposite  sides  of  a  fault  are  different; 
the  strata  are  then  said  to  "strike  at  each  other."  This  suggests 
a  rotation,  but  it  may  be  due  to  local  variation  of  strike  or  to 
an  unconformity. 

The  word  "displacement"  should  receive  no  technical  mean- 
ing, but  is  reserved  for  general  use;  it  may  be  applied  to  a  relative 
movement  of  the  two  sides  of  the  fault,  measured  in  any  direction, 
when  that  direction  is  specified;  for  instance,  the  displacement 
of  a  stratum  along  a  drift  in  a  mine  would  be  the  distance 
between  the  two  sections  of  the  stratum  measured  along  the 
drift.  The  word  "dislocation"  will  also  be  most  useful  in  a 
general  sense. 

There  are  two  methods  of  defining  the  displacement  due  to  a 
fault;  we  may  define  the  apparent  relative  displacement  of  a  bed 
by  naming  the  distance  between  its  two  disrupted  branches 
measured  in  any  chosen  direction,  such  as  the  vertical  distance 
between  the  branches,  measured  in  a  shaft,  or  the  perpendicular 
distance  between  the  lines  of  intersection  of  the  two  branches 
with  the  fault  plane;  or  we  may  define  the  actual  relative  dis- 
placement and  its  components  in  important  directions.  The  ap- 
parent displacements  are  those  usually  measured;  the  actual 
displacement  must  be  worked  out  for  a  complete  understanding 
of  the  fault. 

Only  four  important  technical  words  are  used  to  denote  the 
various  displacements  caused  by  faulting,  qualifying  words  being 
added  to  indicate  the  component  of  the  displacement  referred  to. 
These  words  are: 

Slip,  which  indicates  the  relative  displacement  of  formerly 
adjacent  points  on  opposite  sides  of  the  fault,  measured  in  the 


126  MINERAL  DEPOSITS 

fault  surface.  The  qualifying  words  relate  to  the  strike  and  dip 
of  the  fault  surface. 

Shift,  which  indicates  the  relative  displacement  of  regions  on 
opposite  sides  of  the  fault  and  outside  of  the  dislocated  zone. 
The  qualifying  words  relate  to  the  strike  and  dip  of  the  fault 
surface,  except  in  the  expression  "vertical  shift,"  which  is  self- 
explanatory. 

Throw,  which  indicates  a  displacement  not  related  to  the  strike 
or  dip  of  the  fault  plane. 

Offset,  which  indicates  the  horizontal  distance  between  the 
outcrops  of  a  dislocated  bed. 

By  keeping  in  mind  the  general  meaning  of  these  four  words, 
all  confusion  in  the  uses  of  the  proposed  nomenclature  can  be 
avoided. 

There  is  no  generally  accepted  word  in  present  use  to  denote  the 
slip.  Willis  and  Tolman  use  "displacement;"  Spurr  uses 
"throw."  We  have  reserved  "displacement"  for  general  use, 
and  the  word  "throw"  is  here  used  in  quite  a  different  sense. 
The  word  "shift"  also  suggests  the  meaning  attached  to  it;  there 
is  no  distinctive  word  now  in  use  to  describe  the  shift. 

In  mines,  where  the  fault  surface  itself  its  visible,  the  slip  will 
generally  be  determined;  it  is  of  paramount  importance  in  min- 
ing. In  field  surveys,  where  the  fault  is  studied  by  the  dislocation 
of  the  outcrop  of  strata,  or  dikes,  often  at  a  considerable  distance 
from  the  fault,  the  shift  is  determined.  In  the  larger  problems  of 
geology  the  shift  is  of  greater  importance  than  the  slip.  The  dis- 
tinction between  the  slip  and  the  shift  is  important;  it  has  not 
heretofore  been  recognized  in  the  nomenclature  of  faults.  The 
perpendicular  throw  is  of  the  greatest  importance.  It  is  fre- 
quently the  only  displacement  determined,  and  in  strike  faults  all 
the  displacements  in  a  vertical  plane  at  right  angles  to  the  fault 
strike — that  is,  all  the  displacements  which  have  heretofore 
received  the  most  attention — can  be  expressed  in  terms  of 
perpendicular  throw.  The  offset  is  often  the  most  important 
surface  measurement  made. 

Faults  in  Stratified  Rocks.— Among  stratified  rocks  the 
character  of  the  displacement  of  the  strata  due  to  a  fault  is  so 
much  influenced  by  the  relation  of  the  strike  of  the  fault  to  the 
strike  of  the  strata  that  syecial  subclasses  are  generally  recognized. 

A  strike  fault  is  one  whose  strike  is  parallel  to  the  strike  of 
the  strata. 


FOLDING  AND  FAULTING 


127 


A  dip  fault  is  one  whose  strike  is  approximately  at  right  angles 
to  the  strike  of  the  strata. 

An  oblique  fault  is  one  whose  strike  is  oblique  to  the  strike  of 
the  strata. 

These  terms  are,  of  course,  not  directly  applicable  in  regions  of 
unstratified  rocks;  but  they  might  be  used  in  such  regions  with 
respect  to  the  strike  of  a  system  of  parallel  dikes  if  this  were 
distinctly  stated  in  the  description  of  the  faults. 

Similarly  with  regard  to  the  general  structure  of  the  region: 

A  longitudinal  fault  is  one  whose  strike  is  parallel  with  the 
general  structure. 

A  transverse  fault  is  one  whose  strike  is  transverse  to  the  gen- 
eral structure.1 

Slip. — The  word  "  slip  "  indicates  the  displacement  as  measured 
on  the  fault  surface;  the  qualifying  words  refer  to  the  strike  and 
dip  of  the  fault. 

The  slip  or  net  slip  is  the  maximum  relative  displacement  of  the 
walls  of  the  fault,  measured  on  the  fault  surface,  along  the  line 
of  the  movement;  it  is  given  by  ab  in  Figs.  19  and  20.2 


FIG.  19.— The  slip. 


FIG.  20.— The  shift. 


The  strike-slip  is  the  component  of  the  slip  parallel  with  the 
fault  strike  or  the  projection  of  the  net  slip  on  a  horizontal 
line  in  the  fault  surface;  ac  in  Figs.  19  and  20.3 

The  dip-slip  is  the  component  of  the  slip  parallel  with  the  fault 
dip,  or  the  projection  of  the  slip  on  a  line  on  the  fault  surface 
perpendicular  to  the  fault  strike;  be  in  Figs.  19  and  20. 4 

Shift. — It  frequently  happens  that  a  fault  has  not  a  single  sur- 
face of  shear,  but  consists  of  a  series  of  small  slips  on  closely 

1  See  the  word  "flaw"  further  on. 

Spurr  and  Tolman  call  this  the  "total  displacement." 

2  Tolman  calls  this  the  "horizontal  displacement." 
Tolman  calls  it  the  "normal  displacement." 


128 


MINERAL  DEPOSITS 


spaced  surfaces,  and  in  some  faults  the  strata  in  the  neighbor- 
hood of  the  fault  surface  are  bent,  so  that  the  relative  displace- 
ments of  the  rock  masses  on  opposite  sides  of  the  fault  may  be 
quite  different  from  the  slip  and  not  even  parallel  with  it.  The 
word  "shift"  is  used  to  denote  the  relative  displacements  of  the 
rock  masses  situated  outside  of  the  zone  of  dislocation;  the  quali- 
fying words  relate  to  the  strike  and  dip  of  the  fault,  with  one  ex- 
ception, in  which  the  meaning  is  clear. 

The  shift,  or  net  shift,  is  the  maximum  relative  displacement 
of  points  on  opposite  sides  of  the  fault  and  far  enough  from  it  to 
be  outside  of  the  dislocated  zone;  de  in  Figs.  20  and  21,  where  d 
is  the  position  of  a  selected  point  before  and  e  after  the  faulting. 

The  strike-shift  is  the  component  of  the  shift  parallel  with  the 
fault  strike;  df  in  Figs.  20  and  21. 


FIG.  21.— The  shift. 


FIG.  22.— The  throw. 


The  dip-shift  is  the  component  of  the  shift  parallel  with  the 
fault  dip;  fe  in  Figs.  20  and  21.  (The  triangle  def  is  parallel 
with  the  fault  surface  in  Fig.  20.  *) 

The  bending  of  the  strata  near  the  fault  may  be  so  great  that 
the  direction  of  the  shift  is  no  longer  even  nearly  parallel  with  the 
fault  surface;  it  is  better  then  to  use  the  three  following  terms 
for  the  components  of  the  shift: 

The  strike-shift  is  the  horizontal  component  of  the  shift  parallel 
with  the  fault  strike,  as  already  defined. 

The  normal  shift  is  the  horizontal  component  of  the  shift 
at  right  angles  to  the  fault  strike.  It  equals  the  horizontal 
shortening  or  lengthening  of  the  earth's  surface  at  right  angles  to 
the  fault  strike,  due  to  the  fault. 

The  vertical  shift  is  the  vertical  component  of  the  shift.     These 

1  The  dip-shift  and  strike-shift  are  not  accurately  shown  in  Fig.  20, 
because  the  net  shift,  de,  is  not  parallel  with  the  fault  plane,  and  the  lines 
de,  df,  and  fe  would  not  lie  in  one  plane.  But  the  definitions  are  clear  and 
the  figure  illustrates  them  fairly  well. 


FOLDING  AND  FAULTING  129 

terms  may  evidently  be  used  equally  well  when  the  shift  is 
parallel  with  the  fault  plane. 

Throw. — The  word  "throw"  will  apply  to  components  of  the 
displacement  having  no  immediate  bearing  on  the  strike  or 
dip  of  the  fault  plane. 

The  throw  is  the  vertical  component  of  the  slip;  eg  in  Figs.  20 
and  22,  de  in  Figs.  23  and  24.  The  word  is  almost  universally 
used  in  this  sense,  but  A.  Geikie  uses  it  to  designate  the  vertical 
distance  between  the  two  parts  of  a  dislocated  bed,  projected  if 
necessary — a  very  different  thing.  Geikie's  "throw"  would  be 
represented  by  df  in  Figs.  23  and  24.  Spurr  uses  "throw"  to 
designate  the  distance  between  the  two  parts  of  a  dislocated  bed 
measured  on  the  fault  plane. 


FIG.  23. — Section  of  a  normal  fault.  FIG.  24. — Section  of  a  reverse  fault- 
The  heave1  is  the  horizontal  component  of  the  slip,  measured  at 
right  angles  to  the  strike  of  the  fault;  bg  in  Figs.  20  and  22,  eg 
in  Figs.  23  and  24.  The  word  "heave"  has  been  used  in  many 
senses;  J.  Geikie,  Willis,  Scott,  and  Fairchild  use  it  as  denned 
above;  A.  Geikie  and  Spurr  use  it  to  designate  what  we  have 
called  the  "offset"  of  a  bed  (see  below);  Jukes-Brown  apparently 
used  it  for  the  strike-slip  (De  Margerie  and  Heim,  page  72) ; 
so  did  Ransome;  and  Scott  also  uses  it  in  this  sense  when  he  re- 
fers to  "heave  faults." 

The  perpendicular  throw  of  a  bed,  dike,  vein,  or  of  any  recog- 
nizable surface,  is  the  distance  between  the  two  parts  of  the 
disrupted  bed,  etc.,  measured  perpendicularly  to  the  bedding 
plane  or  to  the  plane  of  the  surface  in  question.  It  is  measured, 
therefore,  in  a  vertical  plane  at  right  angles  to  the  strike  of  the 
disrupted  surface.2  The  importance  of  the  perpendicular  throw 

1  Sometimes  called  the"horizontal  throw." 

2  Spurr  calls  it  the  "perpendicular  separation."     Tolman's  "perpendicu- 
lar  throw"  would  under  certain  conditions  correspond  in  meaning  with  our 
expression. 


130  MINERAL  DEPOSITS 

of  the  strata  is  so  great  that  it  is  convenient  to  have  special  terms 
for  it;  these  are  given  below. 

The  stratigraphic  throw  is  the  distance  between  the  two  parts  of 
a  disrupted  stratum  measured  at  right  angles  to  the  plane  of  the 
stratum;  oh  in  Figs.  23  and  24.  The  stratigraphic  throw  is  in 
general  the  simplest  throw  to  determine;  it  can  be  found  from 
the  distance  between  the  outcrops  of  the  two  parts  of  the  same 
stratum,  the  dip  of  the  stratum,  and  the  slope  of  the  ground. 

The  dip  throw  is  the  component  of  the  slip  measured  parallel 
with  the  dip  of  the  strata;  cb  in  Figs.  23  and  24. 

The  throws  have  been  defined  as  components  of  the  slip. 
Where  we  are  dealing  with  a  simple  fault  in  plane  strata,  the 
shifts  will  be  the  same  as  the  slips,  and  the  term  throws  will 
apply  to  both  equally  well;  it  is  only  in  plane  strata  that  the  per- 
pendicular throw  is  important.  Where  there  is  a  dislocated  zone 
about  the  fault,  the  term  perpendicular  throw  would  necessarily 
apply  to  the  shift;  but  we  cannot  detach  the  word  throw  from  its 
accepted  meaning  and  apply  it  generally  to  the  shift. 


FIGS.  25   and    26.— Plan   of   an   oblique    slip. 

Offset.— The  offset  of  a  stratum  is  the  distance  between  the 
two  parts  of  the  disrupted  stratum  measured  at  right  angles  to  the 
strike  of  the  stratum  and  on  a  horizontal  plane.1 

If  Figs.  25  and  26  represent  the  ground  plans  of  oblique  faults 
on  a  level  surface,  ab,  and  not  ac,  would  be  the  offset  of  the 
stratum;  ac  would  be  the  horizontal  displacement  of  the  stratum 
parallel  with  the  fault  strike. 

Some  confusion  of  nomenclature  results  from  the  non-observ- 
ance of  the  fact  that  the  distance  between  the  dislocated  parts 
of  a  stratum  measured  in  a  certain  direction  is  not  the  same  as 

1  A.  Geikie  and  Spurr  use  the  term  "heave"  for  this  offset. 


FOLDING  AND  FAULTING 


131 


the  component  of  the  slip  in  the  same  direction;  for  instance,  let 
Fig.  27  represent  a  reverse  strike  dip-slip  fault1  in  section. 
A.  Geikie  calls  ad  the  throw  and  ef  the  heave,  whereas  the  most 
general  usage  seems  to  be  to  call  ac  the  throw  and  be  the  heave, 
as  adopted  above.  The  distance  ad  has  not  been  defined,  but  it 


FIG.  27.— Plan 


a    reverse    fault. 


is  readily  described  as  the  vertical  displacement  of  the  stratum, 
without  limiting  the  word  "displacement"  to  a  technical  mean- 
ing; ef  is  the  offset.  Let  Figs.  28  and  29  lie  in  the  fault  plane  and 
let  the  point  a  move  by  faulting  to  c,  then  ac  will  be  the  net 


FIGS.  28  and   29.— Section   in   a  fault   plane. 


slip,  ad  the  strike-slip,  cd  the  dip-slip,  and  ab  the  displacement  of 
the  stratum  parallel  with  the  fault  strike;  ab  is  not  necessarily 
at  right  angles  to  the  strike  of  the  strata. 

Faults  Classified  According  to  the  Direction  of  the  Movement. — 
Faults  may  be  classified,  according  to  the  direction  of  the  move- 
ment on  the  fault  plane,  into  three  groups,  as  follows: 

1  This  means  a  reverse  fault  whose  strike  corresponds  with  the  strike  of 
the  strata  and  in  which  the  displacement  has  been  in  the  direction  of  the 
fault  dip. 


132  MINERAL  DEPOSITS 

Dip-slip  faults,  where  the  net  slip  is  practically  in  the  line  of 
the  fault  dip. 

Strike-slip  faults,  where  the  net  slip  is  practically  in  the  direc- 
tion of  the  fault  strike.  J.  Geikie  calls  such  faults  "transcurrent 
faults,"  or  "transverse  thrusts."  Jukes-Brown  calls  them 
"heaves."  Scott  calls  them  "horizontal  faults,"  or  "  heave 
faults."  A  vertical  fault  is  one  with  a  dip  of  90  degrees  (see 
below) ;  and,  by  analogy,  a  horizontal  fault  should  be  one  with  a 
zero  dip  and  the  term  should  not  be  applied  to  strike-slip  faults. 

Oblique-slip  faults  where  the  net  slip  lies  between  these 
directions. 

Classes  of  Strike  Faults. — Most  geological  text-books  and 
books  on  field  methods  have  confined  themselves  almost  exclu- 
sively to  the  discussion  of  dip-slip  faults,  and  have  given  little 
attention  to  the  other  two  classes. 

Strike  faults  have  usually  been  treated  as  if  they  were  also 
dip-slip  faults  and  classified  into 

Normal  faults,  where  the  hanging  wall  has  been  depressed 
relatively  to  the  foot  wall. 

Reverse  faults,  where  the  hanging  wall  has  been  raised  relatively 
to  the  foot  wall. 

Vertical  faults,  where  the  dip  is  90  degrees. 

The  relative  displacement  has  usually  been  determined  by 
means  of  a  dislocated  bed.  Although  exception  may  well  be 
taken  to  these  terms,  their  retention  is  recommended,  because 
they  are  in  general  use  and  are  well  understood.  The  word 
"reverse"  is  preferable  to  "reversed"  (which  has  been  almost 
universally  used),  as  the  latter  implies  the  reversal  of  an  action. 

The  horizontal  distance  between  two  points  on  opposite  sides 
of  a  fault,  measured  on  a  line  at  right  angles  to  the  fault  strike, 
is  always  shortened  by  a  reverse  strike  fault,  lengthened  by  a 
normal  strike  fault,  and  unchanged  in  length  by  a  vertical  fault. 
It  can  be  shown  that  normal  faults  may  be  formed  without  the 
existence  of  tension  and  indeed  under  some  pressure,  but  the 
definitions  we  are  giving  are  geometric  and  not  dynamic. 

Extension  of  the  Words  Normal  and  Reverse  to  Diagonal  and 
Dip  Faults. — The  expressions  "normal"  and  "reverse"  may  be 
used  in  connection  with  oblique  and  dip  faults,  even  when  these 
are  strike-slip  or  oblique-slip  faults,  provided  they  are  applied  to 
designate  the  apparent  relative  displacement  of  the  two  parts  of 
a  dislocated  stratum,  or  other  recognized  surface,  in  a  vertical 


FOLDING  AND  FAULTING 


133 


plane  at  right  angles  to  the  fault  strike.  It  does  not  follow,  in 
the  case  of  oblique-slip  faults,  that  a  horizontal  line  at  right 
angles  to  the  fault  strike  would  be  lengthened  by  a  normal  or 
shortened  by  a  reverse  fault.  This  has  been  pointed  out  by 
Ransome1  and  can  be  illustrated  by  Figs.  30  and  31.  In  Fig.  30 
a  reverse  fault,  as  determined  by  the  displacement  of  the  stratum, 
has  caused  an  extension  at  right  angles  to  the  fault  strike.  It 
is  evident  that  if  the  hanging  wall  had  moved,  as  in  Fig.  31,  with 
the  stratum  dipping  as  there  represented,  we  should  have  had 
a  normal  fault  and  a  contraction  at  right  angles  to  the  fault 
strike.  The  relations  of  the  two  parts  of  the  disrupted  stratum 
in  Fig.  30  are  exactly  the  same  as  if  we  had  had  a  simple  reverse 


FIG.  30. — A  reverse  fault  due  to     FIG.  31. — A  normal  fault  due  to  an 
an  oblique  slip.  oblique  slip. 

dip-slip  fault,  and  in  Fig.  31  as  if  we  had  had  a  simple  normal 
dip-slip  fault ;  and  if  there  are  no  disrupted  dikes  or  other  means 
of  determining  the  amount  of  the  strike-slip,  the  movements 
described  could  not  be  distinguished  from  simple  dip-slip  faults.2 
It  very  frequently  happens  that  nothing  more  than  the  apparent 
displacement  of  the  strata  can  be  determined,  and  we  recommend 
that  the  terms  "normal"  and  "reverse"  faults  as  denned  be 
used  purely  for  purposes  of  description  and  not  for  the  purpose 
of  indicating  extension  or  contraction,  tension  or  compression, 
vertical  or  horizontal  forces. 

Special  Classes  of  Faults. — There  are  two  kinds  of  faults  which 
have  played  such  important  roles  in  altering  the  structure  of 
some  regions  that  they  have  received  special  names. 

*Econ.  Geol.  vol.  1,  1906,  pp.  783-787. 

2  The  methods  of  determining  the  complete  displacement  at  a  fault  are 
given  in  Reid's  Geometry  of  faults,  Bull.  Geol.  Soc.  Am.,  vol.  20, 1909,  pp. 
170-196,  and  in  Tolman's  Graphic  solution  of  fault  problems,  op.  cit. 


134 


MINERAL  DEPOSITS 


Overthrusts. — These  are  reverse  faults  with  low  dip  or  large 
hade.  In  some  cases  the  dip-slip  has  been  enormous,  amounting 
to  tens  of  kilometers.  Scott  calls  them  "thrusts"  and  separates 
them  entirely  from  faults  of  high  dip;  but  the  word  "thrust" 
has  been  used  for  ordinary  reverse  faults  of  high  dip.  The  word 
"overthrust"  has  been  generally  used  for  this  kind  of  fault  and 
is  very  descriptive.  It  should  be  adopted. 

Flaws. — Suess  has  described  with  care  certain  faults  in  which 
the  strike  is  transverse  to  the  strike  of  the  rocks,  the  dip  high  and 
varying  from  one  side  to  the  other  in  the  course  of  the  fault,  and 
the  relative  movement  practically  horizontal  and  parallel  with 
the  strike  of  the  fault.  He  has  used  the  word  "Blatt"  (plural, 
"Blatter")  to  designate  them.  Miss  Sollas  has  used  the  word 
"flaw"  in  the  English  translation  of  Suess.  The  gold-quartz 
veins  of  the  Tauern  in  Austria,  investigated  by  Posepny,  follow 
such  dislocations. 


FIG.  32. — Vertical  section  of  a  faulted  vein,  Berlin  mine,  Nevada,  showing 
also  its  probable  original  position.     After  Ellsworth  Daggett. 


Rotatory  Faults. — When  a  rotation  of  one  side  of  the  fault 
occurs,  the  amount  of  the  rotation  and  the  direction  of  the  axis 
should  be  given.  Rotatory  faults  have  been  but  little  studied, 
and  it  is  not  considered  advisable  to  suggest  a  special  nomen- 
clature at  present. 

Mineralization  of  Faults. — Any  fault  may  become  a  fissure 
vein  by  filling  and  replacement  along  its  course.  However,  it 
is  rather  unusual  to  find  large  structural  faults,  normal  or  over- 


FOLDING  AND  FAULTING 


135 


thrusts,  which  have  been  extensively  mineralized.  Shear  zones, 
sheeted  zones,  and  "flaws"  (Blatter)  often  result  in  veins  or 
lodes. 

Complexity  of  Faulting. — During  mining  operations  excellent 
and  detailed  instances  of  the  complexity  of  faulting  are  often 
found.  Normal  and  reverse  faults  may  occur  in  close  proximity . 
A  fault  consists  more  frequently  of  a  series  of  closely  spaced 
breaks  than  of  a  single  fracture.  Displacement  occurs  usually 
along  each  of  these  breaks,  the  result  being  a  distortion  of  the 
deposit  within  the  faulted  zone. 


Fraction_No_[  shaft- 


FIG.  33. — Horizontal  plan  showing  faulted  vein,  Tonopah,  Nevada.     Scale 
50  feet  to  one  inch.     After  J.  E.  Spurr,  U.  S.  Geol.  Survey. 

Fig.  32  shows  a  case  of  complicated  normal  faulting  from 
the  Berlin  vein,  Nevada.1  Besides  the  faults  indicated  there 
are  a  great  number  of  other  dislocations  with  horizontal  dis- 
placement. The  deposit  is  a  filled  quartz  vein,  2  to  3  feet  wide, 
carrying  2  per  cent,  of  sulphides  with  silver  and  gold. 

The  great  complications  ensuing  where  faulting  takes  place 
along  two  intersecting  fault  systems  have  been  described  by 
Spurr2  in  his  report  on  the  Tonopah  district,  Nevada.  The 

1  Ellsworth  Daggett,   The  extraordinary  faulting  at  the  Berlin  mine, 
Eng.  and  Min.  Jour.,  Mar.  30,  1907. 

2  Prof.  Paper,  42,  U.  S.  Geol.  Survey,  1905. 


136  MINERAL  DEPOSITS 

result  of  such  structures  is  likely  to  be  a  zigzag  distribution 
of  the  fragments  of  the  faulted  vein  with  an  average  movement 
determined  by  the  two  components.  Repeated  small  disloca- 
tions practically  result  in  a  deflection  of  the  vein  (Fig.  33). 

Overthrusts  of  great  magnitude,  such  as  are  found  in  the  Alps, 
may  have  had  most  important  results  as  to  the  continuation  in 
depth  of  ore  deposits.  As  these  dislocations  may  be  measured 
in  miles,  it  follows  that  whole  groups  of  deposits  contained  in 
the  overthrust  portion  of  the  strata  may  have  been  cut  off 
entirely  from  their  continuation  in  depth.1  The  relations  of 
ore  deposits  to  dynamic  metamorphism  is  described  in  Chapter 
XXX. 

1  B.  Granigg,  Ueber  die  Erzfuehrung  der  Ostalpen.     Leoben,  1913. 


CHAPTER  X 
OPENINGS   IN   ROCKS 

Chemical  processes  and  alteration  in  general  may  go  on  in  a 
rock  without  cavities  other  than  pore  space  and  capillary  or 
sub-capillary  openings.  Such  processes  are,  however,  metamor- 
phic  rather  than  metasomatic ;  they  simply  effect  a  mineralogical 
rearrangement  without  much  chemical  change;  the  composi- 
tion of  the  rock  remains  constant.  The  formation  of  epigenetic 
mineral  deposits  usually  implies  a  considerable  addition  of  for- 
eign material  by  solutions  and  these  solutions  must  be  guided  to 
the  place  of  deposition  by  open  spaces,  such  as  fissures,  joints, 
or  cracks.  As  a  matter  of  fact  the  great  majority  of  mineral  de^ 
posits  were  formed  where  the  path  of  the  solution  was  prescribed 
by  openings  in  the  rocks  other  than  those  of  ordinary  pore  space. 
After  the  solutions  have  gained  access  to  the  rock  they  may  of 
course  enter  the  pores  and  capillary  openings  and  effect  metaso- 
matic changes.  The  sizes  of  capillary  and  subcapillary  openings 
are  given  on  p.  30. 

The  discussion  which  follows  relates  mainly  to  openings  of 
supercapillary  size.  Such  openings  are  chiefly  found  in  the  zone 
of  fracture  (p.  72).  Few  of  our  mineral  deposits  have  been 
formed  at  depths  much  greater  than  15,000  feet.  Small  openings 
may,  however,  exist  in  hard  rocks  at  a  distance  below  the  surface 
much  greater  than  the  figure  just  indicated  (p.  73).  The  possi- 
bility is,  therefore,  shown  that  solutions  from  great  depths  may 
gain  access  to  the  upper  zone  of  fracture. 

ORIGIN  OF  OPENINGS 

Rock  cavities  may  originate  in  various  ways : 

1.  By  the  Original  Mode  of  Formation  of  the  Rocks. — Many 
volcanic  flows  contain  abundant  gas  pores,  or  blow  holes  produced 
by  the  expansive  force  of  gases  escaping  from  the  magma. 
Zeolites  and  calcite,  sometimes  with  native  copper,  often  accumu- 
late in  these  pores,  and  such  rocks  are  usually  termed  "amygda- 
loids"  and  the  filled  cavities  "amygdules"  (Fig.  34).  Some 

137 


138 


MINERAL  DEPOSITS 


sandstones   and    conglomerates    contain   much   pore   space   in 
which  solutions  may  deposit  ores  or  other  substances. 

2.  By  Solution. — Solution  cavities  are  found  mainly  in  easily 
soluble  rocks,  such  as  limestone,  dolomite,  gypsum,  and  salt. 
Posepny  justly  maintains  that  the  solvent  power  of  water 
suffices  to  produce  long  galleries  or  passages  in  rock  salt  and 
mentions  several  examples.1  Joints  in  limestone  are  often  irregu- 
larly enlarged  by  solution  and  when  subsequently  filled  with  ores 


FIG.  34. — Photomicrograph  of  basalt  showing  blowholes  filled  with 
chlorite,  calcite  and  native  copper.  Black  areas  represent  copper.  After 
Volney  Lewis. 


such  cavities  are  known  as  gash  veins  or  pipe  veins.  Caves  in 
limestone  are  likewise  made  by  atmospheric  water  of  the  upper 
circulation,  containing  dissolved  carbon  dioxide.  Such  caves  are 
generally  formed  above  the  ground-water  level  in  the  zone  of 
oxidation,  though  cases  are  known  which  suggest  that  the  process 


1  Genesis  of  ore  deposits,  1902,  p.  20. 
York. 


Pub.  by  Am.  Inst.  Min.  Eng.,  New 


OPENINGS  IN  ROCKS  139 

can  go  on  also  below  this  level.  Caves  occur  in  all  limestone 
regions  and  are  sometimes  of  enormous  extent;  the  Mammoth 
Cave  of  Kentucky  has  passages  more  than  40  miles  in  length  and 
has  been  formed  by  the  removal  of  millions  of  cubic  yards  of  rock. 
The  extent  of  caves  is  generally  determined  by  faults  and  disloca- 
tions, and  rock  openings  on  a  smaller  scale  are  usually  determined 
by  the  prevailing  joint  systems.  The  breaking  in  of  caves  near 
the  surface  produces  the  "sink-holes"  so  characteristic  of  certain 
limestone  plateaus.  Both  caves  and  sink-holes  have  a  certain 
importance  in  the  origin  of  the  class  of  zinc-lead  deposits  common 
to  many  limestone  areas,  and  caves  of  dissolution  in  the  oxidized 
part  of  ore  deposits  in  limestone  are  sometimes  the  receptacles 
for  a  great  variety  of  secondary  minerals.  The  floors  of  caves 
are  usually  covered  with  red  "cave  earth,"  a  residual  deposit  of 
silica,  kaolin,  limonite,  etc.,  derived  from  the  less  soluble  con- 
stituents of  the  limestone.  Deposits  of  bat  guano  and  nitrates 
are  sometimes  found  in  caves.  Small  solution  cavities  are  often 
found  in  more  resistant  rocks  that  have  been  exposed  to  hot 
solutions  of  great  solvent  power. 

3.  By  Fractures  of  Various  Modes  of  Origin,  (a)  Contraction 
Joints  Produced  by  Tensile  Stress  in  Igneous  Rocks. — When  mag- 
mas congeal  to  igneous  rocks  tensile  stresses  which  result  in  fis- 
sures and  joints  are  developed.  This  is  best  exemplified  in  effu- 
sive rocks,  which  often  show  regular  columnar  structure  and 
which  are  always  full  of  irregular  joints  and  cracks.  No  doubt 
these  open  spaces  may  guide  metal-bearing  solutions.  In  the 
literature  many  authors  attribute  fissure  veins  in  effusive  rocks  to 
contraction,  but  usually  without  sufficient  reason.  The  tensile 
stresses  cannot  produce  long  fissures  with  regular  strike  and  dip. 

According  to  the  views  of  many  geologists,  smaller  irregular 
veins  in  dikes  or  other  -intrusive  rock  masses  fill  contraction 
fissures.  This  explanation  has  been  advanced  for  the  hori- 
zontal tin-bearing  joints  in  the  Zinnwald  granite,  Saxony,  and 
for  other  similar  "stockworks;"  also  for  the  so-called  "lad- 
der veins,"  which  are  short  transverse  fissures  in  dikes,  usually 
extending  only  from  wall  to  wall.  Well-known  examples  of  this 
kind  in  Telemarken,  Norway,1  have  been  described  by  Vogt; 
in  Victoria,  Australia,2  by  Whitelaw;  and  at  Beresowsk,  in  the 

1  Zur  Klassification  der  Erzvorkommen,  Zeitschr.  prakt.  Geol.,  1895,  p. 
149. 

2  Mem.,  Geol.  Survey  Victoria,  vol.  3,  1905,  p.  11. 


140 


MINERAL  DEPOSITS 


Ural  Mountains,1  by  Rose,  Helmhacker,  Karpinsky,  Posepny. 
and  Purington.  In  places,  however,  the  transverse  fissures  may 
extend  over  the  contact  into  the  wall  rock  or  correspond  to  the 
general  joint  systems  of  the  vicinity,  a  fact  which  throws  some 
doubt  on  the  correctness  of  the  explanation  given  (Fig.  35). 
(6)  Contraction  Joints  by  Shrinking  of  Limestone  when  Changed 
to  Dolomite. — Dolomite  is  not  uncommonly  formed  near  certain 
metal  deposits  and  it  is  possible  that  this  process  when  carried 
on  by  rapidly  moving  solutions  and  in  comparatively  free  space 
may  result  in  openings  suitable  as  receptacles  for  ore  minerals. 


FIG.  35. — Section  of  Morning  Star  dyke,  Woods  Point,  Victoria,  showing 
ladder  veins.     After   0.    A.   L.    Whitelaw. 

(c)  Expansion  Joints  Produced  by  Increase  of  Rock  Volume. — 
Peridotite  upon  change  to  serpentine  near  the  surface  and  near 
fissures  is  believed  to  increase  its  volume  greatly  and  such  serpen- 
tine often  breaks  into  smooth  fragments.  Extreme  irregularity  is 
a  characteristic  of  all  expansion  joints  and  they  are  of  little 
importance  in  ore  deposition. 

1  Guide,  Seventh  Int.  Geol.  Congress,  1897,  p.  42. 
F.  Posepny,  Archiv  fur  prakt.  Geologie,  vol.  2,  1895,  p.  499. 
C.  W.  Purington,  Eng.  and  Min.  Jour.,  June  13,  1903. 


OPENINGS  IN  ROCKS 


141 


(d)  Fissures  Produced  by  Torsional  Stress. — The  celebrated  ex- 
periment by  DaubreV  carried  out  by  twisting  a  thick  glass  plate 
has  shown  that  torsional  stress  may  result  in  several  systems  of 
long  and  radiating  fissures.  This  experiment  has  frequently  been 
cited  by  geologists  to  explain  divergent  vein  systems,  but  G.  F. 
Becker  has  pointed  out  that  such  fissures  do  not  follow  approxi- 
mate planes,  like  fissure  veins,  but  are  decidedly  curved  and 
warped.  Becker2  regards  torsional  stress  as  a  system  of  tensions. 


FIG.  36. — Section  through  a  saddle  reef,  Bendigo,  Victoria.     A,  Sandstone; 
B,  shaly  sandstone;  C,  gold-bearing  quartz.     After  T.  A.  Richard. 

(e)  Openings  Produced  by  Folding  of  Sedimentary  Rocks. — The 
bedding  planes  of  sediments  are  primary  structures  which  often 
serve  as  ducts  for  metal-bearing  solutions.  Better  passageways 
for  such  solutions  are  provided  when  a  series  of  sediments  of  un- 
equal resistance  is  folded.  A  sandstone,  for  instance,  will  accom- 
modate itself  to  bending  with  difficulty  and  will  easily  break  at 

1  Etudes  synthetiques  de  geologic  experimentale,  Paris,  1879,  p.  316. 

2  The  torsional   theory  of  joints,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  24, 
1894,  pp.  130-138. 


142  MINERAL  DEPOSITS 

anticlines  or  synclines,  whereas  softer  shales  will  bend  without 
breaking;  the  same  process  may  cause  a  slipping  between  the  vari- 
ous members.  Such  tensional  stresses  may  then  easily  produce 
open  cavities.  The  quartz-filled  so-called  "saddle  reefs"  of  the 
gold  mines  of  Bendigo  and  other  places  in  Victoria  are  believed 
to  have  been  formed  in  this  maner  by  tensile  stresses,  but  they 
are  also  accompanied  by  irregular  masses  or  "makes"  of  quartz 
which  fill  spaces  of  discission  across  the  beds  (Figs.  13  and  36). 

(f)  Openings  Produced  by  Shearing  Stress  under  the  Influence 
of  Gravity. — In  many  disturbed  regions  the  rocks  are  broken  by 
normal  faults  along  which  the  various  blocks  have  settled  down 
under  the  influence  of  gravity.     Such  normal  faulting  is  espe- 
cially characteristic  of  regions  which  do  not  bear  evidence  of 
strong  compressive  stress.     Step-faulting  is  common  and  friction 
breccias  and  crushed  zones  frequently  follow  the  faults;  the  open 
spaces,  more  or  less  continuous,  offer  good  paths  for  the  circula- 
tion of  water  if  above  the  "level  of  discharge;"  the  fault  planes 
are  often  long  and  regular.     But  in  spite  of  all  this,  mineral  de- 
posits, except  some  of  purely  surface  origin,  are  not  common 
along  such  faults.     At  Clifton,  Arizona,  for  instance,  faults  are 
abundant,  but  the  copper  deposits  do  not  ordinarily  appear  in 
them.     There  are  exceptions,  however.     At  Creede,  Colorado,  a 
gold-silver  vein  occupies  an  important  fault  fissure,  and  similar 
cases  are  known  from  the  silver-lead  veins  of  the  Harz  Mountains 
in  Germany. 

In  volcanic  regions,  such  as  Silverton  and  Cripple  Creek 
(Fig.  37),  in  Colorado,  systems  of  nearly  vertical  fissure  veins 
contain  rich  deposits.  They  have  obviously  little  connection 
with  the  main  structural  features  of  the  country,  the  dislocations 
are  usually  small,  and  the  veins  were  formed  shortly  after  the 
close  of  volcanic  activity.  F.  L.  Ransome1  believes  that  these 
fissure  systems  were  generated  by  stresses  resulting  from  slight 
vertical  movements  or  settling,  following  an  enormous  transfer 
of  volcanic  material  from  an  intratelluric  to  a  superficial  position. 
Vertical  upthrusts  of  underlying  magmas  may  have  caused 
faulting  accompanying  or  following  vein  formation.2 

(g)  Openings  Produced  by  Compressive  Stress.- — In  contrast  to 
recently  congealed  lavas,  the  rocks  which  have  formerly  been  far 

1  F.  L.  Ransome,  Bull.  182,  U.  S.  Geol.  Survey,  1901,  p.  66. 

2  J.  E.  Spun-,    Relation  of  ore  deposition  to  faulting,  Econ.  Geol.,  vol. 
11,  1916,  pp.  601-622. 


OPENINGS  IN  ROCKS 


143 


below  the  surface  of  the  earth  but  which  have  been  exposed  by 
erosion  are  usually  traversed  by  more  or  less  regular  joint  systems, 
persistent  over  large  areas.  While  some  of  these  joint  systems 
may  be  caused  by  the  inherent  texture  of  the  rock,  they  are  in 


Scale  of  Feet 
1000      2000      3000 


FIG.  37.— Plan  of  the  principal  veins  of  the  Cripple  Creek  district,  Colo- 
rado, showing  a  roughly  radial  distribution.  G,  granite  and  gneiss;  V, 
tertiary  volcanic  rocks.  After  Lindgren  and  Ransome,  U.  S.  Geol.  Survey. 

most  cases  the  effect  of  compressive  stress.  Closely  spaced  joint 
systems  form  transitions  into  slaty  cleavage,  and  recrystallization 
of  minerals  takes  place  by  preference  along  these  planes.  In  ex- 


144 


MINERAL  DEPOSITS 


treme  cases  fissility  or  cleavage  in  very  thin  laminae  develops. 
Joints  and  cleavage  present  narrow  paths  for  mineralizing  solu- 
tions and  ore  deposits  are  often  determined  by  their  direction. 
There  are  all  transitions  from  joints  to  fissures  along  which 
perceptible  movement  has  occurred.  In  many  districts  the 
fissures  which  have  received  the  ores  are  identical  in  strike  and 
dip  with  the  joint  systems  of  the  country  rock.  A  common 
condition  is  that  two  sets  of  veins  and  joints  occur  which  have 
the  same  strike,  but  dip  in  opposite  directions  (Fig.  38).  Such 
vein  systems  are  termed  conjugated  fractures.  The  explana- 
tion of  such  joints,  fissures,  and  occasionally  accompanying 
schistose  structure  is  furnished  by  certain  experiments  by  Dau- 
breV  and  by  the  mathematical  deductions  of  G.  F.  Becker.2 


FIG.  38. — Vertical  section  of  a  conjugated  system  of  fractures. 


These  show  that  compression  develops  two  systems  of  fractures 
along  the  planes  of  maximum  shear;  these  shearing  planes  are 
inclined  to  the  direction  of  maximum  stress.  The  accompanying 
dislocations  will  largely  be  reverse  faults  in  which  the  hanging  wall 
has  relatively  moved  upward.  In  Daubree's  experiment  on  a 
mass  of  beeswax  and  resin  two  conjugated  systems  of  joints  and 
fissures  were  formed,  making  an  angle  of  about  45°  with  the 
line  of  pressure;  similar  results  have  been  obtained  by  testing 
cubes  of  building  stones.  If  the  stress  is  not  exerted  hori- 
zontally the  dip  of  the  veins  will  be  correspondingly  affected. 
At  Grass  Valley,  California,  and  in  many  other  districts  there 
are  two  such  conjugated  systems  of  fissures  which  have  been 
filled  with  ore. 

1  fitudes  synthetiques  de  geologie  experimentale,  Paris,  1879,  p.  316. 

2  Finite  homogeneous  strain,   etc.,   Bull.,    Geol.   Soc.    America,   vol.   4, 
1893,  p.  13. 

C.  K.  Leith,  Structural  Geology,  New  York,  1913,  p.  16. 


OPENINGS  IN  ROCKS  145 

In  the  locality  just  mentioned  the  majority  of  the  dislocations 
are  small,  but  tangential  stresses  sometimes  produce  great  dis- 
locations. The  Mother  Lode  of  California,  a  vein  system  nearly 
100  miles  in  length,  is  believed  to  represent  a  reverse  fault  or 
system  of  faults  with  considerable  throw. 

When  rocks  are  recrystallized  in  the  deeper  zones  of  the  earth's 
crust  they  may  become  so  plastic  that  deformation  by  rupture 
cannot  take  place.  The  growth  of  crystals  then  probably  takes 
place  predominantly  in  a  plane  perpendicular  to  the  stress  and 
a  close  schistose  structure  like  that  in  many  gneisses  may  de- 
velop which  offers  scarcely  any  interstitial  space  available  for 
the  circulation  of  solutions. 

If  the  fissures  were  perfect  planes  it  would  be  difficult  to  con- 
ceive of  open  spaces  along  them  except  by  tensional  stresses  pull- 
ing the  walls  apart;  but  as  they  are  not,  movement  along  them 
tends  to  produce  a  series  of  openings,  alternating  with  numerous 
touching  points.  As  a  matter  of  fact  the  mode  of  mineral 
deposition  shows  that  open  spaces  existed  and  that  they  some- 
times were  large,  in  exceptional  cases  even  20  feet  or  more  in 
width.  In  mine  workings  in  hard  rock  old  stopes  frequently 
remain  open  for  an  indefinite  length  of  time,  and  it  is  probable 
that  such  large  open  spaces  may  exist  down  to  a  depth  of  at 
least  several  thousand  feet.  Moreover,  it  is  to  be  remem- 
bered that  at  the  time  of  deposition  the  fissures  were  filled  by 
water  under  a  pressure  at  least  equal  to  that  of  the  hydro- 
static column.  The  depositing  solutions  emanating  from  magmas 
under  conditions  of  far  stronger  pressure  may  even  have  made 
way  for  themselves  in  the  manner  of  an  igneous  dike  or  pegma- 
tite vein,  actually  forcing  the  rocks  apart.  Some  of  the  phe- 
nomena of  deep-seated  veins  are  difficult  to  explain  on  any  other 
assumption. 

Gaping  fissures  are  not,  however,  necessary  for  the  circula- 
tion of  solutions.  Water  may  ascend  along  a  number  of 
closely  spaced  fissures — usually  called  a  sheeted  zone — in 
which  very  little  open  space  exists.  But  in  this  case  mineral 
deposition  is  usually  effected  by  replacement.  The  solutions 
are  forced  into  the  adjoining  rock  and  transform  its  minerals 
into  ore. 

The  stresses  set  up  in  a  mass  consisting  of  various  rocks  are 
extremely  complex  and  it  may  only  be  possible  to  ascertain 
the  dominant  mode  of  fracturing. 


146  MINERAL  DEPOSITS 

Force  of  Crystallization. — Minerals  crystallizing  from  solutions 
exert  a  certain  pressure  on  the  walls  which  confine  them.1 
Many  geologists  have  held  that  this  force  is  sufficient  to  enlarge 
cavities  along  fractures  and  thus  make  room  for  mineral  deposits. 
There  is  strong  evidence  in  the  structure  and  texture  of  veins 
which  is  unfavorable  to  such  a  view,  except  where  conditions  of 
light  load  prevail  as  near  the  surface  or  near  open  spaces.  Some 
curious  phenomena  in  regard  to  inclusions  of  rocks  in  veins  may 
find  their  explanation  by  the  action  of  this  force,  for  in  fissures 
filled  with  solutions  a  comparatively  slight  force  might  suffice  to 
detach  fragments  from  the  walls. 

1  G.  F.  Becker  and  A.  L.  Day,  The  linear  force  of  growing  crystals, 
Proc.,  Washington  Acad.  Sci.,  vol.  7,  1905,  pp.  282-288. 

S.  Taber,  Pressure  phenomena  accompanying  the  growth  of  crystals, 
Proc.  Nat.  Acad.  Sci.,  vol.  3,  1917,  pp.  297-302. 

S.  Taber,  Am.  Jour.  Sci.,  4th  ser.,  vol.  41,  1916,  p.  535. 


CHAPTER  XI 

THE  FORM  AND  STRUCTURE  OF  MINERAL 
DEPOSITS 

The  form  of  ore  deposits  is  always  important,  for  the  mining 
methods  used  for  a  body  of  irregular  outline  must,  for  instance, 
be  very  different  from  those  for  a  tabular  vein.  In  the  great 
majority  of  deposits  the  form  is  rudely  tabular,  for  they  usually 
follow  the  planes  of  dislocations  or  tabular  dikes  or  the  bedding 
of  sedimentary  rocks.  Great  weight  was  formerly"  attached  to 
the  form,  both  in  empirical  classification  and  in  genetic  interpre- 
tation. At  present  the  tendency  is  to  regard  form  as  largely  acci- 
dental, and  to  place  more  emphasis  on  the  mineral  association. 

A  convenient  and  fundamental  though  not  strictly  genetic 
classification  divides  mineral  deposits  into  syngenetic,  or  those 
formed  by  processes  similar  to  those  which  have  formed  the  en- 
closing rock  and  in  general  simultaneously  with  it;  and  epigenetic, 
or  those  introduced  into  a  pre-existing  rock. 

Syngenetic  Deposits. — The  syngenetic  deposits  include  the  mag- 
matic  segregations  or  accumulations  of  useful  minerals  formed 
by  processes  of  differentiation  in  magmas,  generally  at  a  con- 
siderable depth  below  the  surface.  Their  form  may  be  wholly 
irregular  or  roughly  spherical,  but  more  often  they  are  rudely  tabu- 
lar or  lenticular,  and  they  are  usually  connected  by  transitions 
with  the  surrounding  rocks.  They  are  either  wholly  enclosed  in 
the  igneous  mass,  or  lie  along  its  margins,  or,  in  some  cases,  form 
dikes  or  offshoots  from  a  deep-seated  reservoir.  The  last  class 
of  ores  may  be  called  epigenetic  with  reference  to  the  rocks 
incasing  the  dikes.  The  width  and  thickness  of  these  deposits 
may  range  from  a  few  inches  to  several  hundred  feet,  and  in 
rare  cases,  their  length  may  exceed  one  mile.  Masses  of  chromite 
in  peridotite  or  titanic  iron  ore  in  anorthosite,  furnish  examples 
of  this  type. 

The  syngenetic  deposits  also  include  sedimentary  beds;  they 
have,  as  a  rule,  a  tabular  or  sheet-like  form;  they  are  horizontal 

147 


148  MINERAL  DEPOSITS 

if  not  disturbed,  but  are  frequently  folded  and  faulted.  Parallel 
to  their  bedding  their  extent  may  be  counted  by  miles,  as  in  the 
case  of  the  Clinton  hematite  ores  of  the  Appalachian  region,  or  the 
French  and  German  limonite  beds;  nevertheless,  each  bed  usu- 
ally thins  out  in  wedge-shaped  form  and  may  be  replaced  by 
others  at  a  slightly  different  horizon.  In  deposits  of  metallic 
ores  the  thickness  is  rarely  more  than  20  feet  and  this  may  include 
intercalated  beds  of  barren  material.  Coal  beds,  especially  those 
of  lignite,  or  brown  coal,  may  attain  a  thickness  of  100  feet  or 
more.  Beds  of  rock  salt,  anhydrite,  and  gypsum  are  in  some 
cases  several  hundred  feet  thick.  In  all  sedimentary  deposits 
displacements  and  folding  may  locally  produce  an  appearance 
of  great  thickness.  In  plastic  material  like  rock  salt  such 
deformation  is  especially  effective. 

Epigenetic  Deposits. — The  epigenetic  deposits  have  various 
forms,  but  among  those  which  follow  fissures  the  tabular  or 
sheet-like  form  is  most  common.  Deposits  concentrated  in  the 
zone  of  weathering  are  often  extremely  irregular  and  of  limited 
extent,  and  several  of  them  are  usually  found  in  close  proximity. 
Some  hematite  ores,  like  those  of  the  Mayari  district  in  Cuba, 
which  are  developed  by  the  weathering  of  serpentine,  may  form 
superficial  sheets  of  great  extent. 

Replacement  deposits  in  limestone  are  extremely  irregular, 
although  their  form  as  a  whole  is  often  dependent  upon  the 
bedding,  the  fissuring,  or  the  contact  with  other  rocks.  They 
are  seldom  large,  but  in  a  few  cases,  like  the  galena  deposits 
in  southeastern  Missouri  or  the  zinc  blende  deposits  in  the  Joplin 
region  of  the  same  State,  they  may  be  followed  at  a  general 
horizon  for  several  miles. 

The  ore  deposits  in  metamorphic  rocks  which  have  undergone 
strong  mechanical  deformation  and  chemical  changes  usually  as- 
sume lenticular  form,  and  the  occurrence  of  successively  overlap- 
ping lenses  is  particularly  characteristic.  In  these  deposits  a 
steep  dip  is  a  common  feature,-  but  the  main  trend  of  the  ore-body 
in  the  plane  of  its  strike  is  usually  not  in  the  direction  of  the 
dip. 

The  strike  of  a  tabular  or  lenticular  deposit  is  the  direction  of 
a  horizontal  line  in  the  plane  of  the  deposit,  measured  with 
reference  to  a  meridian. 

The  dip  is  measured  by  the  vertical  angle  between  a  horizontal 
plane  and  the  plane  of  the  deposit.  Complementary  to  the  dip  is 


FORM  AND  STRUCTURE  OF  MINERAL  DEPOSITS     149 

the  hade  or  underlie,  which  is  measured  by  the  angle  between  the 
vertical  and  the  plane  of  the  deposit. 

The  plunge1  (Fig.  39)  of  an  ore-body  is  the  vertical  angle 
between  a  horizontal  plane  and  the  line  of  maximum  elongation 
of  the  body.  In  lenticular  ore-bodies  in  metamorphic  rocks 
which  have  undergone  strong  mechanical  deformation,  the  plunge 
is  an  important  factor,  and  often  it  is  determined  by  the  direction 
of  the  cleavage  or  schistosity.  In  fissure  veins  the  pitch  of 
the  ore  shoot  is  usually  defined  as  the  angle  between  its  axis 
and  the  strike  of  the  vein,  and  it  is  measured  on  the  plane  of 
the  vein2  (p.  184). 


FIG.  39. — Stereogram  illustrating  strike,  dip,  pitch  and 
plunge   of   an   ore-body. 


Spacial  Relations  of  Veins. — Veins  are  tabular  or  sheet- like 
masses  of  minerals  occupying  or  following  a  fracture  or  a  set  of 
fractures  in  the  enclosing  rock;  they  have  been  formed  later  than 
the  country  rock  and  the  fractures,  either  by  filling  of  the  open 
spaces  or  by  partial  or  complete  replacement  of  the  adjoining 
rock,  or  most  commonly  by  both  of  these  processes  combined. 

Such  alteration  or  replacement  does  not  ordinarily  extend 
far  from  the  fissure.  In  regions  where  the  vein-forming  solu- 

1  Called  "pitch"  or  "rake"  by  many  authors. 

2  See  discussion  in   Trans.,  Am.  Inst.   Min.  Eng.,  vol.  39,   1908,  pp. 
898-916. 


150 


MINERAL  DEPOSITS 


tions  have  acted  with  unusual  intensity  a  partial  alteration  may 
extend  from  the  deposit  over  considerable  areas. 

No  sharp  distinction  can  be  drawn  between  the  filled  veins  and 
replacement  veins.  If  open  spaces  are  available  the  metalliferous 
solutions  which  formed  the  veins  in  most  cases  found  it  easier 
to  deposit  their  load  in  these  spaces  than  to  replace  the  country 
rock.  Quartz  is  more  likely  to  be  deposited  in  the  open  paths, 
and  likewise  most  of  the  heavy  metals,  unless  the  country  rock 
is  one  particularly  adapted  for  replacement,  such  as  limestone. 
Gases  like  carbon  dioxide  and  hydrogen  sulphide  penetrate  the 
wall  rocks  with  ease. 


FIG.  40.— Section  of  Silver  Crown  lode,  Silverton,  Colorado,  showing 
lode  structure,  a,  Andesite;  b,  quartz;  c,  andesite  and  quartz  stringers; 
d,  ore.  After  F.  L.  Ransome,  U.  S.  Geol.  Survey. 

Many  veins  correspond  closely  to  the  old  definition  of  a 
"true  fissure  vein,"  in  which  the  ore  occupies  the  once  open 
spaces  along  the  fracture,  with  some  alteration  spreading  into 
the  wall  rocks.  Of  such  character  are  the  majority  of  the  gold- 
quartz  veins  of  California  and  many  other  occurrences.  When 
the  fissures  are  veiy  small  they  are  referred  to  as  veinlets  or 
seams,  and  all  transitions  to  a  slight  mineralization  of  joint 
planes  are  found.  The  walls  may  be  smooth  and  separated 
from  the  vein  material  by  a  clay  gouge  or  the  filling  may  closely 


FORM  AND  STRUCTURE  OF  MINERAL  DEPOSITS     151 

adhere  to  the  country  rock.     In  the  latter  case  the  vein  is  said 
to  be  frozen  to  the  walls. 

Instead  of  a  single  break  we  may  have  a  fracture  consisting  of  a 
number  of  approximately  parallel  fissures,  irregularly  connected 
and  spaced  over  a  considerable  width,  which  may  attain  100  feet 
or  even  several  hundred  feet.  These  large  fracture  zones,  when 


FIG.  41. — Map   showing  veins   of   Central  City,  Colorado   and  vicinity. 
After  E.  S.  Bastin,  U.  S.  Geol.  Survey. 

filled  with  ore  and  partially  replaced  country  rock,  are  called 
composite  veins  or  lodes  (Fig.  40).  The  Comstock  lode  in  Nevada 
illustrates  this  occurrence;  its  width  in  places  amounts  to  several 
hundred  feet. 

Lodes  often  contain  two  systems  of  fractures,  intersecting  at 
an  acute  angle,  as  shown  roughly  on  Fig.  40.     This  is  sometimes 


152 


MINERAL  DEPOSITS 


referred  to  as  hammock  structure.  A  number  of  adjacent  parallel 
veins  are  called  a  vein  system.  If  connected  by  diagonal  veins 
the  term  linked  veins  (Fig.  41)  is  used. 

When  the  fractures  are  closely  spaced  and  parallel  we  speak  of  a 
sheeted  zone  or  a  shear  zone  (Fig.  42).  Many  of  the  Cripple  Creek 
veins  form  good  illustrations  of  this  mode  of  occurrence.  The 
width  of  a  sheeted  and  mineralized  zone  is  rarely  over  50  feet 
and  ordinarily  much  less. 

A  mass  of  rock  irregularly  fractured  in  various  directions  by 
short  fissures  along  which  mineralization  has  spread  is  called  a 


FIG.  42. — Section  of  the  Howard  vein,  Cripple  Creek,  Colorado,  showing 
a  sheeted  zone.  Ore  follows  the  close  sheeting  in  the  center.  Scale,  1  inch 
equals  13  feet.  After  Lindgren  and  Ransome,  U.  S.  Geol.  Survey. 

stockwork.  Gold-quartz  deposits  sometimes  assume  this  form; 
each  seam  in  the  several  joint  systems  intersecting  the  rock  may 
contain  a  thin  but  often  strongly  auriferous  sheet  of  quartz; 
the  mass  may  be  mined  as  a  whole,  furnishing  low-grade  ore.  In 
deeply  weathered  regions  the  upper  parts  of  such  deposits  may 
be  sufficiently  disintegrated  to  be  washed  by  the  hydraulic 
method.  In  California  such  mines  are  called /'seam  diggings." 

A  shattered  zone  cemented  by  a  network  of  small  non-persist- 
ent veins  is  called  a  stringer  lead  or  stringer  lode. 

Sometimes  ore  deposits  are  wholly  irregular  brecciated  masses, 
the  ores  filling  the  interstices  between  the  fragments.  Again,  the 


FORM  AND  STRUCTURE  OF  MINERAL  DEPOSITS     153 

breccia  may  be  localized  at  the  intersection  of  two  fractures  and  a 
pipe-like  deposit  will  be  formed,  the  ore  cementing  the  frag- 
ments. Or  again,  ore  deposition  may  have  proceeded  in  a 
volcanic  vent  filled  with  fragments  of  rocks  due  to  explosive 
action.  Of  such  character  was  the  celebrated  Bassick  deposit 
in  Custer  County,  Colorado. 


Amphibolite 


Quartz 


Fio.  43. — Vertical  section  of  Schlegel milch  quartz  vein,  South  Carolina, 
showing  lenticular  vein  structure  in  schist  with  offsets  along  joint-planes. 
After  L.  C.  Graton,  U.  S.  Geol.  Survey. 


Brecciation,  shattering,  and  mineralization  often  follow  lines  of 
weakness  along  dikes;  in  such  cases,  illustrated  by  the  Douglas 
Island  mines  in  southern  Alaska,  where  a  dike  of  diorite  intrudes 
metamorphic  clay  slates,  the  mineralized  dike  is  often  referred  to 
as  a  lode. 


154 


MINERAL  DEPOSITS 


Ladder  veins  are  deposits  filling  short  transverse  fissures 
sometimes  occurring  in  dikes  of  intrusive  rocks  (see  Fig.  35). 

Lenticular  veins  (Fig.  43)  are  confined  mainly  to  metamorphic 
schists  and  their  form  is  sometimes  caused  by  deformation  of  an 
older  deposit;  or  again  the  lenticular  shape  may  be  due  to  stresses 


FIG.  44. — Section  of  Snowstorm  bed-vein,  Idaho.     After  F.  L.  Ransome, 
U.  S.  Geol.  Survey. 


FIG.  45. — Vertical  section  of  gash  veins  filled  with  galena  (black),  or  with 
pyrite,  zinc  blende,  and  galena  in  order  of  deposition.  Drusy  cavities  in 
center.  Lead  mines  of  Wisconsin.  After  T.  C.  Chamberlin. 

causing  bulging  of  the  schistose  layers.  It  is  common  to  find  a 
number  of  short  lenses  of  gold-bearing  quartz,  for  instance,  scat- 
tered along  a  certain  line  or  zone.  Their  ends  sometimes  overlap. 

Bed  veins  follow  the  bedding  planes  in  sedimentary  rocks 
(Fig.  44). 

Gash  veins  are  deposits  filling  non-persistent  openings  that  are 
of  fair  width  but  soon  cease  when  followed  along  strike  or  dip; 


FORM  AND  STRUCTURE  OF  MINERAL  DEPOSITS     155 


156 


MINERAL  DEPOSITS 


they  are  particularly  characteristic  of  deposits  of  galena  and  zinc 
blende  in  limestone  and  are  believed  to  have  been  opened  by 
tensional  stress,  often  aided  by  solution. 

Where  soft  sedimentary  beds  have  been  folded  and  crushed, 
irregular  open  spaces  are  more  likely  to  result  than  well-defined 
straight  fissures.  In  such  rocks  ores  may  be  found  in  the  spaces 


LEGEND 

Quartz  Porphyrjr  Bike. 
Anaconda  Vein  System 
Blue  Vein  Fault  System 
Steward  Veint  Fault }  System 
Karue  fault 
Middle  Fault 


FIG.  47. — N.-S.  section  across   Butte  district,  showing  structure  and  ore 
zones.     After  Reno  Sales. 

opened  along  anticlines  and  synclines  or  in  irregular  fractures 
breaking  across  such  folds. 

Veins  and  lodes  rarely  occur  single  but  on  the  contrary  have  a 
tendency  to  cluster  in  vein  systems  such  as  illustrated  in  Figs.  37 
and  41.  In  some  places  may  be  found  several  intersecting  vein 
systems  of  great  complexity  and  differing  ages  and  differing 
mineralization  as,  for  instance,  is  the  case  in  the  great  copper  dis- 


FORM  AND  STRUCTURE  OF  MINERAL  DEPOSITS     157 

trict  of  Butte,  in  Montana,  illustrated  in  plan  and  section  in 
Figs.  46  and  47.  *  The  peculiar  divergent  fractures  at  the 
Leonard  mine  form  what  is  sometimes  called  a  horsetail  structure. 
The  veins  at  Butte  are  moreover  in  many  places  disrupted  by 
later  faults. 

Veins  in  Relation  to  the  Country  Rock. — Veins  crossing  the 
bedding  in  stratified  rocks  are  referred  to  as  cross  veins;  those 
parallel  to  the  stratification  or  schistosity  are  often  called 
bedded  veins  or  bed  veins.  Differences  in  the  texture  and 
hardness  of  the  rocks  traversed  influence  the  form  of  the  vein 
markedly.  In  hard  dikes  crossed  by  the  vein  the  deposit  often 
splits  up  into  stringers,  resuming  its  typical  form  beyond  this 
barrier.  In  fractures  formed  under  light  load  near  the  surface 
there  is  a  great  tendency  to  irregularity  and  brecciation,  espe- 
cially in  the  hanging  wall.  Following  G.  F.  Becker's  proposal 
such  may  be  called  chambered  veins.  In  a  vein  of  strong  dip 
there  will  also  be  a  tendency  for  the  hanging  wall  to  settle  ac- 
companied by  the  development  of  minor  vertical  fissures.  Such 
conditions  were  found,  for  instance,  in  the  Comstock  lode,  Nevada 
(Fig.  165)  and  in  the  El  Oro  mines  of  Mexico  (Fig.  159);  at  both 
places  the  vertical  hanging  wall  veins  were  exceptionally  rich, 
the  richness  being  possibly  caused  by  the  impeded  circulation  of 
the  depositing  waters.  Large  masses  of  country  rock  included 
in  the  vein  material  are  called  horses.  Frequently  the  vein 
follows  a  fissure  along  the  walls  of  a  dike;  the  lamprophyric 
dikes  which  are  the  last  phases  of  batholithic  intrusions  are 
especially  favored  places  for  ore  deposition. 

Clayey  and  soft  rocks  are  most  resistant  to  the  development 
of  regular  fissures;  a  fracture  in  hard  rock  will  suddenly  die  out 
when  encountering  such  material;  many  veins  pinch  immediately 
upon  entering  clay  shales  or  masses  of  clayey  gouge.  One  of  the 
best  examples  of  this  is  furnished  by  the  veins  of  Rico,  Colorado, 
which  do  not  extend  through  the  whole  sedimentary  series  in  that 
district,  but  suddenly  cease  at  a  certain  stratum  of  yielding, 
plastic  rocks,  termed  the  blanket,  under  which  almost  all  the 
ore-bodies  occur.  A  consequence  of  this  peculiarity  of  fissuring 
is  that  in  some  regions  rich  ores  are  often  found  just  below  cer- 
tain horizons  of  shale.  In  southern  New  Mexico  a  persistent 
Devonian  shale  plays  this  part  of  "indicator"  (Fig.  70) 

1  Reno  Sales,  Ore  deposits  of  Butte,  Montana,  Trans.,  Am.  Inst.  Min. 
Eng.,  vol.  46,  1913,  pp.  1-109. 


158  MINERAL  DEPOSITS 

The  vein  solutions  were  arrested  at  this  horizon  and  there 
deposited  their  load. 

When  a  vein  follows  the  contact  between  two  formations,  say 
between  granite  and  andesite,  we  speak  of  it  as  a  contact  vein. 
The  contact  is  usually  caused  by  faulting  movements  in  the  plane 
of  the  fissure,  and  such  veins  are  in  no  wise  different  from 
ordinary  fissure  fillings.  They  should  not  be  confused  with 
contact-metamorphic  deposits,  which  belong  to  a  separate 
class. 

Vein  Walls. — In  a  simple  filled  fissure  vein  we  have  well- 
defined  foot  and  hanging  walls,  which  often  are  smooth  sur- 
faces and  represent  a  single  fissure  opened  by  a  small  or  large 
movement  along  its  slightly  curved  plane.  In  a  replacement  vein 
the  fissures  are  comparatively  tight  and  in  most  cases  appear  to 
have  been  formed  under  stronger  compressive  stress  that  reduced 
the  open  spaces  to  a  minimum.  The  vein-forming  solutions  were 
forced  into  the  country  rock,  and  the  ores  formed  by  replacement 
gradually  merge  into  unaltered  rock.  In  such  cases  we  may 
find  a  single  fissure  plane  with  ore  on  both  sides  and  not  limited 
by  any  well-defined  walls.  The  exact  limits  of  commercial  ore 
can  be  found  only  by  assay  and  are  often  spoken  of  as  "assay 
walls." 

In  a  composite  vein  or  lode  or  in  a  sheeted  zone  there  may 
be  several  smooth  walls  and  if  no  cross-cutting  is  undertaken 
there  is  danger  that  parallel  ore-bodies  separated  by  sheets  of 
country  rock  may  be  overlooked. 

Outcrops. — The  character  of  the  outcrop  of  a  vein,  or  in  fact 
of  any  deposit,  is  determined  by  the  predominant  minerals 
and  by  the  prevailing  surface  conditions.  In  regions  of  long- 
continued  rock  decomposition  and  inactive  erosion,  as,  for  in- 
stance in  some  of  the  Southern  Appalachian  States,  even  the 
most  resistant  outcrops  may  be  reduced  by  weathering  and 
nothing  but  fragments  scattered  over  a  wide  area  may  be  visible 
at  the  surface. 

Under  conditions  of  fairly  active  erosion  veins  with  predomi- 
nant quartz  stand  out  prominently  and  can  be  easily  traced. 

On  the  other  hand,  veins  with  carbonate  gangue  are  likely  to 
weather  more  rapidly  than  the  surrounding  rock,  and  the  deposits 
may  be  indicated  by  little  depressions  or  by  notches  in  the  ridges. 
Where  the  sulphides  are  abundant,  their  oxidation  is  conspicu- 
ously reflected  in  the  outcrops.  Deposits  of  mingled  quartz 


FORM  AND  STRUCTURE  OF  MINERAL  DEPOSITS  159 

and  sulphides  then  form  prominent  outcrops  of  limonite  and 
residual  quartz;  this  is  the  gossan  of  the  Cornish,  the  ironstone 
of  the  Australian,  the  eiserner  Hut  of  the  German,  and  the  colo- 
rados  of  the  Spanish  terminology.  More  details  in  regard  to  the 
weathering  of  ore-deposits  are  given  in  Chapter  XVIII. 

Length  and  Depth  of  Veins. — Where  veins  follow  great  dis- 
locations their  length  may  be  considerable.  One  of  the  more  re- 
cent veins  of  Freiberg,  Saxony,  called  the  Halsbriicker  Spat,  has 
been  followed  for  almost  5  miles.  Some  of  the  lead-bearing 
veins  in  the  Harz  Mountains,  Germany,  are  traceable  for  12 
miles.  Exceptionally  long  single  ore-bearing  fissures  are  found 
in  the  Silverton  quadrangle,  San  Juan  region,  Colorado;  some 
of  them  are  5  miles  long.  Some  of  the  Mother  Lode  veins  in 
California  can  be  traced  for  many  miles.  The  longest  single 
quartz  vein  known  appears  to  be  that  of  the  Pfal,  in  the  Bavarian 
Forest,  which  is  said  to  be  traceable  in  a  straight  line  practically 
without  interruption  for  140  kilometers  through  the  pre-Cam- 
brian  rocks.1  The  quartz  is  said  to  be  barren  of  metals. 

The  great  majority  of  single  ore-bearing  veins  are  short  and 
their  outcrops  can  rarely  be  traced  for  more  than  one  mile;  they 
do  not,  as  a  rule,  occupy  great  dislocations,  but  rather  sub- 
ordinate fissures.  The  great  dislocations  are  formed  during  moun- 
tain building  by  tangential  stresses,  whereas  the  ore-bearing  veins 
are,  as  a  rule,  formed  after  epochs  of  igneous  activity.  In  the 
Coeur  d'Alene  district,  Idaho,  for  instance,  the  rich  galena 
veins  show  little  connection  with  the  principal  structural  faults 
of  the  region  and  were  probably  not  formed  at  the  same  time. 

Veins  do  not  necessarily  continue  to  great  depths.  There 
are  all  kinds  of  fissures,  some  disappearing  within  a  short  distance 
below  the  surface,  others  continuing  down  to  the  greatest  depths 
attained,  or  about  6,000  feet  (Morro  Velho,  Brazil).  Deep 
tunnels  have  been  run  to  intersect  veins  of  favorable  appearance 
on  the  surface  and  have  failed  to  disclose  their  continuation  in 
depth.  There  is  no  definite  relationship  between  depth  and 
length  of  a  fissure,  though  it  is  true  that  fissures  showing  strong 
movement  and  shattering  are  likely  to  continue  to  great  depths. 
The  ore-body  may  be  limited  in  depth,  while  the  barren  fissure 
continues  below  it  as  strong  as  ever. 

Bends  and  curves  in  strike  and  dip  are  common  in  veins,  but 

1  E.  Suess,  Das  Antlitz  der  Erde,  Leipzig,  1883,  vol.  1,  pp.  270-272. 
W.  von  Gumbel,  Geologic  von  Bayern,  Cassel,  1894,  vol.  2,  pp.  461-464. 


160  MINERAL  DEPOSITS 

as  a  rule  a  vein  retains  its  general  angle  of  dip  with  remarkable 
persistence.  The  dip  may  be  at  any  angle,  but  veins  dipping 
from  50°  to  80°  are  most  common.  The  North  Star  vein  at  Grass 
Valley,  California,  is  one  of  the  best  instances  of  a  low-dipping 
vein  of  great  length;  with  a  dip  of  20°  it  has  been  followed  for 
5,000  feet.  Still  natter  veins  are  called  blanket  veins  and  seldom 
are  very  persistent  or  uniform. 


CHAPTER  XII 

THE  TEXTURE  OF  MINERAL  DEPOSITS 
FILLING  AND  REPLACEMENT 

Introduction.— The  ore  minerals  and  gangue  which  make  up 
an  ore  deposit  present  various  types  of  texture.  The  texture 
of  an  ore  is  dependent  upon  many  factors.  Space  available 
for  deposition,  concentration  and  composition  of  the  generating 
solutions,  time,  temperature,  and  pressure — all  are  of  impor- 
tance in  determining  the  primary  texture.  Many  changes  take 
place  in  a  deposit  once  formed;  the  secondary  textures,  so 
far  as  they  are  caused  by  solution  and  redeposition,  are  influ- 
enced by  the  same  factors,  and,  in  addition,  deformation  by 
pressure  plays  a  most  important  role. 

Texture  of  Deposits  of  Igneous  Origin.— The  ores  consolidated 
from  magmas  have  in  general  the  texture  of  igneous  holocrystal- 
line  rock.  The  principal  minerals  comprise  chalcopyrite,  pyrite, 
pyrrhotite,  magnetite,  chromite,  and  ilmenite.  The  texture  is 
ordinarily  coarse  granular,  hypidiomorphic;  the  chalcopyrite  and 
pyrrhotite  are  rarely  crystallized,  but  may  contain  phenocrysts 
of  pyrite  and  magnetite,  both  of  which  are  frequently  developed 
with  crystalline  outlines.  The  ores  may,  of  course,  contain 
phenocrysts  and  anhedrons  of  other  rock-forming  minerals,  par- 
ticularly soda-lime  feldspars,  olivine,  and  pyroxene.  Eutectic 
texture  results  if  the  magma  was  a  eutectic  mixture  from  which 
two  minerals  crystallized  simultaneously  after  the  manner  of 
graphic  granite.  Approximation  at  least  to  such  texture  is 
shown  by  some  intergrowths  of  magnetite  and  apatite. 

If  the  ores  have  been  subjected  to  dynamic  metamorphism, 
granulation  and  metasomatic  development  of  hornblende,  garnet, 
biotite,  and  epidote  in  coarse  or  fine  aggregates  follow  and  the 
ore  may  acquire  schistose  structure. 

Texture  of  Pegmatite  Dikes. — The  pegmatite  dikes  are  believed 
to  have  been  deposited  by  magmatic  solutions  of  great  fluidity 
and  low  temperature  (about  600°  C.).  In  many  cases  the  pegma- 
tites form  transitions  between  igneous  rocks  and  veins  deposited 

161 


1G2  MINERAL  DEPOSITS 

by  hot  solutions.  Their  texture  is  coarsely  crystalline,  often 
drusy,  and  the  minerals  have  a  strong  tendency  to  idiomorphic 
development.  Large  crystals  are  the  rule,  and  sometimes  they 
attain  enormous  dimensions;  crystals  of  spodumene  at  the  Etta 
mine,  South  Dakota,  are  30  feet  or  more  in  length.  Quartz 
crystals  several  feet  long  have  been  observed  in  these  deposits. 
A  rough  tendency  to  crustification  is  often  present,  and  the  walls 
of  the  dikes  are  then  lined  with  crystals  of  feldspar  or  mica. 

Texture  of  Sedimentary  Deposits. — Ores  and  minerals  of 
sedimentary  deposits  are  usually  fine  grained,  and  in  many  cases 
they  have  been  deposited  as  colloids  in  which  subsequent  fine- 
grained crystallization  has  developed.  Coarsely  crystalline, 
allotriomorphic  structure  may  develop  in  deposits  consisting  of 
calcite,  salt,  or  gypsum. 

In  many  cases  the  structure  is  clastic  with  development  of 
new-formed  minerals  between  the  grains.  Newly  formed  quartz, 
if  present,  nearly  always  assumes  a  microcrystalline  or  crypto- 
crystalline  texture.  Subsequent  metamorphism  is  likely  to 
enlarge  the  crystalline  grains  and  result  in  coarser-grained  ores. 

Concretions.1 — Concretions  are  rounded  bodies  of  some 
mineral  aggregate  which  are  often  found  in  shale  and  sandstone. 
Calcite,  silica,  siderite,  pyrolusite,  barite,  pyrite,  marcasite  and 
limonite  are  among  the  minerals  which  most  commonly  form 
concretions.  The  structure  is  often  concentric  or  radial.  In 
some  cases  the  stratification  planes  pass  through  the  concretions, 
while  in  other  cases  they  may  bend  around  them.  These  struc- 
tures are  of  some  economic  importance  as  regards  ores  of  iron 
and  manganese,  especially,  siderite,  limonite  and  pyrolusite. 
They  often  have  a  center  of  a  clastic  grain  or  a  fragment  of  a 
fossil  shell  or  leaf.  The  concretions  result  from  processes  of 
solution  and  precipitation  in  soft  or  semi-consolidated  sediments. 
Accidental  precipitation,  say  of  pyrite  around  decomposing  or- 
ganic material,  may  start  the  action  and  the  laws  of  mass  action 
and  preferred  growth  of  larger  crystals  continue  the  process. 
Concretions  generally  derive  their  substance  from  the  surround- 
ing rock.  Sometimes  the  minerals  simply  fill  pores  and  inter- 
stices; but  in  many  cases  the  original  substance  may  have  been 
removed  by  metasomatic  processes.  Concretions  are  frequently 

1  J.  E.  Todd,  Concretions  and  their  geological  effect,  Bull,  Geol.  Soc. 
Am.,  vol.  14,  1904,  pp.  353-368. 

James  Geikje,  Structural  and  field  geology,  1905,  Chapter  VIII. 


THE  TEXTURE  OF  MINERAL  DEPOSITS        163 

altered  with  volume  changes  and  development  of  cracks  and 
interior  cavities.  When  small,  uniform  and  abundant  they  are 
called  oolites.  The  oolities  usually  result  from  separation  in 
colloidal  solutions,  often  coupled  with  adsorption  of  electrolytes.1 

The  oolitic  texture  is  characteristic  of  many  deposits  of  calcite, 
siderite,  calcium  phosphate,  limonite  and  psilomelane;  pyrite 
rarely  assumes  this  form.  The  oolites  are  often  affected  by 
later  alteration  and  recrystallization. 

Texture  of  Residual  and  Oxidized  Deposits. — In  the  residual 
deposits  of  the  zone  of  oxidation,  the  ore-bodies  are  usually 
very  irregular  in  structure  and  texture.  In  large  part  they 
were  deposited  as  colloids,  which  subsequently  in  part  have 
developed  fine-grained  crystalline  texture. 

Earthy,  clayey  concretionary,  mammillary,  stalactitic,  or  piso- 
litic  textures  are  common,  the  last  being  defined  as  a  coarser 
development  of  the  oolitic  form.  Coarser  crystalline  form  is 
assumed  by  some  minerals  like  calcite,  barite,  zinc  carbonate, 
zinc  silicate,  and  lead  carbonate.  Crustification  or  drusy  struc- 
ture is  common  in  places.  Quartz,  where  developed,  is  usually 
fine-grained  or  cryptocrystalline. 

THE  TEXTURE  OF  EPIGENETIC  DEPOSITS 

Primary  Texture  of  Filled  Deposits. — The  epigenetic  deposits 
are  of  manifold  form  and  origin,  but  the  majority  of  them  result 
from  aqueous  solutions  either  by  filling  of  open  cavities  or  by 
replacement  of  surrounding  rocks.  Precipitation  from  complex 
solutions  in  open  spaces  takes  place  in  a  certain  orderly  succes- 
sion, and  the  deposits  therefore  readily  assume  a  banded  texture ; 
crystallization  is  facilitated  by  the  open  spaces,  but  the  older 
crystals  interrupt  the  development  of  the  products  of  later 
crystallization.  Hence  a  hypidiomorphic  to  panidiomorphic 
texture  is  most  common. 

Banding  by  deposition  is  called  crustification,  a  term  intro- 
duced by  Posepny.  In  many  classes  of  veins,  whether  banded 
or  not,  a  drusy  texture  is  common. 

In  deep-seated  veins  formed  at  a  temperature  but  slightly 
lower  than  that  of  the  pegmatites  the  texture  is  usually  coarsely 
crystalline  and  massive;  sometimes  even  drusy  cavities  are 

1  For  recent  literature  regarding  the  origin  of  oolites  see  Fortschritte  der 
Min.  Krist.,  u.  Petr.  Jena.  1913,  p.  43. 


164  .          MINERAL  DEPOSITS 

lacking.  Delicate  and  repeated  banding  is  absent,  but  a  coarsely 
banded  or  comb  structure  recalling  that  of  the  pegmatite  veins 
is  sometimes  encountered.  It  is  usually  expressed  by  quartz 
crystals  developing  from  the  sides  or  by  metallic  minerals  like 
tourmaline,  wolframite,  or  cassiterite  attached  to  the  walls  of  the 
fissure. 

In  veins  formed  at  intermediate  temperatures  a  coarsely  crys- 
talline 'massive    texture    is  most  common;   combs  and  rough 


FIG.  48. — Thin  section  showing  normal  texture  of  quartz  filling.  Black, 
arsenopyrite ;  remainder,  quartz  with  fluid  inclusions.  Magnified  52  diam- 
eters. Gold  quartz  vein,  Grass  Valley,  California. 

banding  by  deposition  are  by  no  means  unknown,  especially 
where  the  deposit  contains  calcite  or  barite.  In  quartz  veins 
the  filling  appears  to  have  taken  place  rapidly  and  completely, 
so  that  the  resulting  ore  consists  of  an  irregular  massive  mix- 
ture of  quartz  and  sulphides.  That  here  too  the  deposition 
began  from  the  walls  is  indicated  by  some  occurrences  of  par- 
tially filled  veins  which  form  a  loose  aggregate  of  prisms.  Any 


THE  TEXTURE  OF  MINERAL  DEPOSITS        165 

thin  section  of  such  quartz  will  usually  show  long  crystals  of 
earlier  growth  around  which  the  later  quartz  has  been  deposited 
in  large  individuals  (Fig.  48).  Lines  of  inclusions  often  pene- 
trate from  one  grain  into  another.  These  inclusions  consist  of 
aqueous  solutions,  often  with  small  cubes  or  grains  of  trans- 
parent salts  suspended  in  the  liquid.  Inclusions  of  carbon 
dioxide  have  been  reported,  but  are  extremely  scarce.  The 
optical  ^continuity  of  the  crystals  or  grains  is  often  disturbed 
by  a  peculiar  divergent  "flamboyant"  structure  which  appears 
to  be  of  primary  origin,  and  not  caused  by  internal  strains. 

The  sulphides  are  coarsely  crystalline  and  sometimes  roughly 
banded,  parallel  to  the  walls.  Inclusions  of  country  rock  may 
be  surrounded  by  concentric  rings  of  sulphides,  and  a  primary 
brecciated  vein  structure  may  result.  Pyrite  and  arsenopyrite, 
both  among  the  earliest  minerals,  have  a  strong  tendency  to 
crystal  development,  while  galena,  zinc  blende,  chalcopyrite,  and 
tetrahedrite  are  much  less  commonly  found  with  crystal  faces. 

A  banded  structure  sometimes  results  from  the  filling  of 
several  closely  spaced  fissures.  In  quartz  veins  in  fissile  rocks 
a  peculiar  book  structure  may  result  from  numerous  parallel  sheets 
of  slate,  alternating  with  quartz.  It  has  been  thought  that 
this  and  other  features  difficult  to  explain  by  the  assumption  of 
open  cavities  are  due  to  the  opening  of  spaces  by  the  force  of 
crystallization.  Such  views  have  been  expressed  by  E.  Suess, 
W.  O.  Crosby,  E.  J.  Dunn,  S.  Taber,  and  others.  It  is  improb- 
able that  crystallization  could  have  opened  the  cavities.  More 
likely  they  were  supported  by  the  strong  pressure  of  magmatic 
waters.  But  within  such  spaces  a  slight  force  exerted  by  crys- 
tallization could  readily  detach  fragments  of  shale  from  the  walls. 

Stalactites  are  unknown  in  deposits  formed  at  high  or  inter- 
mediate temperature. 

In  veins  formed  at  lower  temperatures  and  comparatively  shallow 
depths  crustified  and  drusy  forms  and  fine  granular  texture  pre- 
dominate. The  quartz  filling  is  usually  fine-grained,  ranging  to 
cryptocrystalline  and  microcrystalline  near  the  surface. 

The  sulphides  are  found  in  small  crystals  or  small  anhedrons; 
large  crystals  of  pyrite,  so  common  elsewhere,  are  rarely  found 
in  these  veins.  On  the  other  hand,  where  calcite,  dolomitic 
carbonates,  rhodochrosite,  fluorite,  or  barite  are  gangue  min- 
erals the  crystals  may  be  much  larger  than  those  found  in  other 
deposits.  An  example  is  furnished  by  the  magnificent  crystals 


166 


MINERAL  DEPOSITS 


of  calcite  at  Joplin,  Missouri,  and  here  galena  also  appears  in 
unusually  large,  well-developed  individuals. 

Symmetrical  and  delicate  crustification  is  often  associated  with 
large  drusy  cavities.  Brecciated  structure  of  primary  origin  is 
common. 

Secondary  Textures  and  Structures  of  Filled  Deposits.— Crush- 
ing and  brecciation  of  the  early  minerals  are  extremely  common; 
indeed,  few  veins  are  entirely  free  from  it.  Repeated  opening 
of  fissures  (Fig.  51)  and  the  deposition  of  new  generations  of 
vein  material  often  take  place  and  the  cementing  ore  may  be 
enriched  at  the  expense  of  the  older  generations. 


FIG.  49. — Specimen  of  quartz  from  Nevada  City,  California,  showing  ribbon 
structure  by  sheeting.    Two-thirds  natural  size. 

A  banded  or  sheeted  structure  often  results  from  the  develop- 
ment of  shear  planes  in  the  old  filling;  examples  of  this  are  seen 
in  many  gold-quartz  veins  of  California  (Figs.  49  and  50). 
Along  these  shear  planes  the  quartz  is  deformed  and  granulated, 
and  gold  may  be  deposited  along  them  by  processes  which  may 
be  called  secondary,  though,  as  a  rule,  they  take  place  shortly 
after  the  vein  formation.  The  shearing  stress  exerted  either 
before  or  after  the  filling  may  affect  the  walls  of  the  vein  and 
render  them  close-jointed  or  even  distinctly  schistose. 


THE  TEXTURE  OF  MINERAL  DEPOSITS        167 


FIG.  50. — Thin  section  of  vein  quartz  from  Nevada  City,  California, 
showing  crushing  and  incipient  ribbon  structure.  Magnified  15  diameters. 
Crossed  nicols. 


FIG.  51. — Cross  section  of  Japan  vein,  Silverton,  Colorado,  showing 
structure  produced  by  repeated  opening  of  original  fissure,  a,  Country 
rock;  6,  quartz;  c,  ore.  After  F.  L.  Ransome,  U.  S.  Geol.  Survey. 


168  MINERAL  DEPOSITS 

In  some  deposits,  especially  those  containing  zeolites,  calcite, 
or  barite,  secondary  replacement  processes  play  an  exten- 
sive part.  A  vein  filled  by  calcite  may  be  replaced  by  quartz, 
which  then  plainly  shows  its  secondary  nature  by  its  hackly  or 
lamellar  texture,  casts  of  cleavage,  pieces  of  calcite,  or  imprints 
of  cleavage  lines.  Such  pseudomorphic  textures  are  sometimes 
accompanied  by  a  marked  enrichment  of  the  metallic  content 
of  the  deposit. 

Metasomatism  in  Mineral  Deposits. — The  nature  of  meta- 
somatism or  replacement  has  already  been  described  on  pages 
26  and  69.  Many  deposits  have  been  formed  by  solutions  con- 
taining various  salts  and  gases  and  capable  of  attacking  certain 
kinds  of  rocks.  Guided  by  fissures  or  other  open  ducts  the 
solutions  deposit  part  of  their  load  in  the  open  supercapillary 
spaces  whenever  supersaturation  takes  place;  thus  is  produced 
the  filling  of  fissures.  As  almost  all  rocks  are  porous  and  as 
the  solutions  are  frequently  under  heavy  pressure  they  will  be 
forced  into  the  rocks  and  will  produce  chemical  and  miner- 
alogical  changes  in  them.  At  the  same  time  the  porous  rock  acts 
undoubtedly  as  a  semi-permeable  membrane  through  which 
various  substances  will  diffuse  at  differing  rates — electrolytes 
and  gases  most  easily,  colloids  and  difficultly  ionized  compounds 
very  slowly.  Thus  any  vein  will  usually  be  accompanied  by  a 
strip  of  altered  country  rock  in  which  the  solutions  have  effected 
certain  metasomatic  changes.  The  minerals  in  the  open  fissures 
will  ordinarily  differ  from  those  formed  in  the  metasomatic  zone. 
We  may  find,  for  instance,  a  quartz  filling  with  various  sulphides 
and  gold,  while  the  minerals  developed  in  the  country  rock  con- 
sist of  pyrite,  sericite  and  calcite  with  little  if  any  gold.  In 
some  cases  no  perceptible  alteration  may  be  observed  in  the 
country  rock.  The  only  difference  between  a  filled  vein  accom- 
panied by  metasomatism  and  a  so-called  replacement  deposit 
is  that  in  the  latter  the  filling  of  the  narrow  open  spaces  is  negli- 
gible and  the  bulk  of  the  ore  has  been  formed  by  metasomatic 
processes. 

Metasomatic  Processes. — In  a  solid  rock  replacement  may  be 
caused  by  many  kinds  of  solutions  the  only  requirement  being  that 
some  or  all  of  the  rock  minerals  must  be  unstable  in  the  penetrat- 
ing fluids.  The  usual  substances,  most  active  in  aqueous  solu- 
tions, are  oxygen,  carbon  dioxide,  sulphuric  acid,  ferric  sulphate, 
hydrogen  sulphide,  alkaline  sulphides,  and  alkaline  carbonates. 


THE  TEXTURE  OF  MINERAL  DEPOSITS        169 

Replacement  may  occur  at  all  temperatures  above  the  freezing 
point  of  the  solution  and  below  the  melting  point  of  the  rock; 
it  is  naturally  most  effective  in  hot  solutions.  Replacement  may 
proceed  at  any  pressure.  It  may  be  effected  by  the  ordinary 
surface  waters,  by  sea  water,  by  hot  ascending  waters  and  by 
magmatic  emanations  whether  gaseous,  fluid  or  above  the  critical 
temperature. 

There  is  no  rock  that  is  proof  against  replacing  natural  solu- 
tions of  some  kind.  Limestone  and  dolomite  are  most  easily 
replaced  and  even  at  ordinary  temperatures,  for  instance,  by  iron 
carbonate  (siderite)  or  by  zinc  carbonate  (smithsonite) .  Granite, 
diorite,  and  in  fact  all  igneous  rocks  are  also  subject  to  replace- 
ment. Even  quartzite,  slate  and  aluminous  shale  may  be  re- 
placed by  other  minerals  though  they  are  more  resistant  than 
others.  Replacement  by  sulphides  may  to  some  extent  take 
place  at  ordinary  temperatures  (for  instance  chalcocite  replacing 
pyrite)  but  large  deposits  of  sulphide  ore  are  usually  formed  by 
hot  solutions. 

Mode  of  Replacement. — As  pointed  out  in  previous  chapters 
replacement  is  effected  by  concentrated  solutions  filling  capillary 
openings  of  extremely  small  size  (sheet  openings  larger  than  0.0001 
mm.,  p.  30),  which  are  just  above  or  below  the  limit  of  micro- 
scopic visibility.  Cases  have  been  noted  when  replacement 
begins  from  a  crack  doubtless  filled  with  a  film  of  solution  and 
connecting  a  series  of  just  visible  fluid  inclusions.  Solution  and 
precipitation  go  on  practically  simultaneously  dependent  upon 
the  constantly  changing  equilibrium,  the  supply  of  solvent  and 
the  facility  of  escape  for  the  dissolved  material.  Two  or  several 
minerals  may  be  dissolved  at  the  same  time  to  make  room  for 
the  new  as  in  the  replacement  of  shale  by  a  pyrite  crystal.  The 
volume  of  the  rock  remains  constant,  held  by  pressure.  The 
moment  a  place  is  available  some  mineral  will  separate  out  from 
the  concentrated  solution.  This  law  fails  to  apply  in  free  crystals 
or  when  rock  pressure  can  be  overcome  by  the  force  of  crystalliza- 
tion, or  when  a  solid  is  replaced  by  a  gel,  or  when  the  solutions 
circulate  so  rapidly  that  there  is  a  strong  balance  in  favor  of 
solution.  As  crystal  grains  develop  they  will  exert  a  different 
amount  of  pressure  in  various  directions  thus  facilitating  solution 
in  the  direction  of  greatest  pressure.  The  development  of 
crystals  in  the  host  mineral  is  a  result  of  this  action. 

The  power  of  crystallization  of  the  different  minerals  varies 


170  MINERAL  DEPOSITS 

greatly,  for  some  are  found  only  as  anhedrons  in  metasomatic 
rocks,  while  others  always  assume  their  crystal  form.  The 
following  list  gives  the  relative  power  of  crystallization  in  solid 
rocks  of  some  minerals,  as  beginning  with  those  of  strongly 
emphasized  individuality:  Rutile,  tourmaline,  staurolite,  arseno- 
pyrite,  pyrite,  magnetite,  barite,  fluorite,  epidote,  pyroxene, 
amphibole,  siderite,  dolomite,  albite,  mica,  galena,  zinc  blende, 
calcite,  quartz,  orthoclase. 

When  a  crystal  has  ceased  to  grow  solution  may  still  continue 
parallel  to  its  faces.  As  no  more  material  for  the  crystal  is  at 
hand  the  voids  are  immediately  filled  by  the  next  precipitate 
available.  Thus  are  explained  the  thin  films  of  quartz  or  calcite 
which  so  frequently  surround  metasomatic  pyrite  crystals. 

When  taking  place  under  the  law  of  constant  volume  replace- 
ment cannot  ordinarily  be  expressed  by  the  simple  chemical 
formulas1  usually  given.  The  reactions  are  likely  to  be  more 
complicated.2 

In  metasomatic  processes  gangue  minerals  like  sericite,  calcite, 
siderite,  barite  and  fluorite  replace  all  silicates.  Ferromagnesian 
silicates  will  be  attacked  first,  then  the  soda-lime  feldspars, 
lastly  orthoclase  and  albite.  The  degree  of  attack  on  quartz 
depends  probably  on  the  amount  of  alkaline  carbonates  in  the 
solution.  All  sulphides  replace  all  silicates  as  well  as  quartz. 
(Fig.  52).  Sulphides  and  sulphosalts  readily  replace  other 
sulphides.  A  succession  common  in  many  ores  is  (1)  pyrite 
(oldest),  (2)  chalcopyrite,  (3)  galena  and  zinc  blende,  (4) 
sulphosalts  (like  tetrahedrite).  Any  of  the  later  minerals  may 
replace  any  of  the  earlier  products  (Fig.  53).  Our  knowledge 
of  these  manifold  replacements  have  been  greatly  increased  by 
the  use  of  the  study  of  polished  sections  in  reflected  light. 
Sulphides  also  easily  replace  gangue  minerals  but  the  latter 

XW.  Lindgren,  Volume  changes  in  metamorphism,  Jour.  Geol.  vol.  26, 
1918,  pp.  542-554. 

2  Smithsonite  often  replaces  calcite  with  preservation  of  structures  in- 
dicating constant  volume.  The  reaction  is  supposed  to  follow  the  formula 
CaCO3  +  ZnSO4  =  CaS04  +  ZnCO3,  both  ZnSO4  and  CaSO4  being  water 
soluble  salts.  One  cubic  centimeter  of  calcite  contains  1.192  milligrams 
CO 2  and  1.518  milligrams  CaO  while  one  cubic  centimeter  of  the  resulting 
smithsonite  contains  1.514  milligrams  CO2  and  2.787  milligrams  ZnO.  It 
is  clear  then  that  the  principle  of  equal  volumes  requires  more  C02  than  is 
available  in  the  calcite.  If  the  process  follows  the  formula,  shrinkage  of 
volume  will  necessarily  result. 


THE  TEXTURE  OF  MINERAL  DEPOSITS        171 

including  sericite,  chlorite,   calcite,  quartz,  fluorite  and  barite 
very  rarely  replace  sulphides. 

Texture  of  Metasomatic  Rocks. — In  metasomatism  new 
minerals  develop  at  countless  points  in  the  old  rock,  some  grow- 
ing with  crystal  form  (metacrystic  or  crystalloblastic  series,  p. 
170)  while  others  grow  into  irregular  grains.  Each  new  grain 
may  be  called  a  metasome,  each  new  crystal  a  metacryst  (pseudo- 
phenocryst).1  The  resulting  textures  will  be  holocrystalline ; 


FIG.  52. — Replacement  veinlets  of  galena  (white)  in  cryptocrystalline  quartz 
(dark  gray)  with  vugs  (black).    Tintic,  Utah.     Magnified  11  diameters. 

the  new  minerals  frequently  contain  inclusions  of  the  old  (sieve 
texture)  and  if  the  replacement  is  incomplete,  as  often  is  the  case, 
enough  of  the  old  texture  may  be  preserved  to  indicate  the  original 
rock  (relict  texture).  It- is  characteristic  of  some  replacements 
that  even  if  the  process  has  been  carried  to  completion  the  original 
texture  may  be  preserved  as  in  silicified  oolitic  limestone  and  in 
silicified  dolomites  (Fig.  56).  In  many  cases,  however,  the 
original  texture  is  wholly  destroyed. 

1  This  term  was  first  introduced  by  A.  G.  Lane,  Bull.,  Geol.  Soc .  Am., 
vol.  14,  1903,  p.  369. 

Grubenmann  and  Becke  use  the  terms  xenoblast  and  idioblast.     N. 
Grubenmann,  Die  kristallinen  schiefer,  Berlin,  1910,  p.  91. 


172 


MINERAL  DEPOSITS 


FIG.  53. — Feathery    geocronite    (5PbS-Sb2S3)     (white)    replacing     galena. 
Tintic,  Utah.     Magnified  24  diameters. 


-  y§ 


FIG.  54. — Galena   (light  gray)   replaced  by  pearceite  (9Ag2S-AsjS3)  (dark 
gray),  in  cryptocrystalline  quartz.    Tintic,  Utah.    Magnified  227  diameters. 


THE  TEXTURE  OF  MINERAL  DEPOSITS        173 

The  structure  of  a  rock  may  be  faithfully  preserved  even 
when  metasomatic  action  has  destroyed  its  texture.  Such 
preserved  structures  are,  for  instance,  stratification,  joints, 
breccias,  folds  and  vesicules  in  lavas.  Preservation  of  texture  of 
limestone  which  has  been  completely  replaced  by  sulphides  are 
mentioned  by  S.  F.  Emmons1  and  J.  M.  Boutwell.2 

It  is  held  by  some  petrographers  that  metamorphic  rocks  show 
no  recognizable  succession  in  order  of  crystallization  but  this  is 
certainly  not  always  correct.  In  many  replacements  gangue 


FIG.  55. — Same  replacement  magnified  690  diame^rs.      Note  that  earlier 
barite  plates  (black)  are  not  replaced  by  galena  but  by  the  later  pearceite. 

minerals   like   quartz   and   barite   may   crystallize   first,    while 
pyrite  comes  next  and  other  sulphides  later. 

Irving3  has  pointed  out  that  in  some  cases  replacement  begins 
from  a  great  number  of  points  in  the  rock  where  metasomes  or 
metacrysts  may  develop  (Fig.  57)  and  by  continuation  of  the 
same  process  (Fig.  58)  the  remainder  of  the  rock  is  finally  re- 
replaced;  the  contact  is  then  indefinite.  In  other  cases  the 
complete  change  occurs  rapidly,  advancing  like  a  wave  over  the 

1  S.  F.  Emmons,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  23,  1893,  p.  602. 

2  J.  M.  Boutwell,  Prof.  Paper   38,  U.  S.  Geol.  Survey,  1905,  p.  193. 

3  J.  D.  Irving,  Jour.  Canadian  Min.  Inst.,  vol.  14,  1911,  pp.  395-471. 


174 


MINERAL  DEPOSITS 


country  rock;  the  contacts  are  then  sharp  and  the  process  prob- 
ably consisted  in  replacement  of  the  original  rock  by  colloid 
silica.  The  replaced  rock  is  usually  dense  and  compact;  in 
places,  however,  drusy  cavities  occur  in  it. 

Under  the  influence  of  the  same  solution  different  results  may 
be  produced  in  different  rocks.  Limestone  may  be  silicified  while 
diorite  may  be  transformed  to  sericite. 

Replacements  at  High  Temperature. — Complete  recrystalliza- 
tion,  development  of  silicate  minerals  with  little  or  no  water, 


FIG.  56. — Thin  section  of  dolomite  completely  silicified,  but  retaining  texture 
and  crystal  form.     After  J.  D.  Irving,  U.  S.  Geol.  Survey. 

and  coarse  texture  are  typical  of  deposits  formed  by  replacement 
at  high  temperatures.  Mineralizers  like  fluorine,  boron  or  phos- 
phorus are  frequently  introduced. 

The  best  examples  of  such  textures  are  found  in  the  replace- 
ment of  limestones  in  contact-metamorphic  deposits  (Figs.  250 
and  251)  resulting  in  coarse  aggregates  of  metacrysts  of  andradite 
garnet  with  metasomes  of  quartz,  calcite,  epidote  and  pyroxene. 
The  limestone  may  be  recrystallized  in  part  to  coarse  calcite. 
Magnetite  and  sulphides  develop  in  large  grains. 


THE  TEXTURE  OF  MINERAL  DEPOSITS        175 

Adjoining  tin-bearing  veins  the  rocks  are  recrystallized  to 
greisen,  consisting  of  coarse  metasomes  of  muscovite,  topaz, 
quartz,  tourmaline,  fluorite  and  cassiterite  (Fig.  226).  Cal- 
careous rocks  or  greenstones  containing  much  lime  are  recrystal- 
lized to  aggregates  of  axinite,  actinolite,  garnet,  etc. 

Replacement  by  apatite  (containing  phosphorus),  scapolite 
(containing  chlorine)  and  pyroxene  occur  adjoining  certain  high 
temperature  veins.  Along  many  deep-seated  gold  quartz  vein 
much  albite,  biotite,  and  zoisite  develop  in  the  wall  rock. 

Replacements  at  Intermediate  Temperature. — Replacements 
at  more  moderate  temperature  are  likely  to  result  in  fine-grained 
textures,  and  hydrous  silicates  like  chlorite  and  sericite  are 
abundant  (Fig.  61).  There  are  exceptions  to  this  where  barite 
or  fluorite  replace  limestone  for  both  of  these  minerals  easily 
develop  as  perfect  metacrysts  (Figs.  62  and  64).  Silicification  of 
limestone,  argillaceous  shale  and  rhyolite  is  a  very  common 
process  taking  place  frequently  with  preservation  of  texture.  The 
quartz  will  usually  be  fine-grained.  Silicified  limestones  are 
called  jasperoids  (Figs.  57  and  58). 

Limestone  may  be  replaced  by  massive  sulphides  (Fig.  63). 
Alteration  by  hot  waters  of  granular  and  porphyritic  igneous 
rocks  as  well  as  of  schists  of  similar  composition  results  in  sericiti- 
zation  of  the  femic  and  salic  minerals,  sometimes  also  of  the 
quartz,  with  development  of  fibrous  aggregates.  Pyrite,  sec- 
ondary quartz,  rutile,  albite  and  adularia  are  sometimes  found 
in  these  rocks.  In  some  classes  of  such  metasomatic  rocks 
carbonates  of  calcium,  magnesium  and  iron  also  occur. 

Serpentine  is  altered  near  some  gold  quartz  veins  to  coarse 
aggregates  of  ankerite,  quartz  and  mariposite  (chromiferous 
sericite). 

Quart zite  and  quartzitic  slates  in  some  lead  deposits  may  be 
extensively  replaced  by  siderite  (Fig.  60). 

In  deposits  which  have  been  formed  by  hot  waters  hear  the 
surface  where  the  rocks  are  permeable  the  incipient  alteration  of 
igneous  rocks  is  often  wide  spread  with  alteration  of  the  femic 
minerals  to  chlorite,  calcite  or  epidote  (propylitization). 

Replacement  of  effusive  rocks  by  alunite,  pyrite  and  kaolinite 
is  characteristic  of  some  deposits  formed  near  the  surface. 

Replacement  at  Low  Temperature. — Under  the  influence  of 
cool  solutions  the  intensity  of  replacement  is  diminished.  The 
minerals  formed  are  strongly  hydrated,  the  texture  fine-grained. 


176 


MINERAL  DEPOSITS 


FIG.  57. — Incipient  silicification  of  limestone.  Aspen,  Colo.  White 
areas  represent  quartz  crystals  with  small  inclusions  of  limestone.  Magni- 
fied 30  diameters. 


FIG.  58. — Silicified  limestone  ("jasperoid").  Aspen,  Colo.  Crossed 
nicols  All  quartz.  Small  inclusions  of  calcite  in  some  of  the  grains. 
Magnified  30  diameters. 


THE  TEXTURE  OF  MINERAL  DEPOSITS        177 

In  igneous  rocks  chlorite,  quartz  and  kaolin,  possibly  also  sericite, 
may  form  by  the  action  of  cool  weak  solutions.     Replacement 


FIG.  59. — Replacement  veinlet  of  tourmaline  in  fresh  andesine  grain. 
Keystone  mine,  Meadow  Lake,  Nevada  County,  Cal.  t,  Tourmaline;  /, 
andesine;  e,  epidote;  s,  sericite.  Magnified  50  diameters. 


FIG.  60. — Siderite  with  pyrite  and  galena,  replacing  quartzite.  Helena 
and  Frisco  mine,  Coeur  d'Alene,  Idaho,  q,  Quartz  grains;  s,  sericite;  si, 
siderite;  black,  galena  and  pyrite.  Magnified  100  diameters. 

by  sulphides  such  as   pyrite,  galena  and  zincblende  may  take 
place     Limestone  may  be  silicified  to  fine-grained  jasperoids. 


178 


MINERAL  DEPOSITS 


To  a  limited  extent  sulphides  may  replace  other  sulphides. 
Chalcocite  for  instance  replaces  pyrite,  chalcopyrite  and  bornite. 
In  acid  descending  waters  kaolin  replaces  sericite  and  other 
silicates. 


FIG.  61. — Andesine  crystal  in  granodiorite,  replaced  by  sericite  and 
calcite.  Pinetree  vein,  Ophir,  Placer  County,  Cal.  q,  Quartz;  m,  musco- 
vite;  c,  calcite;  s,  sericite.  Magnified  80  diameters. 


FIG.  62. — Barite  (B),  replacing  gray,  fine-grained  limestone  (L),  Ouray, 
Colo.     After  J.  D.  Irving,  U.  S.  Geol.  Survey. 

Heated  alkaline  waters  are  not  believed  to  be  capable  of  de- 
veloping kaolin  from  the  aluminum  silicates  of  the  rocks;  alkaline 
silicates  like  sericite  will  result.  On  the  other  hand  the  ordinary 
dilute  ground  waters  will  develop  kaolin  in  the  rocks. 


THE  TEXTURE  OF  MINERAL  DEPOSITS        179 

In  other  words  kaolin  is  confined  to  the  uppermost  metamor- 
phic  zone  and  rarely  ventures  far  below  the  zone  of  weathering.1 


FIG.  63. — Galena,  replacing  crystalline  dolomite.  Elkhorn  mine,  Mon- 
tana, g,  Galena;  p,  pyrite;  c,  calcite  grains  of  limestone;  q,  secondary 
quartz.  Magnified  15  diameters. 


FIG.  64. — Fluorite  replacing  limestone.  Florence  mine,  Judith  Moun- 
tains, Montana.  /,  Fluorite:  I,  limestone;  q,  secondary  quartz.  Magnified  7 
diameters. 

Criteria  of  Replacement. — F.  Posepny  first  established  replace- 
ment as  a  mode  of  origin  of  mineral  deposits.     Shortly  after- 
1  W.  Lindgren,  The  origin  of  kaolin,  Econ.  Geol,  vol.  10,  1915,  pp.  89-93. 


180  MINERAL  DEPOSITS 

ward  S.  F.  Emmons1  demonstrated  it  to  be  a  common  mode  of 
origin  and  illustrated  it  by  the  description  of  many  ore-bodies 
in  Colorado  and  elsewhere.  About  1900  W.  Lindgren  described 
the  principal  modes  of  metasomatism.2  In  1911  J.  D.  Irving3 
published  a  paper  of  great  value  in  which  the  criteria  of 
replacement  ore-bodies  were  summarized. 

Some  of  these  criteria  in  favor  of  replacement  have  already 
been  mentioned  but  they  may  be  briefly  recalled  here: 

1.  Form  of  ore-body,  more  or  less  irregular.     Gradually  fading  limits. 
Not  always  conclusive. 

2.  Presence    of    unsupported    residual    rock    masses.     Sometimes    the 
orientation  of  bedding  may  be  proved  parallel  with  the  surrounding  rocks. 

3.  Absence  of  crustification.     A  banding  may  be  observed  in  places  due 
to  preservation  of  bedding  or  shearing  planes. 

4.  Absence  of  concave  contacts;  in  limestone,  for  instance,  solution  of 
cavities  tends  to  produce  flat  concave  depressions;  a  filled  cave  would  show 
this    whereas    replacement    proceeds    with    convex    outlines    toward   the 
unaltered  rock. 

5.  Preservation  of  textures  and  structures  of  original  rock.     The  last- 
named  criterion  is  the  most  conclusive. 

The  criteria  for  the  determination  of  replacement  are  some- 
times difficult  to  establish;  many  mistakes  have  been  made  along 
this  line.  Replacement  veinlets  crossing  the  older  minerals  and 
dependence  of  the  replacing  mineral  on  minute  fissures  and 
cracks  constitute  good  evidence.  The  projecting  of  crystals  of. 
one  mineral  into  another  is  not  always  a  safe  proof  of  replace- 
ment. The  apparent  host  may  possibly  be  a  later  mineral 
molded  about  the  crystals.  In  many  cases  adjoining  minerals 
may  have  developed  practically  simultaneously.  A  peculiar 
type  of  replacement  results  in  pseudo-eutectic  texture  simulat- 
ing an  intergrowth  (Figs.  54  and  55). 

1  S.  F.  Emmons,  The  genesis  of  certain  ore  deposits,  Trans.,  Am.  Inst. 
Min.  Eng.,  vol.  15,  1887,  pp.  125-147. 

S.  F.  Emmons,  Structural  relations  of  ore  deposits,  idem,  vol.  16,  1888, 
pp.  804-839. 

S.  F.  Emmons,  On  the  origin  of  fissure  veins,  Proc.,  Colorado  Sci.  Soc., 
vol.  2,  1888,  pp.  189-208. 

2  W.  Lindgren,  Metasomatic  processes  in  fissure  veins,  Trans.,  Am.  Inst. 
Min.  Eng.,  vol.  30,  1901,  pp.  578-692. 

3  J.  D.  Irving,  Some  features  of  replacement  ore-bodies  and  the  criteria 
by  means  of  which  they  may  be  recognized,  Jour.  Canadian  Min.  Inst., 
vol.  14,  1911,  pp.  395-471;  Econ.  GeoL,  vol.  6,  1911,  pp.  527-561. 


THE  TEXTURE  OF  MINERAL  DEPOSITS        181 

Role  of  Colloids  in  Filling  and  Replacement. — It  is  well  known 
that  colloid  deposits,  for  instance,  of  silica,  iron  hydroxide  and 
aluminum  hydroxide  play  an  important  part  in  mineral  deposits 
formed  at  or  near  the  surface.  Colloid  minerals  are  also  often 
deposited  during  the  oxidation  of  ore  deposits.  In  the  discussion 
in  this  chapter  the  colloids  have  not  thus  far  been  considered. 

There  is,  however,  an  increasing  mass  of  evidence  that  colloid 
silica  or  silica  gel  is  of  considerable  importance  in  the  origin  of 
deposits  formed  relatively  near  the  surface  by  ascending  waters. 
Some  of  the  quartz  filling  in  such  veins  is  extremely  fine-grained 
and  bears  evidence  of  having  been  deposited  as  a  stiff  jelly  which 
soon  afterward  was  crystallized  in  chalcedonic  or  cryptocrystal- 
line  form.1  Clear  evidence  of  this  is  seen  in  some  filled  veins 
from  the  Tintic  district,  Utah,2  where  the  original  delicate  band- 
ing by  deposition  is  still  seen  though  the  substance  is  now  micro- 
crystalline  quartz. 

There  is  also  good  evidence  presented  in  the  last-named  paper 
on  the  Tintic  district  to  show  that  in  some  deposits  formed  at 
moderate  depths  and  not  very  high  temperatures,  limestone  and 
dolomite  may  be  replaced  by  silica  gel  which  afterward  crystal- 
lized to  chalcedony.  This  type  of  replacement  appears  to  be 
characterized  by  sharp  contacts  with  the  unaltered  rock;  it  does 
not  proceed  from  crystal  nuclei  of  quartz  starting  at  numerous 
points  but  advances  like  wave  and  stops  with  sharp  contacts 
(see  p.  173).  Later  metalliferous  solutions  penetrated  this  gel 
and  deposited  sulphides  in  it.  Sometimes  a  banding  has  been 
produced  which  strongly  recalls  the  so-called  Liesegang  rings3  in 
artificial  gels  and  indicate  a  sort  of  rhythmical  precipitation  of 
sulphides. 

1  W.  Lindgren,  Geology  and  mineral  deposits  of  the  National  District, 
Nevada,  Bull.  601,  U.  S.  Geol.  Survey,  1915. 

2  W.  Lindgren,  Processes  of  mineralization  and  enrichment  in  the  Tintic 
mining  district,  Econ.  Geol.,  vol.  10,  1915,  pp.  225-240. 

3  R.  E.   Liesegang,    Geologische   Diffusionen,    1913,   p.  180.     Reviewed 
by' A.  Knopf  in  Econ.  Geol,  vol.  8,  1913,  p.  803. 


CHAPTER  XIII 
ORE-SHOOTS1 

Form  of  Primary  Ore -shoots. — Commercial  ore  or  mineral  does 
not  ordinarily  occupy  the  whole  volume  of  a  deposit.  The  ore  is 
in  most  cases  surrounded  by  material  of  poorer  grades,  sometimes 
fading  into  the  country  rock,  or  again  sharply  separated  from  it. 
In  replacement  deposits  the  disseminated  grains  of  galena,  for 
instance,  or  zinc  blende,  may  gradually  become  so  few  that  the 
mass  can  no  longer  be  treated  with  profit.  In  veins,  only  certain 
parts  of  the  sheet-like  body  can  be  extracted,  while  the  remainder 
of  the  vein  material  may  consist  of  gangue  minerals  only,  or 
of  clayey  attrition  masses  or  breccias. 

Those  parts  of  a  deposit  in  which  the  valuable  minerals  are 
so  concentrated  that  their  utilization  becomes  possible  are  called 
ore-shoots.  Their  occurrence  and  form  are  exceedingly  variable, 
and  it  is  often  most  difficult  to  ascertain  the  causes  which  have 
guided  their  development.  A  full  discussion  of  this  subject  is 
scarcely  possibly  here,  for  it  involves  the  whole  question  of  gene- 
sis of  mineral  deposits. 

In.  deposits  of  sedimentary  origin  the  ore-shoots  have,  of 
course,  the  general  tabular  form,  but  admixture  with  gangue 
materials  or  valueless  matter  may  so  dilute  the  ore  that  only 
certain  parts  of  the  body  can  be  extracted.  Various  assort- 

1  C.  R.  Van  Hise,  Some  principles  controlling  the  deposition  of  ores, 
Trans.,  Am.  Inst.  Min.  Eng.,  vol.  30, 1900,  pp.  27-177. 

T.  A.  Rickard,  The  formation  of  bonanzas  in  gold  veins,  Trans.,  Am. 
Inst.  Min.  Eng.,  vol.  31,  1902,  pp.  198-220. 

The  localization  of  values  in  ore-bodies,  etc.  Discussion  by  J.  D.  Irving, 
F.  C.  Smith,  Reno  Sales,  F.  L.  Ransome,  H.  V.  Winchell,  H.  Sjogren,  and 
W.^Lindgren,  Econ.  Geol,  vol.  3,  1908,  pp.  143-154,  224-229,  326-330, 
331-336,  425-427,  637-642;  vol.  4,  1909,  pp.  56-61. 

C.  W.  Purington,  Ore  horizons  in  the  San  Juan  Mountains,  Econ.  Geol., 
vol.  1,  1905-06,  pp.  129-133. 

H.  C.  Hoover,  The  valuation  of  gold  mines,  Eng.  and  Min.  Jour.,  May 
19,  1904. 

R.  A.  F.  Penrose,  Jr.,  Some  causes  of  ore-shoots,  Econ.  Geol.,  vol.  5, 
1910,  pp.  97-133. 

182 


ORE-SHOOTS  183 

ments  of  detritus  and  complex  conditions  of  precipitation  from 
waters  of  seas,  lakes,  and  rivers  have  influenced  the  concentra- 
tion of  the  richer  ore  masses.  In  addition,  alterations  by  meteoric 
waters  are  common;  in  the  case  of  phosphate  deposits  and  beds 
of  siderite  they  have  resulted  in  enrichment. 

In  deposits  of  igneous  origin  the  general  form  of  the  deposit 
is  also  that  of  the  ore-shoots.  In  some  deposits,  such  as  the 
magnetite  deposits  of  northern  Sweden  and  the  dike-like  de- 
posits of  ilmenite  at  Iron  Mountain,  Wyoming,  there  is  prac- 
tically no  waste  material  and  the  whole  igneous  body  constitutes 
ore. 

More  commonly  the  irregular  lenticular  or  tabular  masses  of 
igneous  rocks  in  which  ore  minerals  have  developed  by  mag- 
matic  segregation  (for  instance,  gabbro  containing  chalcopyrite) 
have  nuclei  of  richer  material  gradually  fading  into  more  normal 
rock. 

In  the  epigenetic  deposits  the  outlines  of  the  ore-shoots  are 
exceedingly  variable.  In  those  deposits  which  are  formed  by 
replacement  this  is  particularly  true,  and  few  rules  can  be  laid 
down  for  their  occurrence;  the  form  is  determined  by  the  fissures 
giving  access  to  the  solutions,  by  the  presence  of  impermeable 
rocks,  and  by  the  varying  susceptibility  to  replacement  of  the 
original  rocks. 

Most  attention  has  been  given  to  the  shoots  in  fissure  veins. 
Although  the  ore  in  the  main  follows  the  fissure  and  therefore 
has  a  tabular  or  sheet-like  form,  it  rarely  occupies  the  whole 
space  along  this  fissure,  but  is  concentrated  in  bodies  of  vary- 
ing size,  shape,  and  continuity;  smaller  bodies  are  known  as 
bunches,  pockets,  or  kidneys;  in  gold-quartz  veins  these  may  be 
exceedingly  rich.  Narrow  ore-shoots,  greatly  elongated  in  the 
vertical  direction,  whether  occurring  in  fissure  veins  or  independ- 
ently of  them  (for  instance,  in  volcanic  necks),  are  called  chim- 
neys, pipes,  or  necks  (Fig.  68). 

Ore-shoots  may  be  entirely  irregular,  but  commonly  have  a 
more  or  less  well-defined  columnar,  steeply  pitching  shape,  best 
shown  in  projection  upon  the  plane  of  the  vein.  Fig.  65  shows 
the  terminology  proposed1  for  various  dimensions  of  an  ore-shoot 
in  a  vein.  The  pitch  length,  or  axial  length,  is  the  distance  be- 
tween the  two  extreme  ends  of  the  shoot.  The  pitch  is  the  angle 

1  W.  Lindgren  and  F.  L.  Ransome,  Prof.  Paper  54,  U.  S.  Geol.  Survey, 
1906,'p.  206. 


184 


MINERAL  DEPOSITS 


which  the  pitch  length  makes  with  the  strike  of  the  vein,  and  is 
measured  on  the  plane  of  the  vein.  The  stope  length  is  the  hori- 
zontal length  of  the  ore-shoot  on  any  particular  level.  The  thick- 
ness or  width  is  measured  perpendicularly  to  the  plane  of  the  vein. 
The  breadth  of  the  ore-shoot  is  the  stope  length,  multiplied  by 
the  sine  of  the  pitch. 

Fig.  66  shows  the  ore-shoots  of  a  gold-quartz  vein  at  Nevada 
City,  California.  Flat-dipping  shoots  are  not  so  common.  Fig. 
67  shows  an  excellent  example  of  a  flat  shoot  in  the  celebrated 
Eureka-Idaho  vein  at  Grass  Valley,  California. 


FIG.  65. — Diagram  illustrating  the  terms  used  to  describe  the  dimensions  of 
ore-shoots.     After  W.  Lindgren  and  F.  L.  Ransome,  U.  S.  Geol.  Survey. 

In  parallel  veins  the  shoots  are  often,  roughly  speaking, 
coextensive.  Sometimes  the  shoots  in  a  series  of  parallel  veins 
persistently  recur  where  the  veins  cross  a  certain  stratum  or 
dike,  as,  for  instance,  where  the  gold-quartz  veins  of  Gympie, 
Queensland,  intersect  certain  carbonaceous  strata,  or  as  at 
Thames,  New  Zealand,  where  the  veins  intersect  certain  soft- 
ened and  altered  andesites.  Many  shoots  follow  intersections 
of  veins  or  of  veins  with  fissures. 

Shoots,  however  large,  do  not  continue  indefinitely,  but  end 
in  depth,  usually  with  gradual  deterioration.  Small  masses  or 
kidneys  are  likly  to  be  found  below  the  termination  of  a  large 
ore-shoot.  Exploration  may  find  another  shoot  below  the 
first,  either  on  the  same  fissure  or  imbricating  on  a  parallel 


ORE-SHOOTS 


185 


FIG.  66. — Ore-shoots   of  veins  at   Nevada  City,   California. 


Idaho  -Maryland 
Shaft 


Ma.slin  Shaft 


FIG.  67. — Approximate  outline  of  the  Eureka-Idaho  ore-shoot,  Grass  Valley, 
California,  in  projection  on  the  plane  of  the  vein. 


186 


MINERAL  DEPOSITS 


vein.  When  great  depth  is  attained  the  grade  of  the  ore  usually 
decreases  in  the  deeper  levels,  but  this  rule  is  not  without  ex- 
ceptions. Many  shoots  are  lenticular,  that  is,  they  contain  a  rich 


LEVEL 


:'•.'( 

:':!    BURNS    SHAFT 

_ 1 -1 


FIG.  68. — Stereogram  of  Anna  Lee  ore  chimney,  Cripple  Creek,  Colorado. 
Shoot  probably  determined  by  intersection  of  the  basic  dike  with  a  fissure. 
After  V.  G.  Hills. 

nucleus,  outward  from  which  the  ore  gradually  decreases  in  tenor. 
H.    C.   Hoover,   from   an  examination  of  70  mines,  concluded 


ORE-SHOOTS  187 

that  ore-shoots  are  generally  lenticular  and  that  the  probable 
minimum  extension  of  an  ore-shoot  below  any  given  level  would 
be  a  factor  of  not  less  than  a  radius  of  one-half  of  its  breadth. 

At  Cripple  Creek  Lindgren  and  Ransome  found  that  the  shoots 
which  begin  distinctly  below  the  surface  have  a  marked  elon- 
gated form,  the  ratio  between  pitch  length  and  breadth  varying 
from  13^  :  1  to  5  : 1. 

Primary  ore-shoots  rarely  continue  for  more  than  2,000  feet 
along  the  strike,  or  for  more  than  2,000  feet  along  the  pitch 
length. 

In  a  given  district  the  pitch  of  the  ore-shoot  is  often  pre- 
dominantly in  one  direction;  thus  at  Nevada  City  and  Grass 
Valley  the  shoots  pitch  to  the  right  of  an  observer  who  looks 
down  the  dip  of  the  vein.  In  another  district  the  opposite  may 
be  true.  In  some  places  the  tenor  varies  directly,  in  others 
inversely  with  the  swelling  of  the  vein.  According  to  a  rule 
often  quoted,  the  shoots  follow  the  directions  of  the  striations 
on  the  vein  walls,  but  this  again  by  no  means  has  universal 
application. 

Shoots  of  Successive  Mineralizations. — While  in  some  veins 
the  whole  width  consists  of  uniform  ore,  it  is  exceedingly  common, 
especially  in  thick  veins,  to  find  that  there  are  certain  streaks 
which  are  far  richer  than  the  rest.  They  may  follow  foot-wall  or 
hanging-wall,  or  the  center  of  the  vein,  or  may  switch  from  one 
side  to  another.  Such  phenomena  indicate  re-opening  of  the 
vein  or  brecciation  after  the  first  period  of  vein-filling  and 
enrichment. 

Superficial  or  Secondary  Shoots. — Descending  surface  waters 
decompose  and  often  enrich  the  upper  part  of  veins  or  other  de- 
posits. Such  enriched  superficial  portions  of  an  ore  deposit  are 
dependent  upon  the  ground-water  level  and,  when  projected  upon 
the  plane  of  the  vein,  follow  the  surface  of  the  ground  and 
terminate  below  along  an  irregular  and  jagged  line.  Oxidized 
ores,  as  well  as  sulphides  due  to  enrichment,  are  found  in  them, 
usually  at  different  levels.  The  surface  shoots  are  in  fact  char- 
acterized by  horizontal  extension,  in  contradistinction  to  the 
predominance  of  the  vertical  direction  in  the  primary  shoots. 
The  mineralogical  characteristics  of  superficial  shoots  will  be 
discussed  in  detail  in  a  later  chapter.  Their  tendency  is  to 
spread  along  the  strike  of  the  vein,  often  also  out  into  the  wall 
rock.  Thus  pay  ore  may  be  found  for  a  long  distance  along  the 


188  MINERAL  DEPOSITS 

trend  of  the  vein  and  its  appearance  will  be  that  of  the  oxi- 
dized croppings  of  a  long  primary  shoot,  when  in  fact  deeper 
explorations  may  prove  the  existence  of  only  a  few  narrow  primary 
ore-bodies  underneath  the  continuous  surface  ore.  Sometimes, 
as  in  Calico,  San  Bernardino  County,  California,  and  numerous 
other  places,  oxidized  silver  ores  will  be  found  in  croppings  along 
a  vein  which  are  simply  concentrations  of  a  primary  vein  fill- 
ing that  contains  no  workable  shoots.  To  this  class  belong  also 
the  horizontal  or  flat  shoots  of  secondary  copper  sulphides 
(chalcocite  and  covellite)  formed  by  descending  solutions  in 
copper  deposits  at  or  near  the  water  level.  The  primary  material 
may  or  may  not  constitute  commercial  ore.  If  spread  over  wide 
mineralized  areas  such  shoots  are  often  called  chalcocite  blankets. 

Lateral  spreading  is  often  characteristic  of  shoots  of  oxidized 
ores.  Descending  metal  solutions  may  wander  out  in  the  country 
rock  and  here  form  new  bodies. 

Causes  of  Primary  Ore-Shoots. — Ore-shoots  are  due  to  the 
abundant  precipitation  of  valuable  minerals  from  their  solutions. 
The  causes  are  in  part  chemical  and  in  part  mechanical: 

1.  Decrease  of  pressure  and  temperature. 

2.  Favorable  chemical  character  of  wall  rock. 

3.  Favorable  physical  character  of  wall  rock. 

4.  Intersections. 

Decrease  of  Pressure  and  Temperature. — The  fundamental 
reason  for  the  occurrence  of  ores  in  veins  and  allied  epigenetic 
deposits  in  the  upper  crust  is  probably  that  the  metals  were  in 
solution  in  hot  waters  which  were  ascending  and  gradually 
encountered  conditions  favorable  for  precipitation.  First  among 
these  conditions  is  decreasing  temperature.  If  this  is  true  the 
deposits  should  gradually  become  poorer  or  barren  in  depth.1  In 
a  general  way  this  is  doubtless  true,  but  for  many  substances  the 
vertical  space  through  which  deposition  can  take  place  is  very 

1  T.  A.  Rickard,  Persistence  of  ore  in  depth,  Trans.  Inst.  Min.  and  Met., 
vol.  24,  1915,  pp.  3-46,  with  discussion. 

W.  Lindgren,  Ore  deposition  and  deep  mining,  Econ.  Geol,  vol.  1,  1905, 
pp.  34-46. 

F.  L.  Garrison,  Decrease  of  value  in  ore-shoots  with  depth,  Trans.  Cana- 
dian Min.  Inst.,  vol.  15,  1912,  pp.  192-209. 

J.  F.  Kemp,  The  influence  of  depth  on  the  character  of  metalliferous 
deposits,  Compte  Rendu,  12e  Session,  Canada,  Congres  geologique  internat., 
1914,  pp.  253-260. 

Malcolm  Maclaren,  Idem,  pp.  295-304. 


ORE-SHOOTS  189 

large.  We  know  that  gold-bearing  quartz  was  deposited  in  Cali- 
fornia over  a  vertical  distance  of  4,000  feet,  while  in  southeastern 
Alaska  and  at  Bendigo,  Australia,  the  interval  is  not  less  than 
5,000  feet.  This  deposition  took  place  at  considerable  depth 
below  the  surface,  probably  several  thousand  feet  below  it,  and 
as  it  is  known  that  gold-bearing  quartz  may  also  be  deposited 
within  the  upper  zone,  we  have  thus  a  total  vertical  range  of  at 
least  9,000  feet.  At  the  lowest  levels  at  the  places  mentioned 
the  ore  is  of  low  grade,  but  in  Alaska  at  least  there  is  a  large 
quantity  available.  The  richest  ore  was  doubtless  deposited 


MYSORE     GM.  McTaggart> 

Hancock's  G\eu   Riddlesdale'8      Kowse's  Tayl 


Vertical 


FIG.  69. — Pitching  ore  shoots  in  gold  quartz  veins,  Kolar,  India. 
After  T.  A.  Richard. 

close  to  the  surface,  where  we  find  the  bonanzas  of  the  Tertiary 
gold  and  silver  veins;  but  below  this  bonanza  zone  the  decrease 
in  tenor  of  the  ore  is  very  slow  and  rich  shoots  and  pockets  may 
be  found  at  great  depth  below  the  original  surface.  The  most  per- 
sistent gold-bearing  ore  shoots  known  are  those  in  veins  formed 
at  intermediate  or  high  temperatures.  Such  are,  for  instance, 
the  North  Star  vein  at  Grass  Valley,  California,  which  with 
very  slight  impoverishment  has  been  followed  for  6,400  feet  on 
a  dip  of  20°  (p.  569).  The  Kolar  veins  in  India  have  been  mined 
to  a  vertical  depth  of  4,000  feet  in  shoots  of  considerable 
regularity  (Fig.  69),  and  with  little  change  in  tenor  of  ore. 


190  MINERAL  DEPOSITS 

The  most  persistent  ore  body  known  is  that  of  Morro  Velho 
mine  in  Brazil,  where  a  pitching  ore  shoot  has  been  worked  to  a 
vertical  depth  of  6,200  feet  and  a  pitch  length  of  9,000  feet 
(Fig.  236).  For  copper  ores  the  vertical  range  of  deposition 
is  likewise  great,  though  unlike  gold  and  silver  they  seem  to 
be  deposited  in  greatest  quantity  at  lower  levels  and  high 
temperatures.  Lead,  on  the  other  hand,  appears  to  be  precipi- 
tated nearer  the  surface  and  at  lower  temperatures;  while  zinc 
in  this  respect  stands  between  copper  and  lead. 

The  relations  set  forth  explain  why  so  little  decisive  evidence 
of  vertical  succession  in  deposition  is  available  from  observa- 
tions at  any  one  mine. 

In  the  Cornwall  veins  tin  and  tungsten  prevail  in  the  lower 
levels  in  granitic  country  rock,  while  copper  was  deposited  in 
the  cooler  region  of  the  slates  covering  the  granite  batholiths; 
the  lead  ores  are  found  some  distance  away  from  the  intrusive 
granite.  In  many  lead  mines  it  has  been  noted  that  within  a 
distance  of  700  to  3,000  feet  from  the  surface  the  lead  minerals 
give  .way  to  pyrite  and  zine  blende.  In  quicksilver  mines  the 
ore  often  becomes  impoverished  within  1,000  feet  below  the 
surface. 

The  dependence  of  the  deposition  of  various  metals  upon 
temperature  and  therefore  also  upon  the  vertical  and  hori- 
zontal distance  from  the  place  of  origin  of  the  mineralizing 
solutions  has  been  emphasized  lately  by  several  investigators. l 

Character  of  Wall  Rock. — The  character  of  the  wall  rock  has 
sometimes  a  decided  influence  on  the  ore-shoots,  but  it  is  not 
always  easy  to  decide  whether  it  is  due  to  chemical  or  mechanical 
causes.  In  replacement  deposits  limestone  and  lime  shale  are 
usually  favorable,  but  in  the  Coeur  d'Alene  district  of  lead- 
bearing  veins  a  quartzitic  schist  is  the  rock  best  adapted  for 
replacement  by  siderite  and  galena.  At  Freiberg,  Saxony,  the 
gray  gneiss  is  the  favorable  rock,  while  the  veins  split  or  become 
unproductive  in  the  red  gneiss  or  in  the  mica  schists. 

Carbonaceous  rocks  are  believed  to  influence  deposition  favor- 
ably by  their  reducing  action;  the  gold-quartz  shoots  of  Gym  pie, 

1 J.  E.  Spurr,  A  theory  of  ore  deposition,  Econ.  Geol.,  vol.  2,  1907,  p.  790. 

L.  De  Launay,  La  metallogenie  de  1'Italie,  Congres  geologique  internal., 
Mexique,  vol.  1,  1906,  p.  571.  Also  in  Gltes  Mineraux,  vol.  1,  Paris,  1913. 

W.  Lindgren,  Processes  of  mineralization  and  enrichment  in  the  Tintic 
mining  district,  Econ.  Geol.  vol.  10,  1915,  p.  228. 


ORE-SHOOTS  191 

Queensland,  are  often  quoted,  as  well  as  the  supposedly  car- 
bonaceous "indicator"  at  Ballarat,  Victoria.  The  well-known 
replacement  of  fossil  wood  by  chalcocite  in  a  certain  class  of 
copper  deposits  may  be  added  to  these  examples,  as  well  as  the 
supposed  influence  of  certain  oil  shales  on  the  deposition  of 
lead  ores  in  Wisconsin.  The  importance  of  precipitation  by 
carbonaceous  material  has  been  overestimated,  but  in  many 
cases  the  hydrocarbons  have  certainly  favorably  influenced  the 
deposition  of  ores.1 

Rocks  containing  pyrite  or  other  sulphides  often  enrich  trav- 
ersing veins.  Examples  of  this  are  known  from  Kongsberg, 
Norway,  where  the  silver  veins  are  productive  when  crossing 
certain  schists  with  disseminated  sulphides.  At  Ophir,  Cali- 
fornia, gold-quartz  veins  are  enriched  when  crossing  "iron  belts" 
of  pyritic  amphibolites. 

Where  a  vein  cuts  through  a  thick  series  of  sedimentary  rocks 
it  often  widens  and  contains  rich  ore  in  the  limestones,  while 
poor  or  barren  in  shale  or  sandstone.  Similarly,  where  a  thick 
series  of  igneous  rocks,  as  in  the  San  Juan  region,  Colorado,  is 
intersected  by  veins  ore  horizons  will  develop  in  rocks  which  by 
their  physical  and  chemical  character  are  most  favorable  to  con- 
tinuous fissures  or  to  replacement. 

Rhyolites  are  generally  unfavorable  because  fissures  often 
tend  to  splitting  in  such  rocks;  tuffs  likewise  because  the  solu- 
tions tend  to  disperse  through  great  masses  of  rock. 

On  the  other  hand,  rocks  like  andesites  and  latites  are  usually 
favorable.  Purington  (op.  tit.}  has  shown  that  in  the  San  Juan 
Mountains  the  andesitic  breccias  which  contain  abundant  ferro- 
magnesian  silicates  are  most  favorable  to  ore  deposition. 

Impermeable  Barriers. — The  conditions  outlined  above  would 
tend  to  produce  more  or  less  horizontal  ore-bodies.  Such 
ore-bodies  are  most  conspicuous  where  impervious  rocks  inter- 
pose barriers  to  the  solutions.  The  occurrence  of  ore  in  hori- 
zontal extension  below  such  barriers  is  in  fact  one  of  the  best 
indications  that  the  solutions  have  been  ascending  in  the  main. 

Fig.  70  shows  the  occurrence  of  oxidized  silver  ores  below  the 
Devonian  shale  at  Chloride  Flat  (Silver  City),  New  Mexico,  and 
similar  occurrences  are  not  uncommon  in  other  mining  districts 
of  New  Mexico.  The  blanket  veins  of  Rico,  Colorado  (Fig.  71), 

1  W.  P.  Jenney,  The  chemistry  of  ore  deposition.  Trans.,  Am.  Inst. 
Min.  Eng.,  vol.  33.  1903,  pp.  445-498. 


192 


MINERAL  DEPOSITS 


FIG.  70. — Sketch  section  showing  occurrence  of  ore-shoots  in  limestone  at 
contact  of  overlying  Devonian  shale  at  the  Bremen  mine  near  Silver  City, 
New  Mexico,  a,  Limestone;  6,  shale;  c,  ore.  After  R.  A.  F.  Penrose,  Jr. 


FIG.  71. — Diagrammatic  section  across  a  lode,  and  ore-body  formed 
beneath  an  impervious  stratum  (blanket)  of  black  shale,  Rico,  Colorado. 
After  F.  L.  Ransome,  U.  S.  Geol.  Survey. 


ORE'S  HOOTS 


193 


present  another  good  illustration  of  this  principle,  as  do  also 
the  ores  of  the  American  Nettie  mine  near  Ouray,  Colorado,  and 
the  siliceous  gold  ores  replacing  dolomite  in  the  Black  Hills. 


FIG.  72. — Longitudinal  section  along  the  Neu  Hoffnung  vein,  Freiberg, 
Germany,  showing  ore-shoots  along  intersection  with  several  other  veins. 
After  R.  Beck. 

The  impermeable  stratum  is  not  necessarily  shale;  it  may  be 
a  gouge  in  a  fissure,  or  a  sheet  of  volcanic  rock  which,  for  some 
reason,  the  fissures  failed  to  penetrate.  The  same  principle  of 


194  MINERAL  DEPOSITS 

impermeable  barriers  serves  to  explain  why  the  vein  material  is 
often  confined  between  the  clay  seams  of  hanging  and  foot  wall 
without  entering  the  adjacent  country  rock  by  replacement. 

Where  one  fissure  is  faulted  by  another,  deposition  may  occur 
because  the  circulation  becomes  impeded  at  the  fault.  It  is  not 
entirely  clear  why  deposition  of  rich  ores  should  take  place 
when  the  solutions  are  impeded  and  partial  stagnation  follows, 
but  the  conditions  observed  bear  sufficient  testimony  to  the  fact. 

Where  the  solutions  have  moved  downward,  as  in  the  concen- 
tration of  hematite  ore  from  poorer  "iron  formations,"  it  is  often 
observed  that  ores  occur  on  impervious  basements  and  in  troughs 
caused  by  shales,  clayey  fissures,  or  dikes. 

Intersections. — Enrichment  and  ore-shoots  along  intersec- 
tions of  two  veins  or  of  a  vein  and  a  fissure  are  very  common 
phenomena,  well  exemplified  at  Freiberg,  Saxony  (Fig.  72),  and 
at  Cripple  Creek,  Colorado.  Van  Hise  attributes  the  shoots  at 
such  intersections  to  the  mingling  of  two  solutions  and  conse- 
quent precipitation  of  some  constituents.  In  part  they  may  be 
due  to  the  shattering  of  the  rocks  at  the  intersection,  and  Penrose 
notes  that  shoots  are  more  likely  to  occur  where  the  intersection 
takes  place  at  acute  angles,  forming  wedge-shaped  blocks  that 
are  easily  broken  along  their  edges. 

Though  enrichment  at  intersections  is  common  it  is  by  no 
means  a  universal  rule,  and  indeed  sometimes  a  vein  is  impover- 
ished at  the  intersection  with  a  barren  fissure. 

The  occurrence  of  the  large  shoots  such  as  those  in  the  gold- 
quartz  veins  of  California,  at  Cripple  Creek,  and  in  the  Coeur 
d'Alene  lead  mines  cannot  be  fully  explained  by  intersections 
or  by  the  influence  of  the  wall  rock. 

Such  shoots  are  generally  considered  as  the  result  of  decrease 
in  temperature  of  ascending  solutions  in  channels  of  circulation. 


CHAPTER  XIV 
THE  CLASSIFICATION  OF  MINERAL  DEPOSITS 

Classification  by  Form  and  Substance. — A  genetic  classifica- 
tion of  deposits  of  useful  minerals  is  really  equivalent  to  the 
classification  of  "geological  bodies"  as  definied  in  Chapter  I  and 
is  therefore  naturally  beset  with  all  the  difficulties  connected 
with  an  imperfect  knowledge  of  geological  processes.  The  early 
attempts  in  the  way  of  systematic  treatment,  however,  avoided 
this  troublesome  path  by  the  simple  expedient  of  classifying  by 
substance  or  uses,  or  by  form.  These  schemes  are  followed  in 
many  text-books,  even  among  those  of  recent  date;  undoubtedly 
they  have  some  advantages,  especially  for  the  miner,  the  indus- 
trial chemist,  or  the  metallurgist,  who  are  principally  interested 
in  the  form  of  the  deposit  or  in  the  study  of  ores  of  certain  metals. 

By  substance  and  uses  mineral  deposits  may  be  classified  as 
follows : 

1 .  Structural  materials Stone,  glass  sand,  cement  rock,  clay, 

asphaltum. 

2.  Fuels Coal,  petroleum,  natural  gas,  peat. 

3.  Abrasives Corundum,  garnet. 

4.  Fertilizers Potash  salts,  phosphates,  green-sands. 

5.  Precious  stones Diamond,  opal,  tourmaline. 

6.  Various  industrial  uses. . .' Graphite,    barytes,    borax,    asbestos. 

sulphur. 

7.  Metallic  ores Iron  ores,  copper  ores,  gold  and  silver 

ores,  tin  ores,  aluminum  ores,  etc. 

However  convenient,  it  is  evident  that  this  classification  cannot 
lead  to  a  thorough  appreciation  of  the  manifold  processes  by 
which  mineral  deposits  are  formed  in  nature. 

The  first  attempts  at  a  classification  of  the  deposits  themselves 
were  made  by  the  miners  and  thus  the  early  and  not  yet  entirely 
abandoned  schemes  refer  to  the  form  of  the  geological  bodies. 
But  form  is  closely  connected  with  genesis  and  even  in  one  of  the 
earliest  classifications  on  this  basis,  that  of  Bernhard  von  Cotta,1 

1  Die  Lehre  von  den  Lagerstatten,  Freiberg,  1859. 
195 


196  MINERAL  DEPOSITS 

the  difficulty  of  avoiding  genetic  conceptions  is  felt  in  his  defini- 
tion of  a  vein  as  a  "filled  fissure."  He  divides  ore  deposits  as 
follows : 

I.  Regular  deposits. 

A.  Beds. 

B.  Veins. 

a.  Ordinary  fissure  veins  (true  fissure  veins). 

b.  Bedded  veins. 

c.  Contact  veins. 

d.  Lenticular  veins. 
II.  Irregular  deposits. 

C.  Stocks.     (Irregular  masses  with  distinct  limits.) 
a.  Recumbent. 

6.  Vertical. 

D.  Impregnations.  (Irregular  masses,  fading  into  coun- 
try rock.) 

With  variations  this  plan  of  classification  is  followed  in  many 
of  the  older  text-books.  Not  unlike  it  is  a  classification  by 
J.  A.  Phillips  in  his  treatise  on  ore  deposits,  revised  in  1896  by 
H.  Louis. 

Lately  James  Park  has  adopted  the  same  plan  with  some 
modifications  in  a  useful  and  practical  text-book  on  mining 
geology.1  His  classification  is  as  follows: 

I.  Superficial  deposits. 
a.  Fragmentary. 
6.  Massive. 
II.  Stratified  deposits. 

a.  Constituting  beds. 
6.  Disseminated  through  a  bed. 
III.  Unstratified  deposits. 

a.  Deposits  of  volcanic  origin. 
6.  Stockwork  deposits. 

c.  Contact  or  replacement  deposits. 

d.  Fahlbands. 

e.  Impregnations. 
/.   Segregated  veins. 
{/.  Gash  veins. 

h.  True  fissure  veins. 
1  A  text-book  of  mining  geology,  London,  1907,  p.  219. 


CLASSIFICATION  OF  MINERAL  DEPOSITS      197 

Park  states  that  this  classification  is  only  an  empirical  arrange- 
ment to  facilitate  the  study  of  ore  deposits,  and  a  provisional 
classification  by  origin  also  is  given. 

L.  De  Launay1  arranges  the  deposits  according  to  the  prin- 
cipal elements  contained.  This  logical,  though  not  genetic, 
plan  has  been  followed  in  part  in  the  index  appended  to  this 
book. 

Genetic  Classifications. — A  genetic  classification  is  the  most 
desirable  both  theoretically  and  practically.  In  exploring  and 
exploiting  ore  deposits,  the  miner  is  almost  forced  to  form  an 
idea  of  its  origin  in  order  to  follow  up  the  ore-bodies  to  best 
advantage.  Von  Groddeck  and  Stelzner  were  really  the  first 
mining  geologists  who  appreciated  and  applied  the  genetic  prin- 
ciple in  classification.2  Of  course,  the  time  was  hardly  ripe  for  its 
introduction  until  the  conceptions  of  genesis  had  crystallized  into 
fairly  definite  form.  Stelzner  remarks,  with  good  reason,  that  it 
is  only  by  standing  upon  the  ground  of  a  genetic  theory  that  the 
miner  finds  courage  to  sink  deep  shafts  or  drive  long  tunnels. 

We  are  still  in  doubt  as  to  the  true  mode  of  origin  for  many 
deposits.  But,  as  von  Groddeck  and  Stelzner  have  pointed  out, 
this  applies  to  any  classification  and  this  very  uncertainty  is  a 
stimulus  to  further  investigations. 

The  different  classifications  proposed  will  not  be  given  here 
in  detail.  An  excellent  account  is  found  in  Kemp's  "Ore  deposits 
of  the  United  States  and  Canada,"  Appendix  I.  Von  Groddeck 
and  Stelzner,  Posepny,  Wadsworth,  Monroe,  Kemp,  Crosby, 
Hoefer,  Spurr,  Van  Hise,  Weed,  and  several  others  have  more 
or  less  successfully  attacked  the  problem  of  a  consistent  genetic 
classification. 

Von  Groddeck,  followed  by  Stelzner  and  Beck,  makes  the 
primary  distinction  'whether  the  useful  minerals  were  originally 
formed  in  or  with  the  rock  in  which  they  now  occur  or  whether 
they  were  introduced  into  pre-existing  rocks.  Stelzner  called 
the  former  syngenetic,  the  latter  epigenetic.  (Author's  lecture 
notes,  Freiberg,  1881.) 

1  L.  De  Launay,  Gltes  Mineraux  et  Metalliferes,  3  vols.,  Paris,  1913. 

2  The  former  says:  "I  must  confess  that  I  have  never  been  able  to  under- 
stand the  satisfaction  which  many  people  feel  when  they  are  informed  that 
a  certain  deposit,  for  instance,  is  a  'stock.'     This  information  has,  on  the 
contrary,  always  produced  in  me  a  feeling  of  deep  dissatisfaction."     Quoted 
in  Stelzner  and  Bergeat,  Erzlagerstatten,  pt.  1,  1904,  p.  10. 


198  MINERAL  DEPOSITS 

J.  F.  Kemp  divides  the  deposits  into  (I)  those  of  igneous 
origin,  (II)  those  precipitated  from  solutions,  and  (III)  those 
deposited  from  suspension,  or  residues  after  the  decomposition 
of  rocks.  Difficulties  appear  here  too,  for  what  are  igneous  mag- 
mas but  solutions?  j^j*jj 

Beck's  classification  is  in  part  based  on  that  of  Stelzner.  In 
the  first  edition  of  his  hand-book  "  Die  Lehre  von  den  Erzlager- 
statten"  the  syngenetic  or  epigenetic  origin  was  made  the 
principal  basis  of  classification.  In  the  edition  of  1909  this  is 
changed  and  the  deposits  are  classified  as  follows,  on  the  basis  of 
the  various  phases  of  their  genetic  history : 

1.  Magma  tic  segregations. 

2.  Contact-metamorphic  ore. deposits. 

3.  Fissure  veins.          1  ,,      ,    ,     .    .    .       .       .     ,  ,. 

Morphologic  facies  of  a  single  genetic 
J 

6.  Secondary  alterations. 

7.  Sedimentary  ore  deposits. 

8.  Detrital  deposits. 

While  this  is  a  decided  improvement  upon  the  first  classifica- 
tion adopted  by  Beck,  the  description  of  the  various  deposits 
shows  that  many  genetically  different  types  are  forced  into  one 
and  the  same  subdivision. 

Weed1  goes  further  and  gives  the  origin  of  the  ore-forming 
solutions.  His  first  class  includes  igneous  deposits,  segregated 
in  a  magma;  his  second,  igneous  emanations,  including  contact 
deposits,  and  tin  veins;  his  third,  gas-aqueous  or  pneumato- 
hydato-genetic  deposits  formed  by  magmatic  waters  mingled  with 
ground  waters.  His  fourth  and  smallest  division  includes  those 
mineral  masses  formed  by  surface  waters.  This  classification 
has  not  been  generally  accepted  because  it  brings  up  the  admit- 
tedly difficult  separation  of  meteoric  and  magmatic  water. 

The  best  genetic  classification  of  mineral  deposits  would  seem 
to  be  that  according  to  geological  processes.  Mineral  deposits 
must  have  been  formed  by  igneous  processes,  alteration,  cementa- 
tion, deformation,  erosion,  or  sedimentation.  Recognizing  this, 
Van  Hise2  classifies  ores  as  follows:  Those  produced  (1)  by  proc- 

1 W.  H.  Weed,  In  "Ore  deposits,"  a  discussion  republished  from  the 
Eng.  and  Min.  Jour.,  New  York,  1903,  pp.  20-23. 

2  C.  R.  Van  Hise,  A  treatise  on  metamorphism,  Mon.  47,  TJ.  S.  Geol. 
Survey,  1904. 


CLASSIFICATION  OF  MINERAL  DEPOSITS      199 

esses  of  sedimentation;  (2)  by  igneous  processes;  (3)  by  meta- 
morphic  processes,  including  under  this  heading  practically  all 
veins  and  allied  geological  bodies,  conceiving  them  to  be  de- 
posited by  the  circulating  ground  water. 

It  is  probably  impossible  to  produce  a  classification  which  will 
win  the  approval  of  all.  In  the  ultimate  analysis  by  far  the 
larger  number  of  mineral  deposits  have  been  formed  by  physico- 
chemical  reactions  in  solutions,  whether  these  were  aqueous, 
igneous,  or  gaseous.  According  to  this  view  the  only  con- 
sistent division  that  can  be  made  is  that  between  deposits 
formed  by  mechanical  concentration  of  pre-existing  minerals 
and  those  formed  by  reactions  in  solutions. 

A  genetic  classification  should  not  be  confined  to  a  general 
indication  of  the  relative  time  of  ore  deposition — whether  at 
the  same  time  or  later  than  the  country  rock.  Nor  should  it 
confine  itself  to  a  statement  of  the  agents  of  ore  deposition — 
whether  aqueous,  igneous,  or  gaseous  solutions,  or  whether 
sedimentary,  igneous,  or  metamorphic  processes.  The  state- 
ment of  the  place  of  ore  deposition — at  the  surface  or  below  it; 
in  shallow  waters  or  in  deep  seas — is  important  but  not  sufficient. 

Some  authors  have  attempted  a  classification  by  mode  of 
deposition — whether  by  replacement  or  by  filling  of  open  cavities 
— but  all  such  attempts  have  been  failures,  for  the  two  processes 
are  so  closely  associated  that  separation  is  impossible. 

The  genetic  classification  should  ultimately  determine  the 
limits  of  ore  deposition  in  each  class  by  temperature  and  pressure. 
Each  deposit  should  be  considered  as  a  problem  in  physical 
chemistry,  and  the  solution  of  this  problem,  with  the  necessary- 
geological  data,  will  suffice  to  fix  the  mode  of  formation  of  the 
deposit. 

We  are  far  from  having  the  complete  material  for  such  a  classi- 
fication, but  we  have  at  least  a  few  starting  points.  It  is  neces- 
sary to  determine,  by  experiment  or  by  observation  in  nature, 
the  limits  of  existence  of  each  mineral  species.  Some  will  be 
found  to  be  "persistent"  under  widely  differing  conditions  of 
temperature  and  pressure — like  fluorite,  quartz,  or  gold.  For 
others  a  far  more  limited  range  will  be  established.  By  col- 
lecting the  data  of  mineral  association,  sequence  of  deposition, 
and  stability  range  of  the  component  parts  of  the  deposit  it  will 
be  possible  to  ascertain  the  conditions  prevailing  at  the  time  of 
ore  deposition. 


200  MINERAL  DEPOSITS 

An  absolutely  consistent  genetic  classification  is  at  present 
impracticable  for  its  forces  the  geologist  to  take  a  definite  stand 
on  problems  which,  as  yet,  have  not  been  solved.1 

Perhaps  it  is  well  not  to  expect  too  much  from  physical 
chemistry,  magnificent  as  its  services  have  been.  The  com- 
plications, even  in  simple  systems,  become  great  when,  besides 
temperature  and  pressure,  concentration,  mass  action,  and  time 
must  be  considered.  In  multicomponent  systems  the  difficulty 
increases  enormously.  At  the  same  time  it  is  believed  that  the 
direction  indicated  is  the  only  safe  one  to  take  in  classifying  the 
complex  phenomena  of  ore  deposition. 

OUTLINE  OF  PROPOSED  CLASSIFICATION 

Detrital  and  Sedimentary  Deposits. — In  the  scheme  followed 
in  this  book  there  are  two  major  divisions.  The  first  includes 
deposits  formed  by  mechanical  processes  of  concentration.  This 
includes  the  detrital  deposits  such  as  placers  and  quartz  sand 
formed  at  moderate  temperature  and  pressure. 

The  second  division  contains  the  great  majority  of  mineral 
deposits  which  have  been  produced  by  chemical  processes  of 
concentration.  Many  important  processes,  such  as  those  pro- 
ductive of  iron  ores  and  phosphates,  for  instance,  take  place  by 
interactions  of  solutions  in  bodies  of  surface  waters.  These  proc- 
esses may  be  of  inorganic  origin  or  they  may  take  place  through 
the  medium  of  living  bodies,  almost  always  at  moderate  tem- 
peratures. The  products  are  usually  mingled  with  detrital 
matter.  They  may  be  enriched  by  secondary  processes  in 
the  unconsolidated  strata  or  by  processes  of  weathering  after 
their  exposure  to  air. 

Another  class  of  deposits  is  formed  in  bodies  of  surface  waters 
by  their  evaporation  and  consequent  precipitation  of  the  salts 
dissolved  in  them;  these  are  frequently  termed  the  "saline  resi- 
dues." Common  salt,  gypsum,  and  borates  are  among  the  sub- 
stances found  in  these  deposits. 

Concentration  of  Substances  Contained  in  the  Rocks. — 
Instead  of  at  the  surface  or  in  bodies  of  surface  waters  the 
processes  of  concentration  of  useful  substances  may  go  on  in  the 
rocks  themselves.  We  may  distinguish  two  cases:  the  sub- 

1 T.  Crook,  The  genetic  classification  of  rocks  and  ore  deposits,  Mineralog. 
Mag.,  London,  vol.  17,  1914,  pp.  55-85. 


CLASSIFICATION  OF  MINERAL  DEPOSITS      201 

stances  were  originally  contained  in  the  same  geological  body 
in  which  the  deposit  is  found,  or  they  may  have  been  intro- 
duced from  the  outside. 

The  apparent  objection  to  this  basis  of  subdivision,  namely,  the 
difficulty  of  deciding  the  source  of  the  mineral  or  metal,  is  met 
in  many  cases  by  the  knowledge  acquired  during  late  years. 
There  may  be  deposits  for  which  the  qestion  cannot  be  decided, 
but  I  believe  that  in  the  near  future  we  shall  in  most  cases  have 
sufficiently  good  evidence.  No  one  seriously  maintains  that 
the  gold  in  the  quartz  veins  of  California,  for  instance,  has  been 
leached  from  the  surrounding  country  rock,  and  surely  no  one 
denies  that  the  oxidized  nickel  silicate  ores  of  certain  peridotites 
were  originally  contained  in  minute  distribution  in  these  rocks. 

In  the  case  of  substances  contained  in  the  geological  body 
itself,  the  concentration  may  be  effected  by  (1)  rock  decay  and 
residual  weathering — that  is,  by  oxygenated  surface  waters;  (2) 
by  the  ground  water  of  the  deeper  circulation;  (3)  by  processes 
of  dynamic  and  regional  metamorphism,  and  (4)  by  zeolitization 
of  surface  lavas. 

Residual  Weathering. — Rock  decay  tends  to  destroy  the  rocks 
as  units;  to  break  them  down,  mechanically  and  chemically,  and 
to  re-assort  their  constituents  in  new  combinations.  In  the 
decaying  mass  certain  constituents  are  concentrated  or  precipi- 
tated; its  detritus  is  swept  away  and  deposited  in  rivers,  lakes, 
and  oceans;  its  soluble  constituents  are  carried  into  the  larger 
reservoirs  of  water  and  there  perhaps  precipitated  in  various 
forms. 

It  is  true  that  not  quite  all  the  sedimentary  deposits  are 
derived  from  the  decaying  rocks;  the  fossil  coals  are  indirectly 
made  from  the  carbon  of  the  atmosphere;  volcanic  ashes  con- 
tribute a  share  to  the  sediments;  the  exhalations  of  eruptive 
magmas,  as  well  as  ascending  waters,  contribute  some  dissolved 
matter  from  the  lower  part  of  the  earth's  crust. 

Processes  of  sedimentation  and  rock  decay  take  place  at 
moderate  temperatures  and  pressures  and  the  new  minerals 
formed  are,  as  a  rule,  characterized  by  high  hydration.  Below 
0°  C.  mineral  deposits  do  not  form,  except  in  so  far  as  freezing 
of  water  is  retarded  by  rapid  motion  or  dissolved  salts.  Few 
of  the  deposits  have  been  formed  at  temperatures  above  50°,  and 
this  only  exceptionally  during  eruption,  evaporation  in  shallow 
desert  lakes,  or  oxidation  of  pyritic  rocks.  The  pressure  is  in 


202  MINERAL  DEPOSITS 

general  little  different  from  that  of  the  normal  atmosphere,  but 
in  deposits  of  deep  seas  or  lakes  considerably  higher  pressures 
prevailed.  This  increased  pressure,  at  low  temperature,  appears 
to  have  had  little  influence  on  the  mineral  associations  formed. 

Deep  Circulating  Waters. — Under  the  influence  of  the  ground 
water  of  the  deeper  circulation  many  ore  deposits  are  formed, 
concerning  some  of  which  there  may  be  room  for  differing  opin- 
ions. Copper  may  be  leached  from  greenstones  and  the  ores  of 
the  metal  may  be  deposited  in  veins  in  the  same  rock.  Hema- 
tite, like  that  of  the  Lake  Superior  region,  may  be  concentrated 
from  the  surrounding  low-grade  "iron  formation."  Barite, 
magnesite,  and  sulphur  are  other  instances. 

Regional  Metamorphism. — Again,  the  agency  may  be  meta- 
morphism  under  stress  or  regional  metamorphism;  in  such  case 
the  change  takes  place  with  very  little  water  and  it  is  not  con- 
sidered probable  that  a  great  concentration,  say  of  the  metals 
contained,  can  be  effected.  Other  materials  may  form,  such  as 
slate  from  shales,  or  useful  minerals  like  garnets  or  graphite  may 
develop  in  the  rock.  During  static  metamorphism,  temperature 
and  pressure  are  likely  to  be  somewhat  higher  than  at  the 
surface.  Regional  metamorphism  takes  place  under  heavy 
pressure  and  at  fairly  high  temperatures  at  great  depth.  It 
may  merge  into  igneous  metamorphism. 

Zeolitization. — The  processes  of  zeolitization  take  place  shortly 
after  the  consolidation  of  an  igneous  rock  by  the  aid  of  residual 
magmatic  water  or  of  surface  water.  Under  certain  circum- 
stances a  concentration  of  metals  can  be  effected  by  this  process, 
of  which  the  copper  deposits  of  Lake  Superior  offer  an  excel- 
lent instance. 

Introduced  Ores  not  Connected  with  Igneous  Rocks. — Much 
more  common  is  the  case  where  the  valuable  minerals  have  been 
introduced  into  the  rock  from  without,  and  to  this  class  belong 
the  majority  of  the  metal  deposits.  Deposits  of  this  kind  occur 
along  fissures  or  form  replacements  along  fissures  or  are  found  in 
general  where  opportunity  is  offered  for  vigorous  circulation  of 
the  depositing  waters.  For  a  long  time  it  was  held  by  many  that 
the  metallic  contents  of  fissure  veins  were  derived  from  the  sur- 
rounding rock,  but  it  is  now  generally  admitted  that  such  a  view 
in  most  cases  is  erroneous. 

Certain  metallic  ores  occur  entirely  independent  of  igneous 
rocks;  the  mineral  associations  in  these  indicate  a  deposition  at 


CLASSIFICATION  OF  MINERAL  DEPOSITS      203 

moderate  pressure  and  temperature,  the  latter  probably  rarely 
reaching  100°  C.  Of  this  kind  are  certain  lead-zinc  deposits  in 
limestone  or  the  copper  deposits  in  sandstone  which  are  so  com- 
mon in  various  parts  of  the  world.  Most  geologists  agree  that 
such  deposits  have  been  formed  by  surface  waters,  at  moderate 
depths,'  and  that  the  metals  have  been  leached  from  neighboring 
strata  and,  after  a  comparatively  short  wandering,  deposited  in 
fractured  rocks  in  their  present  resting  places.  These  deposits 
are  generally  poor  in  gold  and  silver. 

Deposits  Genetically  Connected  with  Igneous  Rocks. — 
There  is  also  another  and  larger  class  which  appears  only  in 
or  near  igneous  rock  and  whose  epoch  of  formation  usually  can 
be  shown  to  have  followed  closely  after  the  eruption.  This 
class  has  been  clearly  recognized  by  almost  all  geologists.  There 
is  also  general  agreement  that  these  deposits  have  been  laid 
down  by  heated,  ascending  waters,  although  there  is  no  unan- 
imity Las  to  the  source  of  the  water  or  the  source  of  the  metal. 
To  some  the  water  and  the  dissolved  metals  are  simply  igneous 
emanations  from  a  cooling  magma;  to  others  the  waters  are  of 
atmospheric  origin  and,  heated  by  their  passage  through  still 
warm  igneous  rocks,  have  dissolved  the  metals  contained  in 
them. 

Nearly  all  metal  deposits  of  the  American  Cordilleran  region 
belong  to  this  division.  It  is  subdivided  into  several  groups, 
according  to  the  evidence  of  mineral  association  and  geological 
relations.  The  first  group  includes  ores  deposited  at  slight 
depth  below  the  surface;  the  temperature  is  here  relatively  low, 
perhaps  from  50°  to  150°  C.,  and  the  pressure  will  scarcely  ex- 
ceed 100  atmospheres.  Examples  of  this  group  are  found  in 
the  gold  and  silver  veins,  of  Tonopah,  Nevada,  the  Cripple  Creek 
gold  telluride  veins,  and  the  California  quicksilver  veins. 

A  second  group  comprises  the  deposits  formed  by  hot  ascending 
waters  at  moderate  depths,  say  from  5,000  feet  to  10,000  feet  be- 
low the  surface,  at  temperatures  of  perhaps  from  150°  to  250°  C. 
and  correspondingly  increased  pressure.  The  present  outcrops 
are  exposed  by  deep  erosion  and  they  almost  always  appear  in  or 
close  to  intrusive  bodies.  As  examples  may  serve  the  gold- 
quartz  veins  of  California  and  the  metasomatic  pyritic  deposits 
of  Leadville. 

A  third,  deep-seated  group  includes  veins  and  contact-meta- 
morphic  deposits.  During  the  genesis  of  these  the  temperature 


204  MINERAL  DEPOSITS 

was  high,  but  in  most  cases  below  575°  C.,  the  crystallographic 
inversion  point  for  quartz.  The  pressure  was  probably  very 
high.  The  cassiterite  veins,  some  gold-quartz  veins  of  the 
Appalachian  type,  and  the  tourmaline-copper  veins  belong  in 
this  group,  which  with  great  confidence  may  be  ascribed  to 
emanations  from  magmas.  The  deposits  unquestionably 'formed 
by  direct  igneous  emanations  are  the  contact-metamorphic  ores 
appearing  in  limestone  along  igneous  contacts.  They  contain 
oxide  ores,  such  as  magnetite  and  specularite,  together  with 
sulphides  of  copper,  zinc  and  iron,  and  present  an  association 
of  other  minerals  characteristic  of  contact  metamorphism 

The  emanations  from  effusive  bodies  are  deposited  as  subli- 
mates of  little  economic  importance. 

Products  of  Magmatic  Differentiation. — The  last  class  is  that 
of  the  deposits  formed  by  concentration  in  igneous  magmas; 
of  all  types  these  have  formed  at  the  highest  temperature  and 
pressure.  They  include  oxides  or  sulphides  segregated  in  the 
magmas,  like  the  iron  ores  of  Kiruna  in  northern  Sweden,  the 
titanic  iron  ores  of  the  Adirondacks,  or  the  copper-nickel  ores  of 
Sudbury.  They  also  include  the  pegmatite  dikes,  which  contain 
many  gems  and  rare  metals  and  which  are  regarded  as  segrega- 
tions from  cooling  granitic  magmas.  The  pegmatites  were 
formed  at  comparatively  low  temperatures — probably  from  500° 
to  800°  C. — but  during  the  differentiation  of  the  other  deposits 
mentioned  considerably  higher  temperatures  probably  prevailed. 
The  pressure  must,  of  course,  have  been  very  high. 

Metamorphism  and  Surface  Enrichment  of  Deposits. — Tn  the 
proposed  classification  the  mineral  deposits  are  supposed  to  have 
suffered  no  change  from  their  original  condition.  This  is  of  course 
rarely  strictly  true,  for  chemical  changes  as  a  rule  begin  soon  after 
the  cessation  of  the  agency  which  caused  the  deposition.  In  sedi- 
mentary beds  this  is  particularly  the  case,  for  cementation  and 
hardening  and  various  chemical  actions  begin  almost  from  the 
time  of  deposition.  It  is,  however,  not  the  custom  to  refer  to 
these  changes  as  metamorphism. 

Many  mineral  deposits  have  undergone  great  changes  from 
their  original  conditions.  They  may  have  been  reached  by 
igneous  metamorphism  and  thus  a  coal  bed  transformed  into 
anthracite  or  a  bed  of  limonite  into  magnetite.  Or  they  may 
have,  been  sheared  or  crushed  during  regional  metamorphism 
Or,  most  common  of  all  cases,  they  may  have  been  altered  by 


CLASSIFICATION  OF  MINERAL  DEPOSITS      205 

surface  waters.  Such  oxidizing  surface  waters,  as  well  as 
similar  waters  at  somewhat  greater  depth,  when  they  have 
parted  with  their  free  oxygen,  produce  peculiar  modifications 
and  often  most  important  enrichments. 


A   CLASSIFICATION  OF  MINERAL  DEPOSITS1 

I.  Deposits  produced  by  mechanical  processes  of  concentration.    (Tempera- 
ture and  pressure  moderate.) 

II.  Deposits  produced  by  chemical  processes  of  concentration.     (Tempera- 
ture and  pressure  vary  between  wide  limits.) 

A.  In  bodies  of  surface  waters. 

1.  By  interaction  of  solutions. 

a.  Inorganic  reactions. 

b.  Organic  reactions. 

2.  By  evaporation  of  solvents. 

B.  In  bodies  of  rocks. 

1.  By  concentration  of  substances  contained  in  the  geological  body 
itself. 

a.  Concentration  by  rock  decay 

and  residual  weathering  near 
surface. 

b.  Concentration      by      ground 

water  of  deeper  circulation. 

c.  Concentration  by  dynamic  and 

regional  metamorphism. 

d.  Zeolitization  of  surface  lavas. 


Temperature,  0°  to  70°  C.  ± 
Pressure,  moderate  to  strong. 


C.  + 


Temperature,  0°-100C 
Pressure,  moderate. 

Temperature,  0°-100°  C.  ± 
Pressure,  moderate. 
Temperature  up  to  400°  C.  + 
Pressure,  high. 
Temperature,  50°-300°  C.  + 
Pressure,  moderate. 
2.  Concentration  effected  by  introduction  of  substances  foreign  to 
the  rock. 

a.  Origin  independent  of  igneous  activity. 
By     circulating    atmospheric 

waters  at  moderate  or  slight 
depth. 

b.  Origin  dependent  upon  the  eruption  of  igneous  rocks. 

a.  By  hot  ascending  waters  of  uncertain  origin,  but  charged 
with  igneous  emanations. 

1.  Deposition  and  concen- 

tration at  slight  depth. 

2.  Deposition  and  concen- 

tration at  intermediate 
depths. 

3.  Deposition  and  concen- 

tration at  great  depth 
or  at  high  tempera- 
ture and  pressure. 

Presented  before  the  Geological  Society  of  Washington,  May  10,  1911. 


Temperature,  to  100°  C.  ± 
Pressure,  moderate. 


Temperature,  50°-150°  C.  ± 
Pressure,  moderate. 

Temperature,  150°-300°  C.  ± 
Pressure,  high. 

Temperature,  300°-500°  C.  ± 
Pressure,  very  high. 


206  MINERAL  DEPOSITS 

b.  By  direct  igneous  emanations. 

1.  From  intrusive  bodies.     Con-  f  Temperature,      probably 
tact    metamorphic    deposits  j      300°-800°  C.  ± 
and  allied  veins;  [  Pressure,  very  high. 


2.  From  effusive  bodies.     Sub- 
limates, fumaroles. 


Temperature,  100°-400°  C. 
Pressure,    atmospheric    to 


moderate. 
C.  In  magmas,  by  processes  of  differentiation. 

a.  Magmatic    deposits    proper.     Temperature,    700°-1500°    C.    + 

Pressuse,  very  high. 

6.  Pegmatites.     Temperature,   about   575°   C.  ±.     Pressure,   very 
high. 


CHAPTER  XV 

DEPOSITS   FORMED   BY   MECHANICAL  PROCESSES   OF 
TRANSPORTATION   AND    CONCENTRATION; 
DETRITAL   DEPOSITS    , 

INTRODUCTION 

Weathering  tends  to  destroy  rocks  and  mineral  deposits  by 
disintegration  and  chemical  decomposition.  In  part,  new 
minerals,  like  kaolin  and  limonite,  form;  in  part,  the  more  resist- 
ant minerals,  like  quartz,  gold,  platinum,  magnetite,  cassiterite, 
and  garnet,  are  set  free  in  individual  grains.  Erosion  now  steps 
in  and  the  detritus  is  swept  down  the  slopes  and  into  the  water 
channels.  Mechanical  separation  in  running  water  or  along  sea 
or  lake  beaches  sorts  the  detritus  according  to  specific  gravity 
and  size  of  grains.  The  heaviest  particles,  as  those  of  gold, 
magnetite,  and  garnet,  tend  to  collect  in  the  lower  part  of  the 
assorted  detritus;  the  minute  and  easily  moved  scales  of  clayey 
substance  are  carried  far  but  ultimately  deposited  as  sedimentary 
beds;  the  colloids  are  coagulated  by  the  electrolytes  in  the  sea 
water. 

DETRITAL  QUARTZ  DEPOSITS 

The  quartz  grains  are  often  accumulated  as  beds  of  almost 
pure  quartz  sands.  These  are  used  extensively  as  ingredients 
in  pottery  and  glass,  also  for  abrasive  purposes  in  sawing  soft 
rocks,  such  as  marble.  Such  sands  should  contain  99  per  cent, 
silica.  Somewhat  argillaceous  quartz  sands  without  carbonates 
and  carrying  80  to  90  per  cent,  silica  are  used  as  molding 
sands  and  are  mined  on  a  large  scale,  though  occurring  in  thin 
beds.1  When  compacted  by  pressure  and  by  cementation  the 
quartz  sands  are  transformed  into  siliceous  sandstones  and 
quartzites  and  these  are  used  for  millstones,  whetstones,  and 
grindstones.  Comparatively  few  Realities  furnish  good  material. 

1  E.  F.  Burchard,  Requirements  of  sand  and  limestone  for  glass-making, 
Bull.  285,  U.  S.  Gteol.  Survey,  1906,  pp.  473-475. 

L.  Heber  Cole,  The  occurrence  and  testing  of  foundry  molding  sands, 
Trans.,  Canadian  Min.  Inst.,  vol.  20,  1917,  pp.  265-291. 

207 


208  MINERAL  DEPOSITS 

With  the  development  of  modern  methods  of  grinding  the  im- 
portance of  millstones  has  greatly  decreased.  Technical  and 
statistical  information  on  these  subjects  is  contained  in  Mineral 
Resources  of  the  United  States,  1916,  part  2,  under  "Abrasive 
Materials"  and  "Sand  and  Gravel"  and  for  1916,  p.  634,  where 
a  list  of  literature  may  also  be  found. 

In  case  of  very  fine-grained  whetstones  a  doubt  may  exist 
whether  the  material  is  of  detrital  origin  or  formed  by  chemical 
agencies.  The  so-called  novaculite  of  Arkansas,  the  best  whet- 
stone known,  is  a  good  example  of  this.  It  occurs  in  the  Silurian 
beds  of  Garland  and  Saline  counties  in  that  State,  and  is  used  for 
what  are  known,  according  to  color  and  quality,  as  Washita  and 
Arkansas  stones.  The  latter  are  snow-white  and  are  the  harder. 
The  rock  is  much  jointed  and  only  small  pieces  are  obtainable. 
Branner  considers  this  material  a  metamorphosed  chert,  while 
Griswold1  believes  it  to  be  a  fine-grained  sediment. 

DETRITAL  CLAY  DEPOSITS2 

The  fine  material  resulting  from  the  decay  of  rocks  is  carried 
away,  suspended  in  water,  and  deposited  in  river  beds,  lakes, 
and  seas  as  sedimentary  clay.  The  nature  of  clays  is  a  much 
discussed  subject.  Perhaps  the  best  definition  is  given  by  G.  P. 
Merrill,  who  says3  that  the  clays  are  widely  diverse  in  origin  and 
in  mineral  and  chemical  composition  but  have  the  common  prop- 
erty of  plasticity  when  wet  and  that  of  induration  when  dried. 
Clays  are  finely  comminuted  aggregates  of  hydrous  aluminous 
silicates,  detrital  quartz  and  other  mineral  fragments,  often  also, 
iron  hydroxide  and  calcic  and  magnesic  carbonates.  The  sedi- 
mentary clay  is  therefore  to  be  regarded  rather  as  a  rock  than  as 
a  mineral  and  its  principal  use  is  for  structural  purposes;  The 
detailed  description  of  these  deposits,  therefore,  does  not  fall 
within  the  scope  of  this  book. 

The  larger  part  of  the  clays  are  derived  from  decomposition 

1L.  S.  Griswold,  Whetstones  and  the  novaculites  of  Arkansas,  Ann. 
Rept.  Arkansas  Geol.  Survey,  vol.  3,  1890. 

2  For  more  details  in  regard  to  the  important  clay  industry  the  reader 
is  referred  to  H.  Ries,  Clays,  New  York,  1908.     Information  as  to  pro- 
duction, etc.,  is  given  in  Mineral  Resources  of  the  United  States,  published 
annually  by  the  U.  S.  Geol.  Survey.     Further  notes  regarding  residual 
kaolin  deposits  may  be  found  on  pp.  325-328. 

3  G.  P.  Merrill,  Rocks,  rock-weathering,  and  soils,  New  York,  1897,  p.  135. 


TRANSPORTATION  AND  CONCENTRATION     209 

and  hydration  of  feldspathic  minerals;  other  silicates,  however, 
contribute  their  share.  It  has  been  supposed  that  the  mineral 
kaolinite  (H4Al2Si2O9).  is  one  of  the  principal  constituents  of 
clay.  Probably  it  is  present  because  the  formation  of  kaolinite 
from  feldspars  can  easily  be  traced  in  decomposing  rocks  at  the 
surface,  but  in  the  clays  the  mineral  is  so  comminuted  that  it 
cannot  be  readily  identified.  It  is  known  that  colloid  hydrous 
silicates  of  aluminum  exist  and  there  are  also  a  number  of  more 
or  less  indefinite  compounds  of  this  kind  in  nature,  such  as  halloy- 
site,  smectite,  and  pholerite.  The  sedimentary  clays  rarely 
approach  kaolinite  in  composition.  Kaolinite  should  contain 
46.5  per  cent.  Si02,  39.5  per  cent.  A12O3,  and  14  per  cent.  H2O; 
but  by  reason  of  admixture  of  quartz  and  undecomposed  silicates, 
the  sedimentary  clays  usually  contain  much  more  silica  than  the 
amount  indicated. 

Clays  without  carbonates  generally  contain  more  magnesium 
than  calcium,  and  potassium  exceeds  sodium.  Titanium  often 
exceeds  one  per  cent.  Much  of  the  titanium  and  potassium  is 
probably  present  in  colloid  state.  Traces  of  copper,  nickel,  lead, 
zinc  and  vanadium  are  sometimes  found. 

Regarding  residual  clays  derived  from  the  decomposition  of 
rocks  in  place  see  p.  325.  The  clays  formed  by  the  action  of 
sulphuric  acid  on  silicates  in  the  oxidized  part  of  ore  deposits  are 
described  on  p.  480. 

FULLER'S  EARTH ' 

Fuller's  earth  is  the  name  given  to  certain  sediments  of  clay- 
like  material,  originally  used  in  England  by  fullers  for  cleansing 
cloth  of  grease.  At  present  this  substance  is  extensively  used 
for  deodorizing,  decolorizing  and  clarifying  fats  and  oils;  much 
of  it  is  employed  in  the  refining  of  petroleum.  Its_value  thus 
depends  upon  its  adsorbent  qualities. 

The  material  occurs  in  sedimentary  beds  of  Mesozoic,  Cenozoic, 

1  J.  T.  Porter,  Properties  and  tests  of  fuller's  earth,  Bull.  315,  U.  S.  Geol. 
Survey,  1907,  pp.  268-290. 

T.  W.  Vaughan,  Fuller's  earth  of  Florida  and  Georgia,  Bull.  213,  U.  S. 
Geol.  Survey,  1903,  pp.  392-399. 

Mineral  Resources,  U.  S.  Geol.  Survey.  Annual  publication.  Articles 
by  F.  B.  Van  Horn  and  J.  Middleton. 

Charles  L.  Parsons,  Fuller's  earth,  Bureau  of  mines,  Bull.  71,  1913. 

E.  H.  Sellards  and  H.  Gunter,  Second  Annual  Report,  Florida  Geol. 
Survey,  1908-09,  pp.  255-290. 


210  MINERAL  DEPOSITS 

and  Quaternary  age,  but  a  similar  material  is  also  derived  from 
the  weathering  of  basic  igneous  rocks.  Microscopic  examina- 
tion gives  little  evidence  of  its  origin;  in  color  it  ranges  from 
gray  to  dark  green;  it  possesses  little  or  no  plasticity.  The 
chemical  analysis  also  has  little  value  in  determining  its  quality. 
J.  T.  Porter  believes,  and  probably  justly,  that  the  material  owes 
its  quality  to  the  adsorbent  power  of  colloid  hydrous  aluminium 
silicates. 

The  analyses  show  that  the  silica  varies  between  47  and  75 
per  cent.,  alumina  from  10  to  19  per  cent.,  lime  from  1  to  4 
per  cent.,  magnesia  from  2  to  4  per  cent.,  ferric  oxide  from  2  to 
10  per  cent.,  and  combined  water  from  5  to  21  per  cent. 

In  Gadsden  County,  Florida  and  Decatur  County,  Georgia,  it 
occurs  in  Tertiary  strata  and  is  mined  in  open  pits;  in  Arkansas 
it  is  obtained  from  weathered  basic  dikes.  The  further  prepara- 
tion includes  drying,  grinding  and  bolting  to  sizes  from  30  to 
100  mesh  per  inch.  Very  fine  material  clogs  the  filter  presses. 
The  domestic  production  amounts  to  about  70,000  short  tons  and 
about  17,000  tons  were  imported.  Florida  yields  most  of  the 
total  domestic  production.  The  price  of  the  Florida  material  is 
about  $10  per  ton. 

PtACER  DEPOSITS 

Origin  and  Distribution. — The  heavier  and  less  abundant 
minerals  in  the  rocks  are  the  most  resistant  to  decomposition 
and  when  the  weathered  rock  is  eroded  and  sorted  by  water  they 
usually  become  concentrated  in  the  lower  parts  of  the  sand 
and  gravel  beds.  The  gold-bearing  gravels,  which  form  an 
important  source  of  supply  of  this  metal,  were  called  placers1 
by  the  early  Spanish  miners  of  this  continent,  and  this  name  is 
probably  the  best  that  can  be  adopted  for  deposits  of  this  class. 
Instead  of  gold  the  valuable  mineral  may  be  cassiterite,  magnet- 
ite, monazite,  diamonds,  or  other  precious  stones.  Other  terms 
have  been  employed,  as  "gravel  deposits"  or  " gold-bearing 
gravels,"  or  "alluvial  deposits"- — all  equally  objectionable,  for 
the  material  may  be  sand  instead  of  gravel,  and  it  may  be 
deposited  along  the  ocean  beach  instead  of  in  watercourses. 

1  Derivation  uncertain:  Placer,  pleasure;  Plaza,  place.  Stelzner  (Die 
Erzlagerstatten,  p.  1261)  says  placer  is  a  local  Spanish  term  for  sand  bank. 
The  Germans  use  "Seife,"  meaning  washings.  In  French  the  word  "allu- 
vions" is  often  used. 


TRANSPORTATION  AND  CONCENTRATION     211 

The  processes  of  erosion  and  concentration  have  been  active 
since  earliest  geologic  time,  and  so  we  may  have  detrital  depos- 
its or  placers  of  differing  ages.  Land  deposits  are,  however, 
usually  thin  and  easily  removed  and  thus  placers  of  pre-Tertiary 
age  are  comparatively  rare. 

In  the  formation  of  placers  nature  simply  employs  in  her 
own  leisurely  way  the  processes  of  crushing  and  concentration 
which  we  use  in  ore  dressing.  The  rocks  are  broken  and  com- 
minuted by  the  expansion  due  to  alternating  heat  and  cold;  by 
the  growth  of  plants;  or  by  the  impact  of  sliding  and  water- 
carried  rocks;  or  by  the  grinding  action  of  ice;  or  finally  by 
chemical  decomposition  and  hydration.  The  products  are  con- 
centrated in  water  courses  or  along  shores  by  running  water  or 
in  ocean  currents  by  motion  similar  to  that  on  tables  and  jigs. 
Spherical  particles  of  different  substances  fall  in  water  at  a  rate 
proportional  to  their  weight  divided  by  the  resistance.  As  the 
resistance  is  proportional  to  the  area  exposed,  a  fragment  of" 
quartz  the  size  of  a  pea  will  fall  much  more  slowly  than  a  piece 
of  gold  of  the  same  size.  It  will  in  fact  be  carried  along  easily 
in  a  current  of  water  in  which  a  piece  of  gold  of  the  same  size 
will  sink  instantly.  Thus  the  specific  gravities  of  the  valuable 
minerals  play  a  prominent  part  in  the  formation  of  placers. 
The  specific  gravity  of  the  more  important  substances  is  as 
follows:  Quartz,  2.64;  feldspar,  2.55  to  2.75;  ferromagnesian 
silicates,  2.9  to  3.4;  garnet,  3.14  to  4.13;  diamond,  3.54;  corun- 
dum, 4.0;  monazite,  5.0;  magnetite,  5.0;  cassiterite,  6.4  to  7.1; 
gold,  15.6  to  19.33;  platinum,  14.0  to  19.0  (21  to  22  when 
chemically  pure). 

The  shape  of  the  particles  is  also  of  importance.  Flaky 
minerals,  like  molybdenite,  scaly  gold,  or  specularite,  are  difficult 
to  concentrate  in  spite  of  their  high  specific  gravity. 

GOLD  PLACERS 

Introduction. — Gold  is  the  most  important  placer  mineral. 
Roughly  speaking,  about  $70,000,000  out  of  a  world's  pro- 
duction of  about  $450,000,000  are  derived  from  Tertiary  or 
Quaternary  placer  deposits;  discoveries  in  Alaska  and  the  North- 
west Territory  have  lately  increased  the  output.  Gold  placers 
as  a  rule  are  easily  discovered  and  worked;  the  supplies  of  old  and 
long-settled  countries  were  generally  long  ago  exhausted.  Bo- 


212  MINERAL  DEPOSITS 

hernia,  Italy,  Spain,  and  Hungary,  now  almost  barren  of  placers, 
once  furnished  their  share.  New  deposits  are  usually  discovered 
on  the  outskirts  of  civilization,  as  in  Brazil  in  the  eighteenth 
century,  in  Australia  and  California  during  the  middle  of  the 
last  century,  and  in  Alaska  and  Siberia  to-day.  The  production 
of  placer  gold  in  the  United  States,  including  Alaska,  in  1897 
was  $7,800,000;  in  1916  it  was  $22,882,000,  the  increase  being 
due  to  the  recently  discovered  placers  of  Alaska  and  to  the 
development  of  the  dredging  fields  in  California.  Practically 
all  this  gold  comes  from  Quaternary  and  Tertiary  placers, 
some  dating  back  as  far  as  the  Eocene.  A  small  quantity  is 
obtained  from  Cretaceous  conglomerates  in  Oregon  and  north- 
ern California.  Permian  gold-bearing  conglomerates  occur  in 
Bohemia,  according  to  Posepny.1  Permo-Carboniferous  con- 
glomerates containing  detrital  gold  have  been  described  by 
Wilkinson  from  New  South  Wales.2  In  most  cases  the  gold 
content  of  these  older  conglomerates  is  small  and  they  can 
rarely  be  profitably  worked.  Probably  the  best  example  of 
ancient  placers  is  furnished  by  the  Cambrian  basal  conglomerate 
of  the  Black  Hills,  South  Dakota,  which  unconformably  covers 
the  pre-Cambrian  schists  and  gold-bearing  quartz  veins.  It  was 
first  described  by  W.  B.  Devereux3  and  later  by  J.  D.  Irving.4 
This  conglomerate,  which  is  from  2  to  30  feet  thick  and  is  overlain 
by  quartzite,  carries  in  places  much  gold  of  unquestionably 
detrital  origin,  as  indicated  by  the  rounded  grains,  and  has  been 
profitably  worked  in  several  mines.  The  gold  was  derived  from 
the  erosion  of  auriferous  lodes  in  the  pre-Cambrian  rocks  and  was 
deposited  in  depressions  along  the  old  shore  line.  In  part  the 
gold-bearing  conglomerate  is  cemented  by  pyrite,  which  probably 
also  contains  some  gold.  Maclaren5  believes  that  the  scarcity 
of  economically  important  deposits  of  detrital  gold  in  older  forma- 
tions is  due  to  its  solution,  in  depth,  by  alkaline  solutions.  There 
is  little  evidence  in  support  of  this  view. 

Origin  of  Placer  Gold. — In  primary  deposits  gold  is  mainly 
contained  in  veins,  lodes,  or  shear  zones  and  these  appear  in 

1  Genesis  of  ore  deposits,  1902,  p.  163. 

2  Idem,  p.  162. 

3  W.  B.  Devereux,' Trans.,  Am.  Inst.  Min.  Eng.,  vol.  10, 1882,  pp.  465-475. 

4  J.  D.  Irving,  Economic  resources  of  the  northern  Black  Hills,  Prof. 
Paper  26,  U.  S.  Geol.  Survey,  1904,  pp.  98-111. 

6  J.  M.  Maclaren,  Gold,  London,  1908,  p.  90. 


TRANSPORTATION  AND  CONCENTRATION     213 

rocks  of  many  different  kinds.  It  is  often  stated  that  gold  is 
distributed  as  fine  particles  in  schists  and  massive  rocks  and 
that  placer  gold  in  certain  districts  is  derived  from  this  source. 
Most  of  these  statements  are  not  supported  by  evidence,  though 
it  is  not  denied  that  gold  may  in  rare  instances  be  distributed 
in  this  manner.  Even  in  the  Yukon  region,  concerning  which 
such  statements  have  often  been  made,  the  origin  of  the  gold 
from  veins,  lodes,  and  shear  zones  is  beginning  to  be  recognized.1 

The  great  majority  of  gold  placers  have  been  derived  from  the 
weathering  and  disintegration  of  auriferous  veins,  lodes,  shear 
zones,  or  more  irregular  replacement  deposits.  These  primary 
deposits  were  not  necessarily  rich  and  may  not  be  profitable  to 
work.  In  many  regions  the  rocks  contain  abundant  joints,  seams, 
or  small  veins  in  which  the  gold  has  been  deposited  with  quartz. 

Eluvial  Deposits. — Gold  placers  may  be  formed  by  rapid 
erosion  of  hard  rocks,  but  such  placers  are  not  often  rich  and 
highly  concentrated.  In  the  great  placer  regions  the  concen- 
tration has  generally  been  preceded  by  an  epoch  of  deep  secular 
decay  of  the  surface.  It  has  been  supposed  by  many  that  this 
deep  rock  decay  is  peculiar  to  the  tropics,  but  this  is  not  correct. 
The  process  has  been  active  in  the  southern  Appalachian  States, 
in  California,  and  even  in  Alaska,  as  well  as  in  countries  like  the 
Guianas  and  Madagascar.  When  the  outcrops  of  gold-bearing 
veins  are  decomposed  a  gradual  concentration  of  the  gold  follows, 
either  directly  over  the  primary  deposits  or  on  the  gentle  slopes 
immediately  below.  The  vein  when  located  on  a  hillside 
bends  over  (Fig.  73)  and  disintegration  breaks  up  the  rocks  and 
the  quartz,  the  latter  as  a  rule  yielding  much  more  slowly  than 
the  rocks;  the  less  resistant  minerals  weather  into  limonite, 
kaolin,  and  soluble  salts.  The  volume  is  greatly  reduced,  with 
accompanying  gold  concentration.  The  auriferous  sulphides 
yield  native  gold,  hydroxide  of  iron,  and  soluble  salts.  Some 
solution  and  redeposition  of  gold  doubtless  take  place 
whenever  the  solutions  contain  free  chlorine.  The  final  result 
is  a  loose,  ferruginous  detritus,  easily  washed  and  containing 
easily  recovered  gold.  This  gold  consists  of  grains  of  rough 
and  irregular  form  and  has  a  fineness  but  slightly  greater  than 
that  of  the  gold  in  the  primary  vein.  Stelzner  has  applied  to 
such  residual  concentrations,  which  may  be  worked  like  ordinary 

1  A.  H.  Brooks,  The  gold  placers  of  parts  of  Seward  Peninsula,  Alaska, 
Bull.  328,  U.  S.  Geol.  Survey,  1908,  pp.  108  et  seq. 


214 


MINERAL  DEPOSITS 


placers,   the  term  eluvial  gold  deposits.     Their  occurrence  is 
illustrated  in  Fig.  73. 

In  the  gold  region  of  the  southern  Appalachian  States  the 
decomposition  of  the  country  rock,  which  generally  is  a  schist, 
may  reach  a  depth  of  100  feet  or  more.1  The  decomposed 
material  of  the  auriferous  veins  slides  downhill,  mixing  with  the 
weathered  rock,  and  during  this  process  the  gold  in  part  sinks 
deeper  into  the  detritus.  This  has  given  rise  to  a  peculiar 
system  of  mining  by  which  the  whole  mass  is  washed  by  the 
hydraulic  method  and  the  more  resistant  quartz  boulders  crushed 
in  a  stamp  mill  with  coarse  mesh.  This  has  been  practiced  at 
Dahlonega  and  is  often  called  the  Dahlonega  system.  Similar 


FIG.  73. — Diagram  showing  development  of  eluvial  and  stream  placers. 

deposits  were  worked  in  California,  particularly  in  Eldorado 
county,  and  are  here  called  "seam  diggings"  from  the  fact  that 
the  gold  occurs  disseminated  in  quartz  seams  traversing  a  certain 
belt  of  schists.  Such  deposits  frequently  occasion  legal  contests 
owing  to  the  uncertainty  whether  they  should  be  considered  as 
placers  or  as  mineral-bearing  veins. 

In  certain  regions  of  Brazil2  the  schists  and  gneisses  are  covered 
by  auriferous  detritus  accumulated  in  place.  Another  example 
is  the  "Tapanhoancanga"  of  the  same  country.  This  is  a  bed  of 
residual  or  lateritic  iron  ore  up  to  10  feet  thick  covering  the 

1  G.  F.  Becker,  Reconnaissance  of  the  gold  fields  of  the  southern  Appala- 
chians, Twenty-sixth  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  2,  1895. 

2  O.  Derby,  Peculiar  modes  of  occurrence  of  gold  in  Brazil,  Am.  Jour. 
Sci.,  3d  ser.,  vol.  28,  1884,  p.  440. 

O.  Derby,  Notes  on  Brazilian  gold  ores,  Trans.,  Am.  Inst.  Min.  Eng., 
vol.  33,  1892,  pp.  282-283. 


TRANSPORTATION  AND  CONCENTRATION     215 

underlying  hematite  schist  and  containing  gold  throughout. 
The  gold  probably  occurs  in  veinlets  in  the  schists  and  the 
gold-bearing  detrital  material  has  been  concentrated  from  a 
considerable  thickness  of  schist  weathering  in  place. 

Excellent  examples  of  eluvial  deposits  are  reported  from 
Dutch,  British,  and  French  Guiana,1  though  ordinary  stream 
placers  are  the  most  common  deposits  in  these  countries.  Over 
a  great  part  of  this  gold-bearing  territory  secular  decay  of 
crystalline  rocks  has  resulted  in  a  deep  mantle  of  ferruginous 
clayey  earth — laterite — and  in  places  the  gold  has  been  concen- 
trated in  this  material  below  outcrops  of  gold-bearing  veins. 
Many  of  the  stream  beds  are  also  worked  for  placer  gold,  the 
detritus  usually  resting  on  the  clayey  surface  of  the  compact 
laterite. 

It  is  stated  that  many  rocks  in  the  Guianas  centain  gold  and 
that  the  placer  gold  is  derived  from  such  material;  particularly 
are  the  basic  rocks,  diabases  and  amphibolites,  said  to  be  aurif- 
erous. This  conclusion  should  probably  be  accepted  with  some 
reserve.  It  seems  more  probable  that  the  gold  contained  in  the 
greenstones  is  of  secondary  origin  and  that  here,  as  elsewhere, 
granitic  intrusions  have  caused  the  formation  of  a  series  of 
gold-bearing  veins  in  the  surrounding  rocks. 

Processes  of  Concentration. — In  most  cases  the  cycle  has  been 
carried  further  and  the  material  is  not  only  decomposed,  but 
eroded,  transported,  and  redeposited.  This  can  be  effected  by 
wind,  by  streams,  or  by  the  surf  of  the  sea. 

Eolian  Deposits. — Deposits  concentrated  by  eolian  agencies 
can,  of  course,  be  formed  only  in  dry  countries  where  long  sub- 
aerial  decay  has  paved  the  way  for  the  work  of  the  dust' storms; 

1  C.  G.  Dubois,  Geologisch-bergmannische  Skizzen  aus  Surinam,  Freiberg 
i.  S.,  1901,  pp.  112. 

C.  G.  Dubois,  Beitrage  zur  Kenntnisa  der  surinamischen  Latent,  etc., 
Tsch.  Min.  u.  petr.  Mitt.,  22,  1903. 

E.  D.  de  Levat,  The  gold  fields  of  French  Guiana,  Mineral  Industry, 
vol.  7,  1899. 

E.  D.  de  Levat,  Guide  pratique  etc.  de  1'or  en  Guyane  francaise,  Paris, 
1898. 

A.  Bordeaux,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  41,  1910,  pp.  567-593. 

J.  B.  Harrison,  The  geology  of  the  gold  fields  of  British  Guiana,  London, 
1908. 

J.  B.  Harrison,  in  the  Reports  of  the  Instit.  of  Mines,  British  Guiana. 

E.  E.  Lungwitz,  Die  Goldseifen  von  British  Guiana,  Zeitschr.  prdkt. 
GeoL,  1900,  pp.  203-218. 


216  MINERAL  DEPOSITS 

from  the  decomposed  and  crumbled  outcrops  of  the  lodes  the 
winds  blow  away  the  lighter  sand,  leaving  a  mass  of  coarser 
detritus  which  contains  the  gold.  Such  wind-born  placers  have 
been  noted  by  H.  C.  Hoover1  and  T.  A.  Rickard2  near  the  crop- 
pings  of  the  West  Australian  gold  veins.  No  examples  of  this 
kind  are  known  from  the  Cordilleran  States  of  America. 

Stream  Deposits. — Running  water  is  by  far  the  most  important 
agency  in  the  formation  of  gold  placers.  First  of  all,  attention 
must  be  directed  to  the  high  specific  gravity  of  gold,  which 
explains  many  of  the  puzzling  features  of  the  placers.  Placer 
gold  is  six  or  seven  times  as  heavy  as  the  most  common  accom- 
panying minerals — feldspar  and  quartz — and  it  settles  to  the 
bottom  in  flowing  water  with  surprising  rapidity.  It  is  almost 
impossible  to  lose  a  particle  of  gold,  of  the  value  of  one  cent, 
in  a  miner's  pan;  it  sinks  immediately  to  the  bottom  of  the 
gravel  and  sand  after  one  or  two  preliminary  shakes  in  water. 
Once  lodged  at  the  bottom  it  stays  there,  in  spite  of  shaking 
and  rotating.  This  illustrates  the  fundamental  fact  that  (pie) 
gold  is  mainly  on  the  bed-rock.  The  rapid  settling  of  the  gold 
accounts  for  the  partial  failure  of  some  devices  for  placer  mining, 
particularly  the  clam-shell  and  the  suction  dredges. 

The  ease  with  which  some  concentration,  according  to  the 
specific  gravity,  is  effected  is  shown  by  the  well-known  fact 
that  in  powdered  samples  of  ore,  as  well  as  in  dumps  at  the 
mine,  a  settling  of  the  heavier  ore  particles  toward  the  bottom 
can  often  be  observed. 

Suppose  we  have  a  gold-bearing  quartz  vein  deeply  altered 
by  rock  decay;  now  let  the  region  be  raised  say  500  feet  by  one 
of  these  slow  oscillations  which  so  commonly  affect  the  crust. 
A  river  has  excavated  a  valley  to  the  corresponding  depth  in  this 
elevated  plateau,  and  this  valley — under  the  influence  of  a  pause 
in  the  elevating  movement — becomes  filled  with  gravels  to  a  width 
of  about  100  feet.  Let  a  tributary  gulch  with  steep  grade  be 
cut  back  into  the  plateau  to  the  gold  deposit  (Fig.  74) ;  when  the 
gulch  reaches  it  the  eluvial  deposit  will  be  carried  down  by  sliding 
and  washing;  the  clay  and  limonite  are  rapidly  removed  in  sus- 
pension; the  angular  gravel  of  quartz  and  rock,  grinding  the 

1  H.  C.  Hoover,  The  superficial  alteration  of  Western  Australian  ore 
deposits,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  28,  1899,  pp.  762-763. 

2T.  A.  Rickard,  The  alluvial  deposits  of  Western  Australia,  idem,  pp. 
480-537. 


TRANSPORTATION  AND  CONCENTRATION     217 

fragments  of  gold  between  them  and  on  the  bed-rock,  will  be 
moved  downward,  the  fine  grains  in  suspension,  the  coarser  ones 
dragging  and  rolling  on  the  bottom.  There  is  little  deposition; 
the  transporting  power  is  great  and  in  flood  time  the  whole 
gravel  mass,  of  no  great  depth,  will  probably  be  in  motion. 
Heavy  gold  nuggets  may  lodge  in  the  lee  of  little  ridges.  The 
gold  settles  rapidly;  most  of  it, "  continually  hammered ''and 
slowly  shaping  itself  in  flat,  smooth  grains,  will  be  dragged  down 
stream  and  finally  reach  the  edge  of  the  flood-plain  in  the  river. 


100  200  300  FEET 

CONTOUR  INTERVAL  50  FEET 


Strongly  Aurilerous  Gravel 
(on  Bedrock) 


FIG.  74. — Plan  of  quartz  vein  and  placers  below  it,  illustrating  the  develop- 
ment of  pay  streaks. 

At  this  place  the  larger  part  of  the  gold  stops.  It  is  not  washed 
out  with  the  sand  and  gravel  but  stays  on  the  bed-rock  near  the 
margin.  The  finer  particles  will,  of  course,  be  carried  out  a  little 
distance,  but  they  soon  sink  into  the  water-filled  gravel  after  the 
manner  of  grains  of  heavy  ores  in  concentrating  jigs.  Just  as  in 
the  gulch  the  whole  mass  of  detritus  is  transported,  so  it  is 
thought  that  in  larger  streams  the  body  of  water-soaked  gravel 
and  sand  works  downstream  very  slowly.  During  this  process 
the  lighter  gold  contained  in  the  detrital  material  also  works  for- 


218  MINERAL  DEPOSITS 

ward  and  downward,  gradually  joining  the  nuggets  or  coarser 
pieces,  which  have  already  reached  their  final  resting  ground. 

This  mode  of  operation  contains  the  key  to  the  genesis  of 
the  placers.  It  is  not  to  be  expected  that  the  coarse  and  ordinary 
fine  gold  will  be  carried  out  into  the  middle  of  wide  flood-plains. 
As  the  flood-plain  widens  it  will  cover  the  accessions  of  gold 
along  its  margin,  and  the  final  result  will  be  a  streak  of  rich 
gold-bearing  gravel,  resting  on  the  bed-rock  and  extending 
downstream  deep  underneath  the  surface.  When  this  is  traced 
upstream  the  primary  deposit,  the  vein,  will  be  found.  The 
actual  occurrences  of  course  show  infinite  variation.  Let  us 
assume  that,  as  happens  in  the  Creswick  district  in  Victoria, 
Australia,  a  broad  stream  with  moderate  grade  crosses  a  deeply 
decomposed  belt  of  soft  slate  containing  an  abundance  of  small 
veins  or  stringers  of  quartz  with  native  gold,  and  that  in  addition 
a  fair  balance  between  transportation  and  deposition  persists 
for  a  long  time.  The  result  will  be  a  gravel  deposit,  only  a  few 
feet  deep,  but  with  an  abundance  of  gold  concentrated  on  the 
bed-rock  over  the  whole  width  of  the  stream.  Each  freshet  is 
sufficient  to  churn  up  and  move  forward  the  whole  mass  of  gravel, 
continually  adding  to  the  concentrated  gold  on  the  clayey 
bed-rock. 

Again,  we  may  assume  extremely  active  erosion,  as  is  the  case 
in  the  Sierra  Nevada  of  California.  Canyons  several  thousand 
feet  in  depth  have  been  cut  in  an  uplifted  plateau,  veritable 
trenches  or  sluice  boxes,  the  grade  of  which  is  from  60  to  150 
feet  per  mile.  Stretches  of  wild  gorges  with  polished  bottoms 
alternate  with  stretches  of  less  grade  where  shallow  gravel  ac- 
cumulates. These  canyons  receive  for  long  distances  an  abun- 
dant supply  of  gold  of  all  sizes  from  older  hill  gravels  or  from 
decaying  quartz  veins.  The  result  will  be  that  but  little  gold 
will  lodge  in  the  gorges,  while  extremely  rich  shallow  gravel 
bars  will  accumulate  in  the  convex  stream  curves  (Fig.  75). 
Gradient,  volume,  and  load  usually  vary  in  the  same  stream  so 
that  deposition  may  be  going  on  in  one  part  of  its  valley  and 
erosion  in  another.  Continued  corrasion  of  the  stream-bed 
results  in  deepening  the  canyon  and  leaving  the  bars  as  elevated 
benches.  The  miners  of  1849  first  found  these  bars  and  worked 
them.  In  searching  for  the  source  of  the  gold  they  soon  found 
a  trail  of  metal  leading  up  the  gulches  to  great  masses  of  older 
gravels  on  the  hills,  2,000  to  3,000  feet  above.  These  gravels 


TRANSPORTATION  AND  CONCENTRATION     219 


were  washed  by  the  hydraulic  method;  immense  masses  of 
tailings  with  a  little  gold  were  carried  down  to  the  rivers,  totally 
overloading  them.  After  the  prohibition  of  hydraulic  mining  the 
streams  gradually  resumed  active  transportation.  The  whole 
gravel  mass  moved  slowly  downstream  and  a  gradual  recon- 
centration  on  the  bed-rock  took  place.  The  tailings  deposited 
became  enriched  and  will  ultimately  be  reworked.1 

The  torrential  floods  of  the  canyons  scarcely  permitted  the 
lodgment  of  fine  gold.     This  was  swept  out  through  the  narrow 


FIG.  75. — Low  gravel  bars,  American  River,  California,  showing  placer  de- 
posits on  inner  side  of  bends.     After  R.  L.  Dunn. 

portals  into  the  Sacramento  Valley,  where  the  grade  of  the 
streams  suddenly  diminishes.  The  most  minute  particles  may 
have  been  carried  as  far  as  San  Francisco  Bay,  but  the  bulk  of 
the  fine  gold  lodged  in  the  flood-plains  within  a  few  miles  of  the 
mouth  of  the  canyons.  Easily  caught  upon  the  clayey  "false 
bed-rock"  of  a  volcanic  tuff,  this  gold,  the  average  particles  of 
which  are  about  0.3  millimeter  in  diameter,  formed  meandering 
pay  streaks  at  the  base  of  a  sandy  gravel  bed  from  10  to  60 
feet  in  depth.  Such  deposits  are  now  worked  by  dredging. 
By  an  odd  paradox,  gold  is  at  the  same  time  the  easiest  and 
the  most  difficult  mineral  to  recover.  It  is  divisible  to  a  high 

1  G.  K.  Gilbert,  Hydraulic-mining  debris  in  the  Sierra  Nevada,   Prof. 
Paper  105,  U.  S.  Geol.  Survey,  1917. 


220  MINERAL  DEPOSITS 

degree  and  owing  to  its  insolubility  the  finest  particles  are  pre- 
served. A  piece  of  gold  worth  one  cent  is  without  trouble 
divisible  into  2,000  parts,  and  one  of  these  minute  particles  can 
readily  be  recognized  in  a  pan.  In  extreme  subdivision  the  gold 
acquires  a  scaly,  flat  form,  being  known  as  flour  gold  or  flake  gold, 
is  carried  away  very  readily  by  water,  and  does  not  sink  easily 
in  sand  or  gravel.  In  part  the  flour  gold  is  suspended  by  air  films, 
and  it  can  be  carried  away  in  rivers  of  moderate  grade  for  hun- 
dreds of  miles.  The  gold  occurring  in  the  sand  bars  of  Snake 
River,  Idaho,  is  a  good  example  of  this.1  It  will  settle  in  thin 
pay  streaks  at  bars  and  other  favorable  places,  but  the  next 
freshet  will  probably  destroy  the  sand  bars  and  sweep  the  gold 
away.  This  accounts  also  for  the  distribution  of  fine  gold  in 
great  masses  of  gravel  beds  —  for  example,  in  the  wash  600  feet 
thick  deposited  by  glacial  streams  at  Tacoma  and  other  places 
on  Puget  Sound.  Almost  every  pan  of  this  gravel  will  show 
a  "color,"  but  the  material  contains  only  a  fraction  of  a  cent 
per  cubic  yard.  The  fine  colors  along  the  Columbia  River  in 
northeastern  Washington  range  in  value  from  less  than  0.0005  to 
0.02  cent,  the  average  being  about  0.002  cent.2 

The  much-discussed  concentration  of  gold  on  the  bed-rock 
seems,  then,  to  be  due  partly  to  the  natural  jig-like  movement  in 
moderately  deep  gravels,3  during  long-continued  conditions  of 
fair  balance  between  loading  and  erosive  power;  partly  to  slow 
forward  and  downward  motion  of  heavier  gravel  masses,4  of 
which  exact  measurement  as  yet  is  lacking,  and  last  and  largely 
to  the  fact  that  heavier  gold  will  not  be  carried  out  into  the 
gravel  flats  of  rivers  of  gentle  grade  —  the  only  ones  that  have 
extensive  flood-plains  —  but  is  immediately  deposited  on  the 
marginal  bed-rock  of  the  gradually  deepening  and  widening  gravel 


The  best  conditions  for  the  concentration  of  gold  are  found 
injnoderately  hilly  countries  where  deep  secular  decay  of  rocks 
has  been  followed  by  slight  uplifts.  Subsequent  slight  elevations 
would  easily  produce  re-sorting  and  enrichment  of  the  gravels. 
In  regions  of  gold  placers  the  richest  material  is  usually  pro- 

1  J.  M.  Hill,  Gold  of  the  Snake  River,  Bull.  620,  U.  S.  Geol.  Survey, 
1916,  pp.  271-294. 

2  A.  J.  Collier,  Bull.  315,  U.  S.  Geol.  Survey,  1907,  p.  61. 

3  F.  Posepny,  Genesis  of  ore  deposits,  New  York,  1902,  p.  154. 

4  T.  A.  Rickard,  Min.  and  Sci.  Press,  Aug.  15,  1908. 


TRANSPORTATION  AND  CONCENTRATION     221 

duced  by  repeated  reworking  of  gold-bearing  gravels  by  nature. 
Each  reworking  increases  the  richness  of  the  gravels,  eliminates 
easily  decomposed  pebbles,  and  finally  results  in  a  gravel  of 
the  hardest,  most  resistant  rock — quartzite  or  quartz.  Quartz 
is  the  common  gangue  mineral  in  gold  regions;  hence  the  preva- 
lence of  "white  gravels"  or  "white  channels,"  almost  exclusively 
composed  of  white  quartz  pebbles. 


CLASSIFICATION  OF  FLUVIAT1LE  AND  MARINE  PLACERS 

According  to  their  occurrence  the  placers  may  be  conveniently 
divided  as  follows.1 

PLACERS  CLASSIFIED 


Present  topographic 
cycle 


Past  cycles,  elevated        Past  cycles,  depressed 


1.  Gulch  and  creek  i  1.  High  creek  gravels.        1.  Deep  creek  gravels, 
gravels. 


2.  River     and     b 

gravels. 

3.  Gravel  plains. 

4.  Beaches. 


Bench  gravels. 

2.  \   Hill  gravels  or  high 

river  gravels. 

3.  Elevated  gravel 

plains. 

4.  Elevated  beaches. 


2.  Deep  river  gravels. 


3.  Depressed   gravel 

plains. 

4.  Depressed  beaches. 


-^Examples  of  present  gulch,  creek,  and  river  gravels  are  not 
difficult  to  find;  they  occur  in  all  gold-bearing  regions  where 
erosion  is  active  and  where  precipitation  is  abundant  enough  to 
cause  the  sorting  and  carrying  forward  of  the  gravels  in 
the  stream  beds.  In  the  upper  parts  of  the  stream  courses  the 
gravel  will  be  coarse  and  semiangular;  in  the  lower  parts  the 
sands  increase  and  the  pebbles  are  smoother.  Where  the  rivers 
emerge  from  their  narrow  valleys  and  spread  with  gentle  grade 
over  flood-plains,  more  extensive  sand  and  gravel  beds  will 
accumulate,  generally,  however,  with  less  gold  than  in  the  more 
confined  part  of  the  course.  Some  of  the  fine  gold  may  reach  the 
sea  and  is  concentrated  by  the  surf  and  the  oblique  shore  currents 
into  thin  pay  streaks  on  the  sandy  beach. 

1  See  also  A.  H.  Brooks,  The  gold  placers  of  parts  of  Seward  Peninsula, 
Alaska,  Bull.  328,  U.  S.  Geol.  Survey,  1908,  p.  115. 


222 


MINERAL  DEPOSITS 


Marine  Placers. — Beach  placers  occur  along  many  shores  and 
arc  often  produced  by  concentration  from  a  sea  bluff  or  elevated 
gravel  plain.  The  beach  at  Nome,  Alaska  (Fig.  76),  is  a  narrow 
strip  about  200  feet  wide,  from  which  over  $2,000,000  in  fine  gold 
has  been  washed ;  the  flaky  gold  averaged  70  or  80  colors  to  the 


FIG.  76. — Diagrammatic  section  illustrating  development  of  beach  placer. 
After  A.  J.  Collier  and  F.  L.  Hess,  U.  S.  Geol  Survey. 

cent.1  ^Two  older  elevated  beach  lines  are  found  farther  inland. 
The  beach  gold  of  the  Oregon  and  California  coasts  is  much 
finer,  the  colors  ranging  from  100  to  600  to  the  cent. 


FIG.  77.— Gold  dredging  on  the  Solomon  River,  Alaska.     After  P.  S.  Smith, 
U.  S.  Geol.  Survey. 

Buried  Placers. — Subsidence  or  overloading  may  cause  the 
placers  to  be  deeply  covered  by  barren  detritus.  Many  of  the 
streams  of  Alaska,  particularly  in  their  lower  reaches,  are  thus 
covered;  the  process  of  concentration  is  stopped,  the  present 

1  A.  J.  Collier  and  F.  L.  Hess,  Bull.  328,  U.  S.  Geol.  Survey,  1908,  pp. 
140-228. 


TRANSPORTATION  AND  CONCENTRATION     223 

watercourses  having  insufficient  grade  to  effect  the  transporta- 
tion of  detritus.  Fig.  77  shows  the  dredging  operations  on  the 
Solomon  River,  Alaska.  The  depth  of  the  gravel  in  the  river 
bottom  is  about  20  feet.  Fig.  78  shows  a  diagrammatic  section 
of  the  Oroville  dredging  ground,  Butte  County,  California.  The 
depth  of  the  gravel  is  about  30  feet.  At  Fairbanks,  Alaska, 
according  to  Prindle,1  the  placers  occur  in  tributaries  of  moderate 
length,  which  flow  in  open  valleys;  some  of  the  deposits  are  as 
much  as  300  feet  deep.  The  pay  gravels,  in  part  subangular,  lie 
on  the  bed-rock  and  are  from  a  few  inches  to  12  feet  in  thickness; 
these  are  covered  by  10  to  60  feet  of  angular  wash,  evidently 
accumulated  rapidly  without  opportunity  for  concentration,  and 
above  this  rests  a  thick  deposit  of  muck  over  which  the  sluggish 
N  S 

FEATHER  RIVER 
Upper  L  DREDGING  GROUND 


s^gEZ~^r^^e™tionl6°" 
w^s^ 


-About  4  Miles- 


Fio.  78. — Diagrammatic    section   across    Feather    River   below    Oroville, 
California,     a,  Bed-rock;  b,  lone  formation;  c,  tuffs  of  Oroville. 

streams  pursue  their  way.  The  richest  gravel  worked  in  1905, 
containing  from  $5  to  $10  per  cubic  yard,  occupies  pay  streaks 
on  the  bed-rock  150  to  200  feet  wide,  considerably  less  than  the 
average  width  of  the  valley  bottom.  All  the  gravel  on  the 
bed-rock  is,  however,  more  or  less  auriferous.  The  gold  is 
moderately  coarse.  Near  the  head  of  the  stream  deposition 
closely  follows  cutting  and  there  the  deeply  buried,  more  or  less 
permanently  frozen  pay  streaks  of  the  lower  valleys  merge  into 
the  deposits  of  the  present  stream  activity. 

On  a  large  scale  similar  conditions  prevailed  in  Victoria, 
Australia.2  Here  there  existed  in  Pliocene  time  an  extensive 
river  system  with  shallow,  well  washed,  and  locally  extremely 
rich  gravels  which  were  formed  during  a  prolonged  time  of  nice 
balance  between  erosion  and  deposition.  The  region  then 

1L.  M.  Prindle,  The  Fairbanks  and  Rampart  quadrangles,  Bull.  337, 
U.  S.  Geol.  Survey,  1908. 
2  W.  Lindgren,  Min.  Mag.,  2,  1905,  p.  33. 
E^W.  Lindgren,  Eng.  and  Min.  Jour.,  Feb.  16,  1905. 
i  H.  L.  Wilkinson,  Trans.,  Inst.  Min.  and  Met.,  1907,  p.  9. 
Stanley  Hunter,  Mem.  7,  Geol.  Survey  Victoria,  1909. 


224 


MINERAL  DEPOSITS 


became  depressed  and  covered  by  thick  beds  of  sand  and  clay. 
Above  this  were  poured  out  basalt  flows,  in  places  several  hun- 
dred feet  thick  (Figs.  79  and  80).  The  broad  valleys  remain  on 
the  whole  as  before,  but  the  present  streams  are  weak  and  have 
little  power  of  transportation  and  concentration.  The  dis- 
coveries of  gold  were  made  near  the  sources  of  the  old  rivers, 


Surface 


0 
SCALE   (= 


100        200        300 


FIG.  79. — Diagram   illustrating  buried   gravel    channels    (deep   leads)   of 
Victoria,  Australia,  and  method  of  mining  these  deposits. 

where  their  gravels  are  near  the  surface;  they  were  followed  up- 
ward into  the  gullies  of  the  slate  hills,  and  downward  below  the 
level  of  the  basalt  flows.  Such  were  the  conditions,  for  instance, 
at  Ballarat.  South  of  Ballarat  certain  of  the  Pliocene  stream 
gravels  merge  into  coastal  gravel  plains,  soon  becoming  marine  in 
character.  Such  coastal  gravel  beds  are  opened  in  the  Pitsfield 


FIG.  80.— Longitudinal  section  of  the  Chiltern  Valley  and  Rutherglen  deep 
leads,  Victoria,  Australia,  showing  steeper  grade  of  Tertiary  river  beds. 

mines,  where  the  pay  streaks  of  fine  gold,  resting  on  an  almost 
level  bed-rock,  are  worked  beneath  several  hundred  feet  of 
sands  and  gravels. 

The  Sierra  Nevada  of  California,1  on  the  other  hand,  offers  an 
excellent  instance  of  the  result  of  elevation  on  gravel  deposits. 
In  the  early  Tertiary  the  surface  of  this  range  was  comparatively 
gentle,  and  during  long  periods  of  rock  decay  and  well-balanced 

1  W.  Lindgren,  The  Tertiary  gravels  of  the  Sierra  Nevada,  Prof.  Paper 
73,  U.  S.  Geol.  Survey,  1911. 


TRANSPORTATION  AND  CONCENTRATION     225 

conditions  gold  from  the  quartz  veins  had  become  strongly 
concentrated  on  the  bed-rock  of  the  streams.  The  deeper  gravels 
were  then  covered  by  a  considerable  thickness  of  more  rapidly 
accumulated  and  poorer,  but  well-washed  material,  and  this  in 
turn  by  heavy  masses  of  rhyolitic  tuffs  and  andesite  breccias  so 
that  the  old  channels  were  sealed  in  places  by  as  much  as  1,500 
feet  of  superincumbent  barren  material.  The  range  was  elevated 
by  mountain-building  disturbances;  new  rivers  were  laid  out  and 
rapidly  eroded  canons  to  a  depth  of  2,000  or  3,000  feet.  Even- 
tually the  old  gravels  were  exposed  and  now  rest  as  more  or  less 
connected  remnants  on  the  summits  of  the  ridges  between  the 
modern  canons;  the  heavy  gravel  masses  are  worked  by  the 
hydraulic  method,  or  the  pay  streak  on  the  bed-rock  is  extracted 
by  tunneling  operations  in  the  "drift  mines"  (Figs.  81,  82,  83). 


FIG.  81. — Schematic  representation  of  the  four  principal  epochs  of 
Tertiary  gravels  in  the  Sierra  Nevada,  a,  Deep  gravels  (Eocene);  6,  bench 
gravels  (Miocene);  c,  rhyolitic  tuffs  and  inter-rhyolitic  channel;  d,  andesitic 
tuffs  and  intervolcanic  channel. 

The  gold  from  the  destroyed  portions  of  the  old  channels, 
together  with  more  set  free  from  the  quartz  veins  during  the 
erosion,  accumulated  in  the  modern  canons.  Along  their  slopes 
benches  remain  in  places,  indicating  transient  accumulations  of 
gravel  during  the  process  of  canon  cutting. 

Somewhat  similar  conditions  exist  in  some  parts  of  Alaska. 
Near  Nome  on  the  ridges  surrounding  Anvil  Creek  are  "high 
gravels"  600  to  700  feet  above  the  present  rivers.  These  gravels, 
some  of  which  are  rich,  are  the  remnants  of  an  old,  now  almost 
wholly  eroded  system  of  drainage. 

In  the  Klondike  also  high  gravels  occur  a  few  hundred  feet 
above  the  present  creeks,  the  most  conspicuous  instance  being 
the  "White  channel,"  described  by  McConnell1  (Fig.  84). 

1  R.  G.  McConnell,  Klondike  gold  fields,  Ann.  Rept.,  Geol.  Survey  Canada, 
14,  1901. 

R.  G.  McConnell,  Report  on  gold  values  in  the  Klondike  high-level 
gravels,  Ann.  Rept.,  Geol.  Survey  Canada,  1907. 


226 


MINERAL  DEPOSITS 

Elevated  beaches  have  been  mined,  for  in- 
stance, at  Nome,  where  there  are  two  old  beach 
lines  37  and  70  feet  above  the  present  level  of 
the  ocean.  In  Santa  Cruz  County,  California, 
a  similar  elevated  beach  was  mined  for  some 
time.  Gold-bearing  beach  sand  occurs)all  along 
the  Pacific  coast  from  San  Diego  to  Alaska,  and 
in  many  other  parts  of  the  world. 

Size  and  Mineral  Association  of  Placer 
Gold. — Gold  occurs  in  placers  in  all  sizes, 
from  masses  weighing  200  pounds  to  the  most 
minute  flakes.  Large  nuggets  are  recorded 
from  California;  still  larger  specimens,  weigh- 
ing as  much  as  2,184  ounces,  were  obtained 
in  Victoria,  Australia.  It  is  often  stated 
that  heavier  masses  occur  in  placers  than  in 
quartz  veins.  This  is  decidedly  erroneous.  A 
mass  of  native  gold  found  in  the  Monumental 
mine  of  Sierra  County,  California,  weighed 
1,146  troy  ounces,  and  a  quartz  vein  at  Hill 
End,  New  South  Wales,  yielded  a  specimen 
which  contained  about  3,000  ounces.  Every 
one  who  has  had  much  experience  in  gold 
mining  has  noted  the  occurrence  of  thick  sheets 
and  masses  of  gold  in  deposits  of  certain 
kinds — for  instance,  in  the  pockety  quartz 
veins  of  Alleghany,  California.  Almost  all 
the  so-called  placer  nuggets  of  unusual  size 
have  been  obtained  from  superficial  deposits 
at  or  just  below  the  croppings  of  rich  veins. 
This  applies  to  the  Ballarat  nuggets,  weigh- 
ing from  80  to  160  pounds,  which  occurred 
in  small  steep  gulches  underneath  the  basalt 
flows,  but  immediately  below  the  extremely 
rich  outcrops  of  the  quartz  veins.  It  also 
applies  to  the  nuggets  of  Carson  Hill,  Cali- 
fornia, the  Poseidon  nugget  of  Victoria 
(found  in  1906  and  weighing  953  troy  ounces), 
and  other  occurrences.  Some  very  rich  placer 
deposits — for  instance,  those  of  the  Klondike, 
Yukon  Territory,  and  the  Berry  mines  in 


TRANSPORTATION  AND  CONCENTRATION     227 

Victoria,  Australia — contain  no  specially  large  pieces  of  gold. 
The  heaviest  nugget  found  in  the  Klondike  is  said  to  have 
weighed  85  ounces. 

The  angularity  of  the  gold  is  proportional  to  the  distance 
traveled;  the  final  product  is  usually  a  flat,  rounded  grain  from 
a  fraction  up  to  1  millimeter  in  diameter.  Occasionally  crys- 
tallized gold  is  found  in  placers,  but  this  is  unusual  and  indicates 
close  proximity  of  the  primary  deposit. 

There  is  probably  no  authenticated  case  of  crystallized  gold 
occurring  in  the  gravels  of  larger  water  courses  where  there  has 
been  long  transportation,  and  this  is  assuredly  a  strong  argument 
against  the  assumption  that  such  crystals  are  formed  by^second- 
ary  processes  in  the  gravels. 

Fragments  of  quartz  often  adhere  to  the  gold  or  form  part  of 


jsSu^. 

1 

^^___ 

Volcanic 

Capping 

/. 

"~~-~- 

.  

= 

r~jr~~233| 

.r.iu-':  x  sV:i 

3000 

.                    ]>MK%;  2  

FIG.  83. — Longitudinal  section  of  "Blue  gravel  channel,"  at  Breece 
&  Wheeler  mine,  Forest  Hill  divide,  Placer  County,  California.  After 
R.  E.  Browne. 

the  rounded  nugget.  While  the  quartz  pebbles  so  abundantly 
found  in  gold-bearing  gravels  do  not  ordinarily  contain  visible 
gold,  there  are  many  instances  of  such  occurrences — for  ex- 
ample, at  Elk  City  and  Idaho  City,  Idaho,  and  at  Dutch  Flat 
and  Nevada  City,  California.  Some  placer  gold,  more  fre- 
quently the  scaly  variety,  is  covered  by  a  thin  film  of  silica, 
manganese  dioxide,  or  limonite,  and  does  not  amalgamate  easily. 
The  most  abundant  minerals  associated  with  the  gold  in  placers 
are  magnetite  and  ilmenite  ("black  sand"),  garnet,  zircon 
("white  sand");  and  monazite  ("yellow  sand"),  as  well  as  many 
others  of  the  heavy  minerals  occurring  in  the  rocks  which  contain 
the  primary  gold  deposits.  Cassiterite  is  common  in  placers,  and 
some  deep  gold  placers  in  Victoria  contain  enough  to  make  it  a 


228 


MINERAL  DEPOSITS 


valuable  by-product.  Gray  platinum  and  silvery  foils  of  iridos- 
mine  are  present  in  small  quantities  in  many  California  placers 
adjacent  to  areas  of  serpentine.  None  of  the  minerals  mentioned 
are  ordinarily  derived  from  the  gold-bearing  veins,  but  from  the 
surrounding  rocks. 

Pyrite  or  marcasite  may  form  in  the  gravels;  sometimes  this 
pyrite  contains  a  little  gold,  but  contamination  of  the  assay  sam- 


Muck 

Stream  gravels 
Terrace  gravels 
White  gravels  / 


e      Yellow  gravels 
f     High  level  River  gravels 
Klondike  schist 


y  White  Channel  gravels 


FIG.  84. — Sections  across  Bonanza  Valley,  Yukon  Territory,  showing 
several  types  of  gravel  deposits.  After  R.  G.  McConnell,  Geol.  Survey 
Canada. 

pies  by  the  placer  gold  itself  is  always  a  possibility.  Again  the 
pyrite  may  be  clastic  and  derived  from  the  surrounding  rocks, 
for  pyrite  does  not  seem  to  oxidize  readily  in  running  water; 
or,  as  near  Nevada  City,  in  the  Harmony  channel,  the  gravel 
may  contain  undecomposed  pyrite,  rich  in  gold,  and  derived 
directly  from  the  primary  veins  over  which  the  water  course 
flowed. 

Other  occasional  associates  of  gold,  probably  derived  from  its 
primary  deposits,  are  silver  in  nuggets  (Alaska),  native  bismuth 


TRANSPORTATION  AND  CONCENTRATION     229 

(Queensland  and  Alaska),  native  amalgam,  palladium-gold, 
native  copper,  and  cinnabar.  The  presence  of  native  lead  has 
usually  been  explained  by  accidental  admixture  of  hunter's  shot, 
but  J.  Park1  asserts  that  there  is  an  instance  of  its  undoubted 
presence  in  gravel,  the  lead  containing  a  skeleton  of  native  gold. 

Fineness  and  Relation  to  Vein  Gold. — The  fineness  of  placer 
gold  (or  parts  of  gold  per  thousand)  varies  from  about  500  up  to 
999.  Silver  is  always  alloyed  with  the  gold,  but  other  metals 
are  rarely  prominent;  copper  is  occasionally  present.  While 
vein  gold  may  have  a  fineness  of  997  to  999,  this  is  exceptional; 
far  more  commonly  its  fineness  ranges  from  500,  which  corre- 
sponds to  electrum,  to  about  800  or  850.  This  is  assumed  to  be 
determined  on  specimens  of  native  gold,  for  it  is  obviously  not 
fair  to  take  the  usually  lower  figure  of  the  retorted  bars,  which 
become  admixed  with  impurities  during  amalgamation.  The 
placer  gold  in  any  district  will  usually  be  of  higher  grade  than  the 
vein  gold,  and  its  fineness  increases  with  the  distance  transported 
and  with  the  decreasing  size  of  the  grains.  Thus,  while  in  Cali- 
fornia the  vein  gold  averages  850  fine,  the  transported  placer 
gold  in  the  Tertiary  channels  averages  930  to  950.  It  has  been 
shown  that  this  increase  in  fineness  is  due  to  the  solution  of  the  sil- 
ver in  the  alloy  in  the  outer  layer  of  the  grains  by  the  action  of 
surface  waters.  McConnell  has  proved  that  in  the  nuggets 
from  the  Klondike  the  outside  actually  has  a  greater  fineness 
than  the  inside.  The  loss  of  silver  in  the  outer  part  was  from  5 
to  7  per  cent.  This  interesting  result  well  illustrates  the  relative 
insolubility  of  gold.2 

Gold  in  Relation  to  Bed -Rock.— While  the  bulk  of  the  gold 
usually  rests  on  the  bed-rock  or  within  a  foot  or  two  of  it,  this  is 
not  an  invariable  rule.  In  some  gravels  the  coarser  gold  is  oc- 
casionally scattered  through  the  lower  4  to  20  feet.  But  it  is 
never,  except  in  minute  quantities,  distributed  equally  through  a 

1  J.  Park,  Mining  geology,  London,  1907,  p.  18. 

2  W.  B.  Devereux,  The  occurrence  of  gold  in  the  Potsdam  formation, 
Trans.,  Am.  Inst.  Min.  Eng.,  1882,  pp.  465-475. 

Ross  E.  Browne,  Colorado  placer  gold,  Eng.  and  Min.  Jour.,  vol.  59, 
1895,  pp.  101-102. 

W.  Lindgren,  The  gold  belt  of  the  Blue  Mountains  of  Oregon,  Twenty- 
second  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  2,  1901,  p.  637.  See  also  Prof. 
Paper  73,  U.  S.  Geol.  Survey,  1911. 

R.  G.  McConnell,  Report  on  gold  values  in  the  Klondike  high-level 
gravels,  Geol.  Survey  Canada,  1907,  p.  979. 


230  MINERAL  DEPOSITS 

great  thickness  of  gravels.  An  excellent  instance  is  McConnell's 
section  of  the  White  channel  deposit  in  the  Klondike.  (See 
Fig.  84.)  The  washed  gravel  is  here  150  feet  thick.  The 
gold  content  of  the  gravel  is  as  follows: 

0-  6  feet  above  bed-rock,  $4. 13    per  cubic  yard. 

6-12  feet  above  bed-rock,  $0. 18  per  cubic  yard. 
12-18  feet  above  bed-rock,  $0 . 047  per  cubic  yard. 
18-24  feet  above  bed-rock,  $0 . 04  per  cubic  yard. 
24-30  feet  above  bed-rock,  $0.034  per  cubic  yard. 
30-36  feet_above  bed-rock,  $0.032  per  cubic  yard. 
36-42  f  eet'above  bed-rock,  $0 . 032  per  cubic  yard. 
42-48  feet'above  bed-rock,  $0.045  per  cubic  yard. 
48-54  feet  above  bed-rock,  $0.025  per  cubic  yard. 

From  54  feet  above  bed-rock  the  quantity  of  gold  contained 
per  cubic  yard  gradually  and  steadily  diminished  to  $0.006  at 
the  top.  There  is  only  fine  gold  in  the  upper  gravels.  A  local 
enrichment  has  taken  place  on  false  bed-rock,  a  clayey  stratum 
above  the  real  bed-rock;  here  the  gold  is  much  coarser  than 
directly  above  or  below,  but  finer  than  on  the  bed-rock.  Oc- 
casionally rich  gravel  may  be  found  a  few  feet  above  bed-rock 
while  it  is  less  rich  immediately  on  it. 

Coarse  and  moderately  coarse  gold  moves  very  slowly.  Mc- 
Connell  found,  for  instance,  that  the  White  channel,  where  inter- 
sected by  gulches,  has  left  almost  the  whole  amount  of  its  gold 
in  these  immediately  below  the  place  where  the  trenching  has 
occurred.  In  some  cases  the  horizontal  movement  scarcely 
equaled  the  vertical. 

Smooth,  hard  bed-rock  is  poorly  adapted  to  retain  the  gold; 
when  it  is  somewhat  clayey  and  decomposed  much  better  results 
are  obtained.  Schists  and  slates  make  good  bed-rock  when 
decomposed,  especially  when  they  strike  parallel  to  the  channel. 
Serpentine  forms  a  smooth  and  unsatisfactory  bed-rock. 

Gold  works  down  into  bed-rock  in  a  most  'surprising  way.  In 
hard  rock  it  settles  into  the  most  minute  crevices.  In  soft 
rock  it  burrows  to  a  depth  of  1  to  5  feet,  so  that^it  is  always 
necessary  to  mine  this  amount  of  the  bed-rock.  In  limestone, 
irregular  solution  cavities  contain  the  detrital  gold,  and  these 
sometimes  descend  to  a  depth  of  50  feet  or  more.  Compact 
clay  is  good  bed-rock,  also  clayey  sandstone  and  clayey  volcanic 
tuffs,  the  occurrence  of  the  latter  being  exemplified  in  the  Oro- 
ville_dredging  grounds,  in  California. 


TRANSPORTATION  AND  CONCENTRATION     231 

In  glacial  till  and  moraines  there  has  been  little  opportunity 
for  concentration,  and  unless  the  primary  vein  deposits  were 
unusually  rich,  these  gravels  are  of  little  value;  the  gold  contained 
in  them  may,  of  course,  be  concentrated  by  glacial  streams  work- 
ing over  the  morainal  detritus. 

Grade  of  Auriferous  Watercourses. — -All  kinds  of  grade  occur 
in  watercourses  containing  gold-bearing  gravels.  In  steep  creeks 
the  grade  may  be  many  hundred  feet  per  mile,  but  the  placers  in 
these  are  usually  poor.  California  rivers,  in  the  Sierra  Nevada, 
have  grades  of  50  to  100  feet  or  more  per  mile.  Many  of  these 
have  been  extremely  rich  where  gravel  bars  have  had  an  oppor- 
tunity to  accumulate.  The  White  channel,  in  the  Klondike,  has 
a  grade  of  about  30  feet  per  mile.  Many  of  the  present  Alaskan 
streams  have  a  grade  of  100  to  150  feet  per  mile.  In  the  prin- 
cipal Tertiary  channels  of  Victoria,  Australia,  low  grades  down 
to  20  feet  prevailed. 

In  depressed  or  elevated  channels  of  past  epochs,  as  in  Cali- 
fornia, Victoria,  and  the  Klondike,  changes  of  original  grade 
must  be  considered.  This  is  best  established  in  the  California 
channels,  which  now  have  grades  of  100  to  150  feet,  whereas  the 
original  streams  had  much  less,  the  increase  being  due  to  the 
westward  tilting  of  the  Sierra  Nevada.  The  best  results  of  gold 
concentration  are  probably  obtained  in  rivers  of  moderate  grades, 
perhaps  30  feet  per  mile,  under  nicely  balanced  conditions  of  cor- 
rasion  and  deposition.  Whenever  overloading  and  active  de- 
position take  place  concentration  of  coarse  gold  ceases.  On  the 
other  hand,  where  erosion  is  rapid  conditions  for  rich  placers  are 
less  favorable,  unless,  as  in  the  present  streams  of  the  Sierra 
Nevada,  the  gold  supply  is  unusually  abundant. 

The  Pay  Streak  or  "Run  of  Gold."1— Except  in  smaller  creeks 
the  distribution  of  the  gold  in  a  gravel  bed  is  far  from  regular. 
There  is  usually  gold  on  the  bed-rock  over  the  whole  area  of  the 
stream  bed,  but  the  richer  part  makes  a  narrower  streak  which 
follows  a  devious  course,  distinctly  affected  by  the  character  of 
the  bed-rock,  sometimes  splitting  and  re-forming,  following  first 
one  side,  then  crossing  diagonally  to  the  other  side.  It  is  not 
necessarily  in  the  deepest  depression  or  gutter.  Fig.  85  shows 
this  devious  course  of  the  pay  streak  in  comparatively  shal- 
low gravels  at  Maryboro,  Victoria.  It  is  clearly  independent 

1  J.  B.  Tyrrell,  The  law  of  the  pay  streak  in  placer  deposits,  Trans.  Inst.  of 
Min.  and  Met.,  London,  May  16,  1912,  Min.  and  Sci.  Press,  June  1,  1912 


232 


MINERAL  DEPOSITS 


of  the  course  of  the  present  small  stream.  In  broader  gravel 
plains,  of  which  the  Homebush  and  Pitsfield  Tertiary  placers  of 
Victoria  are  examples,  the  "run  of  gold"  follows  a  distinct  and 
\yell-defined  course  on  an  almost  level  country  rock.  All  this 
shows  clearly  enough  the  impossibility  of  the  view  that  the  gold 
was  first  uniformly  distributed  through  the  gravels  and  then 
gradually  settled  to  the  bottom  under  the  influence  of  gravity. 

n 


Goidbearing  Gravels 
covered  by  20-50  It. 

of  Alluvium 
SCALE 


FIG.  85. — Map   showing   position  of   pay   streak   in   alluvial   gravels  of 
Maryboro,  Victoria.     After  S.  B.  Hunter. 

These  pay  streaks  assuredly  indicate  epochs  of  well-balanced  and 
long-maintained  conditions  during  which  the  gravels  could  accu- 
mulate to  only  moderate  depths  and  were  at  all  times  water- 
soaked  and  in  a  condition  of  slow  movement.  With  more 
abundant  loading  of  detrital  material  the  gold-transporting  power 
of  the  stream  diminishes  at  a  very  rapid  rate. 

Solution  and  Precipitation  of  Gold. — Many  of  the  earlier  ob- 
servers, such  as  Genth,  Lieber,  Selwyn,  Laur,  Egleston,  C.  New- 


TRANSPORTATION  AND  CONCENTRATION     233 

bery,  and  Daintree,  concluded  from  observations  in  various  parts 
of  the  world  that  placer  gold,  and  particularly  the  large  nuggets, 
has  been  deposited  by  circulating  solutions.  At  present  the 
mechanical  derivation  of  the  gold  seems  established  beyond  all 
doubt,  although  under  exceptional  circumstances  some  solution 
and  redeposition  may  have  taken  place.1  Even  now,  however, 
some  writers,  like  J.  M.  Maclaren,2  are  inclined  to  place  much  em- 
phasis on  this  secondary  deposition.  It  is  probable,  nevertheless, 
that  this  process  is  absolutely  insignificant  from  an  economic 
point  of  view.  Nuggets,  when  cut  and  polished,  almost  always 
show  a  granular  structure  perfectly  in  accordance  with  vein  gold. 
Liversidge,  in  a  long  series  of  experiments,  found  only  two  speci- 
mens (both  from  New  Guinea)  which  showed  a  concentric  struc- 
ture indicative  of  concretionary  deposition.  Very  rare  instances 
are  quoted  of  quartz  pebbles  with  dendritic  films  of  gold3  or  of 
nuggets  with  minute  gold  crystals  on  their  surface.4  The  col- 
lection of  J.  Edman,  of  San  Francisco,  contained  a  small  crystal 
of  magnetite  coated  with  a  thin  film  of  gold.  This  came  from 
the  Tertiary  deposits  at  Providence  Hill,  Plumas  County,  Cali- 
fornia, and  Mr.  Edman  stated  that  he  had  never  seen  similar 
occurrences  in  the  modern  gravels.  It  seems  to  be  well  estab- 
lished that  pyrite  reduced  by  organic  material  in  the  gravels  may 
contain  some  gold  and  also  that  the  metal  is  occasionally  found 
at  the  roots  of  trees  or  in  the  grass  roots. 

The  gold  crystallized  in  minute  octahedrons  in  the  clay  of 
Kanowna,  Western  Australia,  is,  as  Maitland5  pointed  out,  im- 
mediately above  or  adjacent  to  the  decomposed  croppings  of 
the  veins  and  the  occurrence  can  scarcely  be  called  a  placer.  The 
gold  which  works  down  into  the  soft  bed-rock  of  the  placers  is 
in  all  cases,  where  I  have  observed  it,  of  clearly  detrital  origin. 

It  is  stated  that  the  ashes  of  trees  in  the  gold-bearing  region 
of  the  Guianas  contain  an  appreciable  quantity  of  gold.  Origi- 
nally asserted  by  Lungwitz,  this  has  been  denied  by  Dubois 
and  Kollbeck  and  then  reasserted  by  Harrison  on  the  basis  of 

1  Regarding  the  older  literature,  see  the  text-books  of  Stelzner,  Bergeat, 
and  Beck.     In  more  detail,  see  Liversidge,  Jour.,  Roy.  Soc.  N.  S.  W.,  vol.  27, 
1893,  p.  343;  vol.  31,  1897,  p.  79;  vol.  40,  1906,  p.  161. 

2  J.  M.  Maclaren,  Gold,  London,  1908,  pp.  80-86. 

3  R.  G.   McConnell,  Ann.  Rept.,  Geol.  Survey  Canada,  vol.  14,  1901,  p. 
64-B. 

4  Gordon,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  25,  1895,  p.  294. 

5  J.  M.  Maclaren,  op.  cit.,  p.  83. 


234  MINERAL  DEPOSITS 

careful  investigations.1  His  statement  must  be  accepted, 
although  it  certainly  taxes  the  imagination  to  believe  that  gold- 
bearing  solutions  can  exist  in  a  soil  together  with  organic  matter. 
From  widely  separate  parts  of  the  world  gold  has  been  reported 
in  the  ash  of  coal,  but  in  this  case  it  may  be  detrital  and  con- 
tained in  admixed  sand  and  clay. 

Gold  is  easily  brought  into  the  colloid  state  and  as  such  it  may 
be  transported  in  solutions  of  colloid  silica.  It  is  very  readily 
precipitated  by  electrolytes  but  this  mode  of  solution  may 
account  for  some  cases  of  secondary  gold  in  placers.  According 
to  the  most  recent  investigations2  gold  is  soluble  in  superficial 
waters  only  when  free  chlorine  becomes  liberated  by  the  inter- 
action of  sulphuric  acid,  sodium  chloride,  and  manganese  dioxide, 
a  combination  that  must  sometimes  occur  in  ore  deposits  subject 
to  oxidation;  in  the  presence  of  oxidizing  pyrite  some  gold  may 
therefore  be  taken  into  solution,  as  chloride  but  it  would  probably 
not  remain  long  before  encountering  reducing  substances.  While 
gold  is  slightly  soluble  in  sodium  carbonate,  sodium  sulphide,  and 
other  similar  compounds,  these  would  not  ordinarily  be  encoun- 
tered in  the  waters  of  the  zone  of  oxidation. 

Relation  to  Primary  Deposits. — That  placer  gold  is  directly 
derived  by  mechanical  processes  from  vein  deposits  or  analogous 
occurrences  is  absolutely  certain,  and  examples  of  convincing 
character  are  present  everywhere.  This  does  not  imply  that  the 
primary  deposit  can  be  worked  at  a  profit.  In  most  cases  the 
gold  is  traceable  up  to  the  deposit.  On  this  principle  the  pocket 
hunter  proceeds,  panning  the  detritus  and  working  up  hill  until 
the  source  of  the  scattered  gold  has  been  found.  The  area 
in  which  the  detritus  occurs  has  the  shape  of  a  triangle,  the  apex 
of  which  is  the  pocket. 

It  is  a  common  experience  that  rivers  or  creeks  crossing  a 
vein  or  a  mineral  belt  are  enriched  immediately  below  it, 
the  coarseness  of  the  gold  increasing  upstream  to  the  place  where 
the  outcrops  are  crossed.  As  examples  may  serve  the  great 
accumulations  of  placer  gold  in  the  Neocene  gravels  of  Eldorado 
County,  California,  where  the  Mother  Lode  crosses  them,  and  the 
rich  channels  in  upper  Nevada  County,  just  below  the  belt  of 

1  J.  B.  Harrison,  Geology  of  the  gold  fields  of  British  Guiana,  London, 
1908,  p.  209. 

2  W.  H.   Emmons,  Enrichment  of  ore  deposits,  Bull.  625,  U.  S.  Geol. 
Survey,  1917,  p.  305. 


TRANSPORTATION  AND  CONCENTRATION     235 

quartz  veins  at  Washington  and  Graniteville.  There  are  fine 
examples  in  Victoria,  where  the  gravels  are  rich  only  where 
they  cross  or  follow  systems  of  veins  or  "reef  lines."  The  White 
channel  of  the  Forest  Hill  divide,  California,  follows  a  belt  of 
quartz  stringers  in  clay  slate.  The  Idaho  Basin1  presents  an 
excellent  instance  of  large  gravel  bodies  the  gold  content  of 
which  is  traceable  up  to  certain  auriferous  vein  systems. 

Economic  Notes. — The  world's  annual  production  of  placer 
gold  is  about  $70,000,000.  To  this  the  Alaska  and  Yukon  dis- 
tricts contribute  $20,000,000,  California  $9,000,000,  Victoria 
$2,000,000,  and  Siberia  $18,000,000.  While  placers  are  found 
in  almost  all  gold-  and  silver-producing  regions,  Brazil,  the  Ural 
Mountains,  Siberia,  California,  Alaska,  and  Victoria  have  had  by 
far  the  greatest  total  production. 

Gold-bearing  gravel  is  often  measured  by  the  ton,  but  more 
commonly  by  the  cubic  yard.  Still  another  measure  is  by  sur- 
face area,  sometimes  by  the  square  foot,  in  Australia  commonly 
by  the  square  fathom;  this  is  especially  applicable  to  deep  mining 
when  only  the  richest  bottom  layer  is  mined;  at  least  2  feet  of 
gravel  and  1  foot  of  soft  bed-rock  are  extracted,  making  one 
square  fathom  equivalent  to  a  minimum  of  4  cubic  yards.  . 

In  river  bars  gravels  are  worked  by  wing  dams  and  pits  kept 
dry  by  simple  pumping  devices.  On  a  large  scale  they  may  be 
ground-sluiced  or  washed  by  the  hydraulic  method,  with  the  aid 
of  elevators  when  the  natural  fall  is  insufficient. 

The  elevated  gravels  of  earlier  periods  are  worked  in  California 
by  tunnels  and  drifting  operations  on  the  bed-rock.  The  mini- 
mum cost  of  working  under  the  most  favorable  conditions  is  50 
cents  per  cubic  yard,  but  is  commonly  $1  to  $2  per  cubic  yard; 
most  of  the  gravels  actually  worked  contain  at  least  $1.50,  and 
often  much  more.  The  whole  gravel  body  may  be  washed  by  the 
hydraulic  method,  when  the  expense  may  be  reduced  to  2  to  5 
cents  per  cubic  yard;  of  course  the  cost  of  preliminary  work  like 
ditches,  etc.,  is  often  great.  Some  creek  gravels  in  the  Seward 
Peninsula  contain  from  $2  to  $6  per  cubic  yard;  the  width  of  the 
deposit  may  be  about  50  feet,  the  depth  3  to  6  feet. 

The  depressed  gravels  of  earlier  periods  are  worked  by  drifting 
from  shafts,  as  in  Victoria,  where,  however,  the  preliminary 
pumping,  to  permit  access,  is  an  extremely  heavy  expense,  often 

1  W.  Lindgren,  The  mining  districts  of  Idaho  Basin  and  the  Boise  Ridge, 
Eighteenth  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  3,  1898,  pp,  617-744. 


236  MINERAL  DEPOSITS 

indeed  prohibitory.  Some  of  these  Australian  channels  have 
been  extremely  rich,  the  workable  portions  ranging  from  $2  to 
$15  per  cubic  yard.  Some  of  the  channels  are  in  places  several 
hundred  feet  in  width. 

Of  late,  gravels  have  been  extensively  worked  in  California, 
Alaska,  the  Klondike,  and  elsewhere  by  the  dredging  process.  In 
California,  where  this  method  has  reached  its  highest  develop- 
ment, $7,769,000  was  obtained  from  58  dredges  in  1916  from  the 
flood-plains  of  the  rivers  at  the  foot  of  the  Sierra  Nevada,  and 
the  cost  has  been  reduced  from  about  10  cents  to  3  or  4  cents  per 
cubic  yard  handled.  In  Alaska  the  cost  is  of  course  much  higher 
and  gravels  containing  less  than  50  cents  per  cubic  yard  are  rarely 
worked.  The  dredge  will  probably  prove  to  be  the  most  efficient 
placer-mining  machine  of  the  future,  replacing  the  hydraulic 
method,  which  offers  difficulties  in  the  disposition  of  the  tailings. 
In  1917  Alaska  yielded  $2,500,000  from  dredging. 

Certain  gravels  in  the  dry  regions  of  Arizona  and  northern 
Mexico  are  treated  by  pneumatic  concentration  in  so-called  dry 
washers,  but  the  output  of  these  placers  is  insignificant.1 

Yields  of  placer  deposits  are  often  calculated  in  dollars  per 
lineal  foot  of  channel.  Good  channels  for  drifting  may  produce 
from  $70  to  $500  per  foot.  The  richest  drift  mine  worked  was 
probably  "Madame  Berry"  in  Victoria,  with  average  width  of 
450  feet,  yielding  $1,293  per  foot  along  channel.  The  two  claims 
below  this  produced,  respectively,  $843  and  $443  per  foot,  the  last- 
named  channel  being  mined  1,000  feet  wide.  The  White  Channel 
in  the  Klondike  gave  $380  per  foot;  the  Red  Point  channel  in 
Placer  County,  California,  $72,  the  width  being  120  feet;  the 
American  Hill  hydraulic  mine,  Nevada  County,  1,000  feet  wide, 
$414;  the  Nome  creeks,  Alaska,  50  feet  wide,  about  $100. 
By  drifting  operations  alone,  only  a  part  of  the  gold  will  be 
extracted,  say  one-fifth  to  one-half,  dependent  upon  the  thickness 
of  overlying  gravels. 

THE  GOLD-BEARING  CONGLOMERATES  OF  SOUTH  AFRICA 

Of  the  extensive  literature  the  following  principal  papers  are 
quoted : 

G.  F.  Becker,  The  Witwatersrand  Banket,  etc.,  Eighteenth  Ann.  Rept., 
U.  S.  Geol.  Survey,  pt.  5.  1896, 

1  T.  A.  Rickard,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  28,  1899,  p.  480. 
F.  J.  H.  Merrill,  Min.  and  Sci.  Press,  July  13:  1912. 


TRANSPORTATION  AND  CONCENTRATION     237 

G.  A.  F.  Molengraaff,  Die  Reihenfolge  der  geol.  Form,  in  Siid  Afrika, 
Neues  Jahrbuch,  1900,  B.  1,  pp.  113-119. 

F.  H.  Hatch  and  G.  S.  Corstorphine,  Petrography  of  the  Witwatersrand 
conglomerates,  etc.,  Proc.,  Geol.  Soc.  S.  A.,  vol.  7,  pt.  3,  1904,  pp.  140-145. 

F.  H.  Hatch  and  G.  S.  Corstorphine,  The  geology  of  South  Africa,  London. 
1905. 

J.  W.  Gregory,  The  origin  of  the  gold  in  the  Rand  Banket,  Trans.,  Inst. 
Min.  and  Met.,  London,  vol.  17,  October,  1907.  Also  Econ.  Geol.,  vol.  4, 
1909,  pp.  118-129. 

Discussion  in  Econ.  Geol,  vol.  4,  1909,  by  G.  F.  Becker  and  G.  A.  Denny 

R.  B.  Young,  The  Rand  Banket,  London,  1917,  pp.  125. 

See  also  description  by  R.  Beck,  Erzlagerstatten,  2,  1909,  pp.  183-200. 

F.  H.  Hatch,  The  conglomerates  of  the  Witwatersrand  hi  types  of  ore 
deposits,  San  Francisco,  1911,  pp.  202-219. 

C.  B.  Horwood,  The  Rand  Banket,  M in.  and  Sci.  Press,  Oct.  to  Dec.,  1913. 

E.  T.  Mellor,  The  upper  Witwatersrand  System;  the  East  Rand,  Trans., 
Geol.  Soc.  S.  A.,  vol.  18,  1915,  pp.  11-71. 

E.  T.  Mellor,  The  conglomerates  of  the  Witwatersrand  with  discussion, 
Trans.,  Inst.  Min.  and  Met.,  London,  vol.  25,  1916,  pp.  226-348. 

Hugh  F.  Marriott,  Mining  on  the  Rand,  Trans.  Inst.  Min.  and  Met., 
London,  1918,  Min.  and  Sd.  Press,  July  20,  1918. 

The  development  of  the  gold-bearing  conglomerates  of  the 
Witwatersrand  district,  in  the  Transvaal,  is  one  of  the  most 
wonderful  chapters  in  the  history  of  mining.  From  an  in- 
conspicuous beginning  in  1887,  the  production  of  these  unique 
deposits  has  steadily  increased.  In  1917  the  ore  production 
amounted  to  about  28,000,000  tons,  with  a  yield  of  $180,000,000. 
The  total  production  to  the  end  of  1917  exceeds  $2,500,000,000, 
which  is  more  than  the  total  gold  production  of  California,  Colo- 
rado and  Alaska.  The  average  content  of  the  ore  has  decreased, 
probably  mostly  on  account  of  reduction  in  mining  and  metal- 
lurgical costs,  from  $12  to  $6  or  $7  per  ton,  and,  according  to 
Hatch,  it  is  probable  that  in  the  future  ore  of  $5  grade  will  be 
utilized.  The  increase  in  production  continued  to  1916  but  it  is 
probable  that  the  flood  tide  of  output  has  been  reached.  A 
depth  of  over  5,000  feet  has  now  been  reached,  and,1  owing  to 
a  favorable  geothermic  gradient  (p.  84),  it  will  be  possible  to 
go  considerably  deeper.  The  ore  is  reduced  by  a  combination 
of  amalgamation  and  the  cyanide  process;  stamp  mills  and  tube 
mills  are  the  grinding  machinery  most  commonly  employed. 

South  Africa  is  in  the  main  a  plateau  of  thick  sedimentary  beds 
which  are  poor  in  fossils  and  in  part  of  sub-aerial  origin. 

1  In  1918  the  Village  Deep  had  attained  a  vertical  depth  of  5,350  feet  and 
a  depth  along  the  dip  of  9,800  feet. 


238 


MINERAL  DEPOSITS 


The  oldest  rocks  known  are  the  Swaziland  crystalline  schists 
,  and  the  granites  intruded  in  them. 
On  their  eroded  surface  rest  the 
upper  and  lower  Witwatersrand  sys- 
tem of  slates,  quartzites,  and  con- 
glomerates, aggregating  19,000  feet 
in  thickness,  and  on  top  of  these  in 
turn  a  thick  series  of  volcanic  flows, 
called  the  Ventersdorp  system  (Fig. 
86). 

H  The  age  of  the  Witwatersrand  sys- 
&3  tern  is  not  definitely  known;  it  is 
tcj  probably  Cambrian  or  pre-Cambrian. 
fei  Next  higher  in  the  succession  of  rocks 
j§  is  the  Potchefstrom  system,  includ- 
^  ing  the  Black  Reef  (oldest) ,  Dolomite, 
TO  and  Pretoria  series.  This  again  is  cov- 
j§  J  ered  by  the  Devonian  Waterberg  sys- 
^oo  tern  (Table  Mountain  sandstone  of 
§  I  the  Cape)  and  the  most  recent 
J2  "f  Karroo  system,  which  is  coal-bearing 
j|'f  and  considered  to  be  of  Permo-Car- 
£  |  boniferous  age  Each  system  is  sep- 
j§.g  arated  by  an  unconformity  from  the 
g  f  next. 

The     Witwatersrand     system     is 
folded  in  a  syncline  extending  about 
•J      120  miles  east  to  west  and  45  miles 

f      north  to  south.     At  Johannesburg, 
at  the  north  side  of  the  syncline,  the 
">      dip  is  to  the  south,  steep  near  the 
2      surface,   but  flattening  in  depth  to 
about  30°.     Faulting  is  common  and 
there    are    a    number    of   intrusive 
diabase  dikes,  thought  to  belong  to 
the  overlying  Ventersdorp  volcanic 
system. 

Auriferous  conglomerates  occur  at 
.several  horizons  in  the  Witwaters- 
rand system  and  also  in  the  Black 
Reef  series.  The  productive  beds 


% 


TRANSPORTATION  AND  CONCENTRATION     239 

are,  however,  in  the  upper  part  of  the  Witwatersrand,  including  a 
thickness  of  about  7,000  feet  of  quartzites  and  conglomerates, 
among  which  the  following  are  distinguished,  beginning  from 
the  top:  Kimberley  group,  Bird  Reef  group,  Livingstone  Reef 
group  and  Main  Reef  group.  The  first  two  are  each  about  500 
feet  thick  but  the  conglomerates  contained  are  of  low  grade, 
rarely  exceeding  $3  per  ton  in  gold.  The  Main  Reef  group, 
about  90  feet  thick,  includes  several  conglomerate  beds  more 
or  less  persistent. 


FIG.  87. — Gold-bearing  conglomerate,  Johannesburg,  South  Africa.  Peb- 
bles of  quartz,  crushed  in  places.  Cement  of  sericite,  quartz,  and  a  little 
chlorite.  Black  areas  are  concretions  of  pyrite,  replacing  groundmass  and 
quartz.  B,  prisms  of  chloritoid.  Drawn  by  J.  D.  MacKenzie. 

The  usual  subdivision  of  the  Main  Reef  group  includes  from 
top  to  bottom: 

South  Reef  (3  feet). 

Bastard  Reef  (scattered  pebbles)  and  quartzite  (20  to  40 

feet). 

Main  Reef  Leader  (2  feet). 
Quartzite  (2  to  20  feet). 
Main  Reef  (4  feet). 

Of  these  the  Main  Reef  Leader  is  the  most  productive;  the 
pebbles  in  the  conglomerate  are  small,  averaging  2  inches  in 
diameter,  and  consist  of  well-rolled  fragments  of  glassy  quartz 
with  fewer  pebbles  of  more  angular  quartzite,  banded  chert,  and 
slate.  The  pebbles  lie  in  a  matrix  of  sandy  material,  which  has 


240 


MINERAL  DEPOSITS 


become  hardened  by  infiltration  of  silica.  Pyrite  occurs  in 
abundance  in  the  cement,  averaging  about  3  per  cent,  of  the  rock 
and  being  present  both  in  crystalline  form  and  as  rounded 
replacements  after  quartz  probably  representing  two  generations, 
both  subsequent  to  the  sedimentation.  Chloritoid,1  sericite, 
calcite,  and  graphite  are  other  authigenetic  minerals.  The  gold 
is  not  contained  in  pebbles,  but  only  in  the  cement,  and  forms 
minute  angular  crystalline  aggregates,  very  seldom  rounded 
particles.  It  is  usually  closely  connected  with  pyrite,  either 
enclosed  by  it  or  covering  the  surface  of  pyrite  aggregates.  As 
a  rule  it  is  not  visible  to  the  naked  eye. 


FIG.  88.— Section  through  Village  Deep  No.  3  shaft.     After  H.  F.  Marriott. 

In  spite  of  a  long-continued  discussion  there  is  no  unanimity 
among  geologists  as  to  the  genesis  of  these  remarkable  deposits. 
It  is  evidently  necessary,  for  a  satisfactory  discussion  of  the 
question,  to  go  beyond  the  limits  of  the  Johannesburg  occurrences 
and  consider  the  geological  relations  of  the  Transvaal  and  South 
Africa  as  a  whole. 

The  first  suggestion  that  the  conglomerate  may  be  simply  an 
alluvial  or  littoral  placer  is  refuted  by  the  character  of  the  gold 
and  its  close  association  with  the  pyrite.  Detrital  pyrite  may,  of 

1 A  prismatic  colorless  mineral  usually  described  as  chloritoid  is  common 
(Fig.  §7),  but  its  idejitification  appears  questionable. 


TRANSPORTATION  AND  CONCENTRATION     241 

course,  occur  in  gravels,  but  there  should  always  be  some  mag- 
netite and  ilmenite  present.  Their  absence  is  a  strong  argument 
against  the  theory  of  direct  placer  deposition.  It  is  clear  that  if 
this  is  a  placer  deposit  there  has  been  extensive  recrystallization 
and  some  migration.  Equally  untenable  is  the  hypothesis  of 
F.  W.  Voit1  that  the  gold  has  been  brought  to  the  surface  by  hot 
springs  which  discharged  into  the  ocean. 

The  advocates  of  the  placer  theory,  among  whom  are  G.  F. 
Becker,  J.  W.  Gregory,  G.  A.  Denny,  R.  B.  Young  and  E.  T. 
Mellor,  are  compelled  to  admit  a  recrystallization  of  the  gold 
and  a  transformation  of  magnetite  and  ilmenite  into  pyrite. 

Many  geologists  and  engineers,  impressed  with  the  difficulties 
confronting  the  placer  theory,  hold  that  the  deposits  are  epigen- 
etic — that  the  gold  and  pyrite  have  been  introduced  by  a  post- 
sedimentary  infiltration,  perhaps  after  the  intrusion  of  the 
diabase  dikes.  Such  views  are  held  by  H,  Louis,  J.  H.  Ham- 
mond, R.  Beck,  F.  H.  Hatch,  G.  S.  Corstorphine  and  R.  B. 
Horwood. 

Small  and  irregular  quartz  veins  which  in  some  places  contain 
a  little  gold  and  sulphides  intersect  the  Witwatersrand  series. 
Whether  there  is  any  enrichment  along  the  few  diabase  dikes  is  a 
disputed  question.  No  doubt  these  quartz  veins  are  related  to 
the  dikes. 

E.  T.  Mellor  has  recently  approached  the  subject  from  the 
wider  geological  viewpoint  and  his  papers  contain  very  strong 
arguments  in  favor  of  the  original  deposition  of  the  gold  in  alluvial 
gravels.  He  considers  the  quartzite  and  conglomerate  series  as 
large  delta  deposit  rather  than  shore  gravels  and  shows  the 
existence  of  many  horizons  of  gold-bearing  conglomerates. 

Against  the  infiltration  theory  stands  a  long  array  of  strong 
arguments:  1.  The  absence  of  channels  followed  by  the  solutions; 
2.  the  regular  distribution  of  the  gold  in  the  conglomerate; 
often  it  is  concentrated  in  its  lower  layers;  3.  the  practical 
confinement  of  the  gold  to  the  conglomerates,  though  the 
quartzites  are  equally  permeable;  4.  the  conglomerates  were 
deposited  in  an  alluvial  plain  skirting  the  deeply  eroded  Swaziland 
schists  with  their  lenticular  gold  quartz  veins  and  would  thus 
certainly  contain  some  gold. 

The  fine  flake  gold  would  be  recrystallized  and  pressed  between 

1  F.  W.  Voit,  Der  Ursprung  des  Goldes  in  den  Randconglomeraten,  Mon- 
atsber.  Deutsch.  geol.  Gesell.,  vol.  60,  Nog.  5  and  7,  1908. 


242  MINERAL  DEPOSITS 

secondary  growths  of  quartz  grains.  The  original  black  sand 
would  be  recrystallized  to  pyrite  by  action  of  meteoric  waters 
in  sediments  which  contained  sulphates  and  organic  matter. 
The  difficulty  not  yet  fully  explained  lies  in  the  abnormal  rich- 
ness and  extent  of  the  conglomerates.  It  is  pointed  out,  how- 
ever, that  large  areas  of  the  conglomerate  are  practically  barren. 
The  rich  beach  sands  of  Nome,  Alaska,  have  been  cited  as  an 
analogous  case  but  the  analogy  is  by  no  means  perfect.  Very 
fine  detrital  gold  would,  of  course,  be  expected  in  a  delta  deposit 
near  the  shore  line. 

Similar  conglomerates  of  considerable  geological  antiquity 
are  found  in  West  Africa  at  Tarkwa  and  Abosso1  and  these  have 
been  worked  on  a  fairly  large  scale.-  Instead  of  pyrite  these  con- 
tain magnetite,  and  ilmenite  with  chloritoid. 

PLATINUM  PLACERS2 

It  is  known  that  platinum  occurs  as  a  primary  constituent  of 
peridotites,  and  specimens  showing  its  intergrowth  with  olivine 
and  chromite  have  been  described.  Almost  the  entire  world's 
production  is  obtained  from  placers  and  95  per  cent,  of  it  is 
extracted  from  the  placers  on  the  eastern  slope  of  the  Ural 
Mountains,  where  detrital  platinum  occurs  in  the  gravels  of  the 
stream  courses,  which  head  in  certain  areas  of  peridotite  and 
pyroxenite.  It  is  associated  with  iridosmine,  iridium,  chromite, 
and  often  also  with  gold.  The  crude  platinum  forms  small 
rounded  grains,  very  rarely  nuggets  up  to  20  pounds  in  weight, 
and  its  fineness  (per  thousand)  ranges  from  750  to  850,  the 
remainder  being  iron,  copper,  and  various  metals  of  the  platinum 
group,  particularly  iridium. 

Platinum-bearing  gravels  occur  also  in  Colombia,  South 
America,  in  river  beds  and  Tertiary  conglomerates,  and  the  pro- 
duction from  them  is  increasing.  In  the  United  States  the 
metal  occurs  in  small  quantities  together  with  gold  in  almost 
all  the  gold-bearing  districts  in  northern  and  central  California 

1  Edward  Halse,  Trans.,  Fed.  Inst.  Min.  Eng.,  vol.  2,  1891,  p.  69. 
R,  Beck,  Erzlagerstatten,  2,  1909,  p.  200. 

2  J.   F.   Kemp,   Geological  relations  and  distribution  of  platinum  and 
associated  metals,  Butt.  193,  U.  S.  Geol.  Survey,  1902. 

Louis  Duparc,  Le  platine  et  les  g'ites  platiniferes  de  1'Oural.  Geneve,  1911. 
D.  T.  Day,  W.  Lindgren  and  J.  M.  Hill,  in  successive  issues  of  Mineral 
Resources,  U.  S.  Geol.  Survey. 


TRANSPORTATION  AND  CONCENTRATION     243 

and  in  southwestern  Oregon,  where  serpentine  or  peridotite  is 
found.  From  400  to  700  ounces  are  annually  recovered,  chiefly 
from  the  black  sands  of  the  dredges.1  Platinum  also  occurs  in 
the  beach  sands  of  southern  Oregon,  together  with  more  or  less 
gold;  a  small  quantity  of  this  is  usually  also  recovered.  The 
Tulameen  district,  British  Columbia,  formerly  yielded  some 
production.  The  normal  world's  production  of  platinum  before 
the  war  was  about  300,000  troy  ounces,  but  it  is  much  less  at  the 
present  time.  /Regarding  platinum  and  palladium  in  vein  de- 
posits and  in  peridotite  see  p.  790,  where  the  uses  of  the  platinum 
metals  are  also  described?)  For  a  long  time  the  price  of  platinum 
was  less  than  that  of  gold;  a  gradual  rise  increased  the  value 
to  $20  per  ounce,  and  in  1911  it  had  reached  $45.  Increasing 
scarcity  forced  the  price  up  to  $105  in  1917.  Crude  platinum 
with  70  per  cent,  to  85  per  cent.  Pt  is  sold  from  $30  to  $60  per 
ounce. 

CASSITERITE  PLACERS2 

The  original  home  of  cassiterite  (or  oxide  of  tin)  is  either  in  the 
granites,  in  pegmatite  dikes,  or  in  quartz  veins.  From  any  of 
these  sources  it  may  be  set  free  by  weathering  and  disintegra- 
tion, and,  on  account  of  its  high  specific  gravity,  it  easily  becomes 
concentrated  in  gravel  deposits  of  different  types.  Among 
the  accompanying  minerals  tourmaline,  topaz,  and  wolframite  are 
the  most  common.  Grains  of  metallic  tin  are  reported  to  occur 
with  cassiterite  in  Nigeria  and  Australia.  Eluvial  deposits  imme- 
diately below  the  croppings  are  numerous  and  are  worked  on  a 
large  scale  at  Mount  Bischoff,  in  Tasmania.  A  small  deposit  of 
this  kind  resting  in  a  shallow  gully  immediately  below  a  pegmatite 
dike  was  mined  near  Gaffney,  South  Carolina,  in  1905.  The 
earliest  production  of  stream  tin  came  from  gravels  below  the  tin- 

1 J.  M.  Hill,  Platinum,  Mineral  Resources,  U.  S.  Geol.  Survey,  pt.  1,  1917. 

James  W.  Neil,  Recovery  of  platinum  in  gold  dredging,  Min.  and  Sci. 
Press,  Dec.  8,  1917. 

2  Sydney  Fawns,  Tin  deposits  of  the  world,  London,  1907. 

H.  W.  Kayser  and  R.  Provis,  The  Mt.  Bischoff  tin  mine,  Proc.,  Inst.  C. 
E.  (London),  vol.  123,  1896,  pp.  377-387. 

O.  H.  Van  der  Wyck,  The  occurrence  of  tin  ore  in  the  islands  of  Banca 
and  Billiton,  Seventeenth  Ann.  Rept.,  U.  S.  Geol.  Survey,  1896,  pt.  3,  pp. 
227-242.  Mineral  Resources,  U.  S.  Geol.  Survey,  1895. 

L.  C.  Graton,  Reconnaissance  of  some  gold  and  tin  deposits  of  the 
southern  Appalachians,  Bvll.  293,  U.  S.  Geol.  Survey,  1906. 


244  MINERAL  DEPOSITS 

bearing  lodes  of  the  Erzgebirge,  in  Saxony,  and  of  Cornwall,  both 
sources  now  practically  exhausted.  Considerably  more  than 
one-half  of  the  world's  production  of  about  123,000  short  tons  of 
tin  is  still  obtained  from  placers,  mainly  in  the  Malay  Peninsula 
and  the  islands  of  Banca  and  Billiton,  near  Sumatra.  New 
South  Wales  and  Victoria  furnish  minor  amounts  and  in  the 
latter  state  some  cassiterite  is  saved  in  working  Pliocene  auri- 
ferous stream  channels.  In  this  case  the  tin  ore  appears  to  be 
sparsely  disseminated  in  granite  and  is  liberated  after  its  disin- 
tegration. At  the  Briseis  mine,  in  Tasmania,  the  deposit  worked 
consists  of  14  to  45  feet  of  river  gravel,  covered  by  20  to  40  feet 
of  decomposed  basalt  and  containing  from  2  to  4  pounds  of  cassi- 
terite per  cubic  yard. 

In  the  United  States  small  amounts  of  stream  tin  are  recov- 
ered in  Alaska  near  the  extreme  western  point  of  the  American 
continent,  in  the  Black  Hills  of  South  Dakota,  and  in  North  and 
South  Carolina. 

As  tin  is  worth  from  25  to  90  cents  per  pound  and  the  easily 
reduced  cassiterite  contains  78.6  per  cent,  of  the  metal,  it  is 
clear  that  a  small  quantity,  say  2  pounds  per  cubic  yard  of  gravel, 
might  suffice  for  profitable  working.  Some  gravels  in  Tasmania 
now  worked  average  only  0.6  pound  per  cubic  yard. 

MONAZITE  PLACERS1 

Monazite,  an  anhydrous  phosphate  of  cerium,  lanthanum,  and 
other  cerium  metals,  usually  contains  also  from  3  to  8  per  cent, 
of  thoria,  making  it  valuable  for  the  production  of  nitrate  of 
thorium,  which  is  utilized  in  the  manufacture  of  incandescent 
gas  mantles. 

The  mineral  has  a  specific  gravity  of  5.203,  a  resinous  luster, 
and  a  yellow  to  brown  color;  when  occurring  in  placers  it  is  found 
together  with  gold,  zircon,  magnetite,  ilmenite,  garnet,  etc., 
after  concentration  in  sluices.  From  its  associated  minerals  it 
is  cleaned  in  electromagnetic  separators,  the  final  product  being 
about  90  per  cent.  pure.  The  source  of  the  monazite  is  in  the 
granites,  gneisses,  and  pegmatites,  where  it  occurs  as  a  primary 
mineral.  As  its  value  (changing  with  the  percentage  of  thoria) 

1  J.  H.  Pratt  and  D.  B.  Sterrett,  Monazite  and  monazite  mining  in  the 
Carolinas,  Trans.,  Am.  Inst.  Min.  Eng.,  40,  1909,  pp.  488-511. 

D.  B.  Sterrett,  Mineral  Resources,  U.  S.  Geol.  Survey,  1906,  pp.  1195- 
1209.    W.   T.   Schaller,  idem,  pt.  2,  1916,  pp.  223-237. 


TRANSPORTATION  AND  CONCENTRATION     245 

is  about  8  cents  per  pound,  monazite  gravels  may  in  places  form 
workable  deposits,  especially  where,  as  often  happens,  gold  is 
present.  Monazite  is  now  obtained  from  marine  and  fluviatile 
placers  in  Brazil  and  India,  but  it  is  also  obtained  from  similar 
deposits  in  North  and  South  Carolina  and  has  lately  been  found 
in  Idaho,  where  a  large  intrusive  batholith  of  granite  or  quartz 
monzonite  evidently  carries  the  mineral  sparsely  distributed 
throughout.  The  principal  occurrence  in  Idaho  is  at  the  old 
placer  district  of  the  Idaho  Basin.  In  1910  about  100,000  pounds 
of  monazite  was  mined  in  the  United  States,  chiefly  from  placer 
deposits  in  the  Carolinas.  The  total  value  is  stated  as  $12,000. 
Since  1910  there  has  been  no  production  in  the  United  States, 
the  supply  being  obtained  from  the  extensive  deposits  in  Brazil. 

OTHER  PLACERS 

Magnetite,  or  "black  sand,"  has  been  frequently  mentioned 
above  as  a  product  of  concentration  in  gravels  and  sands  and  is 
usually  derived  from  the  disintegration  of  igneous  rocks.  Along 
the  beaches  and  the  bars  of  some  rivers  it  may  accumulate 
in  considerable  masses — for  instance,  on  the  lower  St.  Lawrence 


BIG  MOUNTAIN 

.  i!«nn 

-£S          1187                          1200 
^S^SSS*!  1130  1001     £3?*>>viiaL-^.     .Shaft  No.  1 

Clay 
/            Sandy  Limestone 

1200 

-1100 
1000 
-   900 
800 

1 

'  /vv  ^/'C*»Jv*' 

700 

Conglomerate 

Calcareous  and 

600 

Ore 

Gritty  Clay 

500 

400 

500  -100  300  200  100   0  Feet 

FIG.  89. — Section  of  Iron  Mountain,  Missouri,  showing  mining  of  detrital 
ore  underneath  limestone  and  sandstone,  and  of  hematite  ore  in  the  por- 
phyry. After  G.  W.  Crane. 

River,  Canada,  and  along  the  Columbia  River,  Oregon — but  it  is 
exceptional  that  such  deposits  have  been  utilized.1  More  or 
less  ilmenite  is  usually  mixed  with  the  magnetite. 

There  are  several  examples  of  eluvial  deposits  of  iron  ore 
(magnetite,  hematite,  or  limonite),  formed  below  croppings  of 
iron  deposits,  and  also  of  such  detrital  masses  in  the  debris  slopes 

1  The  magnetite  sands  of  Japan  appear  to  have  been  rather  extensively 
utilized;  also  those  occurring  along  the  coast  of  New  Zealand. 


246  MINERAL  DEPOSITS 

of  older  formations.  At  Iron  Mountain,  Missouri,1  Paleozoic 
rocks  rest  upon  a  deposit  of  boulders  of  iron  ore  and  porphyry, 
which  in  turn  lie  upon  pre-Cambrian  porphyry.  The  porphyry 
itself  also  contains  deposits  of  hematite  (Fig.  89). 

Similar  eluvial  masses  of  copper  and  lead  ores  are  found  in 
places.  We  may  recall  the  great  debris  mass  of  chalcocite  below 
the  croppings  of  the  Bonanza  mine2  in  the  Copper  River  region, 
Alaska,  and  galena  beds  on  the  slopes  below  the  Elkhorn  mine, 
Wood  River,  Idaho.3 

Some  placers  yield  precious  stones.  Diamonds  are  believed 
to  occur  as  a  primary  mineral  of  some  peridotites,  possibly  also 
in  other  rocks.  Diamond  placers  have  been  worked  in  Brazil, 
India,  and  South  Africa.  In  the  last-named  region  fine  stones 
are  found  in  the  gravel  of  the  Vaal  River,  and  small  diamonds 
have  lately  been  washed  from  the  beach  sands  of  Liideritz  Bay, 
German  West  Africa.  In  a  few  places  sapphires,  more  rarely 
rubies  (both  aluminum  oxide),  are  recovered  from  gravels. 
Along  the  Missouri  River  near  Helena,  Montana,4  a  bar  was 
worked  for  several  years  for  the  sapphires  it  contained.  They 
were  plentiful,  but  a  large  proportion  were  of  yellowish  or  pale 
blue  color. 

1  F.  L.  Nason,  Report  on  the  Iron  Mt.  sheet,  Geol.  Surv.  Mo.,  vol.  9,  1912. 

2F.  H.  Moffit  and  S.  R.  Capps,  Bull.  448,  U.  S.  Geol.  Survey,  1911, 
p.  89. 

8  W.  Liudgren,  Twentieth  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  3,  1900,  p. 
210. 

4  D.  B.  Sterrett,  Mineral  Resources,  U.  S.  Geol.  Survey,  pt.  2, 1910,  p.  877. 


CHAPTER  XVI 

DEPOSITS  PRODUCED  BY  CHEMICAL   PROCESSES 

OF  CONCENTRATION  IN  BODIES  OF  SURFACE 

WATER  BY  REACTIONS  BETWEEN  SOLUTIONS 

LIMESTONE 

Definition  and  Origin. — The  limestones  are  sedimentary 
rocks,  composed  of  carbonate  of  calcium,  usually  calcite,  but  in 
recent  deposits  also  aragonite;1  they  contain  minor  amounts  of 
magnesium  and  iron,  also  varying  amounts  of  alumina  and 
silica,  and  by  the  increase  of  these  constituents  transitions  to 
shale  or  sandstone  result.  Phosphate  of  calcium  and  organic 
matter  also  enter  into  the  composition  of  most  limestones.  The 
rocks  are  always  crystalline,  for  there  is  no  such  mineral  in  nature 
as  amorphous  carbonate  of  calcium,  but  the  grain  varies  between 
the  widest  limits. 

When  water  containing  bicarbonate  of  calcium  is  discharged 
into  the  ocean  or  bodies  of  fresh  water  the  calcium  carbonate  is 
often  precipitated  because  of  changing  equilibrium  in  the  solu- 
tions.2 This  is  exemplified  by  deposits  along  the  shore  of  the 
Great  Salt  Lake  in  Utah.  Such  limestones  often  form  "oolitic" 
beds  of  small,  rounded  concretions. 

Generally,  organic  life  plays  a  most  important  part  in  the  de- 
position of  calcite  either  indirectly  by  precipitation  by  ammonium 
carbonate  generated  by  decaying  organisms,  or  directly  by  life 
processes.  Bacteria3  may  be  the  agent,  very  often  also  algae,  the 
latter  both  in  the  sea  and  in  fresh  water  deposits.  Mollusks, 
corals,  Crustacea  and  echinoderms  segregate  calcite  and  aragonite4 
in  their  shells  which  accumulate  on  the  bottom  at  moderate  depths. 

1 J.  Johnston,  E.  M.  Merwin  and  E.  D.  Williamson,  The  several  forms 
of  calcium  carbonate,  Am.  Jour.  Sci.,  4th  ser.,  vol.  41,  1916,  pp.  473-512. 

2J.  Johnston  and  E.  D.  Williamson,  The  role  of  inorganic  agencies  in  the 
deposition  of  calcium  carbonate,  Jour.  Geol,  vol.  24,  1916,  pp.  729-750. 

8  T.  W.  Vaughn,  Chemical  and  organic  deposits  of  the  sea,  Bull.  Geol. 
Soc.  Am.,  vol.  28,  1917,  pp.  933-944. 

G.  H.  Drew,  Publ.  182,  Carnegie  Inst.  Washington,  1914,  pp.  1-78. 

4  Aragonite  is  the  unstable  form  of  calcium  carbonate  and  always  tends 
to  change  to  calcite.  It  is  most  common  in  recent  or  Tertiary  deposits. 

247 


248  MINERAL  DEPOSITS 

Many  organisms,  such  as  sponges,  secrete  silica  from  the  sea  water 
and  thus  cherty  deposits  may  be  admixed  with  the  limestones. 
Many  limestones  are  almost  wholly  made  up  of  shell  remains 
but  in  others  no  trace  of  organic  structure  may  be  visible. 
Metamorphism  tends  to  increase  the  grain  and  destroy  the  fossils. 
Evaporation  of  ordinary  surface  waters  in  dry  climates  may 
produce  thick  beds  of  porous  limestone.  This  is  known  in 
Mexico  as  "caliche." 

Calcite  and  aragonite  are  often  deposited  in  large  masses  by 
hot  springs  containing  bicarbonate  of  calcium,  and  such  deposits 
may  closely  simulate  limestones.  Certain  beautiful  banded  and 
translucent  spring  deposits  are  called  onyx  and  are  used  for 
ornamental  stones. 

Among  the  many  varieties  of  limestone  the  following  may  be 
mentioned : 

Chalk. — This  is  a  white,  fine-grained,  loosely  coherent  lime- 
stone of  comminuted  shells  of  mollusks  and  also  of  foraminifera. 
Its  occurrence  in  the  Cretaceous  along  the  English  coast  is  well 
known.  Extensive  beds  are  reported  from  Texas,  New  Mexico, 
Arkansas,  and  Kansas.  Chalk  is  used  as  fertilizer,  for  whiting, 
for  marking,  for  polishing  powder,  and  for  many  other  purposes. 
"Paris  white"  is  a  pigment  made  by  grinding  "cliffstone,"  a 
hard  variety  of  chalk.  Much  of  this  is  imported. 

Lithographic  Stone. — The  variety  of  limestone  used  for  engrav- 
ing and  the  reproduction  of  colored  plates  is  a  fine-grained  rock 
with  imperfect  conchoidal  fracture,  gray  or  yellowish  in  color, 
and  uniform  in  texture.  It  must  be  porous,  to  absorb  the  grease 
in  the  printer's  ink,  and  soft  enough  to  work  readily  under  the 
engraver's  tool.  Lithographic  stone  of  good  quality  is  difficult 
to  find.  The  product  from  the  Solenhofen  quarries  in  Bavaria 
is  a  Jurassic  limestone  of  unusual  excellence.  The  material  is 
variable  in  composition  and  its  value  is  ascertained  only  by 
trial.1 

Lithographic  stone  is  reported  to  occur  in  several  States  of  the 
Union,  but  none  of  it  appears  to  be  as  good  as  Solenhofen  rock. 
The  plates  used  are  22  or  28  by  40  inches,  and  3  inches  thick. 
The  better  grades  are  expensive,  selling  at  about  22  cents  per 
pound.  The  best  grade  quarried  in  the  United  States  is  said  to 
come  from  Brandenburg,  Kentucky.2  The  demand  is  limited. 

1  S.  J.  Ktibel,  Mineral  Resources,  U.  S.  Geol.  Survey,  1900,  pp.  869-873. 
2E.  O.  Ulrich,  Eng.  and  Min.  Jour.,  vol.  73,  1902,  p.  895. 


CHEMICAL  PROCESSES  IN  SURFACE  WATERS   249 

Hydraulic  Limestone. — Certain  argillaceous  limestones  or 
dolomitic  limestones  are  used  for  the  manufacture  of  natural 
cement.  Such  rock,  crushed  and  burned,  hardens  or  "sets" 
when  mixed  with  water,  owing  to  hydration  and  crystallization  of 
the  silicate  and  aluminate  of  calcium  formed  during  the  burning. 
"  Portland  'C  cements  are  mixtures  of  limestone  and  argillaceous 
rocks,  subjected  to  a  similar  process  of  grinding  and  burning.1 

Lime. — Pure  limestones  are  changed  by  burning  and  conse- 
quent decarbonation  to  quicklime  which  is  usually  shipped  as 
"lumplime;"  this  "slakes"  to  a  calcium  hydroxide  when  mixed 
with  water  and  is  used,  with  the  addition  of  sand,  as  mortar  in 
brick  constructions.  The  "slaking"  is  retarded  by  the  presence 
of  magnesia  and  argillaceous  impurities.2  Slaked  lime,  finely 
powdered  is  a  recent  product  used  extensively  for  water  proofing 
concrete. 

Uses. — While  limestone  is  mainly  a  structural  material  it  is 
also  used  as  a  flux  in  smelting  operations.  Only  the  pure  varieties 
are  acceptable,  though  magnesian  limestones  are  also  used  in  iron 
furnaces. 

Burnt  lime  is  probably  employed  for  more  purposes  than  any 
other  natural  product.  It  is  used  for  the  manufacture  of  bleach- 
ing powder,  ammonia,  calcium  carbide,  fertilizers,  wood  alcohol, 
soap,  glycerine,  glue,  glass,  pottery,  paints,  paper  and  sugar. 
Also  for  tanneries  and  as  insecticide  and  fungicide. 

DOLOMITE3 

Pure  dolomite  contains  54.35  per  cent.  CaC03  and  45.65  per 
cent.  MgCOs.  Beds  of  dolomite  and  dolomitic  limestone  are 
common  in  sedimentary  deposits.  They  may  often  be  distin- 
guished by  a  fine-grained  sugary  texture,  due  to  a  development 

1  E.  F.  Burchard,  Mineral  Resources,  U.  S.  Geol.  Survey,  1916,  with  list 
of  literature. 

2  E.  F.  Burchard  and  W.  E.  Emley,  The  source,  manufacture  and  use  of 
lime,  Mineral  Resources,  U.  S.  Geol.  Survey,  pt.  2,  1913,  pp.  1509-1593. 

3  F.  M.  Van  Tuyl,  New  points  on  the  origin  of  dolomite,  Am.  Jour.  Sci., 
4th  ser.,  vol.  42,  1916,  pp.  249-260. 

F.  M.  Van  Tuyl,  The  origin  of  dolomite,  Ann.  Rept.,  Iowa  Geol.  Survey, 
vol.  25,  1914,  pp.  251-422. 

For  an  extended  discussion  of  dolomite  see  F.  W.  Clarke,  Geochemistry, 
Butt.  616,  U.  S.  Geol.  Survey,  1916,  pp.  559-571. 

E.  Steidtmann,  Origin  of  dolomite,  etc.,  Butt.  Geol.  Soc.  Am.,  vol.[28, 
1917,  pp.  431-450. 


250  MINERAL  DEPOSITS 

of  uniform  rhombohedral  crystals.  Dolomite  is  somewhat 
harder  than  limestone  and  is  insoluble  in  dilute  hydrochloric  acid. 
Magnesium  carbonate  is  much  less  soluble  in  water  than  calcium 
carbonate,  as  shown  by  the  fact  that  stalactites  in  magnesian 
limestone  caves  are  almost  wholly  CaCO3.  Some  travertines 
from  mineral  springs  are  rich  in  MgCO3  and  may  contain  up  to  29 
per  cent,  of  this  compound.  Dolomite  is  doubtless  deposited  by 
direct  precipitation  in  sea  water,  but  much  of  the  dolomite 
has  been  formed  by  alteration  of  the  limestone  by  sea  water, 
or  by  subsequent  dolomitization  by  surface  waters. 

Deep  borings  in  coral  reefs  have  shown  that  the  limestone, 
somewhat  magnesian  at  the  surface,  passes  into  dolomite  in 
depth.  Certain  algae  deposit  much  MgC03  with  CaC03;  some 
shells  also  contain  magnesium  carbonate  but  seldom  more  than 
7  per  cent.  In  warm  waters  the  percentage  of  MgCO3  in  shells 
tends  to  increase. 

Instances  are  known  of  the  deposition  of  thin  beds  of  pure 
magnesite  in  bodies  of  water. 

IMPORTANCE  OF  CARBONATE  ROCKS  AS  RELATED  TO 
ORE  DEPOSITS 

Within  the  zone  of  oxidation  the  carbonate  rocks  are  often 
dissolved,  residual  clays  being  then  developed.  Accessory  con- 
stituents such  as  barium  (and  strontium),  probably  present  in 
most  limestones  but  rarely  determined,  or  zinc  and  lead  in  the 
form  of  sulphides,  or  admixed  phosphates  may  then  become 
concentrated  and  acquire  economic  importance. 

Limestone  is  easily  silicified  by  waters  containing  silica;  the 
silica  usually  appears  as  irregular  masses  of  fine-grained  quartz 
or  chert.  It  is  quite  as  easily  dolomitized  by  dilute  waters  con- 
taining some  magnesia,  and  this  is  often  observed  near  ore 
deposits  formed  at  slight  or  moderate  depth.  Limestone  and 
dolomite,  under  the  influence  of  heated  waters,  are  subject  to 
replacement  by  quartz,  dolomite,  barite,  and  fluorite  or  by 
metallic  ores  such  as  pyrite,  blende,  and  galena.  At  high  tem- 
perature and  pressure  pure  limestones  recrystallize  to  marble. 
Silicates,  such  as  garnet,  diopside,  or  wollastonite,  form  in 
argillaceous  or  siliceous  limestone  from  the  impurities  contained 
or  from  the  introduction  of  solutions  rich  in  silica  and  iron. 
Lastly,  the  limestones  are  easily  soluble  and  caves  develop  along 
fractures,  forming  receptacles  for  the  deposition  of  ores. 


CHEMICAL  PROCESSES  IN  S  URFA  CE  WA  TERS   25 1 
CHERTS  AND  DIATOMACEOUS  EARTH 

The  silica  accumulated  by  detrital  processes  as  sandstone  and 
quartzite  has  already  been  mentioned.  Silica  may,  however, 
also  be  extracted  from  water  and  deposited  as  a  sediment  by 
means  of  organisms,  such  as  radiolarians,  diatoms,  and  sponges. 
In  part  this  silica  forms  cherty  masses  included  in  limestone;  part 
is  deposited  as  distinct  beds.  Diatomaceous  earth1  is  a  deposit 
formed  in  lakes  and  swamps,  as  well  as  in  the  sea,  and,  when  pure, 
consists  of  the  delicate  tests  of  diatoms,  a  class  of  algae  (see 
Arnold  and  Anderson,  Bull  315,  U.  S.  Geol.  Survey,  p.  438). 
Such  deposits  accumulate  abundantly  where  siliceous  volcanic 
tuffs  were  deposited  in  lakes,  as  occurred  at  many  places  in  the 
Cordilleran  region  during  the  Tertiary  period.  Thick  beds  are 
found  in  the  Miocene  of  Santa  Barbara  County,  California. 
The  diatomaceous  earth  is  frequently  more  or  less  admixed  with 
rhyolitic  glass  and  other  detritus;  the  tests  consist  of  hydrated 
opaline  silica.  The  earth  forms  light-colored  beds  of  extremely 
fine  texture  and  it  finds  extensive  use  as  a  polishing  powder, 
a  steam-pipe  packing,  and  an  absorbent  for  various  liquids.  It 
contains  up  to  87  per  cent.  Si02  and  5  to  9  per  cent.  H2O. 

SEDIMENTARY  SULPHIDE  DEPOSITS 

As  the  sedimentary  rocks  largely  consist  of  the  detritus  of  the 
continents,  it  is  self-evident  that  they  may  contain  the  metals 
of  the  rocks  and  ore  deposits  of  the  land  areas.  Iron  is,  of  course, 
abundant,  also  in  a  lesser  degree  manganese;  concretions  of 
hydrous  oxides  of  manganese  are  found  in  the  deep  sea  deposits 
and  analysis  shows  that  these  contain  notable  amounts  of 
nickel,  cobalt,  copper,  zinc,  lead,  and  molybdenum.  Many 
limestones  have  been  shown  to  contain  minute  amounts  of  zinc, 
lead,  and  copper.  River  sands  and  gravels  and  even  littoral 
ocean  sands  may,  locally,  contain  some  detrital  pyrite,  but  it  is 
extremely  unlikely  that  a  sufficient  quantity  of  these  sulphides 
would  escape  oxidation  to  form  important  deposits. 

Two  very  important  analyses  of  composite  samples  have  been 
published  recently  by  the  U.  S.  Geological  Survey  and  are  given 

1  W.  C.  Phalen,  and  F.  J.  Katz,  Mineral  Resources,  U.  S.  Geol.  Survey, 
1908-1916,  under  heading  "Abrasives." 

For  the  preparation  and  uses,  see  Percy  A.  Boeck,  Met.  and  Chem. 
Eng.,  vol  12,  1914,  pp.  109-113. 


252 


MINERAL  DEPOSITS 


below.1  The  rarer  elements  determined  are  of  special  interest. 
There  can  be  no  doubt  now  that  copper,  lead  and  zinc  as  well  as 
nickel  and  cobalt  are  contained  in  marine  and  fluviatile  deposits 
but  the  quantities  seem  to  average  smaller  than  these  determined 
in  paleozoic  carbonate  rocks,  and  considerably  smaller  than  those 
obtained  from  pre-Cambrian  igneous  rocks  (p.  10). 

ANALYSES  OF  SILT  AND  DELTA  DEPOSITS 

1  2 

SiOj 46.64  69.96 

AliOs 14.08  10.52 

Fe2Os 4.14  3.47 

FeO 1.88 

MgO 1.95  1.41 

CaO 7.20  2.17 

NasO 2.98  1.51 

K»O , 1.84  2.30 

H2O- 4.73  3.78 

H2O+ 5.86  1.96 

Ti02 1.84  0.59 

ZrOz None  0.05 

COi 4.05  1.40 

P»Oi 0.17  0.18 

SO. 0.32  0.03 

Cl 2.25  0.30 

F '     0.07 

S 0.11  0.07 

BaO 0.05  0.08 

SrO 0.025  Trace 

MnO 0.10  0.06 

V,Oi 0.028  0.02 

CnOt 0.044  0.01 

MoOi None  

AsiO» .  Trace  0.0004 

(Ni,  Co)0 0.080  0.017 

CuO 0.009  0.0043 

PbO 0.0004  0.0002 

ZnO 0.0087  0.001 

C 1.38  0.66 

101.775         100.6229 
LessO. 0.56  0.12 


101.215 


100.5029 
George  Steiger, 


1.  Terrigenous  blue  mud;  composite  of  52  samples 
analyst. 

2.  Mississippi  delta  mud;  composite  of  235  samples.     George  Jsteiger, 
analyst. 

Pyrite,  and  infrequently  other  sulphides,  may  be  precipitated 
by  chemical  reactions  in  sediments.  Beds  of  oolitic  pyrite  are 
known  (p.  269);  iron  disulphide  is  formed  in  places  in  bogs 

1  C.  E.  Siebenthal,  Zinc  and  lead  deposits  of  the  Joplin  region,  Bull.  606, 
U.  S.  Geol.  Survey,  1915,  p.  72.  With  extensive  discussion  of  results. 


CHEMICAL  PROCESSES  IN  SURFACE  WATERS   253 

and  streams  or  in  oceanic  sediments  where  hydrogen  sulphide 
developed  by  decaying  organic  matter  reacts  on  the  sulphates 
of  iron.  If  these  sediments  are -brought  to  the  surface  by  oro- 
genic  movements  and  slightly  metamorphosed,  the  sulphide, 
originally  in  fine  dissemination,  may  recrystallize  in  more 
prominent  form.  As  a  matter  of  fact,  the  deep  sea  muds 
thus  far  analyzed  contain  little  or  no  pyrite.  In  the  special 
case  of  the  Black  Sea,  often  quoted  of  late  from  N.  And- 
roussof  s  description,1  microorganisms  assist  in  liberating  hydro- 
gen sulphide,  part  of  which,  by  reaction  with  iron  from  the 
sediments,  develops  pyrite. 
P 


Fia.  90. — Vertical  section  through  the  pyritic  deposit  at  Meggen,  Ger- 
many. P,  Pyrite;  B,  barite;  k,  limestone;  bs,cs,ls,  Devonian  slates.  After 
Strauss. 

Although  pyritic  clays  are  abundant  in  the  unmetamorphosed 
sedimentary  formations,  there  is  little  evidence  of  extensive 
sedimentary  beds  of  pyrite. 

The  deposit  at  Meggen,  in  Westphalia,  is  often  referred  to  as 
tending  to  prove  the  existence  of  sedimentary  beds  of  pyrite. 
Oolitic  pyrite  with  barite  and  zinc  blende  occurs  here  in  De- 
vonian beds  and  is  worked  on  a  fairly  extensive  scale  the  bed 
being  from  12  to  20  feet  thick  (Fig.  90).  Alfred  Bergeat2  ap- 
pears to  have  demonstrated  the  sedimentary  origin  of  the  oolitic 
pyrite.  The  barite  and  the  zinc  blende  are  somewhat  later  than 
the  pyrite  and  their  origin  is  still  doubtful.  Bergeat  holds  that 
all  these  minerals  form  a  local  marine  deposit. 

Barite  when  appearing  in  sandstone  is  probably  deposited 

1  N.  Androussof,  Guide  des  excurs.  du  VII  Cong.  ge~ol.  int.,  29,  1907,  p.  6. 

2  A.  Bergeat,  Zeitschr.  prakt.  Geol,  vol.  22,  1914,  pp.  237-249. 


254  MINERAL  DEPOSITS 

by  hot  springs;  it  is  difficult  to  conceive  its  origin  from  sea 
water.  For  barite  and  manganese  carbonates  as  marine  shore 
deposits  see  p.  269. 

Evidence  is  scant  as  to  the  sedimentary  deposition  on  a  large 
scale  of  sulphides  other  than  pyrite  or  marcasite.  "The  Kup- 
ferschiefer"  of  Mansfeld,  in  which  the  sulphides  may  be  of 
syngenetic  origin,  will  be  described  elsewhere  (p.  413). 

SEDIMENTARY  IRON  ORES 

It  is  conceded  that  iron  ores,  such  as  magnetite,  can  be  de- 
posited by  mechanical  concentration  as  placers  along  rivers  or  the 
seashore  (p.  245),  or  again  we  may  easily  conceive  hematite  or 
limonite  derived  from  deep  decay  of  rocks  along  the  littoral,  or 
from  the.  oxidation  of  pyrite  deposits,  as  at  Rio  Tinto,  Spain, 
swept  out  into  the  sea  and  deposited  close  to  the  shore.  Iron 
ores  are  also  formed  by  chemical  reactions  in  bodies  of  water,  and 
these  yield  a  notable  proportion  of  the  iron  production  of  the 
world.  In  the  latter  cases  the  iron  has  been  supplied  from  the 
land  areas  in  form  of  solutions.  In  many  instances  both  dis- 
solved iron  salts  and  detrital  minerals  of  iron  contribute  to  the 
genesis  of  the  deposits. 

The  surface  waters  extract  iron  from  ferromagnesian  silicates 
as  well  as  from  oxides  or  other  minerals;  this  extraction  proceeds 
most  energetically  in  regions  covered  by  a  deep  mantle  of  decayed 
rock.  !  Both  iron  and  manganese  are  contained  in  springs  and 
streams.  An  example  of  such  spring  water,  rising  underneath  a 
deposit  of  bog  iron  ore  in  Holland,  is  quoted  by  Clarke: 

ANALYSIS  OF  SPRING  WATER  AT  EDERVEEN,  NETHERLANDS  1 
(Analyst,  G.  Moll  van  Charante) 

(Parts  per  million) 
Ca 107.6  A12O3 3.3 


Mr 

5  6 

Cl 

15  2 

Fe 

.      19.6 

H,PO, 

10  9 

Mn  
K  
Na 

11.4 
0.9 
10  0 

SO4  
CO3  
SiO 

0.9 
207.6 
18  0 

Organic  

56.0 

467.0 

1  Cited  by  F.  W.  Clarke,  Geochemistry,  Bull.  616,  U.  S.  Geol.  Survey, 
1916,  p.  530. 


CHEMICAL  PROCESSES  IN  SURFACE  WATERS    255 

A  larger  part  of  the  iron  dissolved  by  the  surface  water  is 
precipitated  after  a  short  journey,1  but  some  of  it  is  carried 
down  by  the  streams  to  lakes  and  seas,  in  which  it  then  may 
be  deposited  on  an  extensive  scale.  The  sea  water  contains  about 
0.6  milligram  per  liter  of  iron  and  probably  more  at  some  places 
near  the  shore. 

LIMONITES  IN  SWAMPS  AND  LAKES  (BOG  IRON  ORES) 

Occurrence. — The  bog  iron  ores  are  found  in  swamps,  lakes, 
or  even  in  sluggish  water  courses,  and  they  are  especially  abun- 
dant in  the  recently  glaciated  regions  of  northern  Europe,  Asia, 
and  North  America.  They  consist  of  dark-brown,  rough  and 
cellular  masses  or  loose  particles,  sometimes  oolitic  in  structure 
and  then  designated  as  "shot  ore,"  and  form  a  layer  of  varying 
thickness  at  the  bottom  of  the  swamp  or  lake.  Plants  and 
roots  may  be  replaced  by  limonite.  Such  ores  are  usually  mined 
by  means  of  primitive  dredges  or  scoops. 

The  ore  occurs  mainly  in  shallow  waters  along  the  shore,  to  a 
depth  of  about  12  feet.  After  removal  a  new  layer  is  formed 
within  a  few  years;  according  to  A.  Geikie2  several  inches  of 
limonite  accumulated  in  26  years  in  a  Swedish  deposit.  The 
rate  would  naturally  be  subject  to  great  variations  according  to 
local  conditions. 

The  bog  iron  ores  are  now  of  slight  importance  to  the  mining 
industry,  but  the  easily  traceable  processes  of  their  formation 
give  us  a  most  welcome  key  to  the  origin  of  other  and  more 
obscure  deposits. 

Composition. — These  ores  are  always  mixed  with  sand  and  clay 
and  rarely  contain  as  much  as  50  per  cent,  of  iron.  The  principal 
mineral  contained  is  limonite,  but  carbonate  of  iron  is  commonly 
present,  also  phosphate  as  vivianite;  soluble  silica  is  sometimes 
recorded.  In  some  low-grade  ores  from  the  Netherlands,  the 
analyses  of  which  are  quoted  by  Clarke,3  there  is  much  more 
ferrous  carbonate  than  limonite.  Varying  quantities  of  manga- 
nese are  present  in  ores  from  Sweden,  Finland,  and  Holland.  The 
Swedish  ore  contains  traces  of  vanadium,  molybdenum,  copper, 
lead,  zinc,  arsenic,  nickel,  and  cobalt.  All  bog  iron  ores  contain 
phosphorus,  but  there  is  rarely  much  sulphur. 

1  The  clogging  of  water  supply  pipes  by  hydroxides  of  these  metals  is  a 
common  occurrence. 

2  A.  Geikie,  Text-book  of  geology,  4th  ed.,  1903,  p.  187. 

3  F.  W.  Clarke,  Geochemistry,  Bull.  616,  TJ.  S.  Geol.  Survey,  1916,  p.  530. 


256  MINERAL  DEPOSITS 

According  to  Svanberg,  cited  by  Zirkel,1  the  average  of  30 
analyses  of  Swedish  bog  iron  ores  gave: 

Fe203 62.57  MgO 0.19 

Mn2O3 5.58  P2O6 0.48 

SiO2 12.64  SO3 0.07 

A12O3 3.58  Ignition 13.53 

CaO 1.37 

Total 100.01 

Origin.2 — The  agents  by  which  iron  is  carried  into  solution 
are  (1)  carbon  dioxide  from  the  air  and  decomposing  organisms; 
(2)  sulphuric  acid  from  the  weathering  of  pyrite,  and  (3)  organic 
acids  derived  from  decomposing  vegetable  matter.  In  the  ab- 
sence of  air  ferric  oxide  is  reduced  to  the  ferrous  state  and  forms 
soluble  double  salts  with  ammonia  and  humic  acid. 

Precipitation  is  effected  in  bicarbonate  solutions  by  the  escape 
of  carbon  dioxide  in  the  air  or  through  its  absorption  by  plant 
cells.  The  ferrous  carbonate  is  easily  oxidized  to  ferric  hy- 
droxides. In  the  presence  of  much  organic  matter  ferrous 
carbonate  remains  in  the  precipitate. 

From  ferrous  sulphate  solution  iron  is  precipitated  as  limon- 
ite  by  oxidation  and  hydrolysis,  or  by  reaction  with  calcium 
carbonate  solution,  in  which  case  siderite  and  gypsum  will  result, 
the  former  oxidizing  to  limonite,  or  the  iron  may  be  precipitated 
by  ammonium  humate,  always  present  in  swamp  waters,  or 
finally  by  soluble  calcium  phosphate,  in  which  case  vivianite  or 
other  iron  phosphates  result.  Less  commonly  the  iron  is  pre- 
cipitated as  pyrite  by  alkaline  sulphides  or  hydrogen  sulphide. 

From  soluble  humates  iron  is  also  precipitated  by  organisms, 
called  iron  bacteria,  which  take  up  these  humates,  as  well  as 
ferrous  carbonate,  and  coat  their  cell  walls  with  the  segregated 
limonite,  but  regarding  the  real  importance  of  this  process  we 
have  few  data. 

In  these,  as  in  so  many  other  surface  reactions,  the  ferric 

1  F.  Zirkel,  Lehrbuch  der  Petrographie,  vol.  3,  1894,  p.  574. 

2  R.  Beck,  Die  Lehre  von  den  Erzlagerstatten,  3d  ed.,  vol.  2,  1909,  p.  397. 
F.  W.  Clarke,  Geochemistry,  Bull.  616,  U.  S.  Geol.  Survey,  1916,  p.  529. 
Ossian  Aschan,  Zeitschr.  prakt.  Geol.,  1907,  pp.  56-62. 

C.  R.  Van  Hise,  Metamorphism,  Mon.  47,  U.  S.  Geol.  Survey,  1904,  p. 
826. 

J.  M.  van  Bemmelen,  Zeitschr.  anorg.  Chemie,  vol.  22,  1906,  p.  313. 
J.  H.  L.  Vogt,  Zeitschr.  prakt.  Geol.,  1894,  p.  30,  and  1895,  p.  38. 
F.  M.  Stapff,  Zeitschr.  Deutsch.  geol.  Gesell.,  vol.  18,  1866,  p.  86. 


CHEMICAL  PROCESSES  IN  SURFACE  WA  TERS   257 

hydroxides  are  probably  precipitated  as  colloidal  complexes  of 
indefinite  composition,  or  "gels,"  which  in  time  tend  to  change  to 
crystalline  bodies.  Much  of  the  ferric  hydroxide  is  doubtless 
transported  for  considerable  distance  in  colloid  form.  Five 
species  of  ferric  hydroxide  are  recognized.  Arranged  by 
increasing  water  they  are: 

Turgite 2  Fe2O3.H2O  94.6  per  cent.  Fe2O3 

Goethite 2  Fe2O3.2H2O  89.9     "     " 

Limonite 2  Fe2O3.3H2O  85 . 5     "     " 

Xanthosiderite....  2  Fe2O5.4H2O  81.6    "     " 

Limnite 2  Fe2O3,6H2O  74 . 7     "     " 

Both  gothite  and  turgite  are  red  and  may  be  mistaken  for  hema- 
tite. Limonite  and  gothite,  and  probably  also  the  other  com- 
pounds, soon  acquire  crystalline  properties  and  then  show  strong 
double  refraction  and  fibrous  texture.  There  are  several  complex 
ferric  silicates  and  sulphates,  which  look  somewhat  like  limonite. 

Examples. — Many  occurrences  of  bog  iron  ores  are  known  in 
the  United  States.  At  Radnor  and  Drummondville,  Three  Rivers 
district,  Quebec,  the  occurrences  are  extensive.  The  ores  con- 
tain 0.3  per  cent,  phosphorus  and  less  than  0.1  per  cent,  sul- 
phur. This  iron  ore  was  utilized  until  1911,  being  dug  in  the 
swamps  or  dredged  in  the  lakes;  23,000  short  tons  were  mined 
in  1907,  but  in  1911  the  operations  ceased.  The  production 
in  this  district  began  in  1733. 

One  of  the  most  famous  deposits  formerly  mined  is  at  Katahdin, 
Maine.  Small  deposits  are  found  at  very  many  places  in  New 
England  and  have  been  worked  on  a  small  scale.  Near  Portland, 
Oregon,  at  the  Prosser  mine,  limonite  ore  was  found  in  the  sur- 
ficial  hollows  of  a  basalt  flow,  covered  by  a  later  flow  of  the  same 
rock;  it  was  6  to  15  feet  thick  and  contained  roots  and  trunks 
of  trees.1  The  earlier  basalt  was  rich  in  iron  and  its  decom- 
position furnished  the  iron  to  the  swamps  which  covered  its 
surface. 

THE  SIDERITES  OF  MARINE  AND  BRACKISH-WATER  STRATA 

Occurrence. — Siderite  (FeCO3)  is  an  iron  ore  of  some  impor- 
tance, both  in  epigenetic  and  syngenetic  deposits.  It  occurs  in 
fissure  veins  and  as  a  replacement  of  limestone,  but  is  also  found 
in  the  sedimentary  rocks  as  a  product  of  the  sedimentary  proc- 

1  J.  F.  Kemp,  Ore  deposits  of  the  United  States  and  Canada,  1900,  p.  92. 


258  MINERAL  DEPOSITS 

esses.  The  sedimentary  siderite  ores  are  called  clay  ironstone, 
spherosiderite,  or  black  band.  A  dense  or  fine-grained  con- 
cretionary structure  is  characteristic  of  the  "clay  ironstone" 
occurring  in  clays  or  shales  and  these  concretions,  more  or  less  ad- 
mixed with  clay  and  sand  and  often  inclosing  vegetable  remains, 
are  found  abundantly  at  certain  horizons.  The  variety  called 
"black  band"  forms  continuous  beds  of  dark-colored,  compact 
appearance,  in  the  shales  of  the  coal  measures,  and  often  directly 
underneath  or  above  the  coal  beds. 

These  ores  contain  less  than  48  per  cent,  of  iron  and  must  be 
calcined  before  smelting.  Both  sulphur  and  phosphorus  are 
present,  sometimes  in  considerable  quantities. 

Marcasite,  pyrite,  arsenopyrite,  millerite,  galena,  blende,  and 
chalcopyrite  are  sometimes  found  along  cracks  in  the  concre- 
tions of  siderite,  indicating  that  the  iron  solutions  carried  small 
amounts  of  the  less  common  metals,  probably  as  sulphates. 
After  the  deposition  of  the  siderite  these  metals  were  leached  and 
redeposited  as  sulphides  along  available  openings.  An  analysis 
of  siderite  ore  from  Maryland1  showed  36.05  per  cent.  Fe,  13.53 
per  cent.  SiO2,  6.47  per  cent.  A1203,  0.94  per  cent.  Mn,  0.08 
per  cent.  P,  and  0.42  per  cent.  S. 

The  economic  importance  of  these  ores,  formerly  great,  is  now 
small.  Near  the  surface  they  are  sometimes  changed  to  limonite. 
*  The  origin  of  sedimentary  siderites  is  explained  along  the  same 
lines  as  that  of  the  bog  iron  ores.  Solutions  of  .ferrous  bicarbon- 
ate were  supplied  to  the  marshes  along  the  sea  coast  or  to  the 
shallow  sea  where  organic  matter  was  abundant.  Precipitation  of 
the  normal  insoluble  carbonate  took  place  through  absorption  of 
the  solvent  CQ%  by  vegetation.  Free  oxygen  was  absent,  for 
otherwise  the  carbonate  would  have  been  transformed  into  limon- 
ite. Even  if  the  iron  had  originally  been  deposited  as  limonite  a 
reduction  and  carbonation  to  siderite  may  have  been  effected 
by  the  limonitic  precipitate  being  covered  by  mud  containing 
organic  matter. 

The  concretionary  ores  are  not  the  products  of  primary  pre- 
cipitation, but  are,  probably  in  all  cases,  segregated  into  nodu- 
lar form  by  the  action  of  percolating  solutions  around  a  suit- 
able nucleus  while  the  sediments  were  still  soft. 

1  J.  T.  Singewald,  Econ.  Geol,  vol.  4,  1909,  pp.  530-543. 

J.  T.  Singewald,  Report  on  the  iron  ores  of  Maryland,  Maryland  Geol. 
and  Econ.  Survey,  vol.  9,  pt.  3,  1911. 


CHEMICAL  PROCESSES  IN  SURFACE  WATERS   259 

Examples.  —  In  the  United  States  sedimentary  siderites  are 
known  in  Pennsylvania,  Ohio,  West  Virginia,  Maryland,  and 
Kentucky.  Their  present  industrial  importance  is  slight,  but 
they  were  formerly  mined  on  a  more  extensive  scale.  The 
production  in  1916  was  only  1,800  long  tons,  chiefly  from  Ohio. 

In  Pennsylvania  and  adjacent  States  the  upper  barren  Coal 
Measures  contain  abundant  nodules  of  siderite  in  the  shales  and 
sandstones,  but  no  valuable  deposits  In  the  upper  productive 
Coal  Measures,  or  Monongahela  River  series,  black  band  ore 
occurs,  for  instance,  just  below  or  above  the  Pittsburg  coal  bed. 
In  the  lower  Coal  Measures  the  siderites  are  especially  abundant; 
in  Ohio  12  horizons  of  black  bands  and  concretionary  ores  are 
distinguished  by  Orton.1 

Siderite  ores  also  occur  in  the  Tertiary  Claiborne  formation  of 
Mississippi. 

The  black  bands  are  common  in  Germany,  but  are  not  mined 
extensively.  They  were  formerly  actively  worked  in  Westphalia 
and  near  Saarbriicken,  where  the  ore  formed  flat  lenticular 
masses  as  much  as  1%  meters  thick  and  sometimes  several 
hundred  meters  in  extent. 

In  England  the  black  bands  were  formerly  of  the  highest  im- 
portance and  40  years  ago  furnished  four-fifths  of  the  total 
iron  output.  They  are  now  mined  only  in  North  Staffordshire 
and  in  Scotland.  In  Wales  the  black  bands  occur  in  the  lower 
Coal  Measures.  Kendall2  enumerates  75  horizons  of  siderite  ore. 

In  Scotland  (Ayrshire)  the  black  bands  occur  both  in  Coal 
Measures  and  in  Carboniferous  limestone.  The  ores  contain 
25  to  40  per  cent.  Fe,  and  occur  as  thin  strata,  1^  feet  or  less 
thick;  several  of  them  are  usually  close  together. 

THE  JURASSIC  SIDERITES  OF  ENGLAND 

The  carbonate  ores  of  the  Jurassic  "oolite"  in  England  have 
a  much  greater  importance  than  formerly,3  the  mine  production 


Geol.  Survey,  vol.  5,  1884,  p.  378. 

2  J.  D.  Kendall,  The  iron  ores  of  Great  Britain  and  Ireland,  London,  1893, 
pp.  145-199. 

3  Henry  Louis,  The  iron  ore  resources  of  the  world,  Int.  Geol.  Congress, 
Stockholm,  1910. 

W.  G.  Fearnsides,  British  iron  ore  resources,  Min.  Mag.,  London,  Nov., 
1917,  pp.  241-243. 

W.  Barnes,  Mining  iron  ore  in  the  Midlands,  idem,  March,  1918,  pp. 
120-126. 


260  MINERAL  DEPOSITS 

being  about  11,000,000  metric  tons,  out  of  a  total  output  of 
15,000,000  tons.  While  the  ores  are  of  low  grade  they  are 
cheaply  mined,  largly  in  open  cuts.  The  largest  yield  comes  from 
the  Cleveland  Hills,  in  the  Yorkshire  district.  The  ores  form 
three  or  four  beds,  in  the  shales  and  sandstones  of  the  Lower 
Oolite,  Upper  Lias,  and  Middle  Lias;  the  thickest  bed  attains 
13  feet  with  several  minor  clay  streaks.  The  ore  is  changed 
to  limonite  near  the  surface,  but  the  bluish  green  unaltered  rock 
is  composed  largely  of  oolitic  siderite;  a  little  glauconite  is  present. 
Its  percentage  composition  is  approximately  as  follows:  SiC>2, 
10  to  20;  FeO,  40;  Fe203,  1.4;  CaO,  1.5;  CO2,  25;  P205,  0.5 
to  2.  There  is  little  sulphur  and  the  metallic  iron  varies  between 
29  and  35  per  cent.  A  little  magnetite  is  reported  in  the  ore. 

THE  OOLITIC  MARINE  LIMONITES  AND  HEMATITES 

The  oolites  (name  derived  from  the  semblance  to  fish-roe)  con- 
sist of  small  rounded  grains  of  concretionary  origin,  each  grain 
often  being  formed  around  a  small  sand-grain  or  around  a  small 
fossil  fragment.  They  are  found  in  shallow  water  near  the 
shore,  where  the  action  of  waves  and  currents  is  strong.  Oolitic 
limestones  are  common  occurrences  in  some  sedimentary  series. 
The  oolitic  iron  ores  consist  of  limonite,  of  hematite,  of  siderite 
or  of  iron  silicates.  Frequently  all  these  occur  together.  The 
concretions  are  cemented  by  calcite  or  siderite  or  more  commonly 
by  an  argillaceous  substance. 

THE  OOLITIC  LIMONITES 

Occurrence. — The  oolitic  limonites  form  well-defined  and 
extensive  beds  in  purely  sedimentary  series  of  sandstones,  shales, 
and  marls.  Though  several  ore  beds  are  usually  present  in  each 
district  they  are  not  always  persistent,  but  may  thin  out,  other 
beds  appearing  at  different  levels.  The  ores  have  no  relation  to 
volcanism,  though  in  many  cases  the  decay  of  volcanic  rocks  may 
have  supplied  the  iron.  Though  not  particularly  characteristic  of 
any  one  formation  the  ores  are  most  abundant  in  Jurassic  strata. 
The  percentage  of  iron  is  low  and  that  of  phosphorus  high; 
favorable  features  are  the  presence  of  calcium  carbonate,  which 
makes  the  ores  self-fluxing,  and  the  great  extent  of  the  beds. 
The  great  iron  industries  of  Germany  and  France  are  largely- 
dependent  upon  the  oolitic  limonites. 


CHEMICAL  PROCESSES  IN  SURFACE  WATERS   261 

Examples. — The  so-called  "minettes,"1  or  oolitic  limonites  of 
the  German  and  French  Lorraine  and  of  Luxembourg,  are  of  the 
highest  importance  as  present  and  future  resources  of  European 
iron.  In  France  there  are  at  least  50  mines  with  an  annual 
ore  production  of  nearly  20,000,000  metric  tons  (1913).  The 
proved  reserves  are  estimated  at  3,000  million  tons.  In  German 
Lorraine,  now  ceded  to  France,  the  production  attains  similar 
figures  and  the  estimated  reserves  are  over  2,000  million  tons. 
Dipping  gently  westward  the  strata  attain  a  depth  of  3,000  feet 
or  more  in  France.  The  present  mining  is  done  at  a  depth  of 
700  feet  or  less,  and  in  part  by  tunneling  or  open  cuts  (Fig.  91), 

The  ores  lie  in  the  Middle  (Dogger)  part  of  the  Jurassic  systems 
and  occur  with  shales,  sandstones,  and  marls  as  distinct  bed. 


FIG.  91. — Section  through  the  minette  measures  at  Esch;  8,  Calcar- 
eous layer  with  Harpoceras  humphriesianum;  7,  calcareous  layer  with 
Harpoceras  sowerbyi;  6,  marl  with  Harpoceras  murchisonae;  5,  the  minette 
measure  group  (see  legend);  4,  sandstone  with  Trigonia  natris;  2  and  3, 
upper  and  lower  clays  with  Harpoceras  striatulum;  1,  Lias  (micaceous  marl). 
After  W.  Branco. 

within  a  vertical  distance  of  75  to  150  feet.  The  strata  are  not 
absolutely  persistent  at  the  same  level,  but  are  local  accumula- 
tions, thinning  out  in  lenticular  manner.  The  several  beds 
known  are  of  different  thickness,  the  maximum  being  15  feet. 
A  low  percentage  of  iron,  varying  from  31  to  40,  is  characteristic, 
likewise  a  high  percentage  of  phosphorus,  varying  from  1.6  to  1.8, 
the  latter  making  the  ores  available  for  the  basic  process.  From 
5  to  12  per  cent.  CaO  and  from  7  to  33  per  cent.  Si02  are  present. 
The  ores  are  earthy  and  soft  and  are  of  brown,  gray,  or  yellow 

1  P.  M.  Nicou,  in  "iron  ore  resources  of  the  world,"  Int.  Geol.  Congress, 
Stockholm,  1910. 

L.  van  Werveke,  Zeitschr.  prakt.  Geol,  1895,  p.  497,  1901,  pp.  396-403. 


262 .  MINERAL  DEPOSITS 

tints.  Limonite  forms  the  bulk  of  the  ore,  but  there  is  always 
calcite  and  some  siderite,  often  also  a  little  secondary  magnetite, 
and  more  rarely  grains  of  pyrite,  zinc  blende,  galena,  and  chalco- 
pyrite.  The  small  concretions  of  the  size  of  a  pin-head,  or  a 
little  larger,  consist  of  limonite  but,  like  the  Clinton  oolites  of 
hematite,  have  a  skeleton  of  silica,  which,  according  to  van 
Werveke,  points  to  a  probable  derivation  by  alteration  from 
glauconite.  The  cement  consists  of  silica,  lime,  or  clay  shale, 
and  grains  of  glauconite,  a  ferri-potassic  silicate,  occur  in  it. 

Less  important  oolitic  ores  occur  in  other  parts  of  Europe, 
likewise  in  the  Jurassic  system. 

Cretaceous  oolites  in  which  the  limonite  is  probably  derived 
from  glauconite  or  siderite  have  been  described  from  Texas.1 

Origin. — The  origin  of  these  limonites  is  a  disputed  question. 
As  already  indicated  some  observers  doubt  the  direct  deposition 
of  limonite  in  the  sea  water,  but  hold  that  the  mineral  resulted 
from  the  oxidation  of  oolitic  siderite  or  glauconite. 

In  a  recent  detailed  monograph  L.  Cayeux2  emphasizes  the 
peculiar  fact  that  the  limonitic  oolites  of  France  are  confined 
to  the  older,  pre-Cretaceous  formations,  while  in  the  Cretaceous 
or  later  beds  the  glauconites  predominate.  He  believes  that 
the  older  oolites  are  in  all  cases  derived  from  siderite  by  replace- 
ment and  oxidation. 

THE  MARINE  OOLITIC  SILICATE  ORES 

A  number  of  silicates  of  iron  are  distinctly  sedimentary  prod- 
ucts and  common  in  many  waterlaid  series  of  rocks;  the  most 
important  are  glauconite  (greensand),  chamosite,  thuringite, 
and  greenalite.  Glauconite  seems  especially  abundant  in  the 
Cretaceous,  chamosite  and  thuringite  in  the  Silurian,  and  green- 
alite in  the  Algonkian,  but  none  of  them  are  confined  to  any 
definite  horizon.  Their  composition  is  uncertain  and  variable, 
the  glauconite  KFeSi206.H20  alone  being  distinguished  by  a 
large  percentage  of  potash. 

Glauconite3  forms  in  marine   deposits   on   the   present   sea 

1  R.  A.  F.  Penrose,  Bull.  Geol.  Soc.  Am.,  vol.  3,  1891,  p.  47. 

2  L.  Cayeux,  Les  minerals  de  fer  oolithique  de  France,  Paris,  1909. 

3  C.  W.  von  Giimbel,  Sitzber.  Akad.  Mlinchen,  vol.  16,  1886,  p.  417;  vol. 
26,  1896,  p.  545. 

L.  Cayeux,  Contribution  6tude  microgr.  des  terr.  se'd.,  Lille,  1897. 
W.  A.  Caspari,  Contributions  to  the  Chemistry  of  the  Marine  Glauconite, 
Proc.  Edinb.  Roy.  Soc.,  vol.  30,  1909,  p.  364. 


CHEMICAL  PROCESSES  IN  SURF  A  CE  WA  TERS   263 

bottom  and  also  occurs  scattered  in  marine  sands  of  older  forma- 
tions from  the  Cambrian  to  the  present  time,  sometimes  so 
abundantly  that  the  rocks  are  termed  greensands.  The  Creta- 
ceous greensands  of  New  Jersey  form  a  good  example  of  rocks  con- 
taining abundant  glauconite;  they  are  rich  in  both  phosphorus 
and  potassium.  The  mineral  occurs  as  dark-green  granules, 
often  in  the  interior  of  shells. 

According  to  Murray  and  Renard,  glauconite  is  formed  just 
beyond  the  limits  of  wave  and  current  action,  where  the  muddy 

APPROXIMATE  COMPOSITION  OF  SEDIMENTARY  IRON  SILICATES 

Glauconite  Chamosite  Thuringite       Greenalite 

SiO2 53  29  24                     30 

A12O3 10  13  17 

Fe2O3 21  6  15                     35 

FeO 2  42  33                    26 

CaO 1 

MgO 3 

K20 4 

H20 6  10  11                      9 

deposits  begin.  Organic  matter  is  believed  to  reduce  the  iron 
in  the  mud  to  sulphide,  which  later  oxidizes  to  limonite;  at  the 
sa'me  time  colloidal  silica  is  set  free  and  the  colloidal  limonite 
absorbs  this  as  well  as  potash,  forming  ferric  silicate.1 

In  part,  however,  as  shown  by  Cayeux,  the  glauconite  has  been 
formed  somewhat  later  than  the  original  deposition  of  the  beds 
and  without  the  intervention  of  organic  matter. 

Greenalite  occurs  abundantly,  according  to  Leith2  in  the 
Algonkian  ferruginous  cherts  of  the  Mesabi  and  other  iron  dis- 
tricts; it  was  formerly  mistaken  for  glauconite.  It  is  believed 
to  be  a  marine  deposit. 

Neither  glauconite  nor  greenalite  rocks  form  iron  ores,  but 
the  former  may  be  transformed  by  alteration  into  limonite, 
and,  according  to  Van  Hise  and  Leith,  the  greenalite  is  the 
source  from  which  the  hematites  of  the  Lake  Superior  region 
were  in  part  derived. 

Chamosite  and  thuringite3  form  the  principal  ore  minerals  in 

1  F.  W.  Clarke,  Geochemistry,  Bull.  616,  U.  S.  Geol.  Survey,  1916,  p.   516. 
The  extensive  literature  is  here  summarized. 

2  C.  K.  Leith,  Mon.  43,  U.  S.  Geol.  Survey,  1903,  pp.  237-279. 

3  A.  W.  Stelzner  and  A.  Bergeat,  Die  Erzlagerstatten,  1,  1904,  p.  201. 
E.  R.  Zalinski,  Neues  Jahrb.  1904;  Beil.  B.,  19,  pp.  40-84. 


264  MINERAL  DEPOSITS 

a  number  of  interesting  deposits  in  Thuringia  (Germany)  and 
Bohemia,  which  formerly  were  mined  extensively  and  which  are 
still  being  mined  on  a  small  scale  in  the  latter  region.  These  sili- 
cates form  oolitic  grains  in  slightly  metamorphosed  clay  slates  of 
the  Lower  Silurian.  The  beds  still  retain  fossils.  In  Germany 
the  ores  occur  as  lenticular  beds  as  much  as  7  feet  thick.  In 
Bohemia  the  Silurian  series  consists  of  slates,  graywacke,  and 
diabase  tuffs.  These  contain  beds  of  oolitic  hematite,  one  bed 
being  16  feet  thick,  while  other  beds  consist  of  oolitic  chamosite 
of  considerable  thickness.  The  latter  show  a  groundmass  of 
siderite  or  chamosite,  in  which  are  embedded  oolites  of  dark-gray 
chamosite.  The  ores  are  rich  in  phosphorus  and  also  carry  a 
little  magnetite. 

Many  believe  that  the  iron  is  derived  from  the  decomposition 
of  the  associated  diabase  tuffs.  Be  that  as  it  may,  these  iron 
ores  are  certainly  of  sedimentary  origin. 

THE  MARINE  OOLITIC  HEMATITE  ORES 

Occurrence. — Oolitic  hematite  ores  of  undoubted  sedimentary 
origin  are  common  in  many  parts  of  the  world,  as,  for  instance, 
Germany,  France,  and  Bohemia.  They  are  usually  associated 
with  Paleozoic  rocks,  but  appear  to  be  lacking  in  Mesozoic  and 
Tertiary  sediments.  Siderite  and  calcite  usually  accompany 
them.  Rarely,  if  ever,  do  they  contain  magnetite  or  metallic 
sulphides.  Differing  opinions  are  expressed  as  to  their  origin; 
they  have  been  explained  as  replacements  of  limestone  or  of  sid- 
erite, or  again  as  primary  sediments,  the  tendency  in  the  United 
States  being  in  favor  of  the  latter  theory  of  origin. 

The  Clinton  Ores.1 — The  most  important  oolitic  ores  in  the 
United  States  are  those  of  the  Clinton  formation  in  the  Appala- 
chian States;  they  persist  with  remarkable  regularity  wherever 

1  C.  H.  Smyth,  Jr.,  On  the  Clinton  iron  ore  Am.  Jour.  Sci.,  3d  ser., 
vol.  43,  1892,  p.  487. 

E.  F.  Burchard,  The  Clinton  iron  ore  deposits  of  Alabama,  Trans.,  Am. 
Inst.  Min.  Eng.,  vol.  39,  1908,  pp.  997-1055. 

E.  F.  Burchard,  The  red  iron  ores  of  East  Tenn.,  Bull.  16,  Geol.  Survey 
Tennessee,  1913. 

Burchard,  Butts,  and  Eckel,  The  Birmingham  district,  Alabama,  Bull. 
400,  U.  S.  Geol.  Survey,  1910. 

D.  H.  Newland  and  C.  A.  Hartnagel,  Iron  ores  of  the  Clinton  formation, 
Bull.  123,  New  York  State  Mus.,  1908. 


CHEMICAL  PROCESSES  IN  SURFACE  WATERS   265 

this  formation  appears.  The  Clinton  (Silurian)  lies  between 
the  Trenton  limestone  and  the  Devonian  shale,  and  it  invariably 
contains  one  or  several  beds  of  hematite  ore  alternating  with 
limestone  and  shale.  The  succession  of  sedimentary  rocks  in 
the  Birmingham  district  is  as  follows.  In  a  general  way  the 
section  applies  to  the  entire  southern  Appalachian  region. 

Carboniferous :  Feet 

Pennsylvanian:  Pottsville  formation  ("Coal  Measures").  2,600  to  7,000 

Unconformity. 

Mississippian: 

Parkwood  formation 0  to  2,000 

Pennington  shale  (30-300  feet)    \  „. 

>  Floyd  shale 1,000  ± 

Bangor  limestone  (670  feet)         J 

Fort  Payne  chert 200  to  250 

Unconformity. 
Devonian : 

Chattanooga  shale  x  to  25 

rrog  Mountain  sandstone     J 
Unconformity. 

Silurian:  Clinton  (Rockwood)  formation 250  to  500 

Unconformity. 

Ordovician:  Chickamauga  (Pelham)  limestone 200  to  1,000 

Unconformity. 

Cambro-Ordovician :  Knox  dolomite 3,300 

Cambrian : 

Conasauga  (Coosa)  limestone 1,000  + 

Rome  (Montevallo)  shale  (great  thickness). 

The  Clinton  ores  extend  from  western  New  York,  through 
Pennsylvania,  Virginia,  West  Virginia,  Kentucky,  Tennessee,  and 
northwestern  Georgia,  into  Alabama,  where,  near  Birmingham, 
they  attain  their  greatest  development.  The  ores  constitute 
beds  or  lenses  at  various  horizons  in  the  Clinton  formation, 
which  forms  a  striking  unit  of  red  shallow  water  deposits  under- 
lain and  covered  disconformably  by  great  thicknesses  of  limestone 
of  the  Cambrian  and  Mississippian  ages.  Thin  beds  of  ferrugi- 
nous sandstone,  shale  and  oolitic  hematite  make  up  the  formation, 
with  frequent  cross  bedding  and  some  conglomerates.  The  ores 
contain  calcite  and  in  some  places  show  gradual  transition  to 
limestones. 

The  average  thickness  of  the  ore  beds  is  only  two  or  three  feet 
but  in  Alabama  they  reach  20  feet  of  merchantable  ore  with 
occasional  thin  shale  or  sandstone  partings.  Single  ore  beds  may 
extend  for  many  miles.  In  the  Birmingham  district  the  Clinton 


266  MINERAL  DEPOSITS 

beds  outcrop  on  the  east  flank  of  an  anticline  and  they  can 
be  traced  continuously  northward  into  Tennessee,  but  the  ac- 
tively working  mines  extend  only  for  15  miles  along  the  outcrop. 
Good  ore  beds  have  been  found  by  drilling  for  several  miles 
eastward  but  toward  the  west  the  formation  becomes  more 
calcareous. 

Four  beds  are  known  within  80  feet  in  the  upper  part  of  the 
formation  two  of  which  are  worked,  with  a  thickness  of  from  9  to 
20  feet.  The  iron  ores  are  generally  sharply  bounded  by  shale 
or  sandstone;  in  places  they  form  transitions  into  ferruginous 
sandstone. 

An  important  iron  industry  is  based  upon  the  deposits  in 
Alabama  and  the  annual  production  of  ore  has  now  attained  about 


FIG.  92. — Section  showing  Clinton  iron  ores,  Birmingham,  Alabama;  Sc, 
Clinton  (Rockwood)  formation,  Silurian  Oc,  Chickmauga  (Pelham)  lime- 
stone, Ordovician;  Cfp,  Fort  Payne  chert,  Mississippian.  After  E.  F. 
Burchard. 

5,000,000  long  tons,  or  about  8  per  cent,  of  the  total  output  of 
iron  ore  in  the  United  States.  Mining  has  been  carried  4,000 
feet  on  the  dip  in  some  of  the  properties,  and  entirely  similar  ore 
has  been  shown  to  exist  by  borings  at  a  vertical  depth  of  2,000 
feet,  2  miles  eastward  from  the  outcrop.  Large  reserves  of  ore 
are  available  in  this  district. 

Clinton  ores  are  also  mined  north  of  Alabama  in  Tennessee 
though  the  operations  are  generally  confined  to  the  enriched 
surface  ore.  North  of  Tennessee  the  mining  is  profitable  in  few 
places. 

There  are  several  types  of  Clinton  ores;  most  of  them  are 
fairly  rich  in  calcium  carbonate. 

One  common  type  is  a  fine-grained  pebbly  conglomerate  or 
sandstone,  each  pebble  or  grain  coated  with  hematite  and  the 
rock  cemented  with  that  mineral  and  with  calcite.  Another  type 
consists  largely  of  fragments  of  bryozoa,  shells,  trilobites, 


CHEMICAL  PROCESSES  IN  S  URFA  CE  WA  TERS   267 

etc.,  partly  coated  or  replaced  by  ferric  oxide,  besides  an  abun- 
dance of  oolitic  grains,  usually  with  a  grain  of  sand  as  the  center. 
Still  another  type  consists  entirely  of  oolites  of  hematite  in  calcite 
matrix,  averaging  1  or  2  millimeters  in  diameters.  ^J  fourth 
type  shows  small  flattened  hematite  concretions  with  fragments 
of  fossils  changed  to  hematite;  this  is  the  "flax  seed  ore"  which  is 
very  common  at  Birmingham.  There  is  very  little  siderite  or 
chlorite.  Fragments  of  quartz  and  other  minerals  are  common. 
The  beds  vary  along  the  strike  in  their  calcareous  or  siliceous 
admixtures.  Phosphorus  is  present  above  the  Bessemer  limit 
that  is  above  0.05  per  cent. 

At  the  surface  and  down  to  a  depth  of  about  200  feet  the 
calcium  carbonate  is  in  part  dissolved  and  the  ore  correspondingly 
enriched.  Such  ore  is  called  "soft,"  in  contrast  to  the  unaltered 
or  "hard"  variety.  The  poorest  ores  used  carry  25-30  per  cent. 
iron. 

ANALYSES  OF  CLINTON  ORES 
(E.  C.  Harder,  Mineral  Resources,  U.  S.  Geol.  Survey,  1908) 

Hard  ore  Soft  ore 

Fe 37.00  50.44 

SiO2 7.14  12.10 

A12O3 3.81  6.06 

CaO 19.20  4.65 

Mn 0.23  0.21 

S 0.08  0.07 

P 0.30  0.46 

The  origin  of  the  Clinton  ores  is  a  much  discussed  subject. 
The  principal  views  demand  either  a  direct  sedimentary  origin 
or  a  derivation  by  replacement  of  limestone.  The  latter  ex- 
planation is  supported  by  Rutledge,1  who  states  that  progressive 
steps  in  the  transformation  of  limestone  to  ore  may  be  followed 
in  the  field,  in  thin  sections,  and  in  analyses.  In  view  of  the 
constant  character  of  the  ore  at  great  depth  it  is  clear  that  if 
replacement  has  occurred  at  a  comparatively  late  date  it  has  at 
least  not  proceeded  from  the  surface. 

The  theory  of  direct  sedimentation  is  held  by  C.  H.  Smyth,  who 
contributed  a  notable  paper  to  the  question  of  origin.  Similar 
views  are  advocated  by  Newlands,  Eckel,  and  Burchard.  Smyth 
thinks  that  the  iron  was  carried  out  into  shallow  marine  basins 
and  was  there  slowly  oxidized  and  precipitated  ^mechanically 

1 J.  J.  Rutledge,  The  Clinton  iron  ore  deposits  of  Stone  Valley,  Pennsyl- 
vania, Trans.,  Am.  Inst.  Min.  Eng.,  vol.  39,  1908,  p.  1057. 


268  MINERAL  DEPOSITS 

around  the  shells  or  replaced  them.  S.  W.  McCallie  believes  that 
the  original  ore  was  glaticonite  or  greenalite,  citing  as  evidence 
the  delicate  skeleton  of  silica  remaining  when  the  oolite  is  dis- 
solved in  acid. 

The  Brazilian  Hematites. — In  the  pre-Cambrian  metamor- 
phosed sediments  of  Minas  Geraes  in  Brazil1  there  are  thick 
beds  of  rich  hematite  in  a  formation  of  ferruginous  sandstone 
(itabirite)  underlain  by  heavy  quartzite.  The  origin  of  this 
undoubtedly  sedimentary  hematite,  which  as  yet  has  not  been 
mined,  is  in  doubt.  There  is  no  oolitic  structure,  nor  are  there 
fossils.  Harder  and  Chamberlin  state  that  "not  having  much 
confidence  in  the  hypothesis  that  the  iron  oxide  was  precipitated 
directly  from  sea  water  by  ordinary  chemical  means  we  prefer 
to  turn  to  the  iron  bacteria  as  perhaps  forming  a  better  hypo- 
thesis." 

The  Oolitic  Hematite-chamosite-siderite  Ores. — Ores  contain- 
ing hematite,  chamosite  and  siderite  have  been  described  from 
several  places2  and  are  in  all  cases  of  marine  shallow  water 
origin.  The  description  by  A.  O.  Hayes  of  the  Wabana  ores  in 
Newfoundland  is  of  particular  interest.  The  ores  occur  in 
the  upper  1,000  feet  of  flat  dipping  Ordovician  sandstone  and 
shale  and  contain  several  workable  beds  from  10  to  30  feet  in 
thickness,  one  of  which  has  been  mined  for  a  distance  of  one 
and  one-half  miles  under  the  sea.  The  ores  look  like  hematitic 
oolites  and  contain  some  fragments  of  marine  shells  but  there  is 
little  calcite  and  no  limestone.  In  average  there  is,  in  per- 
cent., 50-70  hematite,  15-25  chamosite,  0-50  siderite,  0-1  calcite 
and  1-10  quartz.  The  oolites  consist  often  of  concentric  shells 
of  hematite  and  chamosite  such  as  shown  in  Fig.  93,  and  are 
frequently  embedded  in  a  matrix  of  siderite.  The  hematite  con- 
cretions, upon  treatment  with  HC1,  yield  a  residual  skeleton  of 
silica. 

It  is  shown  that  borings  of  alga3  penetrate  both  oolites  and 

1 E.  C.  Harder  and  R.  T.  Chamberlin,  The  geology  of  Central  Minas 
Geraes,  Brazil,  Jour.  Geol,  vol.  23,  1915,  Nos.  4  and  5. 

2L.  Cayeux,  Les  minerals  de  fer  oolithique  de  France,  Ministere  des 
Travaux  publiques,  Paris,  1907. 

W.  T.  Dorpinghaus,  Erzlagerstatten  vom  Chamosittypus  ...  in  der 
nordspanischen  Provinz  Leon,  Archiv  fiir  Lagerstatten-forschung,  Berlin, 
1914. 

A.  O.  Hayes,  The  Wabana  iron  ore  of  Newfoundland,  Mem.  78,  Geol. 
Survey  Canada,  1915, 


CHEMICAL  PROCESSES  IN  SURFACE  WATERS   269 

matrix  and  that  thus  the  ore  was  practically  in  its  present  con- 
dition when  covered  by  later  sediments.  Oxygen  given  off  by 
these  algae  may  have  caused  oxidation  of  chamosite  to  hematite. 
Direct  precipitation  of  all  three  iron  minerals  is,  therefore, 
advocated,  though  siderite  is  believed  to  be  the  latest  and 
may  replace  chamosite. 

Of  exceptional  interest  are  thin  beds  of  pyritic  oolite  above 
the  "Dominion"  bed.  This  contains  graptolites,  and  the 
small  pyrite  concretions  lie  in  an  argillaceous  matrix  with  some 
crystalline  quartz. 


FIG.  93. — Ore  from  Silurian  beds  at  La  Ferriere-aux-Etaugs,  France. 
Magnified  22  diameters.  The  oolites  are  chlorite  With  a  kernel  of  siderite; 
the  fine-grained  cement  is  chlorite  and  siderite.  a,  Oolite  of  chlorite,  in 
center  of  lighter  color;  partly  converted  into  hematite  on  the  outside,  b, 
Nucleus  of  corroded  pure  siderite.  c,  Same  of  yellow,  altered  siderite.  d, 
Grains  of  siderite  in  the  cement,  e,  Chloritic  oolite,  partly  crushed  and 
invaded  by  cement.  /,  Blackish  cement  of  chlorite  and  siderite.  After 
L.  Cayeux. 

REVIEW  OF  THE  SEDIMENTARY  IRON  ORES 
The  descriptions  given  above  show  that  in  marshes,  lakes  and 
rivers  the  hydroxides  of  iron,  mainly  limonite,  are  deposited, 
and  that  smaller  quantities  of  ferrous  carbonate  (siderite),  iron 
disulphide  and  iron  phosphates  may  be  precipitated, 


270  MINERAL  DEPOSITS 

Regarding  the  marine  ores,  it  is  certain  that  glauconite  and 
allied  iron  silicates  are  deposited  in  the  sea  and  that  under 
special  reducing  conditions  siderite  and  iron  disulphide  may  also 
form.  The  probability  is  also  very  strong  that  hematite  is 
developed,  in  part  from  oxidation  of  siderite  and  glauconite,  in 
part  by  detrital  processes.  Whether  limonite  is  ever  formed 
in  sea  water  is  much  more  doubtful  for  the  salt  solutions  have  a 
strong  dehydrating  effect.1  It  is  more  likely  that  the  so-called 
marine  limonites  are  products  of  oxidation  of  siderite  and  iron 
silicates. 

The  marine  iron  ores  are  all  shallow  water  deposits  and  the 
frequent  oolitic  structure2  is  in  part  at  least  due  to  accompany- 
ing action  of  waves  and  currents.  Many  of  the  replacements 
observed  have  certainly  occurred  immediately  after  deposition. 
Some  geologists  like  Cayeux  hold  that  the  ore  was  a  limestone 
of  organic  origin  which  has  been  later  transformed  into  hematite 
and  siderite  by  successive  replacements  but  there  seems  to  be 
little  to  support  this  view. 

The  part  played  by  micro-organisms  is  as  yet  difficult  to 
evaluate.  It  seems  certain  that  many  of  the  blue-green  algse 
develop  oxygen  in  their  life  processes  which  would  of  course 
promote  oxidation;  it  is  also  known  that  sea  water  at  all  depths 
contains  air  enriched  in  oxygen.  It  is  also  certain  that  some 
bacteria  of  the  Crenothrix  type3  segregate  hydroxide  of  iron  by 
oxidation  of  dilute  solutions  of  FeCO3,  but  this  process  can 
probably  not  go  on  in  sea  water.  Certain  other  bacteria  accord- 
ing to  Drew  of  the  de-nitrifying  type  seem  to  promote  the  forma- 
tion of  calcareous  oolites  in  the  sea,  and  similar  processes  may 
possibly  under  favorable  conditions  result  in  the  precipitation  of 
siderite. 

That  either  siderite  or  pyrite  can  be  deposited  in  large  bodies 
in  the  open  sea  must  be  considered  very  unlikely. 

Wherever  iron  disulphide  is  formed  reducing  conditions  pre- 

1  W.  Spring,  Neues  Jahrbuch,  pt.  1,  1899,  pp.  47-62. 
2G.  Linck,   Die   Bildung    der    Oolite  und   Rogensteine,   Neues    Jahrb. 
Beil.  B.  16,  1903,  p.  495. 

O.  Reis,  Geognostische  Jahreshefte,  vol.  22,  1909,  p.  58. 
3S.  Wienogradski,  Ueber  Eisenbakterien,  Botan.  Zeit.,  vol.  46, 1888,  p.  261. 

E.  C.  Harder,    Iron  depositing  bacteria   and  their  geologic  relations, 
Prof.  Pap.  113,  U.  S.  Geol.  Survey,  1919.     A  recent  paper  containing  many 
new  and  valuable  data. 

F.  Lafar,  Technical  mycology,  vol.  1,  1910,  p.  272. 


CHEMICAL  PROCESSES  IN  S  URFA  CE  WA  TERS   27 1 

vailed;  the  sulphide  has  probably  precipitated  as  a  colloid  and  is, 
therefore,  neither  pyrite  nor  marcasite.1 

Any  of  these  oolitic  deposits  may,  of  course,  have  been  en- 
riched, after  uplift  and  erosion,  by  solution  of  calcite  but  the  iron 
was  certainly  not  introduced  by  atmospheric  waters. 

It  is  very  significant  that  the  oolitic  concretions  in  all  these  ores 
yield  a  delicate  concentric  skeleton  of  soft  silica,  upon  treatment 
with  dilute  hydrochloric  acid.  This  may  indicate  that  a  silicate 


FIG.  94. — Clinton  ore,  Wolcott,  Wayne  County,  New  York.  Magnified 
20  diameters.  Ore  essentially  formed  of  remains  of  bryozoans  and  crinoids. 
a,  Fragments  of  bryozoans,  calcareous  walls  preserved,  interstices  filed  with 
ferric  oxide;  b,  fragment  of  bryozoan  encrusted  with  ferric  oxide,  the  walls 
partially  replaced  by  ferric  oxide;  c,  bryozoan  structure  almost  obliterated 
by  ferric  oxide;  d,  crinoid  stalk  replaced  by  ferric  oxide,  cells  filled  with 
calcite  of  uniform  optical  orientation;  e,  same,  almost  entirely  replaced; 
/,  calcite  cement.  After  L.  Cayeux. 

had  been  present  in  all  cases,  but  a  possible  alternative  is  that 
gelatinous  silica  and  iron  ore  were  precipitated  together. 

So  we  arrive  at  the  conception  of  shallow  bays  in  which  coral 
reefs  flourished  or  the  detritus  of  older  fossiliferous  limestone 

1  Bruno  Doss,  Melnikovit,  ein  neues  Eisenbisulfid,  Zeitschr.  prakt.  Geol. 
vol.  20,  1912,  pp.  453-467. 


272  MINERAL  DEPOSITS 

was  spread.  Into  these  bays  were  swept,  at  intervals,  masses 
of  finely  divided  detritus  from  the  deep  mantle  of  decayed  rock 
of  adjacent  tropical  land  areas,  undoubtedly  rich  in  hematite 
as  such  products  always  are.  The  water  discharged  from  the 
land  certainly  contained  ferrous  bicarbonate.  In  this  mud 
agitated  by  the  waves  progressed  numerous  and  complicated 
reactions.  Oolites  and  shells  of  calcite  were  replaced  by  siderite, 
which  almost  simultaneously  oxidized  to  hematite.  In  the 
deeper  water  glauconite  was  probably  deposited,  and  it  also  toay 
soon  have  been  altered  to  hematite.  Somewhat  similar  condi- 
tions are  found  to-day,  for  instance,  on  the  south  side  of  Molokai, 
Hawaiian  Islands,  where  such  hematite  mud  is  spread  out  over 
a  large  area  of  shallow  coral  reef. 

SEDIMENTARY  MANGANESE  ORES 

There  is  much  less  manganese  than  iron  in  the  earth's  crust,  the 
average  of  analyses  of  igneous  rock  calculated  by  Clarke  showing 
but  0.078  per  cent,  of  manganese.  Deposits  of  manganese  ore  are 
also  much  less  common  than  those  of  iron  ore.  Nevertheless, 
many  spring  waters  carry  manganese  and  a  minute  amount  of  it  is 
contained  in  sea  water.  Sedimentary  deposits  of  manganese  are 
known,  marine  and  lacustrine  as  well  as  fluviatile. 

According  to  experiments  by  E.  C.  Sullivan1  the  manganese 
in  rocks  is  taken  into  solution  more  easily  than  iron,  both  by  car- 
bonated water  and  by  dilute  sulphuric  acid.  He  also  finds  that 
from  mixed  ferrous  and  manganese  sulphates  almost  all  of  the 
iron  is  precipitated  first  by  carbonate  of  calcium  before  any 
manganese  is  thrown  down.  Fresenius,  many  years  ago,  also 
found  that  from  spring  water  iron  is  precipitated  first  as  limonite, 
while  the  manganese  remains  in  solution  much  longer.  This 
accounts  for  the  very  general  separation  of  the  two  metals  in  the 
oxidized  zone. 

Manganese  is  dissolved  mainly  as  bicarbonate  and  sulphate, 
possibly  also  as  phosphate.  It  is  easily  precipitated  by  oxida- 
tion, generally  as  MnOg  in  the  form  of  pyrolusite  (63.2  per  cent. 
Mn),  or  as  slightly  hydrous  psilomelane  or  wad  (an  impure 
mixture  of  manganese  oxides),  or  more  rarely  as  manganite 
(Mn2O3.H20).  The  precipitate  is  generally  a  "gel,"  which 

1  E.  C.  Sullivan,  quoted  by  W.  H.  Emmons  in  Bull.  46,  Am.  Inst.  Min. 
Eng.,  1910,  p.  803. 


CHEMICAL  PROCESSES  IN  SURFACE  WATERS   273 

crystallizes  in  time,  but  which  appears  to  have  a  tendency  to 
adsorb  certain  oxides,  especially  those  of  barium  and  potassium. 
According  to  F.  P.  Dunnington1  an  acid  solution  of  ferrous 
sulphate  dissolves  manganese  from  the  carbonate,  as  sulphate, 
with  the  separation  of  ferric  sulphate  and  limonite;  from  the 
compound  solution  calcium  carbonate  precipitates  the  iron, 
but  the  manganese  is  precipitated  only  upon  access  of  air. 

MnS04  +  CaCO3  +  0  =  CaS04  +  Mn02  +  C02. 

Bog  Manganese  Ore. — It  has  been  stated  above  that  many 
bog  iron  ores  contain  manganese;  pure  bog  manganese  ores  are 
also  known,  though  the  deposits  are  not  abundant.  The  material 
is  generally  earthy  and  soft,  approaching  wad  in  composition. 
In  part  the  bog  manganese  consists  of  a  skeleton  of  hard  and 
glossy  black  ore  containing  cavities  filled  with  a  black  powder. 
The  deposits  are  rarely  more  than  a  few  feet  in  thickness;  a  small 
occurrence  near  Wiekes,  Montana,  described  by  Harder,2  lies  in 
the  flat  bottom  of  a  gulch  covered  by  soil  and  underlain  by  ochery 
bog  limonite. 

A  much  larger  and  thicker  deposit  occurs  at  Hillsborough, 
New  Brunswick;  it  is  said  to  extend  over  17  acres  with  a  thick- 
ness of  6K  feet.  An  analysis  shows  Mn,  45.81 ;  Fe,  9.95;  S,  0.03; 
P,  0.05,  and  SiO2,  5.36  per  cent.3 

J.  H.  L.  Vogt  describes  a  deposit  in  Norway,  about  1  meter 
thick,  in  a  little  valley  above  a  layer  of  sand  and  below  a  cover 
of  peat.  The  manganese  ore  alternates  with  iron  ocher;  it  con- 
tains Mn02,  71.20;  MnO,  8.08;  Fe,  1.90;  P205,  0.10,  and  S,  0.07 
per  cent.  In  many  of  these  occurrences  the  rock  from  which  the 
metal  was  leached  is  a  granite  or  a  quartz  porphyry. 

Manganese  in  Lacustrine  and  Marine  Beds.— Many  sedimen- 
tary beds  in  all  parts  of  the  world  contain  manganese  derived 
from  the  degradation  of  old  land  areas;  it  occurs  as  carbonate  and 
stains  or  concretions  of  dioxide  in  tuffs,  quartzites,  sandstones, 
clays,  shales,  and  limestones.  It  is  frequently  contained  in  beds 
of  jasper  or  radiolarian  chert.  Strongly  manganiferous  sedi- 

1  F.  P.  Dunnington,  Am.  Jour.  Sci.,  3d  ser.,  vol.  36,  1888,  p.  177. 
2E.  C.  Harder,  Manganese  deposits  of  the  United  States,  Bull.  427,  U.  S. 
Geol.  Survey,  1910,  p.  137. 

3  Ann.  Rept.,  Geol.  Survey  Canada,  vol.  2,  1894,  p.  146. 
E.  C.  Harder,  op.  cit.,  p.  171. 


274  MINERAL  DEPOSITS 

ments  recrystallize  to  crystalline  schists,  the  manganese  assum- 
ing the  form  of  rhodonite,  rhodochrosite,  or  manganese  garnet 
(spessartite) .  The  presence  of  manganese  nodules  in  deep  sea 
deposits  is  well  known;  they  are  considered  to  be  rather  a  sub- 
marine product  of  segregation  from  the  red  pelagic  mud  than  of 
chemical  precipitation  from  the  ocean.  Very  rarely,  however,  do 
these  sedimentary  rocks  contain  manganese  of  economic  impor- 
tance, and  it  is  only  by  subsequent  concentration,  especially 
effective  in  regions  of  deep  secular  decay,  that  valuable  deposits 
are  developed  (pp.  338-345). 

An  excellent  example  of  an  undoubtedly  sedimentary  and 
practically  unaltered  deposit  is  described  from  Newfoundland 
by  N.  C.  Dale.1  It  is  of  little  economic  importance.  The  metal 
occurs  as  carbonate,  with  some  MnO2,  in  nodular  form,  in  shaly 
and  calcareous  beds  of  Cambrian  age  and  is  associated  with 
calcium  phosphate  in  nodular  form,  hematite  spherules,  and 
barite  in  crystals  and  blades;  the  psilomelane  in  the  deposit  also 
contains  baryum.  Such  deposits  could  probably  only  form  in 
shallow  water  mud  near  land  areas  subjected  to  secular  rock 
decay. 

The  great  manganese  deposits  of  the  province  of  Kutais,  in 
Trans-Caucasia,2  are  apparently  sedimentary,  if  judged  from 
descriptions,  but  it  is  not  impossible  that  here,  too,  enrichment 
by  decomposition  has  taken  place.  These  deposits,  said  to  be  the 
largest  in  the  world,  are  beds  in  Eocene  clays,  marls,  and  sand- 
stones, the  last  resting  on  Cretaceous  limestone,  on  the  top  of  an 
extensive  plateau.  The  ore  beds,  at  the  base  of  the  Eocene,  are  7 
to  16  feet  thick,  and  consist  of  several  strata  of  oolitic  pyrolusite 
with  cementing  earthy  manganese  ore.  They  are  said  to  extend 
over  an  area  of  22  square  miles.  The  ores  average  40  to  50  per 
cent.  Mn  and  0.16  per  cent.  P.  Drake  gives  a  complete  analysis 
of  an  ore  containing,  Mn02,  86.25;  Mn304, 0.47;  Fe203,  0.61 ;  NiO, 
0.3  per  cent.,  and  a  trace  of  copper.  Barium  is  present  as  usual  in 
these  ores.  The  annual  production  before  the  war  was  about 
1,000,000  metric  tons. 

XN.  C.  Dale,  The  Cambrian  manganese  deposits  of  Conception  and 
Trinity  Bays,  Newfoundland,  Proc.,  Am.  Philos.  Soc.,  vol.  54,  1915,  pp. 
371-456. 

2C.  F.  Drake,  The  manganese  ore  industry  of  Caucasus,  Trans.,  Am.  Inst. 
Min.  Eng.,  vol.  28,  1898,  p.  191. 
E.  C.  Harder,  op.  ait.,  p.  208. 


CHEMICAL  PROCESSES  IN  S  URFA  CE  WA  TERS   275 

SEDIMENTARY  PHOSPHATE  BEDS1 

Composition  of  the  Calcium  Phosphates. — Phosphorus  enters 
in  the  average  composition  of  igneous  rocks,  according  to  F.  W. 
Clarke,  to  the  extent  of  only  0.11  per  cent.,  and  the  analyses 
of  sediments  show  smaller  percentages.  Nevertheless,  it  plays 
a  most  important  part  in  the  life  processes  of  plants  and  animals, 
in  the  sea  and  on  the  land,  and  in  places  its  compounds  accumulate 
in  large 'masses.  Its  most  common  salt  is  a  calcium  phosphate; 
the  phosphates  of  iron,  aluminum,  lead,  and  other  metals  are 
entirely  subordinate. 

Apatite,  the  most  common  calcium  phosphate,  also  con- 
tains CaF2  or  CaCl2.  The  formulas  may  be  written  Ca5(PO4)3F 
and  Ca5(P04)3Cl,  or  3Ca3(PO4)2.Ca(F,Cl)2,  the  first  part  of  the 
latter  formula  being  the  tri-basic  calcium  phosphate.  Fluorine 
apatite  contains  42.3  per  cent.  P205;  chlorine  apatite,  41.0  per 
cent.  The  pure  tri-basic  phosphate,  which  is  used  as  a  standard 
to  express  the  tenor  of  phosphate  rocks,  contains  45.8  per  cent. 
P2O5.  The  phosphate  in  sedimentary  rocks  approaches  more 
or  less  closely  the  tri-basic  phosphate,  but  sometimes  is  almost 
identical  with  a  fluorine  apatite. 

In  deposits  of  guano  a  considerable  number  of  acid  hydrous 
phosphates  such  as,  monetite  (CaH.PO4)  and  brushite  (CaH.PO4.- 
2H20)  have  been  found,2  but  they  have  little  practical  impor- 
tance. In  the  same  deposits  various  complex  phosphates  of 

1 R.  A.  F.  Penrose,  Jr.,  Nature  and  origin  of  deposits  of  phosphate  of  lime, 
Bull.  46,  U.  S.  Gepl.  Survey,  1888.  (Gives  bibliography.) 

David  Levat,  Etude  sur  1'industrie  des  phosphates,  Ann.  des  Mines,  7, 
1895,  135. 

X.  Stainer,  Bibliographic  ge'ne'rale  des  gisements  de  phosphates,  Ann. 
des  Mines  de  Belgique,  vol.  7,  1902,  et  seq. 

F.  W.  Clarke,  The  data  of  geochemistry,  Bull.  616,  U.  S.  Geol.  Survey, 
1916,  pp.  519-528. 

O.  Stutzer,  Die  wichtigsten  Lagerstatten  der  Nicht-Erze,  Berlin,  1911, 
pp.  265-461. 

Eliot  Blackwelder,  The  geologic  role  of  phosphorus,  Am.  Jour.  Sci.,  4th 
ser.,  vol.  42,  1916,  pp.  285-298. 

W.  C.  Phalen,  The  conservation  of  phosphate  rock  in  the  United  States, 
Trans.,  Am.  Inst.  Min.  Eng.,  vol.  57,  1918,  pp.  99-132. 

Mineral  Resources,  U.  S.  Geol.  Survey,  Annual  publication;  various 
authors. 

Mineral  Industry,  New  York,  Annual  publication,  various  authors. 

2F.  W.  Clarke,  Geochemistry,  Bull.  616,  U.S.  Geol.  Survey,  1916,  p.  520. 


276  MINERAL  DEPOSITS 

iron,  magnesium,  sodium,  and  ammonium  occur,  but  these 
also  are  unimportant. 

The  mineralogical  composition  of  the  marine  and  residual 
phosphates  is  complex.1  Apatite  is  essentially  a  high  tem- 
perature mineral  and  has  not  been  recognized  in  the  marine  phos- 
phates; in  the  latter  hydrous  carbono-phosphates  play  the 
principal  part.  The  latter  are  amorphous  and  doubtless  hard- 
ened colloid  precipitates;  they  are  referred  to  two  species: 
collophanite  (9CaO.3P205CaO.CO2.H2O  +  nH20)  and  fluocol- 
lophanite.  The  crystalline  minerals  which  in  part  are  altered 
colloids,  in  part  crusts  and  mammillary  structures  comprise 
dahllite  and  francolite  both  of  which  are  similar  carbono-phos- 
phates with  or  without  fluorine. 

The  marine  phosphate  rocks,  aside  from  detrital  impurities, 
contain  thus  calcium  carbonate  and  calcium  phosphate;  shell 
fragments  and  glauconitic  granules  are  frequently  present. 
The  poorer  kinds  may  be  classified  as  phosphatic  sands,  marls, 
or  limestones.  The  richer  varieties  are  usually  oolitic,  dark- 
colored  rocks,  occasionally  with  a  peculiar  whitish  efflorescence, 
and  may  carry  large  amounts  of  organic  matter.  They  are  in- 
conspicuous and  in  places  difficult  to  recognize.  The  specific 
gravity,  averaging  2.9  in  70  per  cent,  phosphate  rock,  is  con- 
siderably higher  than  that  of  limestone  and  may  be  used  to  aid 
in  the  identification.  A  rapid  field  assay  with  ammonium  molyb- 
date  is  the  best  test. 

Other  Phosphates. — Among  the  iron  phosphates,  vivianite, 
Fe3(P04)2.8H20,  is  the  best  known,  and  it  appears  frequently 
in  bog  iron  ores.  Of  the  aluminum  phosphates,  wavellite, 
4A1P04.2A1(OH)3  +  9H2O,  and  turquoise,  A1PO4.A1(OH)3  + 
H20,  are  the  best  known,  the  former  locally  used  as  a  source  of 
phosphorus,  the  latter  a  blue  semi-precious  stone;  both  are  usually 
products  of  the  uppermost  zone  of  the  crust,  sometimes  even 
forming  in  the  zone  of  oxidation.  In  a  similar  geological  position 
occur  the  lead  phosphate,  pyromorphite,  corresponding  in  formula 

1  A.  Lacroix,  Sur  la  constitution  mineralogique  des  phosphorites  francaises. 
Compte  Rendu,  vol.  150,  1910,  p.  1213. 

H.  S.  Gale  and  R.  W.  Richards,  Bull.  430,  U.  S.  Geol.  Survey,  1910,  p.  464. 

W.  T.  Schaller,  Bull.  509,  U.  S.  Geol.  Survey,  1912,  pp.  89-100. 

A.  F.  Rogers,  Am.  Jour.  Sci.,    4th  ser.,  vol.  33,  1912,  p.  475. 

A.  F.  Rogers,  A  review  of  the  amorphous  minerals,  Jour.  Geol.,  vol.  25, 
1917,  pp.  515-541. 


CHEMICAL  PROCESSES  IN  SURFACE  WATERS   277 

to  chlorine  apatite.  Other  phosphates,  like  amblygonite,  a 
fluo-phosphate  of  lithium  and  aluminum,  monazite,  and  other 
phosphates  of  the  rare  earths,  find  their  home  in  the  pegma- 
tite dikes.  This  ^illustrates  the  variety  of  occurrence  of  the 
phosphates. 

Phosphate  Deposits . — The  many  kinds  of  deposits  in  which 
calcium  phosphate  is  of  economic  importance  are  shown  by  the 
following  list: 

1'.  Disseminated  in  igneous  rocks  or  in  their  differentiation 
products  of  metallic  ores. 

2.  Apatite  veins,  closely  allied  to  pegmatitic  dikes. 

3.  Marine  concretionary  beds. 

4.  Sub-aerial  accumulations  of  animal  excrements — bat  caves, 
guano  islands. 

5.  Metasomatic    deposits   by   replacement   of   limestone   by 
means  of  phosphate  solutions,  from  Nos.  3  and  4. 

6.  Residual   concretions,    by   action   of   atmospheric    waters 
on  No.  3. 

Use. — The  principal  use  of  the  calcium  phosphate  is  for  soil 
fertilization,  and  all  the  classes  enumerated  above  are  so  utilized. 
Under  No.  1  comes,  for  instance,  the  apatite  concentrate  from 
the  Adirondack  magnetite  ores;  under  No.  2  the  apatite  veins 
of  Canada  and  Norway;  the  occurrences  of  the  remaining  classes 
are  described  below. 

For  utilization  it  is  necessary  to  transform  the  insoluble  tri- 
basic  phosphate  into  soluble  form  and  this  is  generally  effected 
by  a  60  per  cent,  solution  of  sulphuric  acid;1  hence  the  dependence 
of  the  phosphate  industry  on  an  abundant  and  cheap  supply  of 
sulphuric  acid,  illustrated,  for  instance,  in  the  establishment  of 
large  sulphuric  acid  plants  at  the  pyritic  copper  deposits  of 
Ducktown,  Tennessee,  for  the  treatment  of  the  sedimentary 
phosphates  of  the  Southern  States.  The  treatment  with 
H2S04  results  in  a  partial  decomposition,  with  the  formation 
of  soluble  calcium  phosphate,  also  called  super-phosphate  or 
mono-calcium  phosphate  (Ca.H^PO^.HaO);  and  also  some 
di-calcium  phosphate,  which  is  much  less  soluble.  The  standard 
is  77  per  cent,  of  the  tri-basic  calcium  phosphate  with  less  than  3 
per  cent,  of  alumina  plus  iron,  but  not  all  of  the  production 
reaches  this  grade. 

lfrhe  reaction  is  expressed  by  the  following  formula :  Ca3(PO4)+2H2SO4 
=  CaH4(  PO4)  2 +2CaSO4. 


278  MINERAL  DEPOSITS 

Experiments  show  that  even  the  tri-calcium  phosphate  or 
apatite  is  soluble,  particularly  in  water  containing  carbon  dioxide; 
its  solubility  in  solutions  of  CaCOs  or  in  pure  water  is  slight,  but 
the  presence  of  sodium  chloride  increases  the  solubility.1  The 
marked  absorption  of  phosphoric  acid  by  clays  and  soils  is  held 
to  be  due  to  the  presence  of  colloid  bodies. 

Production.2 — Though  some  phosphates  are  obtained  from 
apatite  deposits  and  from  basic  slags,  the  greater  part  comes 
from  sedimentary  and  residual  beds.  In  the  United  States, 
the  bulk  of  the  production  comes  from  Florida,  Tennessee,  and 
South  Carolina,  in  the  order  named;  by  far  the  most  is  mined  in 
Florida.  The  yield  of  the  United  States  in  1913  was  about 
3,000,000  long  tons.  Large  quantities  were  exported.  In  1917 
owing  to  war  conditions  the  output  was  one-third  less.  Scarcity 
of  sulphuric  acid  and  reduced  exports  contributed  to  this.  The 
average  price  of  Florida  phosphate  was  $3  per  ton  in  1916. 

Of  other  countries  Algeria  and  Tunis  produced  about  2,800,000 
metric  tons,  and  France  about  400,000  tons  in  1913,  but  these 
figures  have  been  greatly  reduced  since  the  war  began.  The 
production  of  the  guano  islands  of  the  Pacific  is  now  compara- 
tively unimportant. 

Origin  of  the  Phosphate  Rocks.3 — As  all  land  animals  absorb 
phosphoric  acid  and  segregate  it  as  calcium  phosphate  in  their 
bones  and  excrements,  it  is  not  difficult  to  understand  the  accu- 
mulation of  phosphates  wherever  animal  life  is  particularly 
abundant  and  undisturbed.  Besides  phosphates,  such  deposits 
contain  much  ammonia  and  nitrogen,  except  where  subjected  to 
leaching  by  heavy  precipitation.  Of  this  kind  are  the  bone  beds 
which  are  found  occasionally  in  various  formations  and  in  caves. 

The  guano  of  commerce  is  deposited  by  sea  birds  congregating 
in  enormous  numbers  on  desert  coasts  and  oceanic  islands,  for 
instance,  along  the  Peruvian  and  Chilean  coast,  on  Christmas 
Island  in  the  Indian  Ocean,  and  in  the  West  Indies.  Some 

1 H.  E.  Patten  and  W.  H.  Waggaman,  Absorption  by  soils,  Bull.  52, 
Bureau  of  Soils,  Dept.  Agriculture,  1908. 

O.  Schreiner  and  G.  H.  Failyer,  Absorption  of  phosphates  and  potas- 
sium by  soils,  Bull.  32,  Bureau  of  Soils,  Dept.  Agriculture,  1906. 

2  Mineral  Resources,  U.  S.  Geol.  Survey,  annual  issues. 

3  Sometimes  described  as  "phosphorites"  (Stelzner  and  Bergeat,  1,  p.  442). 
The  name  phosphate  rock  seems  more  appropriate,   especially  as  some 
authors  (Merrill,  Non-metallic  minerals,  1910,  p.  267)  use  phosphorite  in  a 
somewhat  different  sense. 


CHEMICAL  PROCESSES  IN  SURFACE  WATERS   279 

of  these  deposits  cover  whole  islands  and  in  places  may  accu- 
mulate to  a  depth  of  100  feet,  and  it  is  stated  that  under 
favorable  circumstances  the  rate  of  deposition  is  rapid.  The 
guano  of  dry  climates  varies  greatly  in  texture  and  color,  but 
generally  is  granular,  light  colored,  and  porous.  It  contains 
on  an  average  10.90  per  cent,  nitrogen,  27.60  per  cent,  phosphates, 
and  2  to  3  per  cent,  potash.1 

The  West  Indian  deposits — for  instance,  those  on  Navassa2 
and  Sombrero  islands — have  been  leached  and  are  in  part  hard 
and  compact,  in  part  porous  and  friable.  The  phosphate  has 
been  concentrated  to  70  or  75  per  cent.  The  material  contains 
from  21  to  40  per  cent,  of  phosphoric  acid,  1  to  2  per  cent, 
sulphuric  acid,  20  to  45  per  cent,  lime,  usually  also  much  ferric 
oxide  and  alumina.  The  underlying  limestone  or  igneous  rock 
may  be  locally  replaced  by  the  phosphatic  solutions. 

The  marine  phosphate  beds  also  derive  their  material  from 
animal  life.  Sea  water  contains  phosphoric  acid,  though  the 
quantity  is  extremely  small,  and  likewise  some  fluorine,  each 
amounting  to  about  a  little  less  than  one  part  per  million.3 

According  to  Carnot,  many  shells,  particularly  those  of  the 
older  formations,  are  rich  in  phosphorus  and  fluorine.  A  Cam- 
brian Obohis  contained  36.54  per  cent.  PzQ*,  and  2.78  per  cent. 
F;  a  recent  Lingula  yielded  23.20  per  cent.  PzOs  and  1.52  per 
cent.  F.4  The  shells  of  crustaceans5  contain  up  to  26  per  cent. 
Ca3P208.  Pteropods,  lamellibranchs,  gastropods  and  protozoans 
also  carry  phosphorus.  Corals  likewise  contain  a  small  amount 
of  phosphorus  and  fluorine,  and  the  same  substances  are  found 
in  the  bones  and  teeth  of  fishes.  The  marine  sediments,  then, 
all  hold  more  or  less  of  phosphates,  and  it  is  a  matter  of  some 
surprise  that  fluorite  does  not  more  commonly  occur  in  sedimen- 
tary rocks.6 

1 R.  A.  F.  Penrose,  Jr.,  Bull.  46,  U.  S.  Geol.  Survey,  1888. 

2E.  V.  D'Invilliers,  Phosphate  deposits  of  the  Island  of  Navassa,  Bull, 
Geol.  Soc.  Am.,  vol.  2,  1891,  p.  71. 

3  A.  Carnot,  Ann.  des  Mines,  9th  ser.,  vol.  10,  1896,  p.  175. 

4Andersson  and  Sahlbom,  Ueber  den  Fluorgehalt  schwedischer  Phos- 
phorite, Bull.  4,  Geol.  Inst.  Upsala,  1900,  p.  79.  Neues  Jahrb.,  ref.,  1903,  1, 
pp.  195,  197. 

6  F.  W.  Clarke  and  G.  Steiger,  Proc.  Nat.  Acad.  Sci.,  vol.  5,  1919.  pp.  6-8. 

6K.  AndrSe,  Ueber  einige  Vorkommen  von  Flusspath  in  Sedimenten, 
Tsch.  M.  und  p.  Mitt.,  vol.  28,  1909,  pp.  535-562. 

H.  S.  Gale  and,R.  W.  Richards,  Bull.  430,  U.  S.  Geol.  Survey,  1910,  p.  463. 


280  MINERAL  DEPOSITS 

In  some  beds  the  phosphates  occur  disseminated  in  small  quan- 
tities, in  part  as  small  concretions,  in  part  remaining  in  the  shell 
fragments.  In  the  more  valuable  deposits  the  phosphates  appear 
in  more  concentrated  form  and  characteristically  assume  the 
forms  of  nodules,  or  concretions  (sometimes  of  large  size),  or 
oolitic  rocks  built  up  of  small  oolites  in  part  of  concentric  and 
fibrous  structure.  The  nodules  have  often  a  shell  nucleus  and, 
as  a  result  of  enrichment,  may  contain  more  phosphate  in  the 
peripheral  than  in  the  central  parts. 

While  phosphate  nodules  have  been  brought  up  by  the  dredge 
from  great  oceanic  depths,  the  conditions  for  their  formation  are 
probably  best  at  moderate  depths,  near  shores,  where  the  marine 
life  is  most  abundantly  developed  or,  as  pointed  out  by  some 
authors,  where  sudden  changes  of  temperature,  owing  to  con- 
flicting currents,  kill  large  numbers  of  marine  organisms. 

The  origin  of  the  oolitic  and  nodular  phosphate  rocks,  in  some 
of  which  recognizable  organic  remains  are  scarce,  has  been 
discussed  extensively,  but  is  as  yet  not  fully  explained.  It  is 
believed  that  ammonium  phosphate  may  form  in  the  organic 
matter  and  that  this  reacts  on  shell  remains,  replacing  them  with 
calcium  phosphate,  which  eventually  accumulates  in  larger  con- 
cretions.1 These  processes  are  likely  to  continue  for  some  time 
at  least  after  the  sedimentation,  in  the  yet  soft  sediments.2 

After  the  beds  have  been  uplifted  and  exposed  to  weathering 
enrichment  takes  place  easily,  by  the  removal  of  calcium  car- 
bonate. This  is  especially  effective  in  regions  of  deep  rock 
decay,  as  in  the  Southern  States.  The  rock  phosphates  of 
Utah  and  Idaho  have  remained  almost  unaltered. 

The  cycle  of  migration  of  the  phosphates  is  a  fascinating 
study.  From  their  original  home  in  the  igneous  rocks  they  are 
dissolved  by  surface  waters  and  absorbed  by  all  living  things, 
vegetable  and  animal,  on  land  and  on  sea.  After  the  death  of  the 
organisms  the  phosphates  return  to  the  soil  or  to  the  sedimentary 
beds  to  be  dissolved  and  used  anew  by  other  generations. 

!Renard  and  Cornet,  Bull.  21,  ser.  3,  Acad.  Belgique,  1891,  p.  126. 
L.  Kruft,  Neues  Jahrbuch,  Beil.  Bd.  15,  1902,  pp.  1-65,  Ref.  in  Zeitschr. 
prakt.  Geol,  vol.  10,  1902,  p.  301. 

R.  Delkeskamp,  Zeitschr.  prakt.  Geol,  vol.  12,  1904,  p.  299. 
2  L.  Cayeux,  for  instance,  presents  a  figure,  showing  a  small  concretion  of 
phosphate  molded  against  a  grain  of  glauconite;  the  latter  itself  being  formed 
after  the  sedimentation;  Contrib.  a  l'6tude  micrographique  des  terrains 
sedimentaires,  Mem.  Soc.  g6ol.  du  Nord,  4,  pt.  2,  Lille,  1897. 


CHEMICAL  PROCESSES  IN  S  URFA  CE  WA  TERS   28 1 

Occurrences  of  Phosphate  Rocks. — Deposits  of  phosphate  rock 
are  found  in  the  marine  beds  of  all  ages  and  in  almost  all  countries, 
at  least  from  the  Cambrian,  when  the  segregation  of  phosphoric 
acid  by  the  inhabitants  of  the  sea  appears  to  have  begun,  to  the 
Tertiary,  and  in  the  present  oceans  such  deposits  certainly 
continue  to  form.  In  description  it  is  impracticable  to  separate 
the  primary  marine  deposits  from  those  altered  by  weathering. 

Large  deposits,  enriched  by  weathering,  are  worked  in  the 
Cretaceous  beds  of  northern  France.  In  the  southwestern  part 
of  that  country,  in  the  departments  of  Lot  and  Lot-et-Garonne, 
phosphates  occur  in  irregular  fissures  with  clay  in  Jurassic 
limestone.1  These  deposits  are  probably  formed  by  replacement 
effected  by  descending  solutions  from  sedimentary  phosphate 
beds. 

Phosphate  beds  are  now  mined  on  a  large  scale  along  the 
frontier  of  Algeria  and  Tunis.2  The  beds  occur  in  the  lower 
Eocene,  which  covers  Cretaceous  strata,  and  consist  in  part  of 
larger  concretions  in  marl,  sometimes  carrying  the  rich  phos- 
phate only  as  a  crust;  other  beds  are  formed  by  a  soft  material, 
consisting  of  small  and  smooth  brown  or  yellowish  grains  of 
phosphate  cemented  by  calcite  and  also  containing  many  fossils 
and  much  bituminous  matter.  The  thickness  of  the  richest 
phosphatic  stratum  is  said  to  be  10  to  15  feet. 

The  deposits  found  in  the  United  States  are  mainly  in  three 
regions — (1)  the  Atlantic  coast  belt  of  Tertiary  rocks  in  the 
Carolinas  and  Florida;  (2)  the  Tennessee  area  of  Silurian  and 
Devonian  strata;  (3)  the  Utah-Idaho  region  of  Carboniferous 
beds. 

The  phosphates  of  the  Utah-Idaho  region3  were  discovered 
only  recently,  but  are  of  great  extent  and  prospective  value;  at 

*L.  de  Launay,  Gites  mineraux,  vol.  1,  1913,  p.  679. 
2M,  Blayac,  Description  geologique  de  la  re'gion  des  phosphates  du  Dyr 
et  du  Kouif,  Ann.  des  Mines  (9),  6,  1894,  pp.  319-330. 

L.  de  Launay,  Les  richesses  minerales  de  1'Afrique,  Paris,  1903,  p.  206. 
O.  Tietze,  Die  Phosphatlagerstatten  von  Algier  und  Tunis,   Zeitsc.hr. 
prakt.  Geol,  1907,  p.  229. 

8  F.  B.  Weeks  and  W.  F.  Ferrier,  Bull.  315,  U.  S.  Geol.  Survey,  1907,  pp. 
449-462. 

H.  S.  Gale  and  R.  W.  Richards,  Bull.  430,  idem,  1910,  pp.  457-535. 
Eliot  Blackwelder,  Butt.  430,  idem,  1910,  pp.  536-551. 
R.  W.  Richards  and  G.  R.  Mansfield,  Bull.  470,  idem,  1911,  pp.  371-439. 
G.  R.  Mansfield,  Am.  Jour.  Sci.,  4th  ser.,  vol.  46.  1918,  pp.  591-598. 


282  MINERAL  DEPOSITS 

present,  owing  to  difficulties  and  cost  of  transportation,  they  are 
mined  only  on  a  small  scale  near  Montpelier,  Idaho. 

They  extend  north  of  Ogden,  Utah,  into  Idaho,  Wyoming  and 
Montana,  and  the  best  deposits  are  in  the  ranges  which  constitute 
the  northern  continuation  of  the  Wasatch.  Their  position  is  in 
the  Park  City  formation  of  the  Upper  Carboniferous  (Pennsyl- 
vanian),  which  has  an  average  thickness  of  600  feet  and  consists 
of  limestones,  cherty  in  part,  phosphate  beds,  and  shales.  The 
phosphate  horizon  is  in  the  middle  of  the  formation  and  the  beds 
have  an  average  thickness  of  200  feet.  (See  Figs.  8,  9,  and  95.) 
The  rocks  are  massive  brown  to  gray  phosphatic  shales  and  beds 
of  rock  phosphate  with  some  limestone.  The  richest  bed  mined 
at  Montpelier,  carrying  70  per  cent,  or  more  of  CagPaOs,  lies  at 
the  base  of  the  phosphate  section  and  is  5  or  6  feet  thick.  It  is  a 
black  to  dull-gray  oolitic  rock,  with  grains  of  all  sizes  up  to 
pebble-like  bodies  one-half  inch  in  diameter. 

Large  sections  of  the  phosphatic  beds,  in  places  a  thickness 
of  75  feet,  carry  from  30  to  50  per  cent,  of  Ca3P208.  The  beds 
are  folded  and  locally  have  steep  dips.  The  rock  is  hard  and  the 
mining  is  carried  on  by  underground  operations.  Very  little 
enrichment  is  noted. 

The  phosphates  of  western  Tennessee1  have  been  worked  since 
1894  and  at  present  yield  about  400,000  tons  per  annum.  They 
are  of  three  classes.  1 .  Brown  residual  phosphates,  resulting  from 
leaching  of  Ordovician  phosphatic  limestones;  the  beds  are  from 
3  to  8  feet  thick  and  carry  as  much  as  80  per  cent,  of  tri-calcium 
phosphate.  2.  The  blue  or  black  bedded  phosphates,  which  oc- 
cur in  beds  of  Devonian  age,  show  variations  from  oolitic  through 
compact  and  conglomeratic  to  shaly  forms.  The  high-grade 
rock  is  seldom  more  than  20  inches  thick.  The  nodular  variety, 
which  is  embedded  in  a  green  sand,  carries  about  60  per  cent. 
Ca3P208.  3.  The  white  phosphate,  which  is  a  post-Tertiary 
product  of  replacement  or  filling  of  cavities  of  limestone  of 
Carboniferous  age.  None  of  it  is  now  mined. 

The  phosphate  beds  of  North  and  South  Carolina,2  discovered 

1  C.  W.  Hayes,  The  Tennessee  phosphates,  Sixteenth  Ann.  Rept.,  U.  S. 
Geol.  Survey,  pt.  4,  1895,  pp.  610-630;  Seventeenth  Ann.  Rept.;  U.  S.  Geol. 
Survey,  pt.  2, 1896,  pp.  513-550;  Twenty-first  Ann.  Rept.,  U.  S.  Geol.  Survey, 
pt.  3,  1901,  p.  473. 

2  G.  S.  Rogers,  Phosphate  deposits  of  South  Carolina,  Bull.  580,   U.  S. 
Geol.  Survey,  1914,  pp.  183-220. 


CHEMICAL  PROCESSES  IN  S  URFA  CE  WA  TERS   283 

in  1867,  extend  along  the  coast  for  a  distance  of  60  miles.  They 
are  contained  in  loose  beds  of  Miocene  age,  rich  in  fossils.  The 
land  deposits  lie  at  a  shallow  depth  and  consist  of  so-called  pebble 
rock,  a  solid  mass  from  which  the  calcium  carbonate  has  been 
leached  and  partly  replaced  by  phosphate;  the  solution  cavities 
give  this  material  the  appearance  of  a  mass  of  separate  pebbles. 


8  feet 


FIG.  95. — Section  showing  beds  of  phosphate,  Montpelier,  Idaho. 
After   Weeks  and  Ferrier,    U.  S.  Geol.  Survey. 


The  rock  varies  from  1  to  3  feet  in  thickness  and  is  covered  by  a 
green  sandy  marl.  Similar  deposits  have  been  dredged  in  the 
rivers;  they  consist  essentially  of  water-rounded  fragments  of  the 
land  rock.  The  mining  is  carried  on  by  steam-shovel  or  dredge 
operations. 


284  MINERAL  DEPOSITS 

The  phosphate  deposits  of  Florida1  are  at  present  the  most 
productive  in  the  world  and  large  quantities  are,  under  normal 
conditions,  exported  to  almost  all  European  countries. 

The  deposits  follow  in  the  main  the  northwestern  coast  of  the 
State  but  lie  some  distance  from  the  shore.  There  are  several 
types,  all  contained  in  the  Alum  Bluff  formation  or  above  it,  or 
in  the  underlying  Vicksburg  limestone,  both  formations  of 
Oligocene  age.  The  clays,  marls  and  sandstones  of  the  Alum 
Bluff  contain  in  several  horizons  abundant  smooth  yellowish  or 
brown  nodules  or  ovules  of  phosphate  which  are  considered  by 
Matson  and  Sellards  as  of  primary  deposition  and  the  source  of 


FIG.   96. — Dredging    Florida   phosphates.     Upper   bench  is   sandy   over- 
burden.    Photograph  by  F.  B.  Van  Horn. 

all  the  other  deposits.  These  beds  are  worked  in  some  places 
but  are  not  of  great  importance.  The  material  is  very  similar  to 
the  phosphates  of  Gafsa  in  Tunis. 

The  so-called  "land  pebble"  deposits  which  are  the  most 
important  are  rudely  stratified  detrital  and  residual  masses; 
they  rest  on  the  Alum  Bluff  formation  and  are  believed  to  be 
derived  from  this  source.  They  are  believed  to  be  of  Miocene  or 

»E.  H.  Sellards,  Fifth  Ann.  Rept.,  Florida  Geol.  Survey,  1913,  pp.  23-80. 

E.  H.  Sellards,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  50, 1914,  pp.  901-916. 

G.  C.  Matson,  The  phosphates  of  Florida,  Bull.  604,  U.  S.  Geol.  Survey, 
1915. 

See  also  successive  issues  of  Mineral  Resources,  U.  S.  Geol.  Survey,  and 
Mineral  Industry. 


CHEMICAL  PROCESSES  IN  SURFACE  WATERS   285 

Pliocene  age.  They  contain  concretions  of  white  phosphate, 
averaging  65  to  70  per  cent,  tribasic  calcium  phosphate,  while 
the  finer  matrix  often  contains  20  to  30  per  cent,  of  the  same. 
They  lie  at  elevations  of  about  100  feet  and  form  parts  of  gravel 
beds  with  casts  of  shells,  shark's  teeth,  and  bones  of  mastodon, 
horse  and  rhinoceros.  The  average  depth  of  deposit  is  about  12 
feet,  that  of  the  sandy  overburden  up  to  40  feet.  The  workable 
deposits  average  several  acres,  though  some  cover  as  much  as 
40  acres. 

The  type  of  deposit  called  "rock  phosphate"  occurs  in  the 
Vicksburg  limestone  of  lower  Oligocene  age  "and  rest  in  depres- 
sions on  its  surface.  They  are  of  secondary  origin  and  are 
believed  to  be  leached  from  the  overlying  Alum  Bluff  formation. 
The  concentration  was  affected  by  chemical  and  mechanical 
means;  the  result  is  a  mass  of  rounded  or  subangular  phosphate 
concretions  ("rocks"  or  "pebbles")  in  places  rudely  sorted  in 
layers  of  coarse  and  fine,  and  contained  in  a  matrix  of  sand,  clay 
or  soft  phosphate. 

Pleistocene  sand,  up  to  50  feet  thick,  covers  the  deposits. 
There  are  many  deposits  covering  from  5  to  35  acres,  while  some 
are  mere  pockets  in  the  limestone.  The  average  thickness  is  30 
feet;  transitions  to  the  underlying  limestone  are  sometimes  ob- 
served. The  concretions  or  nodules  vary  from  a  few  inches  to 
10  feet  in  diameter,  are  close  grained  and  light  gray  and  some- 
times show  cavities  lined  with  secondary  and  mammillary  phos- 
phate. Shells  are  rare  but  fragments  of  bones  and  shark's  teeth 
occur  here  and  there.  The  processes  which  have  operated  in 
the  concentrations  from  the  overlying  Alum  Bluff  beds  are  re- 
placement of  limestone  by  phosphate  and  solution  of  residual 
limestone  with  resulting  slumping;  in  places  there  has  been 
mechanical  transportation.  The  soft  phosphates  are  thought 
to  be  formed  by  replacement  of  porous  limestone.  The  per- 
centage of  "recoverable"  phosphate  in  the  deposits  does  not 
average  much  above  15  per  cent. 

The  so-called  "river  pebbles"  are  Pleistocene  deposits  in  the 
present  rivers  but  are  not  worked  now. 

Mineralogically,  the  Florida  phosphates  are  held  by  Matson 
to  consist  of  collophanite  with  francolite  (p.  284). 

The  material  is  mined  by  steam  shovel,  by  hydraulic  method 
or  by  dredge  (Fig.  96);  it  is  then  washed  to  remove  the  clay 
and  afterward  crushed,  screened  and  hand-picked. 


286  MINERAL  DEPOSITS 

The  chemical  composition  of  the  marine  and  residual  phos- 
phates is  shown  in  the  following  analyses  of  which  I  and 
III  represent  unaltered  marine  deposits  and  II  a  residual 
occurrence. 

ANALYSES  OF  PHOSPHATES 

I  II  III 

Insoluble 1.82  6.69  3.05 

SiO2 0.30  

A12O3 0.50  2.14  1.09 

Fe2O3 0.26  0.61  0.64 

MgO , 0.22  0.33  0.57 

CaO 50.97  46.03  48.58 

0.09 


K20 0.47 

H2O- 0.48  0.79             

H2O+ 0.57  3.47             

CO2 1.72  3.93  4.60 

P2OS 36.35  31.50  29.74 

SO3 2.98             2.75 

Cl 0.11 

Fl 0.40  1.86  2.12 

Organic 7 . 45 


99.04  97.35  100.79 

I.  Crawford  Mountains,  Utah.     Geo.  Steiger,  analyst,  Bull.  430,  U.  S. 
Geol.  Survey,  1909,  p.  465.     No  titanium,  organic  matter  not  determined, 
trace  chlorine. 

II.  Florida.    Land  pebble,  G.  H.  Eldridge. 

III.  Gafsa,  Tunis.     O.  Tietze,  Zeitschr.  prakt.  Geol,  1907,  p.  248. 
Analysis  calculated  on  dry  material,  3.81  per  cent.  H^O.     PzOs  equivalent 

to  64.93  per  cent.  Ca3P2O8;  2.35  per  cent.  CaFl2;  4.67  per  cent.  CaSO4;  10.45 
per  cent.  CaCO3. 


CHAPTER  XVII 

DEPOSITS  FORMED  BY  EVAPORATION  OF  BODIES  OF 
SURFACE  WATERS 

THE  SALINE  RESIDUES1 
INTRODUCTION 

The  deposits  thus  far  described  have  been  in  the  nature  of 
insoluble  residues,  or  chemical  precipitates  of  relatively  insoluble 
substances  in  lakes,  rivers,  and  seas.  There  are,  however,  other 
deposits  which  also  may  be  considered  as  chemical  precipitates 
in  surface  waters  but  which  consist  of  soluble  salts  formed 
by  the  evaporation  of  waters  in  closed  or  partially  closed  basins. 
They  contain  the  easily  soluble  substances  leached  from  the 
crust,  brought  down  by  the  rivers  to  oceans  and  lakes,  and 
finally  concentrated  under  certain  characteristic  conditions. 

Closed  basins  are  typical  of  dry  climate  and  of  deserts.  The 
slow  crustal  movements  tend  to  create  them  everywhere,  by 
folding,  subsidence,  and  uplift,  but  in  the  deserts  the  streams 
have  not  the  power  to  cut  outlets  and  to  keep  the  drainage  lines 
established.  On  the  contrary,  the  movement  of  the  debris 
from  the  mountain  ranges  in  broad  alluvial  fans  or  aprons 
increases  the  tendency  toward  closed  basins.  The  dry  climate 
accelerates  evaporation  and  the  precipitation  of  the  salts;  dust 
storms  transport  vast  masses  of  fine  detritus;  blinding  salt  flats 
extend  between  the  barren  mountain  chains.  Thus,  at  present, 
salt  beds  are  found  in  the  Cordilleran  deserts  along  the  western 
side  of  the  whole  American  continent,  in  the  Sahara,  and  in  the 
arid,  central  part  of  Asia.  Similar  conditions  existed  in  the 
past  in  different  parts  of  the  world:  The  Permian  in  central 
Europe,  the  Triassic  in  the  Rocky  Mountain  region,  and  the 
Silurian  in  eastern  North  America — all  these  ages  were  at  times 
characterized  by  arid  wastes  and  deposition  of  salt  and  gypsum. 

Saline  deposits  may  then  form :  (1)  in  bays  of  the  sea ;  (2)  in  lakes  ; 

1  George  P.  Merrill,  The  non-metallic  minerals,  1910. 
F.  W.  Clarke,  Geochemistry,  Bull.  616,  U.  S.  Geol.  Survey,  1916,  pp. 
217-259. 

287 


288  MINERAL  DEPOSITS 

(3)  in  playas  or  intermittent  lakes ;  (4)  on  arid  slopes  by  rapid  evapo- 
ration of  storm  waters. 

In  the  latter  two  classes  capillary  ascent  of  the  solutions  often 
help  to  bring  the  salts  to  the  surface  to  form  "  efflorescences  "  or 
saline  crusts  as  illustrated  by  the  crusts  of  "alkali"  (carbonate, 
chloride  and  sulphate  of  sodium,  sulphate  of  calcium)  which  so 
often  interfere  with  agriculture  in  dry  countries.  . 

In  regions  of  calcareous  rocks,  as  in  the  undrained  basins  of 
Mexico,  soft  or  compact  beds  of  calcium  carbonate  locally  called 
"caliche"  or  "tepetate"  often  cover  the  gentle  slopes  below  the 
mountains.  These  belong  in  class  4.  Minor  saline  deposits 
may  result  from  evaporation  at  the  surface  of  waters  from 
ascending  springs. 

Bodies  of  soluble  salts  are  rarely  formed  below  the  surface; 
but  reactions  may  take  place  in  a  buried  deposit  by  which  new 
salts  are  formed  or  concentrations  of  disseminated  substances  are 
effected.  In  places  it  may  be  difficult  to  distinguish  these 
strictly  speaking  epigenetic  bodies  from  the  syngenetic  salts 
(p.  313). 

No  traces  of  metallic  ores  are  found  in  the  saline  residues. 
Minute  amounts  of  gold  and  silver  have  been  found  in  salt  from 
sea  water  (p.  13).  Regarding  traces  of  gold  in  the  potassium 
deposits  in  Germany  the  evidence  is  conflicting.1 

TYPES  OF  WATER 

From  a  geological  standpoint  there  are  two  types  of  water  in 
the  seas  and  closed  basins.  The  first,  which  may  be  called  the 
oceanic  type,  contains  dominant  sodium  chloride  and  is  char- 
acteristic of  the  sea  as  well  as  of  partly  evaporated  lakes  in 
regions  where  sedimentary  rocks  prevail;  the  Great  Salt  Lake  of 
Utah  is  an  example.  When  such  water  is  subjected  to  extreme 
evaporation,  as  in  the  Dead  Sea,  a  "residual"  type  rich  in 
magnesium  chloride  results.  The  second  main  type  is  that  of 
generally  smaller  closed  basins  in  regions  of  great  volcanic 
activity;  this  type  contains  an  abundance  of  sulphate  and  car- 
bonate of  sodium,  besides  more  or  less  chloride;  it  indicates  the 
result  of  the  first  leaching  of  loose  volcanic  ejecta  and  also 
shows  the  influence  of  the  discharge  of  hot  springs  containing 

1  A,  Liversidge,  Jour.  Chem.  Soc.,  vol.  71,  1897,  p.  298. 
E.  E.  Lungwitz,  Eng.  and  Min.  Jour.,  April  6,  1905. 


DEPOSITS  FORMED  BY  EVAPORATION         289 

sodium  carbonate  and  borate.  The  water  of  Mono  Lake,  Cali- 
fornia, is  a  good  example. 

Certain  sedimentary  series,  such  as  the  Cretaceous  of  the 
Western  States,  contain  abundant  alkaline  sulphates.  Leach- 
ing of  these  beds  by  atmospheric  waters  takes  place  and  these 
products  may  be  carried  down  into  salt  flats  and  small  lakes. 
By  chemical  reactions  (p.  59)  carbonate  of  sodium  forms  from 
other  sodium  and  calcium  salts  and  the  lakes  often  contain  much 
of  this  salt  besides  the  sulphates.  Such  alkali  lakes  occur  in 
Wyoming,  for  example.  Borates  characteristic  of  volcanic 
regions  are  generally  lacking  in  these  lakes. 

The  first,  "oceanic"  type  of  waters  yields  deposits  of  gypsum, 
common  salt,  and  finally  potassium  and  magnesium  salts.  The 
second,  "volcanic"  type  yields  soda,  glauber  salt,  borates, 
probably  also  nitrates,  as  well  as  more  or  less  sodium  chloride. 

COMPOSITION  OF  SALTS  IN  WATER  OF  SEA  AND  CLOSED  BASINS 

I  II  III  IV  V 

10.45         23.34 


Cl. 

55  292 

55  69 

70  25 

Br  .      ... 

0  188 

tr 

1  55 

SO4  
B4O7  

7.692 

6.52 

0.21 

CO3 

0  207 

tr 

Li 

0  01 

Na  

30  593 

32  92 

6  33 

K  ' 

1  106 

1  70 

1  70 

Ca 

1  197 

1  05 

5  54 

Me 

3  725 

2  10 

14  42 

Fe2O3..   . 

tr 

tr 

SiO, 

0  01 

tr 

100 

100 

100 

54.07  12.86 

0.32 

4.24  23.42 

25.88  37.93 

......  1.85 

tr.  0.04 

5.36  0.10 

tr. 

tr. 

0.14 


100  100 

I.  Oceanic  type.     Average  of  77  analyses,   W.   Dittmar,   Challenger 
Kept.,  vol.  1,  1884.     Salinity  3.5  per  cent. 

II.  Oceanic  type.     Great  Salt  Lake,  E.  Waller,  School  of  Mines,  Quart., 
vol.  14,  1892.     Salinity  23  per  cent.,  subject  to  variations. 

III.  Residual  type.     The  Dead  Sea,  Compt.  Rend.,  vol.  62,  1866,  p.  1329. 
A.  Tereil,  analyst.     Salinity  20.7  per  cent. 

IV.  Sulphate  type.     Devil's  Lake,  North  Dakota,  F.  W.  Clarke,  Geo- 
chemistry, 1916,  p.  163;  H.  W.  Daudt,  Analyst.     Salinity  1.1  per  cent. 

V.  Volcanic  type.     Mono  Lake,  Bull.  60,  U.  S.  Geol.  Survey,  1890,  p. 
53.     T.  M.  Chatard,  analyst.     Salinity  5.1  per  cent. 


290  MINERAL  DEPOSITS 

The  general  relation  of  the  salts  dissolved  in  oceanic  waters  to 
those  in  rivers  is  as  follows : 

In  ocean  waters:  C1>SO4>CO3;  Na>Mg>Ca 
In  river  water:  CO3>S04>C1;  Ca>Mg>Na. 

Change  from  river  to  lake  water  involves  a  concentration  of 
chlorides  and  relative  loss  of  magnesia,  silica,  and  lime. 

NORMAL  SUCCESSION  OF  SALTS 

When  water  evaporates  until  precipitation  of  the  dissolved 
salts  begins,  the  least  soluble  salts  will  generally  fall  down  first, 
while  the  most  soluble  salts  will  remain  in  the  solution  until  the 
last.  Experiments  by  J.  Usiglio1  on  sea  water  showed  that  the 
carbonates  of  calcium  and  magnesium,  with  a  little  ferric  oxide, 
were  precipitated  when  one  liter  of  the  water  was  reduced  from 
one-half  to  one-fifth.  Gypsum  was  precipitated  when  the 
volume  was  one-fifth  to  one-seventh,  but  continued  in  lessening 
amounts  until  only  30  cubic  centimeters  of  the  original  liter 
remained.  Sodium  chloride  was  precipitated  abundantly  upon 
reduction  of  volume  to  100  cubic  centimeters,  but  continued  until 
the  volume  of  the  water  was  only  16  cubic  centimeters;  even 
then  some  of  the  salt  remained  in  solution.  Chloride  and  sul- 
phate of  magnesium  fell  down  within  the  same  limits  but  in 
increasing  quantities,  and  the  residual  "bittern"  contained 
mainly  the  chlorides  of  magnesium  and  potassium,  bromide 
of  sodium,  sulphate  of  magnesium,  and  chloride  of  sodium. 
Naturally  the  quantity  of  NaCl  greatly  exceeded  that  of  the 
other  salts. 

The  whole  series  of  these  salts  is  rarely  represented  in  the 
saline  deposits;  the  best  known  and  almost  only  example  of 
such  complete  evaporation  is  found  in  the  great  Prussian 
potash  and  salt  deposits.  Often,  as  in  the  "Red  Beds  "  of  the 
Western  States,  the  process  ceased  after  the  gypsum  was  laid 
down,  and  changes  of  climate  or  invasion  of  the  sea  may  have 
prevented  the  formation  of  sodium  chloride. 

Actually  the  conditions  and  the  results  of  precipitation  are  far 
more  complex  than  the  experiments  mentioned  would  seem  to 
show.  The  influence  of  temperature  and  time  may  vary  the 
details  of  the  precipitation  greatly,  and  double  or  complex  salts 
are  often  formed. 

lAnnales  chim.  phys.,  3d  ser.,  vol.  27,  1849,  pp.  92-172. 


DEPOSITS  FORMED  BY  EVAPORATION         291 

A  saline  solution  containing  the  same  salts  as  sea  water  but 
in  different  proportions  would  yield  materially  unlike  results 
upon  evaporation.  In  brief,  temperature,  concentration,  and 
time  are  always  factors  of  great  importance  in  the  origin  of 
saline  residues. 

The  study  of  the  stability  fields  of  these  salts  has  received 
much  impetus  by  the  labors  of  J.  H.  van't  Hoff  and  his  numerous 
associates,1  undertaken  mainly  to  elucidate  the  problems  of  the 
potassium  deposits  of  Prussia. 

The  occurrence  of  thick  beds  of  anhydrite  is  explained  by  the 
work  of  van't  Hoff  and  Weigert,2  who  established  that  the 
mineral  forms  from  gypsum  in  sodium  chloride  solutions  at  30° 
C.  In  sea  water  the  transformation  takes  place  at  25°  C.  Crys- 
tals of  gypsum,  sinking  through  a  salt  solution  at  that  tempera- 
ture, are  converted  into  anhydrite.  This  is  an  example  of  the 
more  general  rule  of  dehydration  of  minerals  in  contact  with 
salt  solutions,  at  temperatures  considerably  below  their  normal 
inversion  temperature. 

Many  minerals  are  deposited  in  nature  from  solutions  in  a 
lower  state  of  hydration  than  is  produced  at  ordinary  tempera- 
tures in  the  laboratory.  Thus  natron,  the  ordinary  sodium 
carbonate  (Na2CO3.10H2O)  rarely  occurs  as  a  natural  product, 
although  it  is  stable  at  temperatures  below  37°  C.  The  more 
common  product  is  trona  (Na2COs.NaHCO3.2H20),  which  is 
ordinarily  stable  only  above  37°  C. 

From  pure  sodium  sulphate  solution  mirabilite  (Na2S04.10H20) 
is  ordinarily  deposited,  but  in  the  presence  of  sodium  chloride 
thenardite  (Na2S04)  is  formed.  From  a  solution  of  magnesium 
sulphate  in  the  presence  of  magnesium  chloride  kieserite  (MgS04. 
H2O)  is  precipitated  instead  of  the  heptahydrate.  The  presence 
of  a  co-solute,  by  lowering  the  osmotic  pressure,  acts  in  the  same 
direction  as  a  rise  of  temperature.3  This  principle  is  undoubtedly 
also  applicable  to  minerals  in  rocks  and  veins  and  explains 
many  anomalies  of  mineral  occurrence. 

1  Mainly  published  in  the  Sitzungsberichte  K.  preuss.  Akad.  d.  Wiss., 
from  1897  to  the  present  time. 

The  results  are  summarized  by  van't  Hoff  in  a  book  entitled  "Zur  Bildung 
der  Oceanischen  Salz-Ablagerungen,"  Braunschweig,  1905,  and  1909,  and  in 
"Physical  chemistry  in  the  service  of  the  sciences,"  Univ.  Chicago  Press, 
1903. 

*  Sitzungsber.  Akad.,  Berlin,  1901,  p.  1140. 

3  J.  V.  Elsden,  Principles  of  chemical  geology,  1910,  pp.  85-86 


292  MINERAL  DEPOSITS 

STRUCTURAL  FEATURES 

In  desert  valleys  filled  by  temporary  or  permanent  lakes  we 
often  find  a  succession  of  salt  beds  of  no  great  thickness  alter- 
nating with  detrital  matter  of  eolian  or  fluviatile  origin.  If  the 
basin  is  large  and  deep  a  considerable  mass  of  salt  may  accumu- 
late. The  laws  governing  the  deposition  of  saline  residues  in  the 
depressions  of  the  deserts  have  been  ably  set  forth  by  J.  Walther.1 

The  salt  deposits  of  marine  origin  are  frequently  of  great  thick- 
ness. In  some  cases  they  aggregate  1,000  to  2,000  feet,  and  it  will 
be  readily  recognized  that  any  theory  based  on  a  single  cycle  of 
evaporation  of  sea  water,  containing  only  3.5  per  cent,  of  salts 
would  meet  with  great  difficulties. 

The  "bar  theory,"  presented  by  C.  Ochsenius2  in  1877,  but 
already  suggested  by  previously  expressed  views  of  Miller,  Lyell 
and  Bischof,  attempts  to  explain  these  thick  salt  beds.  Och- 
senius believed  that  salt  deposits  of  the  purity  and  thickness  of 
those  in  central  Germany  could  not  have  been  formed  by  the 
flooding  of  a  series  of  shallow  sounds  and  lakes.  A  periodical 
repetition  of  evaporation  and  flooding  would  necessitate  an  im- 
probable number  of  uniform  epochs  of  subsidence  and  elevation. 

Walther's  theory  involving  a  leaching  of  the  small  saline  con- 
tent of  old  sediments  and  its  gradual  concentration  in  desert 
valleys  was  rejected  by  Ochsenius  as  not  properly  representing  the 
conditions  prevailing  in  the  saline  deposits  of  central  Germany. 

The  bar  theory  premises  a  bay  of  the  ocean  separated  from 
the  open  sea  by  a  practically  level  bar  which  permits  only  about 
the  same  quantity  of  water  to  enter  as  is  evaporated  from  the 
surface.  A  dry  climate  and  absence  of  fresh-water  tributaries 
to  the  bay  are  also  premised.  Under  these  conditions  the  sea 
water  entering  over  the  bar  continuously  carries  a  new  supply  to 
the  bay;  the  surface  layers,  becoming  denser,  always  sink  and  the 
concentration  continually  increases  in  the  enclosed  body  of  water. 

The  salt  deposits  on  the  bottom  are  increasing  in  thickness 
and  the  heavy  " bittern"  solution,  with  the  remaining  magnesium 
salts,  correspondingly  rises  toward  the  surface. 

When  these  dense  solutions  reach  the  surface  of  the  bar,  the 

1  J.  Walther,  Lithogenesis  der  Gegenwart,  Jena,  1893,  1894,  pp.  776-800. 
J.  Walther,  Das  Gesetz  der  Wiistenbildung,  Leipzig,  1912. 

2  C.  Ochsenius,  Die  Bildung  der  Steinsalzlager,  Halle,  1877,  p.  172. 

C.  Ochsenius,  Bedeutung  des  orographischen  elementes  "Barre,"  Zeitschr. 
prakt.  Geol..,  1893,  pp.  189-201;  217-233. 


DEPOSITS  FORMED  BY  EVAPORATION         293 

movement  is  reversed  and  the  residual  "bittern"  will  flow  out- 
ward into  the  ocean.  No  accumulation  of  potassium-magnesium 
salts  will  occur.  Should,  however,  the  bar  have  increased  in 
height  just  at  this  time,  the  bittern  would  be  retained  in  the  now 
closed  basin  and  the  deposition  of  the  potassium  salts  would 
follow.1 

The  Gulf  of  Karaboghaz,  on  the  eastern  side  of  the  Caspian 
Sea,  is  frequently  referred  to  as  an  excellent  illustration  of  the 
bar  theory.2 

GYPSUM  AND  ANHYDRITE3 

Occurrence.— Gypsum  (CaSO4.2H2O)  and  anhydrite  (CaSO4) 
usually  occur  in  sedimentary  beds  as  saline  residues.  Both  are 
also  found  occasionally  as  gangue  minerals  in  ore  deposits  and 
gypsum  is  in  places  a  product  of  ascending  springs  or  of  reactions 
of  acid  waters  on  calcareous  beds.  Efflorescences  of  gypsum 
may  be  produced  by  capillary  action  over  gypsiferous  beds  or 
along  saline  lakes.  Anhydrite,  on  account  of  its  slow  transfor- 
mation into  the  hydrous  compound,  has  no  economic  value,  while 
gypsum  is  one  of  the  most  important  non-metallic  minerals. 

Gypsum  in  sedimentary  deposits  frequenty  forms  almost 
pure  beds  of  considerable  thickness.  It  appears  as  snow  white 
fine  grained  aggregates;  characterized  by  softness  (H:2),  low 
specific  gravity  (2.3),  perfect  cleavage  and  great  solubility  in 
dilute  hydrochloric  acid.  Anhydrite  likewise  forms  white 
granular  aggregates,  but  is  easily  distinguished  from  gypsum  by 
its  greater  hardness,  its  greater  specific  gravity  (2.9),  its  pseudo- 
cubical  cleavage  and  resistance  to  weak  HC1.  Anhydrite 

1 H.  Everding,  Deutschlands  Kalibergbau,  Berlin,  1907,  pp.  37-40. 
2F.  W.  Clarke,  Geochemistry,  Bull.  616,  U.  S.  Geol.  Survey,  1916,  p.  165. 
3F.  A.  Wilder,   Eng.  &  Min.  Jour,,  vol.  74,  1902,  p.  276;  Mines  and 
Minerals,  Dec.,  1909;  Mineral  Industry,  Annual  issues. 

G.  P.  Grimsley,  Michigan  Geol.  Survey,  vol.  9,  pt.  2,  1904. 

G.  P.  Grimsley  and  E.  H.  S.  Bailey,  Kansas  Geol.  Survey,  vol.  5,  1899. 

E.  C.  Eckel,  Cements,  limes,  and  plasters,  2d  ed.,  New  York,  1907. 

G.  I.  Adams,  Gypsum  deposits  of  the  United  States,  Bull.  223,  U.  S. 
Geol.  Survey,  1904. 

H.  Ries,  Economic  geology,  New  York,  1916,  pp.  244-259  (with 
references). 

R.  C.  Wallace,  Gypsum  and  anhydrite  in  genetic  relationship,  Geol. 
Mag.,  vol.  1,  1914,  pp.  271-276. 

D.  H.  Newland  and  H.  Leighton,  Bull.  143,  N.  Y.  State  Mus.,  1910. 

A.  F.  Rogers,  Notes  on  the  occurrence  of  anhydrite  in  the  United  States, 
School  of  Mines  Quarterly,  vol.  36,  1915,  pp.  123-142. 


294  MINERAL  DEPOSITS 

slowly  alters  to  gypsum,  and  many  occurrences  of  apparently 
solid  gypsum  contain  remnants  of  anhydrite. 

Beds  of  gypsum  and  anhydrite  occur  in  many  water  laid  forma- 
tions all  over  the  world.  Usually  gypsum  predominates  but 
alternating  beds  of  the  two  are  common.  Beds  of  anhydrite  up 
to  300  feet  in  thickness  are  found  in  the  Permian  of  central 
Germany  in  connection  with  the  potash  salts  (p.  312). 

Anhydrite  is  not  abundant  in  the  United  States  but  occurs 
with  gypsum  overlying  thick  salt  beds  in  Louisiana  and  southern 
Texas;  beds  of  anhydrite  are  also  known  from  southern  Cali- 
fornia, Nevada,  Kansas,  Nova  Scotia,  and  New  Brunswick. 

The  gypsum  beds  of  the  United  States  are  rarely  more  than  30 
or  40  feet  thick  though  there  may  be  several  in  any  one  section. 
They  are  interstratified  with  limestone  or  shale;  in  places  they 
are  of  great  purity  and  snow,  white;  but  frequently  gypsum  is 
also  intergrown  or  interbedded  with  thin  streaks  of  shale  or 
limestone.  The  compact,  translucent  variety  is  called  alabaster 
and  is  used  for  ornamental  objects;  gypsum  in  larger  plates  or 
crystals  is  called  selenite.  Recent  surface  deposits,  mixed  with 
clay  are  known  as  "gypsite." 

A  remarkable  series  of  gypsum  beds,  in  part  alternating  with 
anhydrite  have  been  described  from  the  pre-Cambrian(?)  of 
the  Palen  Mountains1  in  southern  California.  Economically 
important  gypsum  deposits  are  found  in  the  Salina  (Silurian) 
formation  in  northern  New  York  and  extend  parallel  to  the  south 
shore  of  Lake  Ontario. 

Gypsum  beds  are  also  extensively  worked  in  Michigan  where 
they  are  of  Mississippian  age  (Lower  Carboniferous).  Equally 
important  beds  of  the  same  age  are  exploited  in  Nova  Scotia  and 
New  Brunswick.  Iowa,  Kansas,  Texas,  Oklahoma,  New  Mexico 
and  other  states  are  rich  in  gypsum  of  Permian  age;  in  the  western 
part  of  this  region  gypsum  occurs  at  several  horizons  in  the 
"Red  Beds"  whose  age  ranges  from  upper  Carboniferous  to 
Jurassic. 

Exceptionally  thick,  but  not  easily  utilized  deposits  of  un- 
certain age  overlie  the  "salt  domes"  (p.  310)  of  Louisiana 
and  Texas.  Tertiary  deposits  are  known  from  California  and 
Quaternary  "gypsite"  is  abundant  in  Kansas,  Oklahoma 
and  Texas.  The  Tertiary  beds  in  the  basin  of  Paris,  France, 
are  rich  in  gypsum,  hence  the  name  "Plaster  of  Paris." 

1 E.  C.  Harder,  Bull.  430,  U.  S.  Geol.  Survey,  1910,  pp.  407-416. 


DEPOSITS  FORMED  BY  EVAPORATION         295 

Uses. — Gypsum  finds  extensive  use  in  various  industries. 
Ground  in  its  natural  state,  it  is  employed  as  a  fertilizer  (land 
plaster),  to  counteract  alkali  in  soils,  to  retard  the  setting  of 
cement  and  for  numerous  chemical  purposes.  It  is  often  used 
as  a  "filler"  or  adulterant.  Most  important  is,  however,  its  use 
as  structural  material.  For  this  purpose  it  is  calcined  at  350°  F. 
when  a  large  part  of  the  water  is  expelled.  After  grinding  and 
mixing  with  water  gypsum  forms  again  and  the  whole  sets  to  a 
hard  mass  called  stucco  or  plaster  of  Paris.  The  use  of  gypsum 
is  increasing  rapidly.  In  1917  the  production  in  the  United 
States  was  2,700,000  tons. 

Stability  and  Solubility. — As  noted  above  gypsum  is  trans- 
formed to  anhydrite  in  sea  water  at  27°  C.  In  pure  water  it 
begins  to  change  slowly  to  anhydrite  at  66°  C .  At  or  above  27°  C. , 
a  temperature  often  reached  in  salt  lakes  in  tropical  countries 
alternating  beds  of  gypsum  and  anhydrite  may  form,  as  indeed  is 
often  observed.  In  nature  both  hydration  and  dehydration 
takes  place  but  the  changes  are  very  slow. 

The  solubility  of  gypsum  is  a  complicated  problem  owing  to 
the  existence  of  metastable  forms — the  hemi-hydrate  and  the 
soluble  anhydrite — and  it  has  only  lately  been  worked  out  by 
van't  Hoff  and  Meyerhoffer.1  The  solubility  of  gypsum  in  water 
reaches  a  maximum  of  0.21  per  cent,  at  40°  C.,  and  decreases 
slightly  above  this  temperature.  At  66°  C.  the  solubility  of 
anhydrite  is,  of  course,  equal  to  that  of  anhydrite,  but  beyond 
this  point  it  decreases  rapidly  so  that  at  100°  C.  it  is  0.06  per 
cent,  and  at  200°  C.  only  about  0.005  per  cent.2  Other  calcium 
salts,  having  a  common  ion,  depress  the  solubility  of  gypsum,  but 
sodium  chloride  increases  it  about  three  times  owing  to  formation 
of  CaCl2,  so  that  a  saturated  solution  of  NaCl  can  hold  0.54  per 
cent,  at  CaSO4  at  23°  C.  and  0.75  per  cent,  at  82°  C. 

SODIUM  SULPHATE  AND  SODIUM  CARBONATE 

Occurrence.- — Most  of  the  soda  of  commerce  is  an  artificial 
product  from  common  salt,  but  both  the  carbonate  and  the 
sulphate  of  sodium  are  often  contained  in  saline  desert  lakes  or 
in  residues  from  such  lakes.  The  ordinary  white  efflorescence  on 
the  playas  of  the  deserts  consists  of  these  salts  together  with 
more  or  less  sodium  chloride  and  a  little  of  the  chlorides  and 

1  Summarized  by  Cameron  and  Bell,  Bull.  33,  U.  S.  Bureau  of  Soils,  1906. 
*A.  C.  Melcher,  Jour.  Am.  Chem.  Soc.,  vol.  32,  1910,  pp.  50-66. 


296  MINERAL  DEPOSITS 

sulphates  of  potassium  and  magnesium;  the  soda  lakes  contain 
all  these  salts. 

In  the  United  States  the  commercial  utilization  has  been 
attempted  at  Owens  Lake,  in  California,  at  the  Ragtown  lakes, 
in  Nevada,  and  at  the  Wyoming  soda  lakes. 

T.  M.  Chatard's1  work  on  Owens  Lake,  where  sodium  carbonate 
forms  a  little  over  one-third  and  sodium  sulphate  about  one- 
seventh  of  the  dissolved  salts,  showed  that  the  order  of  deposition 
upon  evaporation  is:  (1)  trona  (Na2CO3.NaHCO3.2H20);  (2) 
sodium  sulphate;  (3)  sodium  chloride,  and  (4)  the  easily  soluble 
normal  sodium  carbonate.  The  deposits  at  Ragtown  are  even 
richer  in  carbonate  of  soda,2  but  the  evaporation  by  solar  heat 
did  not  prove  successful  as  a  commercial  process.  One  or  two 
of  the  Wyoming  lake  deposits  are  rich  in  soda.  At  Green  River 
borings  in  the  Wasatch  sandstone  (Eocene?)  at  depths  of  125  and 
700  feet  disclosed  well  water  forming  an  almost  concentrated 
solution  of  sodium  carbonate,  which  for  a  time  was  utilized  for  the 
manufacture  of  caustic  soda;  the  process  was  based  on  reaction 
with  caustic  lime. 

The  alkali  lakes  in  the  arid  regions  of  Wyoming3  and  New 
Mexico  form  deposits  leached  from  surrounding  Mesozoic  and 
Cenozoic  sediments.  The  thickness  of  the  salt  beds  amounts  to 
15  feet  at  most  and  they  extend  over  as  much  as  100  acres.  The 
salts  consist  mainly  of  mirabilite,  epsomite,  natron,  and  halite. 

Sodium  sulphate  is  much  more  soluble  in  warm  than  in  cold 
water,  but  as  the  similar  variation  for  sodium  chloride  is  small, 
"a  mere  change  of  temperature  between  summer  and  winter  in 
salt  lakes  may  cause  mirabilite  (Na2S04.10H20)  to  separate  out 
or  to  redissolve."  The  Great  Salt  Lake,  according  to  Gilbert, 
deposits  sodium  sulphate  during  winter. 

SODIUM  NITRATE 

The  alkaline  nitrates  are  very  soluble  salts  which  are  found  in 
larger  masses  only  under  exceptional  conditions.  Sodium 
nitrate  is  present  in  the  soil  and  is  produced  by  the  so-called 

J  T.  M.  Chatard,  Natural  soda,  Bull.  60,  U.  S.  Geol.  Survey,  1890. 

2F.  W,  Clarke,  .Geochemistry,  Bull.  616,  U.  S.  Geol.  Survey,  1916, 
p.  238. 

SA.  R.  Schultz,  Deposits  of  sodium  salts  in  Wyoming,  Bull.  430,  U.  S. 
Geol.  Survey,  1910,  pp.  570-589. 

W.  C.  Knight  and  E.  E.  Slosson,  Alkali  lakes  and  deposits,  Bull,  Wyo. 
Exper.  Sta.,  1901,  p.  49. 


DEPOSITS  FORMED  BY  EVAPORATION         297 

nitrifying  bacteria1  or  by  reactions  between  organic  nitrogenous 
matter  and  alkaline  salts.  Sodium  and  potassium  nitrates  of 
great  purity  are  sometimes  found  as  efflorescences  and  veinlets 
on  sheltered  cliffs  of  various  rocks  and  in  caves  and  are  in  many 
cases  produced  by  organic  agencies.  Calcium  nitrate  is  known 
from  limestone  caves.  Naturally,  nitrate  deposits  are  most 
common  in  arid  countries.  In  minor  quantities  nitrates  are 
widely  scattered  in  the  Western  States  and  very  frequently  they 
are  associated  with  volcanic  rocks,2  particularly  rhyolite  but 
also  tuffs,  basalts  and  lake  beds  in  regions  of  volcanic  activity. 
The  volcanic  origin  of  these  nitrates  is  not  accepted  by  all  writers 
but  nevertheless  it  is  the  most  probable  theory  advanced. 

There  are  two  sources  of  nitrogen  which  may  be  utilized  by 
nature  for  the  development  of  ammonia  salts  and  nitrates.  1. 
The  nitrogen  in  the  air,  which  may  be  fixed  by  organisms  or  by 
electric  atmospheric  discharges  and  entrainment  in  rain  water. 
2.  The  nitrogen  from  the  interior  of  the  earth,  which  possibly  is 
contained  in  the  magma  as  a  nitride  of  boron  or  of  some  metal. 
At  any  rate  the  volcanic  gases  and  exhalations  frequently  con- 
tain nitrogen  and  ammonia;  it  is  held  by  many  that  a  fixation 
of  nitrogen  from  this  source  as  nitrates  is  well  possible.3 

The  only  place  where  nitrates  are  present  in  abundance  is  in 
the  Atacama  desert  in  northern  Chile.4  These  wonderful  deposits 

1 H.  S.  Gale,  Nitrate  deposits,  Bull.  523,  U.  S.  Geol.  Survey,  1912. 
2G.  R.  Mansfield,  Nitrate  deposits  in  southern  Idaho  and  eastern  Oregon, 
Bull.  620,  U.  S.  Geol.  Survey,  1916,  pp.  19-44. 

Whitman  Cross,  Am.  Jour.  Sci.,  4th  ser.,  vol.  4,  1897,  p.  118. 
W.  Lindgren,  Prof.  Paper  43,  U.  S.  Geol.  Survey,  1905,  p.  121. 
C.  DeKalb,  Min.  and  Sci.  Press,  May  6,  1916. 

3F.  W.  Clarke,  Geochemistry,  Bull.  616,  U.  S.  Geol.  Survey,  1916,  p.  256. 
4  The  literature  is  very  extensive  and  only  part  can  be  quoted. 
L.  Darapsky,   Das  Departement  Tal-tal,   Berlin,  1900.     Ref.  Zeitschr. 
prakt.  Geol.,  1902,  p.  153. 

R.  A.  F.  Penrose,  Jour.  Geol.,  vol.  18,  1910,  pp.  1-32. 
S.  H.  Loram,  Min.  and  Sd.  Press,  Jan.  15  and  29,  1910. 
W.  F.  Clarke,  Geochemistry,  Bull.  616,  U.  S.  Geol.  Survey,  1916,  pp. 
253-259. 

L.  W.  Strauss,  Min.  and  Sci.  Press,  June  13  and  20,  1914. 
J.  T.  Singewald,  Jr.,  and  B.  L.  Miller,  Econ.  Geol,  vol.  11, 1916,  pp.  103-114. 
Lorenzo  Sundt,  Econ.  Geol,  vol.  12,  1917,  p.  89. 

A.  H.  Rogers  and  H.  R.  Van  Wagenen.,  The  Chilean  Nitrate  Industry, 
Bull.  134,  Am.  Inst.  Min.  Eng.,  Feb.,  1918,  pp.  505-522.  Discussion,  Bull. 
136,  pp.  845-848. 

S.  H.  Salisbury,  Jr.,  Mineral  Industry,  Annual  issues. 


298  MINERAL  DEPOSITS 

practically  supply  the  world  with  nitrates;  the  annual  production 
now  (1916)  amounts  to  nearly  3,000,000  metric  tons.  The 
deposits  are  situated  in  the  provinces  of  Tarapaca  and  Anto- 
fagasta  in  the  interior  dry  valleys  between  the  Coast  Range  and 
the  Andes,  at  elevations  ranging  from  1,000  feet  to  3,000  feet, 
and  they  extend  for  300  miles  parallel  to  the  coast.  The  lowest 
depressions  are  often  occupied  by  salt  flats  with  a  little  nitrate 
and,  in  the  higher  region,  by  borax  flats.  The  nitrate  deposits 
lie  on  the  gentle  slopes  of  the  valleys.  The  nitrate  bed  is  a 
superficial  formation  of  considerable  though  irregular  extent;  it 
lies  below  an  overburden  of  a  few  feet  of  loose  crumbly  material 
with  subangular  gravel,  becoming  harder  toward  the  bottom. 
This  overburden  contains  some  nitrate  and  often  much  sodium 
chloride,  sodium  sulphate  and  gypsum,  as  well  as  a  little  sodium 
iodate.  The  "Caliche"  or  nitrate  bed  is  a  reddish  brown  sandy 
gravel  cemented  with  salts;  it  averages  a  few  feet  in  thickness. 
Below  the  "caliche"  lies  rudely  stratified  sand,  gravel  or  clay, 
often  of  considerable  thickness.  The  "caliche"  averages  about 
25  per  cent,  sodium  nitrate  and  the  lower  limit  of  workable 
material  is  placed  at  15  per  cent.  Associated  with  the  nitrate 
are  a  large  amount  of  sodium  chloride,  more  or  less  of  the  sul- 
phate and  borates  or  calcium  and  sodium,  and  a  small  but  con- 
stant quantity  of  sodium  iodate.  Small  quantities  of  the  nitrates 
of  potassium,  calcium  and  barium  as  well  as  a  little  calcium  iodate 
and  iodo-chromate  (lautarite  and  dietzeite)  are  found.  Very 
curious  is  the  occurrence  of  a  small  amount  of  sodium 
perchlorate. 

The  material  mined  is  usually  of  the  following  composition: 

Per  cent. 

Sodium  nitrate 14r-25 

Potassium  nitrate 2-3 

Sodium  chloride 8-50 

Sodium  sulphate 2-12 

Calcium  sulphate 2-6 

Magnesium  sulphate 0-3 

Sodium  biborate 1-3 

Sodium  iodate 0.05-  1 

Sodium  perchlorate 0.1-0.5 

Insoluble 0-50 

The  origin  of  the  nitrate  deposits  of  Chile  is  a  much  debated 
question  and  few  authors  are  in  agreement 


DEPOSITS  FORMED  BY  EVAPORATION         299 

The  theory  advanced  many  years  ago  by  Pissis,  the  Chilean 
geologist,  and  followed  lately,  for  instance,  by  Rogers  and  Van 
Wagenen  accounts  for  the  deposits  by  fixation  of  atmospheric 
nitrogen  by  thunderstorms  and  its  descent  from  the  Andes  in 
the  underground  circulation  and  ascent  to  the  surface  by 
capillarity. 

Penrose  and  others  hold  that  the  nitrate  came  from  beds  of 
bird  guano  accumulated  at  the  time  when  the  Coast  Range  did 
not  exist  and  that  the  nitrates  were  gradually  leached  and  mingled 
with  the  salt  waters  of  a  closed  basin.  Others  are  inclined  to 
consider  the  deposits  caused  by  ordinary  bacterial  fixation  or  by 
oxidation  of  nitrogenous  vegetable  matter.  Singewald  and 
Miller  think  that  the  nitrates  have  been  carried  down  by  the 
ground  water  and  emphasize  that  only  the  usual  processes  in 
operation  everywhere,  have  been  active.  The  accumulation  is 
simply  caused  by  the  abnormally  dry  climate. 

All  these  explanations  appear  inadequate  or  forced.  The 
nitrate  deposits  are  not  marine  or  lacustrine.  Their  extent 
corresponds  in  a  most  remarkable  way  to  the  Jurassic  and  Cre- 
taceous tuffs  and  lava  flows  which  occupy  so  much  space  in  this 
region  and  the  conclusion  is  inevitable  that  there  must  be  some 
causal  connection .  1  It  is  probable  that  the  nitrates  are  of  volcanic 
origin  and  that  the  nitrogen  was  contained  in  the  rocks  men- 
tioned from  which  they  have  been  leached  under  unusual  climatic 
conditions.  This  view  of  the  origin  is  also  supported  by  F.  W. 
Clarke.2  The  •  constant  presence  of  borates  is  an  additional 
suggestive  fact. 

The  world's  need  of  iodine  is  now  supplied  by  the  nitrate 
region  of  Chile.  The  production  was  709,000  kilograms  in  1915. 

BORAXES3 

General  Occurrence.— Borates  and  other  boron  compounds 
appear  in  nature  under  conditions  indicating  widely  differing 
modes  of  origin.  As  complex  and  insoluble  borosilicates  like 

1  Unpublished  observations  by  W.  L.  Whitehead  and  W.  Lindgren. 

2  Geochemistry,  Bull.  616,  U.  S.  Geol.  Survey,  1916,  p.  258. 

3  G.  E.  Bailey,  The  saline  deposits  of  California,  Bull  24,  California 
State  Min.  Bur.,  1902. 

M.  R.  Campbell,  Reconnaissance  of  the  borax  deposits  of  Death  Valley 
and  the  Mohave  Desert,  Bull.  200,  U.  S.  Geol.  Survey,  1902. 

C.  R.  Keyes,  Borax  deposits  of  the  United  States,  Trans.)  Am.  Inst. 
Min.  Eng.,  vol.  40,  1909,  pp.  674-710. 


300  MINERAL  DEPOSITS 

tourmaline  and  datolite  they  are  disseminated  in  igneous  and 
metamorphic  rocks  or  in  pegmatite  dikes  and  fissure  veins,  but 
are  here  of  no  economic  importance  except  that  tourmaline  oc- 
curring in  this  manner  is  sometimes  utilized  as  a  gem  stone. 
As  boric  acid  and  borates  of  calcium  and  magnesium  they  ap- 
pear in  volcanic  exhalations,  of  which  the  most  famous  are  the 
"soffioni"  of  Tuscany,  Italy,  from  which  large  amounts  of  boric 
acid  have  been  recovered.  Borates,  principally  in  the  form  of 
borax  (Na2B4O7.10H2O)  occur  in  hot  springs  and  in  lakes  of  vol- 
canic regions.  Borax  was  first  obtained  from  such  lakes  situated 
in  Tibet.  Von  Schlagintweit  reports  it  as  a  hot-spring  deposit 
in  the  province  of  Ladak,  on  the  headwaters  of  the  Indus.  Ac- 
cording to  A.  Forbes  a  calcium  borate  is  being  deposited  at  the 
hot  springs  of  Banos  del  Toro,  Chile.  The  thermal  waters  of  the 
California  Coast  Ranges  and  Nevada  (page  61)  often  contain 
boron,  sometimes  in  large  quantities.  The  borates  from  these 
springs  are  sometimes  accumulated  in  little  lake  basins  and  there 
deposited  by  evaporation  as  borax  crystals.  About  40  years  ago 
much  borax  was  won  from  the  Borax  Lake,  Lake  County,  Cali- 
fornia. The  evaporated  salts  contained  62  per  cent,  sodium 
carbonate,  20  per  cent,  sodium  chloride,  and  18  per  cent,  borax. 

The  borates  occur  abundantly  in  the  playas,  or  shallow  basins 
intermittently  covered  by  water,  in  southern  California,  Nevada, 
Oregon,  Argentina,  and  Chile;  the  salts  are  borax,  ulexite 
(CaNaB5O9.8H2O),  and  colemanite  (Ca2N6On.5H2O). 

The  Quaternary  borax  beds  are  probably  derived  from  leach- 
ing of  deposits  of  colemanite  in  Tertiary  lake  beds,  formed 
during  volcanic  epochs.  The  deposits  in  southern  California 
now  furnish  most  of  the  world's  boron  salts. 

Finally,  boron  is  contained  in  sea  water  and  appears  in  small 
quantities  in  the  form  of  magnesium  borates,  principally  boracite 
(MgyC^NieOao),  in  saline  residues. 

Marine  Borate  Deposits.- — The  marine  deposits  are  mainly 
confined  to  the  beds  of  potassium  salts  in  central  Germany,  but 


J.  H.  van't  Hoff,  Zur  Bildung  der  ozeanischen  Salzablagerungen,  1905. 

General:   F.  W.  Clarke,  Geochemistry,  Bull.  616,  U.  S.  Geol.  Survey, 
1916,  pp.  243-253. 

C.  G.  Yale  and  H.  S.  Gale,  in  Mineral  Resources,  U.  S.  Geol.  Survey, 
annual  publication. 

1  H.  S.  Gale,  ,  The  origin  of  colemanite  deposits,  Prof.  Paper  85,  U.  S. 
Geol.  Survey,  1913,  pp.  3-9. 


DEPOSITS  FORMED  BY  EVAPORATION          301 


SANTA  j     °-»WT-  — 
RBAR^VENTURA\L°S.AI7E|-E^ 


FIG.  97. — Sketch  map  showing  distributions  of  borate  deposits  in  California 
and   Nevada.     After  H.   S.  Gale,    U.   S.   Geol.   Survey. 


302  MINERAL  DEPOSITS 

borates  have  also  been  observed  in  sodium  chloride,  anhydrite, 
or  gypsum.  The. principal  occurrence  is  as  boracite  and  several 
other  rare  borates  in  the  carnallite  region  (page  313) — that  is, 
in  the  deposits  of  the  last  mother  liquors  of  evaporating  sea 
water.  The  boracite  usually  forms  small  crystals  or  concre- 
tions, but  one  occurrence  is  recorded  of  a  mass  weighing  about 
1,400  pounds.  A  few  hundred  tons  of  borates  are  annually 
obtained  by  crystallization.  The  boron  compounds  .then 
remained  with  the  most  easily  soluble  salts  and  were  finally 
precipitated  as  a  magnesium  salt  because  of  the  predominance 
of  that  metal  over  calcium  in  the  sea  water. 

Borax  Marshes. — The  deserts  of  San  Bernardino  and  Inyo 
counties  in  California,  and  also  those  of  Nevada  and  Oregon,  are 
rich  in  borate  deposits  (Fig.  97).  The  desolate  plains  between 
the  barren  ranges  contain  many  shallow  basins,  which  at  times, 
during  the  brief  seasons  of  rain,  are  covered  with  thin  sheets  of 
water.  The  evaporation  of  this  water  leaves  dazzling  expanses  of 
white  salt  deposit  or  efflorescence,  some  of  which  may  become 
covered  by  the  fine  sand  carried  by  the  desert  storms.  These 
deposits  were  discovered  about  1870  and  for  many  years  yielded  a 
large  production  of  borax  at  Searles  Marsh  (60  miles  north  of 
Barstow),  Death  Valley,  and  other  places.  Though  enormous 
quantities  of  these  salts  remain  they  are  now  of  little  or  no  im- 
portance; the  richer  and  more  easily  worked  colemanite  deposits 
have  replaced  them.  The  crusts  are  rarely  more  than  1  foot 
thick,  the  percentage  of  borax  varying  considerably.  According 
to  Bailey,  the  crude  salt  from  Searles  Marsh  yielded  50  per  cent, 
sand,  12  per  cent,  sodium  chloride,  10  per  cent,  sodium  carbonate, 
16  per  cent,  sodium  sulphate,  and  12  per  cent,  borax.  Borings 
showed  20  feet  of  clay  and  sand  with  crystals  of  calcium  borate, 
underlain  by  a  bed  of  solid  trona  28  feet  thick,  and  below  this  350 
feet  of  clays  impregnated  with  hydrogen  sulphide.  On  ground 
that  had  been  worked  over  a  new  crust  formed  by  capillary  action 
that  was  thick  enough  to  remove  in  3  or  4  years.  The  area  pro- 
ductive of  borax  amounts  to  about  1,700  acres,  slightly  depressed 
below  the  general  level  of  the  playa,  on  which  in  wet  seasons 
stands  about  1  foot  of  water. 

Though  no  ulexite  was  found  at  Searles  Marsh,  it  is  common 
in  many  other  playa  deposits,  both  in  California  and  Nevada  and 
in  Argentina;  it  usually  forms  concretions  of  silky  fiber,  known 
as  "cotton  balls." 


DEPOSITS  FORMED  BY  EVAPORATION 


303 


Tertiary  Lake  Beds. — The  borates  in  the  marshes  and  playas 
have  undoubtedly  been  leached  from  the  older  deposits  in  the 
Tertiary  lake  beds,  which  have  been  recognized  at  many  points 
in  Inyo,  San  Bernardino,  Kern,  Los  Angeles,  and  Ventura 
counties,  California.  These  beds  yield  colemanite  almost 
exclusively  and  it  is  evident  that  the  borax  and  ulexite  of  the 
marshes  are  mainly  products  of  secondary  reactions  of  the 
leached  colemanite  with  the  sodium  salts  of  the  playas. 

These  colemanite  deposits  begin  near  the  Pacific  coast  at  Piru, 
Ventura  County,  and  near  Saugus,  Los  Angeles  County,  where, 
according  to  Keyes,  they  lie  in  a  series  of  yellow  clays  and  sand- 
stones probably  Miocene  in  age  and  several  thousand  feet  in 


FIG.  98. — Lila    C.  borate  mine,  Inyo  County,  California,  closed  in  1915. 

thickness.  The  mineral  is  present  as  nodules  in  clay,  and  above 
the  borate  beds  are  strata  of  gypsum. 

Other  important  beds  are  in  the  foot-hills  of  the  Calico  Moun- 
tains north  of  the  Mojave  River;  they  have  a  steep  dip  and  have 
been  mined  by  shafts  to  a  depth  of  400  feet.  These  large 
masses  of  low-grade  colemanite  shales,  with  7  to  20  per  cent, 
boric  acid,  are  not  mined  now,  attention  being  confined  to 
two  solid  beds  of  the  mineral  7  to  10  feet  thick.  Rhyolite  tuffs 
lie  underneath  the  borate  beds.  f  ' 

The  richest  colemanite  beds  -  a*re,  however,  in  the  Furnace 
Creek  region  of  the  Funeral  Range,  which  overlooks  Death  Valley, 
in  Inyo  County  (Fig.  98).  About  4,000  feet  of  Tertiary  non- 
fossiliferous  sediments  are  recognized  here,  which  form  a  broad 


304  MINERAL  DEPOSITS 

belt  obliquely  crossing  the  range  and  dipping  20°  to  45°  N.  E. 
The  lower  and  thicker  part,  according  to  Keyes,  consists  of  con- 
glomerates and  sandstones,  above  which  are  olive-colored  clays 
interbedded  with  basalts.  The  upper  part  of  the  clay  series 
carries  gypsum,  colemanite,  and  thin  layers  of  limestone.  The 
borate  beds  are  traceable  for  25  miles.  Within  the  colemanite- 
bearing  beds,  which  may  be  as  much  as  50  feet  thick,  the  bluish 
clays  are  thickly  interspersed  with  milky  white  layers  or  nodules 
of  the  white  coarsely  crystalline  mineral,  mingled  with  more  or 
less  gypsum,  according  to  Keyes.  The  solid  layers  may  be  15  feet 
thick.  Near  by  the  clays  are  impregnated  with  fine  particles  of 
colemanite  and  yield  10  to  25  per  cent,  boric  acid,  but  these  low- 
grade  deposits  are  not  utilized  at  present. 

Production  and  Uses. — The  borate  industry  is  now  concen- 
trated in  southeastern  California  and  has  shown  great  expansion 
in  the  last  years.  In  1916,  103,000  tons  of  crude  colemanite 
were  mined,  averaging  about  25  per  cent.  BaOs.  The  richest 
mineral  is  hand-sorted,  and  the  poorer  grades  are  milled,  roasted, 
and  screened,  the  last  process  effecting  the  separation  of  the 
colemanite  from  the  gangue.  The  products  are  shipped  direct 
to  the  sea  board,  where  the  material  is  manufactured  into  borax 
and  boric  acid.  The  further  treatment  involves  boiling  with  soda 
for  the  manufacture  of  borax  or  with  sulphuric  acid  if  boric  acid 
is  desired.  Under  the  influence  of  these  new  discoveries  the  price 
of  borax  has  gradually  decreased;  in  1916  it  was  from  6  to  8  cents 
per  pound. 

Borax  is  extensively  used  in  industrial  chemistry,  in  metal 
enameling,  in  medicine  and  in  the  household. 

Origin. — The  colemanite  deposits  which  in  places  occur  with 
gypsum  and  limestone  are  surely  not  of  marine  origin  and  can 
hardly  be  supposed  to  be  saline  precipitates  from  evaporating 
lake  water.  The  mode  of  occurrence  in  specimens  suggests 
replacement  and  it  has  indeed  been  shown  lately  by  H.  S.  Gale1 
that  some  colemanite  deposits  are  of  epigenetic  nature.  The 
replacing  boron  solutions  may  have  been  leached  from  the 
sediments  or  they  may  have  ascending  waters  in  genetic  connec- 
tion with  basalt  flows  formed  in  close  connection  with  the  beds. 
The  strontianite  deposits  found  in  similar  lake  beds  (p.  380)  form 
a  somewhat  analogous  occurrence.  Further  investigations  of 
the  colemanite  deposits  is  highly  desirable. 

lProf.  Paper  85,  U.  S.  Geol.  Survey,  1913,  pp.  3-9. 


DEPOSITS  FORMED  BY  EVAPORATION         305 

Colemanite  and  the  allied  species  priceite  have  been  formed  in 
recent  borax  marshes,  for  instance,  at  Searles  lake  and  would 
undoubtedly  be  formed  from  alkaline  borate  solutions  in  contact 
with  calcite  or  calcium  carbonate  waters. 

Van't  Hoff1  has  produced  pandermite  (Ca8B2oO38.15H2O)  and 
colemanite  from  the  heptaborate  (Ca2B6On.7H20)  and  states 
that  ulexite,  pandermite,  and  probably  colemanite  can  be  formed 
at  temperatures  from  25°  C.  upward.  Regarding  the  separation 
of  borates  he  states  that  while  the  first  salts  precipitated  in 
oceanic  waters  are  calcium  salts,  different  relations  exist  with 
the  borates;  for  these  the  saturation  point  is  not  reached  until 
carnallite  is  precipitated. 

SODIUM  CHLORIDE 

Occurrence.2 — Sodium  chloride  or  common  salt  forms  beds  in 
sedimentary  rocks  and  in  most  cases  its  derivation  by  evapora- 
tion of  saline  solutions  is  clear.  Only  a  small  part  of  the  four 
million  tons  of  salt  produced  in  the  United  States  is  mined  in 
solid  form.  Most  of  it  is  obtained  from  brines  derived  from 
solution  of  salt  beds  by  natural  waters  or  by  water  forced  down 
into  bore-holes  to  the  saline  strata;  much  also  is  produced  by 
evaporation  of  sea  water  or  water  of  saline  lakes,  such  as  the 
Great  Salt  Lake  of  Utah. 

.  Salt  beds'  are  present  in  formations  of  different  ages,  but  are 
perhaps  most  common  in  the  Permian  and  Triassic  strata;  the 
oldest  saline  rocks  in  the  United  States  are  those  of  the  Silurian 
in  New  York  State.  As  may  be  easily  understood  from  the 
general  statements  on  previous  pages,  strata  of  calcium  sulphate 
are  ordinarily  associated  with  salt  beds  and  should  appear  below 
them;  owing  to  recurrent  and  shifting  epochs  of  desiccation  the 
order  may  be  reversed  and  gypsum  beds  appear  above  .the  salt. 
It  is  also  very  common  to  find  crystals  or  streaks  of  anhydrite 
or  gypsum  with  salt,  as  well  as  streaks  of  clay.  In  thickness 

1J.  H.  van't  Hoff,  Zur  Bildung  der  ozeanischen  Salzablagerungen,  2, 
1909,  pp.  45-75. 

2U.  S.  Geol.  Survey,  Mineral  Resources  (annual  reports).     See  especially 
W.  C.  Phalen,  Mineral  Resources,  1907-1911. 
Mineral  Industry  (annual  issues). 
G.  D.  Harris,  Bull.  7,  Louisiana  Geol.  Survey,  1908. 
J.  O.  von  Buschman,  Das  Salz,  Leipzig,  1906  and  1909,  2  volumes. 
W.  C.  Phalen,  Technology  of  salt  making  in  the  United  States,  Bull. 
146,  U.  S.  Bureau  of  Mines,  1917  (with  description  of  deposits). 


306 


MINERAL  DEPOSITS 


salt  beds  vary  enormously — from  the  thinnest  strata  to  masses 
1,500  feet  or  even  more  in  depth.  A  bore-hole  near  Speeren- 
berg,  in  the  German  potash  region,  penetrated  3,900  feet  of 


FIG.  99.— Sections  of  salt  wells,  Tully,  New  York.     After  F.  J.  H.  Merrill. 

salt,  but  here,  as  in  so  many  other  places,  the  apparent  thickness 
may  be  deceptive,  being  due  to  movements  of  folding  and  fault- 
ing. Besides,  the  plasticity  of  salt  is  remarkably  great,  much 


DEPOSITS  FORMED  BY  EVAPORATION 


307 


greater  than  that  of  the  accompanying  clays  and  anhydrite,  and 
this,  as  the  German  geologists  have  found,  leads  to  most  astonish- 
ing and  confusing  stratigraphic  relations. 

From  the  calcium  sulphate  secondary  sulphur  often  results 
and  may  form  thick  beds.  Hydrocarbons  and  carbon  dioxide  are 
often  contained  as  inclusions  in  the  salt.  The  difficulties  of 


FIG.  100. — Section  of  lower  Michigan  basin.     After  A.  C.  Lane. 

accounting  for  the  great  thickness  of  salt  beds  have  already  been 
ref  erreduto .  It  is  evident  that  by  evaporation  of  sea  water  with  3 . 5 
per  cent,  salt  in  a  basin  100  meters  deep  a  bed  less  than  2  meters 
of  salt  would  accumulate.  The  theories  of  Ochsenius  and 
Walther  attempt  to  explain  this  in  different  ways,  as  described  on 


FIG.  101. — Section   of    Permian    salt    formation   in    Kansas. 
From    Mineral    resources    of    Kansas. 

previous  pages.     The  special  conditions  in  Louisiana  will  be 
referred  to  later. 

Examples. — Salt  beds  occur  in  New  York  State  in  the  red 
Salina  shales  of  the  Silurian  and  underlie  a  considerable  area. 
Much  of  the  salt  is  recovered  from  artificial  brines.  The  salt 
forms  pure  lenticular  masses  and  layers  interbedded  with  soft 
shales,  limestone,  and  gypsum,  the  salt-bearing  formation  having 
a  variable  thickness  up  to  470  feet  (Fig.  99).  At  Ithaca  several 


308  MINERAL  DEPOSITS 

beds  of  salt  occur  at  a  depth  of  2,244  feet  with  a  total  thickness 
of  248  feet.  A  magnesian  limestone,  containing  gypsum,  lies 
above.  Rock  salt  is  mined  at  several  places,  one  shaft  lately 
opened  at  Cuylerville  being  1,100  feet  deep  and  reaching  a  salt 
bed  21  feet  in  thickness.  Salt  has  also  been  mined  near  the 
outcrops  of  the  beds  at  Livonia.  Similar  beds  are  worked  in 
Ohio  by  bore-holes  and  brines. 

The  greatest  salt  production  in  the  United  States  is  derived 
from  Michigan.1  The  salt  occurs  as  large  beds  at  different 
horizons  in  the  Salina  formation  and  also  in  the  sandstones  of 
the  Mississippian  or  Lower  Carboniferous  (Fig.  100) .  The  salt  is 
recovered  by  means  of  natural  and  artificial  brines;  bromine,  in 
which  these  brines  are  unusually  rich,  is  recovered  as  a  by-product 
of  the  final  mother  liquor.  Deep  mining  has  been  undertaken 
under  considerable  difficulties  near  Detroit. 

Kansas  is  likewise  among  the  great  producers.2  Some  salt 
is  obtained  from  salt  springs  in  the  Carboniferous  and  on  the 
"salt  plains"  leached  from  Permian  beds.  From  the  latter  the 
principal  product  is  derived;  it  occurs  interstratified  with  shales, 
the  total  thickness  of  the  salt  beds  being  at  most  500  feet. 
Some  of  the  beds  are  said  to  be  over  200  feet  thick,  but  generally 
they  are  much  less  (Fig.  101). 

In  the  western  arid  States  playa  deposits  of  salt  are  common 
in  the  dry  basins  between  the  ranges;  they  are  usually  thin, 
though  at  Danby,  in  southern  California,  there  are  solid  beds  22 
feet  in  thickness,  according  to  Bailey.3 

The  most  noted  deposit  is  that  of  Salton,  Imperial  County, 
where,  the  basin  lies  below  the  level  of  the  sea.  Before  the  recent 
flooding  by  the  Colorado  River  an  important  production  was 
maintained  here.  A  large  area  is  covered  by  salt  crusts  10  to  20 
inches  in  thickness.  Below  this  lies  a  thin  mud  deposit  covering 
another  salt  crust.  Deeper  borings  encountered  22  feet  of  black 
mud  containing  salt  and  soda,  and  this  covers  270  feet  of  hard 
clay.4 

The  desert  regions  of  northern  Africa  and  central  Asia  offer 
similar  occurrences  in  abundance. 

'A.  C.  Lane,  Mineral  Industry,  vol.  16,  1907;  also  vol.  19,  1910. 
A.  C.  Lane,  Water-Supply  Paper  30,  U.  S.  Geol.  Survey,  1899. 
2  M.  Z.  Kirk,  Mineral  resources  of  Kansas,  Univ.  Geol.  Survey,  1898. 
3G.  E.  Bailey,  Bull.  24,  California  State  Min.  Bur.,  1902,  p.  128. 
4  G.  E.  Bailey,  idem,  p.  126. 


DEPOSITS  FORMED  BY  EVAPORATION 


309 


Large  deposits  of  impure  salt  mixed  with  clay  have  been 
worked  for  a  long  time  in  the  Alpine  Triassic  of  Tyrol;  they  lie 
between  limestone  beds.  Another  important  saline  region 
fringes  the  outside  of  the  Carpathian  chain  in  Roumania, 
Transylvania,  and  Galicia  and  is  contained  in  Miocene  sands  and 
clays.  The  beds  are  generally  greatly  disturbed,  brecciated,  and 
pressed.  The  best-known  place  where  mining  is  carried  on  is 
the  celebrated  mine  of  Wieliczka,  in  Galicia,  now  about  1,000 
feet  deep,  which  is  much  visited  by  tourists  on  account  of  the 


FIG.   102. — Vertical  section  of    salt  dome,  based  on  borings  at  Calcasieu 
parish,  Louisiana.     Black  areas  represent  sulphur.     After  Kirby  Thomas. 


picturesque  and  extensive  workings  with  elaborate  carvings  in 
solid  salt.  The  salt  beds  of  the  Stassfurt  region  will  be  described 
later. 

The  Salt  Deposits  of  the  Gulf  Coast.1 — The  greatest  salt 
deposits  in  the  United  States  have  been  discovered  by  borings  in 

1 G.  D.  Harris,  Bull.  7,  Geol.  Survey  Louisiana,  1908. 

A.  F.  Lucas,  The  possible  existence  of  deep-seated  oil  deposits  on  the 
Gulf  Coast,  Bull.  139,  Am.  Inst.  Min.  Eng.,  1918,  pp.  1119-1134. 

G.  S.  Rogers,  Intrusive  origin  of  the  Gulf  Coast  Salt  Domes,  Econ.  Geol, 
vol.  13,  1918,  pp.  447-485. 

E.  R.  DeGolyer,  The  theory  of  volcanic  origin  of  the  salt  domes,  Bull. 
137,  Am.  Inst.  Min.  Eng.,  1918,  pp.  987-1000. 


310  MINERAL  DEPOSITS 

Louisiana  and  the  adjoining  coast  belt  of  Texas;  they  show  many 
unusual  features  and  some  difficulty  has  been  experienced  in 
explaining  their  genesis.  Above  the  low  and  swampy  coast  west 
of  New  Orleans  rise  a  number  of  low  mounds  or  knolls  and  below 
these  most  of  the  salt  has  been  found.  It  does  not  occur  in 
regular  beds,  but  as  enormous  subterranean  domes,  surrounded  on 
all  sides  by  thick  and  often  steeply  dipping  beds  of  Quaternary 
and  Tertiary  clays  and  sands.  At  some  places  a  thin-bedded 
Cretaceous  limestone  appears  at  the  surface.  Fig.  102  gives  a 
suggestion  of  the  strange  relations  encountered.  At  Petite  Anse, 
according  to  Harris,  the  drill  shows  2,263  feet  of  almost  pure 
salt,  followed  by  70  feet  of  foreign  matter,  below  which  the  drill 
again  enters  rock  salt  of  unknown  thickness.  On  Cote  Carline 
the  drill  entered  salt  at  334  feet  and  continued  in  salt  without 
change  till  the  drilling  ceased  at  2,090  feet.  At  Belle  Isle1 
the  Knapp  Well  No.  1  penetrated  2.000  feet  of  salt  and,  below 
this,  236  feet  of  anhydrite  and  gypsum.  Another  well  at  Humble 
is  said  to  have  penetrated  5,410  feet  of  salt. 

Oil,  gas,  and  sulphur  are  often  met  in  the  drill-holes.  Gypsum 
and  anhydrite,  in  beds  200,  400,  or  even  600  feet  thick  cover 
the  salt  in  some  places,  or  again  the  salt  may  be  overlain  (as  at 
Spindletop,  Texas)  by  several  hundred  feet  of  a  porous  lime- 
stone carrying  oil.  The  dip  of  the  loose  strata  forming  the 
outside  of  the  dome  is  steep  and  bore-holes  only  a  short  distance 
from  those  disclosing  salt  may  fail  to  encounter  it.  Naturally 
the  published  sections,  based  on  a  few  bore-holes,  are  more  or 
less  problematical  as  to  structure. 

These  enormous  salt  resources  are  as  yet  little  utilized.  Rock 
salt  was  mined  in  1915  at  Weeks  Island,  where  the  shaft  is  645 
feet  deep,  and  at  A  very  Island,  at  a  depth  of  about  500  feet. 
The  shafts  are  sunk  in  heavy,  wet  ground  until  the  impermeable 
salt  is  reached.  In  places  there  is  considerable  danger  of  flood- 
ing the  mine  by  driving  into  the  loose  strata. 

According  to  R.  T.  Hill,  these  wonderful  salt  domes  are  de- 
posited by  ascending  solutions;  the  uplift  of  surrounding  strata 
is  caused  by  the  hydrostatic  pressure  of  salt  solutions  and  oil 
rising  through  fissured  rocks.  According  to  L.  Hager  and 
A.  C.  Veatch,  the  domes  are  uplifts  caused  by  laccolithic  intru- 
sions. According  to  G.  D.  Harris,  the  uplifts  are  produced  by 
the  expanding  power  of  growing  salt  crystals,  the  concentrated 

XA.  F.  Lucas,  Trans.  Am.  Inst.  Min.  Eng.,  vol.  57,  1917,  p.  1034. 


DEPOSITS  FORMED  BY  EVAPORATION         311 

solutions  rising  at  the  intersections  of  fissures;  the  salt  is  derived 
from  underlying  Paleozoic  or  Mesozoic  beds. 

None  of  these  views  are  fully  convincing.  A  brief  considera- 
tion of  the  relations  of  solubility  of  sodium  chloride  will  show 
that  only  a  very  small  quantity  could  have  been  precipitated  as 
the  temperature  of  the  ascending  solutions  was  lowered  and 
that,  therefore,  the  quantity  of  primary  salt  required  by  Harris' 
hypothesis  would  be  incredibly  large. 

Recent  literature  has  shown  the  existence  of  many  salt  domes 
along  the  coast  and  some  quite  a  distance  inland.1  The  same 
kind  of  salt  domes  are  also  found  on  the  isthmus  of  Tehuantepec 
back  of  Puerto  Mexico,2  accompanied  in  places  by  oil  and  gas. 

The  foreign  literature  indicates  that  such  salt  domes  also 
exist  in  northern  Germany  and  in  Transylvania.3 

Hopkins  shows  clearly  that  the  salt  dome  at  Palestine,  Texas, 
is  caused  by  a  highly  localized  vertical  uplift  of  quaquaversal 
form.  Rogers  and  DeGolyer  arrive  at  similar  conclusions.  It  is 
probable  that  these  domes  are  Permian  or  Carboniferous  salt 
beds  forced  up  through  the  softer  sediments.  This  is  made  pos- 
sible by  the  extraordinary  plasticity  of  rock  salt,  which  easily 
yields  to  deformation.  The  nature  of  the  force  producing  these 
uplifts  remains  in  doubt. 

Composition,  Production  and  Use. — Rock  salt  is  usually  very 
pure  aside  from  the  occasionally  occurring  admixture  of  clay, 
the  tenor  in  NaCl  ranging  from  96  to  99  per  cent.  Calcium 
sulphate  is  the  principal  impurity  and  is  often  present  to  the 
amount  of  over  1  per  cent.  Salt  from  some  desert  lakes  contains 
sodium  carbonate  and  sulphate.  In  1917,  1,605,000  short  tons 
of  rock  salt  was  mined  in  the  United  States.  The  total  produc- 
tion of  salt  for  the  same  year  was  nearly  7,000,000  short  tons. 
The  average  price  was  $2.86  per  ton. 

The  wide  range  of  uses  of  salt  for  culinary,  preservative, 
and  industrial  purposes  need  not  be  specified.  Large  amounts 
are  used  in  the  manufacture  of  other  sodium  salts,  particularly 
the  carbonate. 

In  1917,  450  tons  of  bromine  were  produced  in  Michigan,  Ohio, 
and  West  Virginia,  the  normal  price  is  50  cents  per  pound. 

1  O.  B.  Hopkins,  Bull.  616,  U.  S.  Geol.  Survey,  1917,  p.  28. 

2  Burton  Hartley,  Econ.  Geol.,  vol.  12,  1917,  pp.  581-588. 

3  F.  F.  Hahn,  Econ.  Geol.,  vol.  7,  1912,  pp.  120-135. 


312  MINERAL  DEPOSITS 

THE  GERMAN  POTASSIUM  SALTS1 

If  carried  to  its  conclusion  the  process  of  evaporation  of  sea 
water  will  result  in  the  deposition  of  the  easily  soluble  chlorides 
and  sulphates  of  potassium  and  magnesium,  also  chloride  of  cal- 
cium. Evidently  this  seldom  takes  place,  in  part  for  the  reason 
given  on  page  292.  Almost  the  only  locality  thus  far  discovered 
where  the  whole  sequence  of  salts  is  present  is  in  central  Ger- 
many, north  and  south  of  the  Harz  Mountains,  in  formations 
of  Permian  age.  These  deposits  are  now  mined  by  about  fifty 
companies  and  yielded  in  1913  about  twelve  million  tons  of 
potassium  salts,  of  which  85  per  cent,  is  used  as  soil  fertilizer  and 
the  remainder  for  general  industrial  purposes.  The  value  of  this 
production  is  about  $45,000,000.  The  mining  is  done  exclusively 
by  shafts  from  1,000  to  2,500  feet  deep.  Circular  shafts  lined 
with  concrete  or  iron  tubing  are  used  and  the  greatest  caution  is 
necessary  to  prevent  influx  of  water  during  sinking  or  working; 
if  the  water  once  breaks  in,  the  mine  will  probably  have  to  be 
abandoned.  The  old  workings  are  filled  with  waste  or  rock  salt. 

Some  of  the  products  are  sold  for  fertilizers  without  further 
chemical  treatment.  Carnallite  is  the  most  important  of  these, 
and  next  to  it  comes  kainite;  as  mined,  both  are  mixed  with 
30  or  40  per  cent,  of  common  salt.  Other  chemical  products 
from  carnallite  and  other  salts  are  chloride  and  sulphate  of 
potassium  and  potassium-magnesium  sulphate.  Kieserite  is 
refined  to  magnesium  sulphate. 

The  larger  part  of  the  bromine  production  of  the  world  is  also 
obtained  from  the  mother  liquor  resulting  from  the  solution  of 
the  Stassfurt  salts.  The  remaining  part  is  derived  from  the 
brines  of  Michigan. 

The  potassium  salts  lie  as  a  relatively  thin  series  of  strata 
over  a  heavy  bed  of  rock  salt  in  the  Permian  and  are  in  turn 
covered  by  Triassic  sandstones  and  limestones,  and  finally 
by  the  Tertiary  and  Quaternary  beds.  They  form  a  series  of 

1  H.  Precht,  Die  Salz  Industrie  von  Stassfurt,  1889. 

R.  Ehrhardt,  Die  Kali-Industrie,  1907. 

E.  Pfeiffer,  Handbuch  der  Kali-Industrie,  1887. 

Beyschlag,  Everding,  Erdmann,  Loewe,  and  Paxmann,  Deutschlands 
Kalibergbau,  1907. 

Summaries:  F.  W.  Clarke,  Geochemistry,  Bull.  616,  U.  S.  Geol.  Survey, 
1916,  pp.  221-228;  JG.  P.  Merrill,  Non-metallic  minerals,  1905;  R.  Meeks, 
Mineral  Industry,  1906.^ 


DEPOSITS  FORMED  BY  EVAPORATION 


313 


faulted  synclines  and  anticlines,  in  places  approaching  closely 
to  the  surface,  but  here  generally  changed  by  secondary  leach- 
ing processes  (Fig.  103). 

The  general  section  is  as  follows,1  counted  from  the  bottom  of 
the  Triassic  sandstone. 


No. 


10 


11 


Thickness  in 
meters 


20  to  30.  .  . 

50 

1  to5 

5  to  15.... 
100  to  150. 
40  to  90.  .  . 

4  to  10 

30  to  40... 


Character  of  strata 


20  to  40.. 
40  to  60 .  . 


300  to  500. 


Red  clay  with  a  little  anhydrite  and  rock  salt. 

Rock  salt. 

Anhydrite  with  salt. 

Red  clay  with  anhydrite  and  rock  salt. 

Younger  rock  salt. 

Main  anhydrite. 

Salt  clay. 

Carnallite    zone.      Carnallite     (KCl.MgCl,+ 

6H20). 

Kieserite  zone.     Kieserite  (MgSO4  +  H2O) . 
Polyhalite  zone.     Polyhalite  (2CaSO4.MgSO4. 

K2SO4  +  2H2O). 
Older    rock  salt,    with    narrow    streaks    of 

anhydrite,  interpreted  as  annual  deposits. 
Older  anhydrite. 
Limestone  (Zechstein  Kalk) .    Marine  deposit. 

(  Black  copper-bearing  shale. 

\  Conglomerate. 
Lower  Permian  and  Carboniferous  beds. 

The  older  series  (Noa.  8  to  11)  closed  with  deposition  of  potassium  salts. 
The  younger  series  (Nos.  1  to '7)  contains  no  potassium  salts. 


12  I  70  to  100. 

13  4  to  10... 

14 


0.5to4.. 


Rock  salt  is  really  present  throughout  the  section,  for  the  car- 
nallite  zone,  which  yields  the  greatest  quantity  of  crude  product, 
contains  only  about  55  per  cent,  of  carnallite,  including  also  25 
per  cent,  rock  salt  and  16  per  cent,  kieserite.  The  kieserite  zone 
yields  65  per  cent,  rock  salt  and  only  17  per  cent,  kieserite. 
Rock  salt,  kieserite,  sylvite  (KC1),  carnallite  (KCl.MgCl2+ 
6H20),  and  kainite  (KCl.MgS04+3H2O)  are  the  main  products. 

The  German  geologists  have  shown  that  extensive  secondary 
changes  have  taken  place  in  the  salt  beds — in  part  immediately 
after  the  deposition,  in  part  much  later,  following  the  Triassic 
sedimentation  and  progressing  even  now.  These  post-Triassic 

XH.  Everding,  Deutschlands  Kalibergbau,  1907,  p.  36. 


314 


MINERAL  DEPOSITS 


changes  have  occurred  both  in  the  croppings  and  at  greater 
depth.  The  minerals  just  enumerated  may  occur  in  all  three 
generations,  but  in  addition  a  large  number  of  more  or  less  com- 
plicated secondary  compounds  were  formed. 

Kainite  is  in  part  a  secondary  product  derived  from  carnallite 
by  the  leaching  of  MgCl2.  It  forms  along  the  crests  of  the 
anticlines.  Under  some  circumstances  a  secondary  mixture  of 


Kochstedt 


0              1 

2 

3           4  Kilometers 

1.  Tertiary  beds 
2.  Triassic  shales  (Keuper) 
3,  Triassic  limestone  (MuscheikalK) 
4.  Upper  sandstone) 
5.  Middle        "         >  Buntsandstem 
6.  Lower        "        J 
7.  Upper  clay 
8.  Secondary  salt 
9.  Younger  salt  beds 
10.  Main  anhydrite 

PERMIAN  TRIASSIC  w 

12 
13. 
14. 
15. 

i& 
17. 

Salt  clay 
Potassium  salts 
Older  salt  beds 
Permian  limestone  (Zechstein) 
Upper  Permian   sandstone 
(  Rotliegendes) 

boniferous  (  Rotliegendes) 

niferous 

FIG.  103.— Section  of  the  Stassfurt-Egeln  anticline.     After  Everding. 

potassium  chloride,  kieserite,  and  salt  would  be  formed  instead 
of  kainite,  and  this  constitutes  an  important  product  under  the 
name  of  "hartsalz."  Secondary  deposits  of  the  older  type  ap- 
pear between  the  carnallite  and  the  salt  clay.  In  all  these 
transformations  the  products  are  very  complex. 

Van't  Hoff1  and  his  associates  have  studied  the  various  com- 

1  Van't  Hoff,  Die  ozeanischen  Salzablagerungen,  1905  and  1909. 


DEPOSITS  FORMED  BY  EVAPORATION         315 

binations  of  salts  in  order  to  ascertain  their  fields  of  existence  at 
temperatures  ranging  from  25°  to  83°  C.  In  this  way  they  have 
arrived  at  the  temperatures  of  stability  of  the  various  salts  and 
consequently  ascertained  the  minimum  temperature  at  which 
they  were  formed.  Sylvite  and  carnallite  are  stable  in  concen- 
trated NaCl  solutions  from  0°  to  85°  C.  Many  of  the  rarer  salts 
(langbeinite  (2MgS04.K2S04),  for  instance)  are  stable  under 
these  conditions  only  from  37°  C.  upward. 

Some  of  the  temperatures  required  may  seem  high;  kieserite 
with  sylvite,  for  instance,  forming  above  72°  C.  There  is,  indeed, 
some  danger  in  using  the  laboratory  results  to  explain  the  proc- 
esses of  nature,  for  the  important  element  of  time  probably 
plays  a  considerable  part  in  these  reactions. 

It  has  been  shown  by  measurements,  however,  that  the  tem- 
perature in  the  middle  depths  of  evaporating  salt  lakes  is 
surprisingly  high.  Interesting  results  were  obtained  by  A.  V. 
Kaleczinsky,1  who  found  the  temperatures  of  certain  Hungarian 
salt  lakes  to  be  as  much  as  71°  C.  during  the  summer  at  a  depth 
of  1.3  meters,  while  the  surface  and  the  bottom  layers  were 
much  cooler,  about  20°  C. 

OTHER  SOURCES  OF  POTASSIUM   SALTS 

The  exhausted  agricultural  lands  of  all  countries  need  potas- 
sium salts,2  together  with  phosphates  and  nitrogen  compounds. 
Germany  is  the  only  country  in  which  potassium  salts  in  easily 
available  form  occur  on  a  large  scale.  The  imports  of  the 
United  States  in  1913  amounted  to  about  1,800,000  metric  tons 
while  the  highly  stimulated  domestic  production  since  the  war 
broke  out  amounts  to  but  20  per  cent,  of  the  former  consump- 
tion. This  brief  statement  indicates  the  acute  situation. 

1  A.  V.  Kaleczinsky,  Ref.  Ann.  Phys.  (4),  7,  1902. 

2W.  C.  Phalen,  Potash  salts,  Mineral  Resources,  U.  S.  Geol.  Survey, 
annual  publication,  1910-16  (.with  literature). 

W.  C.  Phalen,  Occurrence  of  potash  salts  in  thejbitterns  of  the  eastern 
United  States;  Bull.  530,  U.  S.  Geol.  Survey,  1911.  " 

H.  S.  Gale,  The  search  for  potash  in  the  United  States;  Bull.  530, 
U.  S.  Geol.  Survey,  1911.  Also  in  Bull.  580,  U.  S.  Geol.  Survey,  1914, 
pp.  265-317. 

A.  R.  Schultz  and  Whitman  Cross,  Potash-bearing  rocks  of  the  Leucite 
Hills,  Wyo.;  Bull.  512,  U.  S.  Geol.  Survey,  1912. 

B.  S.   Butler  and  H.  S.  Gale,  Alunite,  Bull.  511,  U.  S.  Geol.  Survey, 
1912. 


316  MINERAL  DEPOSITS 

Promising  beds  of  potassium  salts  similar  to  those  of  Ger- 
many have  been  discovered  in  the  Oligocene  of  Alsace.  Other 
deposits  occur  in  Tyrol,  Spain,  Galicia  and  India,  but  none  of 
these  have  as  yet  contributed  to  the  world's  supply.  An  active 
search  for  potassium  salts  has  been  carried  on  in  this  countoy 
since  1910 

Potassium  in  Rocks  and  Minerals. — Granites,  pegmatites, 
some  phonolites,  and  some  leucite  rocks  contain  a  considerable 
amount  of  potassium,  varying  from  5  to  12  per  cent.  Unfor- 
tunately there  is  great  difficulty  in  transforming  the  insoluble 
potassium  silicates  contained  in  the  orthoclase,  leucite,  or  glassy 
base  in  these  rocks  into  soluble  salts.  Some  pegmatite  dikes, 
composed  largely  of  orthoclase,  yield  even  more  than  12  per 
cent,  of  potash.  If  orthoclase  or  any  potassium-bearing  rock  is 
ground  to  fine  powder  and  slimed  with  water  a  certain  small  per- 
centage of  potash  salt  is  converted  into  soluble  form,  probably 
as  a  potassium  silicate,  and  it  is  said  that  such  finely  ground 
powder  has  some  slight  fertilizing  power.  Processes  have  been 
patented  by  A.  S.  Cushman  and  others  based  on  electrolytic 
treatment  of  the  slimed  rock  or  treatment  with  quicklime  and 
calcium  chloride  and  subsequent  calcining,  methods  by  which 
soluble  potassium  salts  are  said  to  be  set  free.  None  of  these 
processes  has  thus  far  been  applied  on  a  large  scale. 

Greensand  marls — for  instance,  the  Cretaceous  beds  in  New 
Jersey — contain  from  3  to  6  per  cent,  of  potash  besides  some 
phosphoric  acid,  the  former  in  glauconite,  the  latter  in  cal- 
cium phosphate.  These  marls  are  used  in  their  crude  state  as 
fertilizers  and  the  recovery  in  soluble  form  of  their  potassium 
content  has  been  proposed,  but  the  practical  application  has 
not  as  yet  been  attempted. 

Another  source  of  potassium  has  been  sought  in  the  mineral 
alunite1,  which  is  a  hydrous  potassium-aluminum  sulphate  of 
inconspicuous  appearance,  white  or  pink,  compact  or  fine  granu- 
lar, rarely  coarse  granular.  The  formula  of  the  mineral  is 
K2O.3Al203.4S03.6H20;  and  it  contains  from  8  to  11  per  cent, 
of  potash.  It  occurs  mainly  in  volcanic  regions,  as  a  product  of 
rock  alteration,  probably  caused  by  waters  containing  sulphuric 
acid,  and  is  much  more  common  than  the  soluble  natural  alum 
which  sometimes  appears  as  efflorescences.  The  alunite  is 
found  disseminated  in  the  rocks  or  in  well-defined  veins.  Nota- 
ble western  occurrences  are  at  Goldfield,  Nevada;  Marysvale, 


DEPOSITS  FORMED  BY  EVAPORATION         317 

Utah;  and  the  Rosita  Hills,  Colorado.  Among  the  foreign  de- 
posits which  have  already  been  utilized  are  those  at  La  Tolfa, 
in  Italy;  Almeria,  in  Spain;  and  Bullah  Delah,  in  New  South 
Wales.  The  transformation  of  alunite  into  soluble  potassium 
sulphate  is  easily  effected  by  calcination;  part  of  the  sulphuric 
acid  and  all  of  the  water  is  volatilized,  leaving  soluble  potas- 
sium sulphate  and  an  insoluble  residue  of  alumina.  The  manu- 
facture of  potassium  sulphate  from  alunite  began  at  Marysvale  in 
1915. 

Potassium  in  Brines. — Potassium  salts  are  easily  soluble  and 
therefore  remain  with  calcium  and  magnesium  chlorides  in  the 
last  residues  or  mother  liquors,  the  so-called  "bitterns."  Many 
natural  brines  pumped  from  bore-holes  in  salt-bearing  beds  con- 
tain some  potassium  and  under  favorable  circumstances  may  be 
used  for  the  recovery  of  these  salts.  Some  of  the  Michigan 
brines  from  the  Marshall  formation  of  the  lower  Carboniferous 
(Fig.  92)  contain  from  3  to  5  grams  per  liter  of  potassium  chlo- 
ride; salt,  calcium  chloride,  and  bromine  are  recovered  from 
these  brines,  but  their  potassium  content  appeals  to  be  too  small 
for  profitable  recovery.  In  places  certain  well-defined  strata 
yield  natural  brines  or  residual  "bitterns."  One  such  bittern 
from  Fairport  Harbor,  in  Ohio,  on  Lake  Erie,  contains,  accord- 
ing to  W.  C.  Phalen,  in  grams  per  liter,  7.4  KC1,  110.1  NaCl, 
134.4  CaCl2,  43.2  MgCl2.  Such  a  brine  could  possibly  be  utilized 
for  the  recovery  of  potassium.  This  stratum  is  almost  400  feet 
above  the  topmost  salt  bed  from  which  artificial  brines  are 
pumped  in  Ohio. 

Lakes  in  dry  regions,  especially  in  areas  of  former  volcanic 
activity,  contain  appreciable  quantities  of  potassium.  The 
water  of  Owens  Lake,  in  eastern  California,  yields  almost  3  grams 
of  potassium  chloride  per  liter.  Both  soda  and  potash  are  now 
recovered  from  the  new  plant  at  this  locality. 

Evaporation  in  the  Quaternary  lakes  of  the  Lahontan  basin 
in  Nevada  and  California  has  at  many  places  resulted  in  deposits 
of  salt  of  moderate  thickness.  Changes  in  drainage  among  these 
basins  sometimes  resulted  in  the  residual  brines,  /richer  in 
potash,  being  drawn  off  into  a  neighboring  depression,  and  thus 
it  happens,  as  at  Searles  Marsh,  in  San  Bernardino  County, 
California,  that  the  salt  bed,  which  here  is  almost  60  feet  in 
thickness  and  covers  an  area  of  at  least  11  square  miles,  is 
saturated  with  a  strong  brine  unusually  rich  in  potassium.  In 


318  MINERAL  DEPOSITS 

the  dissolved  salts  of  Searles  Marsh  there  is  almost  7  per  cent. 
of  K20.  A  large  plant  is  now  under  construction  for  the 
recovery  of  potash  at  this  place. 

Some  small  lakes  in  the  "sand  hill  country"  of  Nebraska  and 
Colorado  contain  a  remarkably  high  percentage  of  potassium 
which  it  is  thought  may  have  become  concentrated  from  the 
potash  resulting  from  repeated  burnings  of  the  grass  in  the  sur- 
rounding country.  The  Nebraska  lakes  now  (1917)  yield  about 
one-half  of  the  potash  production  of  the  United  States. 

The  earliest  source  of  potassium  was,  as  is  well  known,  ashes 
of  vegetable  matter.  Seaweed  is  especially  rich  in  this  metal 
and  also  contains  iodine.  Potash  salts  are  now  also  recovered 
from  the  flue  dust  of  cement  plants. 

Bromine  and  Calcium  Chloride. — The  very  soluble  salts  of 
bromine  and  calcium  chloride  are  recovered  from  residual  salt 
brines  in  Michigan  and  West  Virginia.  The  principal  production 
before  the  war  came  from  the  Stassfurt  salts  of  Germany. 


CHAPTER  XVIII 

MINERAL  DEPOSITS  RESULTING  FROM  PROCESSES  OF 
ROCK  DECAY  AND  WEATHERING1 

GENERAL  CONDITIONS 

The  uplifted  sedimentary  beds,  the  lavas  of  the  volcanoes,  the 
granular  crystalline  rocks  uncovered  by  erosion — all  these,  when 
exposed  at  the  surface  of  the  earth  are  subject  to  a  series  of 
changes,  the  sum  total  of  which  is  called  weathering.  The  agents 
are  water,  air,  heat,  and  vegetable  and  animal  life.  Water  is 
essential — without  it  very  little  decomposition  could  take  place. 
Oxygen  is  also  essential,  and  indeed  we  often  speak  of  weathering 
as  synonymous  with  oxidation.  Carbon  dioxide  dissolved  in 
water  decomposes  the  minerals  and  hastens  the  process  of  solu- 
tion. Change  of  temperature  acts  mainly  by  promoting  disin- 
tegration, most  powerfully  by  the  expansion  of  water  when 
freezing  in  cracks  and  crevices,  a  force  sufficient  to  break  and 
dislodge  heavy  rock  masses.  Vegetable  life  furnishes  carbon 
dioxide  and  disintegrates  the  soil  by  the  vital  energy  in  the  roots, 
and  bacterial  life  changes  its  composition.  Animals  burrow  in 
the  ground,  loosening  it  and  effecting  chemical  changes. 

Weathering  differs  from  alteration  and  metamorphism  in  that 
its  ultimate  result  is  the  destruction  of  the  rock  as  a  unit;  its 
chemical  processes  are  far  more  radical  and  intense  than  those  of 
the  depths.  Weathering  effaces  the  texture  of  the  rocks  and 
segregates  their  chemical  compounds  in  ways  wholly  different 
from  those  of  the  other  processes  mentioned.  Metals  closely 
associated  in  the  primary  rocks  part  company  and  seek  new 
associates.  Segregations  of  large  masses  of  single  minerals 
are  usually  a  result  of  the  process.  The  ordinary  silicates  and 

1  N.  S.  Shaler,  The  origin  and  nature  of  soils,  Twelfth  Ann.  Rept.,  U.  S. 
Geol.  Survey,  pt.  1,  1891. 

G.  P.  Merrill,  Rocks,  rock-weathering,  and  soils,  1897. 
C.  R.  Van  Hise,  Metamorphism,  Mon.  47,  U.  S.  Geol.  Survey,  1904. 
F.  K.  Cameron  and  J.  M.  Bell,  The  mineral  constituents  of  the  soil  solu- 
tion, Bull.  30,  Bureau  of  Soils,  Washington,  D.  C.,  1905.  j 

T.  L.  Lyon,  E.  O.  Fippin  and  H.  O.  Buckman,  Soils,  New  York,  1916, 
p.  764. 

319 


320  MINERAL  DEPOSITS 

the  carbonates  of  iron,  magnesium,  and  calcium  are  unstable  in 
the  belt  of  weathering.  The  uppermost  thin  mantle  of  the 
products  of  weathering  we  call  the  soil;  in  it  the  disintegration 
and  chemical  changes  are  carried  to  their  limit;  it  is  mixed 
with  the  products  of  life,  and  its  constituents  and  reactions  are, 
of  course,  of  more  interest  to  the  agricultural  chemist  than  to 
the  student  of  ore  deposits. 

The  depth  to  which  weathering  extends  differs  greatly;  in  some 
desert  regions,  recently  glaciated  areas,  or  areas  covered  by 
fresh  lava  flows  it  is  practically  absent,  disintegration  being  the 
only  visible  effect.  In  regions  of  heavy  vegetation  and  rainfall 
the  weathering  may  extend  to  a  depth  of  100  or  even  200  feet; 
along  fractures  in  particularly  permeable  and  soluble  rocks  like 
limestone  oxidation  may  be  carried  to  still  greater  depth;  in 
mineral  deposits  its  effects  are  in  places  felt  for  several  hundred 
or  in  extreme  cases  as  much  as  2,000  feet.  As  a  rule,  however, 
weathering  does  not  extend  deeper  than  50  feet,  and  its  more 
intense  effects  are  usually  limited  to  the  zone  above  the  surface 
of  underground  water. 

Disintegration  and  decomposition  work  together,  but  the 
former  is  likely  to  extend  deeper  than  the  latter.  The  upper 
layers,  ordinarily  colored  red  or  brown  by  ferric  iron,  gradually 
change  into  paler-colored,  more  or  less  softened  and  disintegrated 
rock.  In  some  areas,  notably  over  limestone  strata,  there  is  a 
sharp  change  to  the  unaltered  rock — so  sharp,  indeed,  that  the 
red  clayey  soil  has  often  been  taken  for  a  different  formation 
resting  on  the  calcareous  rock. 

Erosional  transportation  attends  disintegration,  and  removal  of 
material  by  solution  accompanies  decomposition,  both  tending 
strongly  to  reduce  the  volume  of  the  rock.  On  the  other  hand, 
hydration  and  the  peculiar  quality  of  adsorption  which  the  soils 
possess  tend  to  increase  the  volume.  On  the  whole  weathering 
lessens  the  volume.  According  to  G.  P.  Merrill  the  granites  of 
the  District  of  Columbia  may  lose  by  weathering  13.5  per  cent, 
of  their  volume;  T.  L.  Watson  calculates  the  loss  of  granites  of 
Georgia  at  7  to  72  per  cent.  The  most  marked  loss  is  the  shrink- 
ing in  residual  clays  derived  from  limestone;  often  it  is  more  than 
95  per  cent.  Whitney  long  ago  arrived  at  the  conclusion  that 
1  meter  of  residual  clay  in  Wisconsin  was  derived  from  a  thick- 
ness of  35  to  40  meters  of  limestone  or  shale. 

Except  in  the  easily  soluble  rocks  the  decomposition  is  never 


ROCK  DECAY  AND  WEATHERING  321 

complete,  for,  as  brought  out  by  Cameron  and  Bell,  even  in 
the  fine  soils  abundant  grams  of  the  original  minerals  remain 
unaltered.  Other  conditions  being  equal,  weathering  is  most 
complete  in  tropical  and  moist  countries.  In  the  United  States 
the  most  intense  action  of  this  kind  has  taken  place  in  the  Ap- 
palachian region  south  of  the  glaciated  area,  and  this  region 
contains  the  majority  of  ore  deposits  caused  by  weathering. 

Air  contains  approximately  by  volume  21  per  cent,  oxygen, 
slightly  less  than  79  per  cent,  nitrogen  (with  argon),  and  0.03 
per  cent,  carbon  dioxide.  In  the  air  contained  in  rain  water 
both  oxygen  and  carbon  dioxide  are  greatly  concentrated.  In 
the  soils  carbon  dioxide  and  air  are  absorbed;  soils  and  clays  of 
various  kinds  contain  from  14  to  40  cubic  centimeters  of  gas  per 
100  grams,  with  14  to  34  per  cent,  of  carbon  dioxide  and  consid- 
erably less  oxygen  than  the  air — indeed,  in  some  soils  oxygen 
appears  to  be  absent.1  Decaying  vegetation  still  further  in- 
creases the  percentage  of  carbon  dioxide.  As  the  ground-water 
level  is  approached  the  oxygen  decreases  rapidly,  as  shown  by 
the  measurements  made  by  B.  Lepsius2  in  bore-holes,  and  below 
this  level  there  is  probably  little  left. 

Naturally  the  processes  of  weathering  are  hastened  by  the 
presence  of  sulphuric  acid  derived  from  the  decomposition  of 
pyrite  or  exhaled  from  solfataric  vents.  W.  Maxwell3  has  shown 
interestingly  how  extensive  a  part  this  acid  plays  in  the  develop- 
ment of  soils  on  the  slopes  of  volcanoes. 

The  processes  characteristic  of  weathering  are  oxidation,  hy- 
dration,  and  solution.  In  the  surface  waters  calcium  and  mag- 
nesium carbonates  ordinarily  prevail,  with  a  considerable  amount 
of  alkaline  carbonates  and  relatively  much  soluble  silica,  both 
derived  from  the  decomposition  of  the  silicates.  Under  special 
conditions,  as  in  volcanic  regions  or  in  sediments  rich  in  salts, 
the  surface  waters  may  be  materially  different  in  composition, 
being  predominantly  sulphate  solutions.  The  ground  waters 
contain  in  addition  small  amounts  of  iron  and  manganese, 
carried  mainly  as  bicarbonates,  also  phosphoric  acid  and  sodium 
chloride. 

In  the  weathered  zone  will  remain  the  residual,  almost  insol- 

1  CameroA  and  Bell,  op.  cit.,  p.  26. 

2  Quoted  in  F.  W.  Clarke,  Geochemistry,  Bull.  616,  U.  S.  Geol.  Survey, 
1916,  p.  477. 

3  W.  Maxwell,  Lavas  and  soils  of  the  Hawaiian  Islands,  Honolulu,  1898. 


322  MINERAL  DEPOSITS 

uble  minerals,  like  quartz,  hydrated  aluminum  silicates  more 
or  less  closely  approaching  kaolinite  in  composition,  ferric  oxides 
(as  limonite,  gothite,  or  hematite),  and  manganese  dioxide,  all 
mingled  to  form  a  red  or  brown  clayey  soil. 

All  these  reactions  involve  the  development  of  colloid  bodies 
like  aluminum  silicates  and  hydroxides  of  iron,  which  before  their 
transformation  into  crystalline  minerals  are  characteristic 
absorbents  of  many  salts.  The  colloids  of  manganese,  for 
instance,  have  a  tendency  to  adsorb  potassium  and  barium. 
The  zone  of  weathering  has  indeed  been  called  the  realm  of  the 
colloids. 

DECOMPOSITION  OF  MINERALS 

The  silicates  of  the  rocks  are  decomposed  by  water  rather 
than  dissolved,  for  the  resulting  solution  does  not  usually  contain 
the  elements  in  the  same  proportions  as  the  original  mineral. 
Owing  to  hydrolysis  the  solution  in  most  cases  gives  an  alkaline 
reaction.1 

Cameron  and  Bell  believe  that  the  more  rapid  decomposition 
shown  to  take  place  in  the  presence  of  CC>2  is  in  large  part  due 
to  its  combination  with  the  hydrolyzed  bases  by  the  formation 
of  bicarbonates  rather  than  to  direct  solvent  action.  Orthoclase, 
for  instance,  would  give  KAlSi3Os+HOH  =  KOH+HAlSi3O8, 
and  the  carbon  dioxide  would  form  potassium  bicarbonate  with 
KOH.  Kaolin  would  form  from  the  unstable  silicate  HAlSi3O8. 

The  biotite,  amphibole,  and  pyroxene,  perhaps  previously 
altered  to  chlorite  below  the  water-level,  break  up  into  soluble 
earthy  carbonates,  with  limonite,  hydrous  aluminum  silicate,  and 
silica,  the  latter  three  in  colloidal  state.  These  ferromagnesian 
minerals  are  attacked  first,  so  that  the  ordinary  surface  waters 
contain  more  of  the  carbonates  of  calcium  and  magnesium  than  of 
other  salts.  The  soda-lime  feldspars  come  next  while  the  alkali 
feldspars  are  more  resistant.  Quartz  is  only  partially  attacked. 
The  decomposition  of  orthoclase  is  usually  expressed  in  the 
following  equation: 


The  ultimate  product  is  kaolin,  or  allied  colloidal  bodies. 
Muscovite  or  sericite  do  not  result  from  weathering,  although  the 
colloidal  aluminum  silicate  may  adsorb  potassium  'and  form 
amorphous  compounds  related  in  composition  to  the  white  micas. 

i  F.  W.  Clarke,  Bull.  167,  U.  S.  Geol.  Survey,  1900,  p.  156. 


ROCK  DECAY  AND  WEATHERING  323 

Zeolites  are  undoubtedly  unstable  in  the  zone  of  weathering. 
Muscovite  or  sericite  is  slowly  attacked;  Cameron  and  Bell1 
treated  2  grams  of  powdered  muscovite  with  a  liter  of  pure  water 
for  14  months  in  paraffine  cylinders  and  obtained  in  the  solution 
10.4  parts  K  per  million;  when  treating  it  with  carbon  dioxide 
in  water  they  obtained  18.3  parts  per  million.  The  same  quan- 
tity of  orthoclase  with  pure  water  yielded  a  solution  with  1.7 
parts  per  million  of  K;  when  saturated  with  COg  it  yielded 
2.5  parts  per  million.  Muscovite  thus  yields  its  potassium 
more  easily  than  orthoclase.  Albite  treated  in  the  same  way 
gave  1.0  and  1.1  parts  of  sodium  per  million  respectively.  Earlier 
experiments  leading  to  similar  results  have  been  undertaken  by 
A.  Daubree,  R.  Miiller,  A.  S.  Cushman  and  F.  Henrich.2 

Magnetite  is  soluble  with  difficulty,  but  finally  yields  hematite 
and  limonite.  Pyrite  easily  yields  limonite  and  sulphuric  acid. 
Apatite  appears  to  be  rather  easily  soluble,  especially  in  car- 
bonated water.  Cameron  and  Bell3  treated  powdered  chlo- 
rine apatite  suspended  in  water  at  25°  C.  for  7  days,  passing  C02 
through  the  liquid.  The  solution  showed  0.0378  gram  CaO, 
0.0096  gram  P206,  and  0.0026  gram  hydrochloric  acid  per  liter 
of  solution.  In  soils  and  clays  the  phosphates  are  decomposed 
or  hydrolyzed,  soluble  phosphates  being  formed,  but  the  per- 
colating water  contains  these  only  in  minimal  quantities.  It  is 
stated4  that  humus  suspended  in  water  can  adsorb  calcium  and 
a  considerable  amount  of  phosphoric  acid  from  the  calcium 
phosphates. 

The  reactions  of  the  iron  phosphates  are  in  the  main  similar  to 
those  of  the  calcium  salts.  Lachowicz6  found  that  organic  matter 
in  soil  is  a  distinct  solvent  for  ferric  phosphate.  Cameron  and 
Bell  ascertained  that  carbon  dioxide  greatly  aided  the  solu- 
tion of  ferric  phosphate,  5  grams  of  which,  shaken  for  5  days 
with  100  c.c.  H20  and  later  with  100  c.c.  of  saturated  solutions 

1  Op.  cit.,  p.  33. 

2  A.  Daubre"e,  fitudes  synthe"tiques  de  ge"ologie  experimentale. 
i  R.  Miiller,  Jahrb.  K.  k.  geol.  Reichsanstalt,  vol.  27,  1887. 

F  A.  S.  Cushman,  Bull.  92,  Bureau  Chemistry,  U.  S.  Dept.  Agr.,   1905. 
F.  Henrich,  Ueber  die  Einwirkung  von  Kohlensaurehattigen  Wasser 
auf  Gesteine  Zeitschr.  prakt.  Geol,  1910,  pp.  84-94. 

3  Bull.  41,  Bureau  of  Soils,  U.  S.  Dept.  Agr.,  1907. 

4  Patten  and  Waggaman,  Absorption  by  soils,  Bull.  52,  Bureau  of  Soils, 
U.  S.  Dept.  Agr.,  1908. 

6  Gesteins  und  Bodenkunde,  1877,  p.  329. 


324  MINERAL  DEPOSITS 

of  C02  and  CaS04,  yielded  respectively  74,  171,  and  118  mil- 
ligrams of  phosphoric  acid. 

Zircon,  pyrope  garnet,  tourmaline,  and  similar  minerals  are 
almost  insoluble. 

Quartz  also  shows  great  resistance  and  appears  practically 
insoluble  in  the  zone  of  weathering,  except  when  exposed  to  the 
action  of  a  stronger  solution  of  alkaline  carbonates.  C .  W.  Hayes, 1 
M.  L.  Fuller,2  and  C.  H.  Smyth3  have  observed  a  marked  corro- 
sion of  quartz  pebbles  in  conglomerates,  but  the  exact  nature  of 
the  reaction  is  uncertain.  Cherty  and  fine-grained  quartz  is  a 
little  more  soluble.4  The  theory  of  the  origin  of  the  Lake  Superior 
iron  ores,  supported  by  Van  Hise  and  Leith,  is  based  on  the 
supposed  solubility  of  such  material.  It  was  formerly  thought 
that  certain  organic  acids  had  the  power  of  dissolving  quartz,  but 
this  is  now  considered  very  questionable. 

TOTAL  CHEMICAL  CHANGES  BY  WEATHERING 

The  studies  and  analytical  work  of  G.  P.  Merrill  have  greatly 
advanced  our  knowledge  of  weathering,  and  many  others  have 
contributed  valuable  data.  A  compilation  of  a  number  of 
characteristic  gradational  analyses  is  given  in  F.  W.  Clarke's 
"Data  of  geochemistry"6  and  allows  an  estimate  of  the  final 
effects  of  weathering.  The  analyses  show  consistently  an  appar- 
ent increase  in  alumina  and  water  and  decreases  in  SiOg,  CaO, 
MgO,  K2O,  and  Na2O;  in  short  the  composition  tends  toward  that 
of  a  ferruginous  kaolin,  except  that  in  the  weathering  of  acidic 
rocks  residual  quartz  prohibits  the  decrease  of  silica  to  the 
amount  characteristic  of  kaolin.  Comparing  equal  volumes  we 
find  little  actual  change  in  the  quantity  of  alumina,  and  for 
purposes  of  comparison  this  oxide  is  assumed  to  be  constant. 
By  recalculating  the  analyses  on  this  basis,  the  percentage  lost 
or  gained  by  each  constituent  may  be  ascertained.  In  the 
analyses  quoted,  the  loss  of  silica  is  the  largest,  varying  from  8  to 
32  per  cent,  by  weight  of  the  original  rock  and  from  15  to  52  per 
cent,  of  the  original  quantity  of  silica.  The  abstraction  of  silica 

1  Bull.  Geol.  Soc.  Am.,  vol.  8,  1897,  p.  213. 

2  Jour.  Geology,  vol.  10,  1902,  p.  815. 

3  Am.  Jour.  Sci.,  4th  ser.,  vol.  19,  1905,  p.  277. 

4  G.  Lunge  and  C.  Millberg,  Zeitschr.  angew.  Chemie,  1897,  p.  393. 

6  Butt.  616,  U.  S.  Geol.  Survey,  1916,  pp.  486-490.  See  also,  C.  K.  Leith 
and  W.  J.  Mead,  Metamorphic  geology,  New  York,  1915,  pp.  3-24. 


ROCK  DECAY  AND  WEATHERING  325 

as  soluble  silicates  is,  then,  the  dominant  factor  in  the  weathering 
of  silicate  rocks. 

Compared  with  the  decrease  in  silica,  the  losses  of  bases  in 
silicate  rocks  are  small.  Calcium,  magnesium,  sodium,  and  potas- 
sium are  removed,  but  the  loss  is  ordinarily  only  partial.  The 
leaching  of  both  potassium  and  sodium  is  characteristic  and  is 
markedly  different  from  certain  processes  in  the  alteration  of 
wall  rocks  of  ore  deposits,  where  sodium  is  completely  removed 
and  potassium  enriched. 

The  analyses  quoted  show  that  from  one-seventh  to  one-fifth  of 
the  iron  oxides  are  carried  away.  The  water  invariably  increases. 

RESIDUAL  CLAY1 

Occurrence. — The  residual  clays,  as  might  be  expected,  are 
found  mainly  where  decay  of  rocks  has  proceeded  unchecked  for 
a  long  time  and  where  the  products  have  not  been  swept  away 
by  strong  erosion  or  by  glacial  action. 

Such  clays  are  found  in  all  parts  of  the  world;  in  the  United 
States  they  occur  chiefly  in  the  southern  Appalachian  region. 
Acidic  granular  rocks  like  granite  and  gneiss — particularly  those 
rich  in  feldspar  and  poor  in  ferromagnesian  silicates,  like  peg- 
ma'tite  dikes — yield  the  best  and  purest  clays,  but  even  these  mast 
often  be  purified  by  washing  in  order  to  remove  residual  quartz 
and  limonite.  At  varying  depths,  100  feet  at  most,  these  clays 
gradually  change  into  unaltered  rocks. 

Uses  and  Properties. — The  ordinary  varieties  of  residual  clays 
are  used  for  brickmaking,  the  purer  for  fire-bricks,  the  finer 
grades  for  pottery;  for  the  last  use  the  deposits  of  the  United 

1  The  literature  of  clays  is  exceedingly  voluminous.  For  information 
more  detailed  than  can  be  given  here,  consult: 

H.  Hies,  Clays,  occurrence,'properties,  and  uses,  1908. 

H.  Ries,  A  review  of  the  theories  of  origin  of  white  residual  kaolins, 
Trans.  Am.  Ceramic  Soc.,  vol.  13,  1911. 

I.  E.  Sproat,  Refining  and  utilization  of  Georgia  kaolins,  Bull.  128, 
U.  S.  Bureau  of  Mines,  1916. 

H.  O.  Buckman,  The  chemical  and  physical  processes  involved  in  the 
formation  of  residual  clay,  Trans.  Am.  Ceramic  Soc.,  vol.  13,  1911. 

J.  H.  Watkins,  White-burning  clays  of  the  southern  Appalachian  states, 
Trans.  Am.  Inst.  Min.  Eng.,  vol.  51,  1916,  pp.  481-501. 

A  full  bibliography  of  the  older  literature  by  H.  Rosier  is  contained  in 
Neues  Jahrb.,  Beil.  Bd.  15,  1902,  p.  231. 


326 


MINERAL  DEPOSITS 


States  are  insufficient,  hence  large  quantities  are  imported  from 
England.  These  purer  grades  of  white  burning  clays  are  usually 
called  kaolin,  ball  clay,  paper  clay  or  china  clay,  and  are  also 
used  in  manufacturing  paper  and  as  fillers  in  paints,  putty  and 
crayons.  About  200,000  tons  of  these  fine  clays  are  produced 
in  the  United  States,  the  average  price  being  $8  per  ton.  Their 
composition^before  and  after  washing,  is  indicated  by  the  follow- 
ing analyses. 

ANALYSES  OF  CRUDE  AND  WASHED  KAOLIN,  WEBSTER  COUNTY, 
SOUTH  CAROLINA 


Crude 

Washed 

Kaolinite 

Si02  
A1203  
Fe2O3  

62.40 
26.51 
1.14 

45.78 
36.46 
0.28 

46.5 
39.5 

FeO 

1  08 

CaO 

0  57 

0  50 

MgO 

0  01 

0  04 

Alkalies 

0  98 

0  25 

H2O  
Moisture  

8.80 
0.25 

13.40 
2.05 

- 

14 

Total  
Clav  substance  .  .  . 

100.66 
66.14 

99.84 
93.24 

100 

It  is  sometimes  difficult  to  determine  with  the  microscope  the 
particles  of  kaolinite  (H4Al2Si209)  in  an  altered  rock,  on  account 
of ,  their  minute  flaky  size  and  low  double  refraction.  It  is 
probable  that  kaolinite  is  present  in  the  residual  clays,  but 
besides  this  well-defined  mineral  there  may  be  other  hydrated 
silicates  of  alumina  separated  in  colloidal  form,  as  gels,  during  the 
weathering.  Among  these  hardened  gels  there  are  some  highly 
hydrated  forms  like  halloysite  and  allophanite  which  are  de- 
composed by  HC1.  Others  corresponding  approximately  to 
kaolinite  are  only  attacked  by  H2SO4.  Both  classes  occur  in 
sediments  as  well  as  in  residual  deposits.  Paper  clays  are 
mined  in  Cretaceous  beds  of  South  Carolina  and  Georgia, 
plastic  kaolins  or  ball  clays  are  obtained  in  Tertiary  beds  in 

1  H.  Ries,  Economic  geology,  1917,  pp.  170-186. 


ROCK  DECAY  AND  WEATHERING  327 

Florida  and  Western  Tennessee.  Residual  clays  are  mainly 
mined  in  North  Carolina. 

The  most  important  property  of  clay  is  plasticity,  by  means  of 
which  it  can  be  kneaded  or  molded  into  a  desired  shape,  which 
it  retains  when  dry.  Not  all  the  residual  clays  are  plastic, 
nor  is  the  pure  mineral  kaolinite.  It  is  now  generally  be- 
lieved that  the  plasticity  of  clay  depends  upon  the  presence  of 
colloids.1 

The  tensile  strength  of  air-dried  clays  varies  from  15  to  400 
pounds  or  more  per  square  inch,  according  to  Ries.  The  fusi- 
bility varies  according  to  the  impurities  present.  In  low  grades 
of  clay  incipient  fusion  may  occur  at  about  1,000°  C.,  while  in 
refractory  clays,  which  are  low  in  fluxing  impurities,  it  may  not 
occur  until  1,300°  or  1,400°  C.  is  reached.  The  melting-point  of 
kaolin  is  about  1,800°  C. 

Origin. — The  best  residual  clays  are  derived  largely  from  the 
decomposition  of  the  feldspars  as  indicated  on  p.  322  by  carbon 
dioxide.  The  process  is  hastened  by  sulphuric  acid,  as  is  attested 
by  the  great  development  of  pure  kaolin  in  the  upper  levels  of 
pyritic  mineral  deposits. 

The  decrease  in  volume  by  decomposition  of  orthoclase, 
if  the  silica  were  liberated  in  soluble  form,  would  be  54.44  per 
cent.  In  kaolinization  anorthite  simply  loses  its  calcium  oxide 
and  takes  up  C02  and  H^O.  Pure  orthoclase  loses  43.24  per  cent. 
Si02  and  all  of  its  potassium;  albite  loses  45.87  per  cent.  SiO2 
and  all  of  its  sodium. 

The  origin  of  kaolin  has  been  the  subject  of  much  discussion. 
About  10  years  ago  H.  Rosier2  published  a  long  and  important 
paper  which  gave  rise  to  an  animated  discussion.3  Rosier  con- 
cludes that  kaolin  is  not  formed  by  weathering,  but  only  by 
pneumatolytic  or  allied  processes  by  the  action  of  thermal  waters. 
It  is  impossible  to  accept  these  results  and  they  have  been 
vigorously  contested  by  Stremme  and  Barnitzke;4  the  latter 
showed  that  the  celebrated  deposit  at  Meissen,  in  Saxony,  where 
a  high  grade  of  chinaware  is  made,  is  decidedly  a  product  of 

1  H.  E.  Ashley,  The  colloid  matter  of  clay  and  its  measurement,  Bull. 
388,  U.  S.  Geol.  Survey,  1909,  p.  65. 

*Neues  Jahrb.,  Beil.  Bd.  15,  1902,  p.  231-393. 

3SeeH.  Rosier.  Zeitschr.  prakt.Geol.,  1908,  p.  251;  Stremme,  idem,  p.  122. 

4  Ueber  das  Vorkommen  der  Porcellanerde  bei  Meissen  und  Halle. 
Zeitschr.  prakt.  Geol.,  1909,  pp.  457-472. 


328  MINERAL  DEPOSITS 

weathering,  gradually  changing  in  depth  into  unaltered  porphyry 
and  syenite.  The  chemistry  of  kaolin  is  given  in  full  by  H. 
Stremme,1  and  has  lately  been  summarized  by  Doelter. 2 

It  has  been  shown  by  W.  Lindgren3  and  others  that  kaolin  does 
not  form  in  deposits  that  are  due  to  ascending  thermal  waters, 
except  possibly  very  close  to  the  surface,  where  they  may  mingle 
with  atmospheric  waters.  The  idea  that  the  mineral  may  form 
by  pneumatolysis,  or  the  action  of  water  or  gases  liberated  at 
high  temperature  from  igneous  magmas,  is  assuredly  untenable; 
a  strongly  hydrous  mineral,  parting  with  its  water  at  the 
comparatively  low  temperatures  of  300°  to  400°  C.,  could  not 
possibly  originate  together  with  such  minerals  as  topaz  and 
tourmaline.  The  frequent  association  of  kaolin  with  cassiterite 
veins — for  instance,  in  Cornwall — has  been  held  by  L.  v.  Buch,  A. 
Daubre"e,  J.  H.  Collins,4  and  H.  Rosier  to  indicate  a  derivation 
by  the  action  of  hydrofluoric  acid  on  feldspars,  but  as  the 
kaolin  deposits,  during  the  metallogenetic  epoch,  were  under 
the  same  general  conditions  of  pressure,  temperature,  and  depth 
as  the  tin  deposits,  this  view  must  be  abandoned. 

Extensive  observations  in  the  United  States  have  shown  that 
in  mineral  deposits  kaolin  is  scarcely  ever  a  primary  mineral, 
but  has  been  derived  largely  by  the  action  of  sulphuric  acid  on 
the  feldspar  minerals  of  the  rocks  and  on  sericite,  which  is  often 
abundantly  developed  in  ore  deposits  formed  under  widely  differ- 
ing physical  conditions.  In  view  of  this,  it  seems  odd  that  Rosier 
expresses  astonishment  at  finding  a  large  amount  of  muscovite 
with  the  kaolin  from  Cornwall  and  suggests  that  the  former 
may  be  a  secondary  product.  G.  Hickling5  has  investigated 
the  china  clays  of  Cornwall  and  shows  that  they  form  essentially 
a  sheet  covering  the  corroded  surface  of  the  granite  and  that 
they  have  resulted  from  the  weathering  of  sericitic  granite,  the 
sericite  being  due  to  previous  alteration  by  thermal  waters. 

Kaolin,  then,  is  formed  abundantly  in  the  zone  of  weathering 
and  in  smaller  .amounts  for  a  considerable  distance  below  this 
zone. 

1  Die  Chemie  des  Kaolins,  Fortschritte  der  Min.,  Krist.  u.  Petr.  Jena,  1912. 
2C.    Doelter,   Handbuch  der  Mineralchemie,  vol.  2,   1914,  pp.  31-91; 
125-137. 

3The  origin  of  kaolin,  Econ.  Geol,  vol.  10,  1915,  pp.  89-93. 
4  J.  H.  Collins,  Min.  Mag.,  vol.  7,  1886-1887,  p.  217. 
6  Trans.  Inst.  Min.  Eng.  (England),  voL  36,  1908-1909,  p.  10. 


ROCK  DECAY  AND  WEATHERING 
RESIDUAL  IRON  ORES  (LIMONITE  AND  HEMATITE) 


329 


Origin. — During  the  processes  of  weathering  only  a  small  part 
of  the  iron  is  carried  away  in  solution;  the  greater  part  remains  in 
the  rock  altered  to  limonite  (2Fe203.3H2O),  to  gothite  (Fe203. 
H20),  or  to  indefinite  colloidal  mixtures  of  various  hydroxides  of 
iron;  hematite  may  also  be  present.  In  places  basic  sulphates  or 
phosphates  may  remain,  as  well  as  somewhat  indefinite  and 
unstable  ferric  silicates.  Nontronite,  H4Fe2Si2Og,  the  equivalent 
of  kaolin,  is  said  to  be  present  in  weathered  rocks.  In  the  zone 
of  weathering  the  iron  shows  a  strong  tendency  to  move  out- 
ward and  segregate  in  irregular  or  mammillary  masses,  separated 
by  clayey  material,  though  much  of  it,  of  course,  remains  inti- 


10  FEE* 


FIG.  104. — Section  showing  oxidation  of  iron  carbonate  to  limonite  in 
Tertiary  beds,  Cass  County,  Texas.  After  E.  F.  Burchard,  U.  S.  Geol. 
Survey. 

mately  mixed  with  clay.  The  same  is  true  of  manganese,  some  of 
which  may  be  associated  with  the  limonite,  though  when  much 
manganese  is  present,  it  also  tends  to  separate  by  itself. 

The  "centrifugal"  tendency  of  iron  hydroxide  is  well  seen  in 
many  oxidized  mineral  deposits,  often  also  in  the  weathering  of 
pebbles.  A  fine  instance  was  observed  in  the  cobbles  of  andesite 
in  the  Tertiary  river-bed  at  Iowa  Hill,  California.  The  outside 
of  these  cobbles  is  hard  and  consists  of  an  impure  limonite;  the 
center  contains  soft  yellowish  kaolin. 

During  the  concentration  the  ferric  hydroxides  (see  p.  257) 
were  probably  transported  as  colloids,  which  hardened  and  be- 
came crystalline,  as  shown  by  the  radial  structure  of  many  con- 
cretions. The  chemical  character  of  these  ores  has  rarely  been 


330  MINERAL  DEPOSITS 

studied  in  detail;  probably  it  will  be  found  that  barite,  oxidized 
zinc  minerals,  and  compounds  containing  manganese,  nickel,  and 
cobalt  are  present.  Many  of  the  limonites  are  rather  pure  and 
they  are  of  considerable  economic  importance. 

Classification.— One  class  of  residual  brown  iron  ores  is 
derived  from  the  decomposition  of  deposits  of  siderite  or  pyritic 
ores,  both  usually  formed  by  ascending  waters,  or  from  the 
weathering  of  black  bands  or  glauconite  beds  (Fig.  104). 
Such  limonites  in  places  reach  considerable  depths,  dependent 
on  the  penetrating  power  of  oxygenated  waters.  The  decom- 
posed croppings  of  pyritic  ores  are  not  often  used  as  iron  ores. 

Another  class  consists  of  local  segregations  of  limonite  or 
allied  hydroxides  in  the  decayed  rock  and  residual  clay  near  the 
surface.  These  masses  are  particularly  common  in  limestone 
areas.  Little  or  no  siderite  is  found  near  the  surface,  but  it  may 
appear  in  the  limestone  at  greater  depth.  When  oxygen  is 
exhausted  the  iron  is  more  easily  transported  as  a  bicarbonate 
and  the  metasomatic  replacement  of  calcite  by  siderite  may  then 
occur.  There  are,  however,  few  deposits  of  limonite  which 
change  in  depth  to  large  irregular  replacements  of  siderite,  so 
that  it  may  be  assumed  that  the  rate  of  solution  and  downward 
transportation  of  the  precipitated  limonite  is  slow. 

Finally,  a  third  class  of  residual  iron  ores,  consisting  of 
limonites  mixed  with  hematite,  occurs  as  widespread  sheets 
formed  by  the  gradual  decay  of  strongly  ferriferous  rocks. 

Brown  Hematites  of  the  Appalachian  Region.1 — In  the  United 
States  the  residual  iron  ores  are  most  abundant  in  the  Appala- 
chian region,  mainly  in  Alabama,  Georgia,  Virginia,  and  Ten- 
nessee. The  annual  production  of  such  ores  is  about  2,000,000 
long  tons,  a  small  part,  of  course,  of  the  yearly  output  of  iron 
ores  in  the  United  States.  These  so-called  "brown  hematites" 

1 C.  W.  Hayes  and  E.  C.  Eckel,  Iron  ores  of  the  Cartersville  district, 
Georgia,  Bull.  213,  U.  S.  Geol.  Survey,  1902,  pp.  233-242. 

E.  C.  Eckel,  Limonite  deposits  of  eastern  New  York,  etc.,  Bull.  260, 
U.  S.  Geol.  Survey,  1904,  pp.  335-342. 

R.  J.  Holden  in  Mineral  resources  of  Virginia,r1908. 

E.  C.  Harder,  The  iron  ores  of  the  Appalachian  region  in  Virginia,  Bull. 
3£0,  U.  S.  Geol.  Survey,  1908,  pp.  215-254. 

E.  C.  Harder  and  E.  F.  Burchard,  Mineral  Resources,  U.  S.  Geol. 
Survey,  particularly  pt.  2,  chapter  on  Iron,  1908. 

E.  F.  Burchard  and  E.  C.  Eckel,  Birmingham  district,  Bull.  400,  U.  S. 
G  ol.  Survey,  1910,  pp.  145-167. 


ROCK  DECAY  AND  WEATHERING 


331 


are  mined  in  many  small  deposits;  their  content  in  iron  ranges 
from  38  to  52  per  cent,  (limonite  59.89  per  cent.  Fe);  most  of 
them  are  comparatively  rich  in  phosphorus. 

Most  of  the  southern  limonites  lie  in  Cambro-Silurian  strata 
and  extend  along  the  "Great  Valley,"  between  the  pre-Cam- 
brian  on  the  east  and  the  Paleozoic  rocks  on  the  west.  They  are 
classed  as  vaUey  ores,  mountain  ores,  and  Oriskany  ores. 

The  valley  ores  appear  as  irregular  deposits  of  shallow  pockets 
in  clay  derived  from  the  decomposition  and  solution  of  Cambro- 
Silurian  limestone  or  dolomite.  The  ores  lie  as  scattered  lumps 
in  the  clay,  not  so  much  on  the  eroded  surface  of  the  limestone, 
but  rather  higher  up  (Fig.  105).  Each  deposit  is  soon  exhausted, 


FIG.  105. — Vertical  section  showing  structure  of  the  valley  brown  ore 
deposits  of  the  Rich  Hill  mine,  Virginia.  After  E.  C.  Harder,  U.  S.  Geol. 
Survey. 

and  few  extend  below  a  depth  of  50  feet.  The  ores  are  mixtures 
of  limonite,  gothite,  and  clay;  the  composition  ranges  from  40  to 
56  per  cent.  Fe,  5  to  20  per  cent.  SiO-2,  0.05  to  0.5  per  cent.  P,  and 
0.3  to  2.0  per  cent.  Mn. 

Many  of  these  ores  were  evidently  concentrated  under  con- 
ditions different  from  those  of  to-day;  most  of  them  are  probably 
of  Tertiary  age  as  shown  particularly  in  the  deposits  south  of 
Birmingham,  Alabama.  It  is  not  unlikely  that  the  same  applies 
to  many  "mountain  ores." 

The  mountain  ores,  according  to  Harder,  show  greater  varia- 
tion in  occurrence  and  appearance.  They  are  found  as  small 
discontinuous  pockets  in  residual  material  above  the  Lower 


332 


MINERAL  DEPOSITS 


Cambrian  quartzite  at  or  near  the  contact  with  the  overlying 
formation,  which  is  generally  a  limestone.  While  these  ores 
are  mainly  superficial,  they  are  sometimes  worked  to  a  depth  of 
several  hundred  feet.  The  composition  ranges  from  35  to  50 
per  cent.  Fe,  10  to  30  per  cent.  SiO2,  0.1  to  2  per  cent.  P,  and 
0.5  to  10  per  cent.  Mn.  These  limonites  are  often  glassy  and 
concretionary. 
The  occurrences  are  classed  by  Harder  as  follows: 

1.  Pocket  deposits  in  clay,  in  part  replacements  of  limestone, 
in  part  manganiferous  (Fig.  106). 

2.  Small  replacement  deposits  in  shale,  along  fractures. 

3.  Deposits  in  quartzite  or  sandstone,  not  abundant,  including 

a.  Breccia  deposits  accompanied  by  replacement. 
6.  Vein  deposits  along  faults. 


—  Brown-clay -with -ore-fragments: — T- 


FIG.  106. — -Vertical  section  showing  the  structure  of  mountain  brown  ore 
occurring  as  mammillary  masses  in  clay.  Mary  Creek  mine,"  Virginia. 
After  E.  C.  Harder,  U.  S.  Geol.  Survey. 

The  sandstones  of  the  Cambro-Silurian  are  often  ferruginous 
in  this  region.  Some  of  the  varieties  rich  in  hematitic  cement 
change  along  the  strike  to  beds  of  siliceous  hematite,  several 
feet  thick  and  of  possible  economic  importance. 

The  Oriskany  ores1  are  mined  in  Virginia  and  form  irregular 
replacements  along  local  folds  or  fracture  zones  on  the  flanks  of 

1C.  M.  Weld,  The  Oriskany  iron  ores  of  Virginia,  Earn.  Geol,  vol.  10, 
1915,  pp.  399-421. 


ROCK  DECAY  AND  WEATHERING 


333 


greater  anticlines.  They  occur  in  the  calcareous  Oriskany 
sandstone  which  is  overlain  by  the  Romney  shale  (Devonian)  and 
underlain  by  the  Helderberg  limestone  (Silurian).  The  ore 
largely  replaces  the  sandstone,  sometimes  also  the  limestone 
and  may  be  from  10  to  100  feet  wide.  The  greatest  depth 
reached  is  600  feet;  at  this  or  lesser  depth  the  ore  grades  into 
unaltered  rock.  The  iron  is  considered  by  some  authors  to  be 
derived  from  the  Romney  shale  but  is  more  likely  derived  from 
the  sandstone  itself.  The  ore  is  made  up  of  earthy  masses  and 
rounded  concretions  of  fibrous  limonite  filled  with  clay  or  sand. 
The  ore  from  the  Callie  mine  contains  about  0.2  per  cent,  of 
zinc.1  At  the  Callie  mine  the  ore  production  is  about  2,700  tons 


FIG.  107. — Vertical  section  showing  the  Oriskany  brown  ore  deposit  at  the 
Callie  mine,  Virginia.     After  E.  C.  Harder,  U.  S.  Geol.  Survey. 


per  month.  Probably  hematite  or  turgite  are  also  present  for 
the  ore  does  not  contain  enough  water  for  limonite;  it  averages 
43  per  cent.  Fe,  10  to  25  per  cent.  Si02,  0.06  to  0.5  per  cent.  P, 
and  0.5  to4  per  cent,  manganese.  Cobalt  and  nickel  are  reported 
to  be  present  in  traces. 

The  Oriskany  ores,  like  the  other  "  brown  hematites,"  are 
subjected  to  a  rough  concentration  in  log  washers  in  order  to 
remove  the  clay. 

irThe  foot- wall  limestone  is  said  to  contain  the  same  amount  of  Zn. 
Letter  from  S.  E.  Doak. 


334  MINERAL  DEPOSITS 

Iron  Ores  of  Bilbao,  Spain.1 — -The  great  deposits  of  Bilbao, 
in  northern  Spain,  have  for  many  years  yielded  several  million 
tons  annually,  the  ores  being  exported  to  England. 

According  to  Adams,  both  replacement  and  residual  ores  are 
present.  The  ores  are  superficial  and  limited  to  areas  of -Cre- 
taceous limestone,  which  is  250  feet  thick  and  dips  northeast. 
The  white  siderite  ore,  which  is  found  at  some  depth,  is  altered 
near  the  surface  to  red  hematite  with  80  to  90  per  cent.  Fe2O3. 
The  ores  are  of  Bessemer  grade.  Adams  believes  that,  during  the 
progress  of  denudation,  the  calcareous  beds  became  replaced 
by  siderite  by  the  aid  of  downward-percolating  solutions,  de- 
rived partly  from  the  overlying  calcareous  shale.  Through  long- 
continued  rock  decay  the  siderite  was  altered  to  hematite  and 
limonite,  which  now,  with  much  clay,  cover  the  limestone  areas 
like  a  sheet.  One  of  the  largest  iron-bearing  areas  is  2  miles 
long  and  3,300  feet  wide;  the  iron  ore  in  this  area  had  a  thick- 
ness of  about  100  feet.2 

Residual  Ores  of  Cuba.3 — Iron  ores  have  been  mined  for  a 
number  of  years  in  the  vicinity  of  Santiago,  Cuba,  but  these  ores, 
of  contact-metamorphic  origin,  consist  of  hematite  with  some 
magnetite  and  contain  a  high  percentage  of  sulphur.  The  three 
new  districts  described  by  Spencer  and  others  are  likewise  in  the 
eastern  part  of  the  island,  but  are  of  an  entirely  different  type. 
They  are  the  Mayari  and  Moa  districts  in  Oriente  province, 
and  the  San  Felipe  in  Camaguey  (Fig.  108).  The  ores  occur  as 

1 F.  D.  Adams,  Notes  on  the  iron  deposits  of  Bilbao,  Jour.,  Canadian  Min. 
Inst.,  1901. 

John,  Zeitschr.  prakt.  Geol.,  1911,  pp.  208-212. 

P.  Grosch,  Geol.  Rundschau,  vol.  5,  1914-15,  pp.  392-400. 
2Stelzner  and  Bergeat,  Die  Erzlagerstatten,  vol.  2,  1906,  p.  1049,  with 
list  of  literature. 

3  A.  C.  Spencer,  Three  deposits  of  iron  ore  in  Cuba,  Bull.  340,  U.  S. 
Geol.  Survey,  1907,  pp.  318-329. 

C.  M.  Weld,  The  residual  iron  ores  of  Cuba,  Trans.,  Am.  Inst.  Min.  Eng., 
vol.  40,  1909,  pp.  299-312. 

J.  F.  Kemp,  The  iron  resources  of  the  world,  Int.  Geol.  Congress, 
Stockholm,  1910,  pp.  793-795;  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  51,  1916, 
pp.  3-30. 

See  also  seven  papers  on  the  same  subject  by  J.  S.  Cox,  Jr.,  C.  K.  Leith, 
W.  J.  Mead,  A.  C.  Spencer,  C.  W.  Hayes,  W.  L.  Cumings,  B.  L.  Miller, 
D.  E.  Woodbridge,  and  J.  E.  Little,  in  Trans.,  Am.  Inst.  Min.  Eng.,  vol. 
42,  1911,  pp.  73-152.  Also  C.  K.  Leith  and  W.  J.  Mead,  idem,  vol.  53, 
1916,  pp.  75-78. 


ROCK  DECAY  AND  WEATHERING 


335 


residual  mantles  resulting  from  the  weathering  of  serpentine 
and  for  the  most  part  lie  on  plateaus  at  rather  high  elevations. 
They  were  probably  formed  during  the  Tertiary  before  the  uplift 
of  the  present  plateaus.  Near  the  surface  the  material  is  earthy 
and  dark  red,  sometimes  cemented  with  shot-like  lumps  of  hema- 
tite scattered  over  the  surface;  underneath  lie  yellowish  ores 
changing  rather  abruptly  into  decomposed  and  soft  serpentine. 
In  places  a  layer  of  cherty  material  is  found  immediately  above 
the  !  serpentine.  In  the  Mayari  district  the  average  depth 
of  the  ore  is  about  15  feet  and  it  extends  over  an  area  of  lOjby 
4  miles.  Hundreds  of  millions  of  tons  are  said  to  be  available, 


Map  of  the 

EASTERX  PART  OF  CUBA 
Showing  Iron  Ore  Districts 


FIG.  108. — Sketch  map  of  eastern  part  of  Cuba.     After  W.  L.  Cumings 
and  B.  L.  Miller. 

allowing  for  parts  of  the  area  which  are  below  the  workable 
grade.     The  ore  is  removed  by  drag-line  steam  shovels. 

According  to  analyses  the  ore  is  fairly  uniform,  the  metallic 
iron  varying  in  percentage  from  40  to  50.  It  is  remarkably  free 
from  phosphorus  and  evidently  contains  hematite,  limonite, 
a  little  magnetite,  and  also  some  free  aluminum  hydroxide. 
It  is,  in  brief,  a  typical  iron-rich  laterite  (see  p.  351).  There  is 
much  water;  according  to  Kemp  the  Moa  ores  yield  25  to  30  per 
cent,  hygroscopic  and  10  to  12  per  cent,  combined  water;  silica 
is  low  and  alumina  high.  The  concentration  of  nickel  and  chro- 
mium is  also  remarkable;  the  latter  metal  is  removed  during  the 
smelting;  the  former  is  favorable  to  the  quality  of  the  iron. 


336  MINERAL  DEPOSITS 

ANALYSES  OF  SERPENTINE  AND  ORE  FROM  THE  MAYARI 
DISTRICT,  CUBA 
(After  C.  K.  Leith) 

2.90 
10.24 
72.35 
50.56 


SiO2  
A12O3  

39.80 
1.39 

Fe2O» 

10  14 

Fe  
MgO  
Cr  
Ni+Co  

7.10 
33.69 
0.20 
0.97 

p 

0  001 

s 

0  06 

H.O  +  .. 

.  .13.31 

1.66 
0.84 
0.016 
0.20 
10.96 


99.561       99.166 

1.  Serpentine,  at  depth  of  29  feet. 

2.  Iron  ore,  at  depth  of  6  feet. 
Analyses  by  Spanish- American  Iron  Co. 

The  porosity  of  the  ore  is  exceedingly  great  amounting  to  75 
per  cent,  of  its  volume  but  lessens  near  the  surface. 

In  considering  the  alteration  of  serpentine  to  ore  in  terms  of 
weight  it  is  found  that  the  alumina  has  remained  nearly  constant. 
The  changes  in  the  composition  of  the  serpentine  during  its 
alteration  to  ore  is  shown  by  Leith  and  Mead  in  Fig.  109,  which  is 
based  on  many  analyses  at  uniform  intervals.  The  diagram 
illustrates  the  rapid  destruction  of  the  serpentine  by  leaching  of 
Si02  and  MgO,  the  marked  relative  increase  of  iron  and  alumina 
and  a  gradual  loss  of  nickel.  Toward  the  surface  hematite 
(with  a  little  magnetite)  develops  from  limonite  and  bauxite  from 
kaolin.  In  the  middle  part  of  the  ore  body  iron  has  increased  in 
proportion  to  the  alumina,  owing  probably  to  re-deposition  and 
oxidation  of  ferrous  iron  dissolved  by  the  reducing  action  of  the 
vegetation.  Silica  is  lost  throughout  and  magnesia  is  wholly 
removed. 

In  100  pounds  of  typical  serpentine  there  are  1.5  pounds  of 
alumina  and  10  pounds  of  ferrous  oxide.  When  the  magnesia 
and  silica  are  removed  in  solution  and  the  iron  oxidized  there 
remain  approximately  11.75  pounds  of  limonite,  3.8  pounds  of 
bauxite  and  kaolin,  and,  at  the  most,  2  pounds  of  minor  constitu- 
ents. This  residual  of  17.55  pounds  contains  7.8  pounds,  or  44.4 
per  cent.,  of  metallic  iron  and  is  an  iron  ore. 

Distribution  and  Stability  of  Residual  Iron  Ore.— The  residual 
iron  ores  are  widely  distributed  in  countries  of  warm  climate, 


ROCK  DECAY  AND  WEATHERING 


337 


where  secular  decay  has  progressed  without  interruption  for  a  long 
time.  It  seems,  however,  that  great  concentration  has  been 
effected  only  from  relatively  soluble  rocks  like  limestone  and 
serpentine.  Many  of  the  laterites  of  India,  Africa,  and  other 
tropical  countries  are  rich  in  ferric  oxide  and  have  the  same 


characteristic  concretionary  pellets  and  shots  on  the  surface. 
Extensive  limonite  deposits  similar  to  those  of  Cuba  have  lately 
been  discovered  in  Borneo  and  on  Mindanao,  in  the  Philippines. 
Vegetation  plays  an  important  part  in  the  origin  of  many  of 
these  deposits.  Underneath  the  mat  of  roots  and  decayed 
vegetation  the  soil  in  tropical  countries  is  often  white.or  yellowish, 


338  MINERAL  DEPOSITS 

indicating  that  the  iron  is  in  the  ferrous  state,  probably  as  car- 
bonate. When,  as  happened  on  the  high  volcanic  plateau  of 
Molokai,  Hawaiian  Islands,1  the  vegetation  is  destroyed  the  soil 
immediately  turns  red  and  hard  and  shows  characteristic  pellets 
of  ferric  oxide.  In  part  at  least  the  rock  is  thus  changed  directly 
to  hematite  without  passing  through  the  intermediate  stage  of 
limonite. 

According  to  H.  Wolbling,2  the  natural  ferric  hydroxides  have 
great  stability  and  cannot  readily  be  changed  to  ferric  oxide, 
probably  not  by  exposure  to  air  and  salt  solutions.  The  freshly 
precipitated  hydroxides  are,  however,  easily  converted  to  ferric 
oxide  and  these  colloids  may  easily  be  crystallized.3  His  experi- 
ments show  that  by  the  precipitation  of  ferric  solutions  with 
calcite  or  siderite  at  100°  C.,  Fe2O3  is  easily  formed,  containing 
only  1  or  2  per  cent.  H20,  while  during  slow  and  wet  oxidation 
of  ferrosalts,  ferric  hydrates  of  iron  are  obtained.  Wolbling  also 
asserts  that  there  are  yellow  forms  of  FegOs,  as  well  as  red  forms 
of  the  hydroxides. 

It  is  certain,  at  any  rate,  that  the  ferric  oxide,  as  well  as  the 
hydrates,  is  very  stable  when  once  formed  and  is  not  easily 
altered. 

No  one  can  fail  to  be  impressed  by  certain  similarities  of 
the  Cuban  residual  ores  to  those  of  the  Mesabi  range  (p.  366). 
Similar  large  expanses  of  rock,  weathered  under  a  tropical  sun 
and  covered  by  residual  ferric  oxide,  undoubtedly  yielded  the 
material  for  the  sedimentary  hematite  deposits. 

RESIDUAL  MANGANESE  ORES* 

The  minerals  of  the  residual  manganese  ores  consist  of  pyrolu- 
site  (MnC>2,  63.2  per  cent.  Mn),  psilomelane  (MnO2,  with  H2O, 
K20,  and  BaO;  49  to  62  per  cent.  Mn),  wad  (perhaps  MnO2. 

*W.  Lindgren,  The  water  resources  of  Molokai,  Water-Supply  Paper, 
77,  U.  S.  Geol.  Survey,  1903,  p.  19. 

2H.  Wolbling,  Bildung  der  oxydischen  Eisenerzlager,  Stahl  und  Eisen, 
1909,  p.  1248;  also  Zeiischr.  prakt.  Geol,  vol.  17,  1909,  p.  495. 

3C.  Doelter,  Ueber  Umwandlung  amorpher  Mineralkorper  in  Krys- 
talline,  Tsch.  M.  und  p.  Mitt.,  vol.  28,  1909,  pp.  556-559. 

*R.  A.  F.  Penrose,  Jr.,  Manganese,  its  uses,  ores,  and  deposits,  Ann.  Rept. 
Arkansas  Geol.  Survey,  vol.  1,  1890. 

T.  L.  Watson,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  34,  1904,  p.  207. 
T.  L.  Watson,  Preliminary  report  on  the  manganese  deposits  of^Geor- 
gia,  Bull.  14,  Georgia  Geol.  Survey,  1908. 


ROCK  DECAY  AND  WEATHERING  339 

nMnO+H20,  varying  percentage  of  metal),  more  rarely  braunite 
(3Mn2O3.MnSi03  (?) ;  69.7  per  cent.  Mn),  and  manganite  (Mn2O3. 
H2O,  62.4  per  cent.  Mn). 

The  most  common  ores  are  pyrolusite  and  psilomelane,  both 
occurring  frequently  in  botryoidal,  renif orm,  or  mammillary  con- 
cretions. Harder  has  shown  that  these  two  minerals  may  form 
alternating  layers  in  the  concretions.  Earthy  or  rough,  slaggy 
forms  are  also  common.  Like  limonite  they  are  largely  colloid 
deposits,  later  converted  into  crystalline  minerals. 

Primary  Sources. — Nearly  all  workable  manganese  deposits  are 
of  secondary  formation — that  is,  they  are  concentrated  from 
manganese  minerals  more  sparsely  distributed  in  rocks.  Pyrolu- 
site, psilomelane,  and  wad  are  always  secondary,  formed  under 
the  influence  of  weathering,  even  where  they  descend  to  con- 
siderable depths  below  the  water  level. 

In  igneous  rocks  manganese  is  always  present  but  only  in 
small  amounts.  The  largest  percentages  (about  0.36  percent.) 
are  found  in  syenite  and  its  porphyries  and  in  basalts. 

Sedimentary  rocks  may  contain  manganese  in  the  form  of 
oxide  and  carbonate.  Manganese  nodules  occur  in  some  deep- 
sea  deposits. 

Analyses  of  limestones  often  show  a  small  amount  of  manga- 
nese. In  many  cherts  and  jaspers  of  the  sedimentary  series 
manganese  is  characteristically  present  as  rhodonite  or  rhodo- 
chrosite.  On  previous  pages  it  has  been  shown  that  important 
deposits  of  manganese  may  be  produced  by  sedimentation. 

In  crystalline  schists,  especially  in  those  of  more  basic  composi- 
tion, small  quantities  of  manganese  are  found. 

In  some  crystalline  schists  spessartite  (manganese  garnet), 
rhodonite,  and  piedmonite  (manganese  epidote)  appear  in 
considerable  quantities. 

Finally,  rhodochrosite  and  rhodonite  are  rather  common  in  ore 
deposits  of  hydrothermal  or  contact-metamorphic  origin,  and 
much  manganese  is  present  in  some  metamorphic  specularite 
and  magnetite  deposits. 

E.  C.  Harder,  Manganese  deposits  of  the  United  States,  Bull  427, 
U.  S.  Geol.  Survey,  1910  (with  bibliography  and  notes  on  foreign  occurrences). 

E.  C.  Harder  and  D.  F.  Hewett,  Mineral  Resources,  U.  S.  Geol.  Survey, 
annual  publication. 

D.  F.  Hewett,  Some  manganese  ore  in  Virginia  and  Tennessee,  Bull. 
6iO,  U.  S.  Geol.  Survey,  1916,  pp.  37-71. 


340 


MINERAL  DEPOSITS 


Manganese  Deposits  in  the  United  States. — From  the  rocks 
above  mentioned  manganese  may  be  concentrated  by  processes 
of  weathering,  and  its  ores  are  found  in  concretions  embedded  in 
residual  clay  or  ocher  and  accompanied  more  or  less  closely  by 
limonites.  During  this  process  some  other  metals,  notably 
nickel,  cobalt,  zinc,  and  barium,  have  a  tendency  to  accompany 
the  pyrolusite  and  psilomelane.  In  general  such  deposits  are 
superficial  or  of  slight  depth  and  closely  parallel  the  residual 
limonites  already  described. 

In  California  small  deposits  of  secondary  manganese  ores  occur 


FIG.  110. — Generalized  section,  showing  the  occurrence  of  manganese  ore 
at'Batesville,  Arkansas,  a,  Boone  chert  (Mississippian) ;  b,  Cason  shale  with 
manganese_deposits  (Ordovician) ;  c,  Polk  Bayou  limestone  (Ordovician) ;  d, 
surface  clay  with  manganese  deposits.  After  E.  C.  Harder,  U.  S.  Geol. 
Survey. 

in  areas  of  the  radiolarian  cherts  or  jaspers  of  the  Franciscan 
formation  (Jurassic  ?) 

In  Arkansas  residual  ores  have  been  mined  at  Bates ville,1 
where  they  occur  both  in  the  Cason  manganiferous  shale,  of 
upper  Ordovician  age,  and  in  clay  derived  from  this  formation 
(Fig.  110).  Penrose  believed  that  the  manganese  was  derived 
from  the  pre-Cambrian  area  in  southeast  Missouri  and  de- 

1  R.  A.  F.  Penrose,  Jr.,  op.  tit. 


ROCK  DECAY  AND  WEATHERING 


341 


posited  in  the  sedimentary  formation,  but  the  later  work  of 
Ulrich  and  others  has  shown  that  erosional  epochs  have  inter- 
vened within  the  formation  period  assumed  by  Penrose  and 
that  the  ores  are  original  marine  deposits,  reconcentrated  dur- 
ing two  subsequent  land  stages,  first  during  the  late  Silurian 
and  Devonian  partial  emergence,  and  second  during  the  post- 
Paleozoic  erosion  of  the  Boone  chert.1 

In  the  Appalachian  region  small  deposits  occur  in  granites  and 
schists  of  the  Piedmont  region,  but  chiefly  in  the  Paleozoic  sedi- 

/-  Manganese  ore  kidneys 

Ir 

i 


White  sandy 
mass 


FIG.  111. — Sketch  showing  distribution  of  manganese  ore  lumps  in  clay  at 
the  Crimora  mine,  Virginia.     After  E.  C.  Harder,  U.  S.  Geol.  Survey. 


ments  of  the  Cambro-Silurian  belt — that  is,  in  the  general  area  of 
the  residual  iron  ores.  At  the  Crimora  deposit,  in  "Virginia 
(Fig.  Ill),  the  ore  is  found  as  "masses  of  various  sizes  scattered 
through  variegated  clays  in  an  elliptical  basin  in  a  canoe-shaped 
syncline  of  the  Cambrian  quartzite,"  into  which  the  manganese 
penetrates  as  dendritic  forms  and  crystalline  coatings.2 


1  E.  C.  Harder,  Bull.  427,  U.  S.  Geol.  Survey,  1910,  p.  117. 
*  E.  C.  Harder,  idem,  p.  60. 


342  MINERAL  DEPOSITS 

ANALYSIS  OF  BEST  QUALITY  CRIMORA  ORE 
[T.  L.  Watson,  Mineral  resources  of  Virginia,  p.  248] 

MnO2 81.703  BaO 0.829 

MnO 7.281  CaO 0.880 

Fe2O3 0.533  MgO 0.630 

CoO 0.354  P2O5 0.171 

NiO 0.096  (NaK)20 0.467 

ZnO 0.623  H.,01 3.405 

A12O3 0.896  SiO2 2.132 


Total 100.000 

Mn 57.297 

The  manganese  deposits  of  the  Appalachian  region  occur  in  a 
decomposed  surface  zone  of  many  different  rocks  (Figs.  112  and 
113),  but  most  of  the  deposits  are,  according  to  Harder,  asso- 
ciated with  the  top  stratum  of  an  impervious.  Cambrian  quartz- 
ite  overlain  by  limestone.  Penrose2  holds  that  they  were 
laid  down  in  local  basins  during  the  deposition  of  the  rocks 
in  whose  residual  clays  they  are  now  found.  Harder3  believes 
that  the  metal  was  in  the  first  place  obtained  from  the  crystal- 
line rocks  of  the  Piedmont  region  and  that  since  the  emergence 
of  the  sediments  repeated  concentration  by  rock  decay  has 
been  going  on. 

In  central  Texas,4  in  Mason,  Llano,  and  San  Saba  counties, 
oxidized  manganese  ores  occur  as  products  of  weathering  of 
crystalline  schists  containing  spessartite,  piedmontite,  and 
tephroite. 

As  stated  above,  many  ore  deposits  contain  manganese  as 
carbonate  and  silicate,  and  in  the  oxidized  zone  the  metal  is 
often  highly  concentrated  in-  the  form  of  psilomelane,  etc.,  mixed 
with  limonite;  these  ores  often  contain  gold  and  silver,  but  rarely 
much  copper,  lead,  or  zinc.  Considerable  quantities  of  such 
ores,  used  in  part  as  flux  for  lead  smelting  and  in  part,  if  of  high 
grade,  for  the  manufacture  of  spiegeleisen,  are  mined  at  Lead- 
ville,  Colorado.5  Here  the  oxidized  ore  is  apparently  derived 
from  a  manganiferous  siderite. 

1  Probably  by  difference. 

2  R.  A.  F.  Penrose,  Jr.,  op.  tit, 

3  E.  C.  Harder,  op.  tit.,  pp.  99-101. 

4  R.  A.  F.  Penrose,  Jr.,  op.  tit.,  p.  432;  Sidney  Paige,  Bull.  450,  U.  S. 
Geol.  Survey,  1911. 

6  S.  F.  Emmons  and  J.  D.  Irving,  Bull.  320,  U.  S.  Geol.  Survey,  1907, 
p.  26. 


ROCK  DECAY  AND  WEATHERING 


343 


The  largest  part  of  the  manganese  obtained  in  the  United 
States  is  derived  from  ores  of  the  Lake  Superior  region,  where 
manganese  occurs  as  oxides  associated  with  specularite,  and  from 
the  zinc  residues  of  the  great  zinc  deposit  of  Franklin  Furnace, 


FIG.  112. — Sketch  showing  occurrence  of  manganese  breccia  ore  at  Reynolds 
Mountain,  Virginia.     After  E.  C.  Harder,  U.  S.  Geol.  Survey. 


FIG.  113. — Sketch  showing  development  of  breccia  ore  by  replacement. 
White  areas,  chert  or  sandstone;  black,  manganese  ore.  One-fifth  natural 
size.  After  T.  L.  Watson. 

New  Jersey,  where  the  manganese  is  contained  in  the  franklinite 
[(Fe,Zn,Mn)O.(Fe,Mn)2O3]  associated  with  zincite  [(Zn,Mn)O] 
in  a  deposit  of  deep-seated,  probably  contact-metamorphic, origin. 


344  MINERAL  DEPOSITS 

Brazil. — The  high-grade  manganese  deposits  of  Minas  Gerses, 
Brazil,  have  been  described  by  J.  C.  Branner  and  0.  A.  Derby.1 
In  the  main  they  appear  to  be  residual  ores  derived  from  the 
weathering  of  lenses  in  the  crystalline  schists  containing  rhodo- 
chrosite,  tephroite,  and  spessartite.  The  ores  are  concretions, 
masses,  and  vein-like  deposits  of  psilomelane  in  the  soft  decom- 
posed rock. 

India. — Manganese  ores  are  extensively  distributed  in  India 
and  their  occurrence  and  origin  have  recently  been  described  in 
a  detailed  manner  by  L.  L.  Fermor.2  To  a  large  extent  these 
rich  ores  are  formed  by  the  combined  replacement  and  decom- 
position of  Archean  rocks  containing  manganese  silicates.  In 
part  the  rocks  are  crystalline  schists  with  spessartite  and  rhodo- 
nite, in  part  probably  non-metamorphosed  peculiar  igneous  rocks, 
one  of  which,  for  instance,  consists  of  spessartite  (spandite)  and 
orthoclase  with  3.70  per  cent,  apatite.  To  a  smaller  extent  the 
ores  are  contained  in  jaspery  quartzites  and  also  in  laterite, 
which  is  purely  residual. 

Many  deposits  of  the  first  class  contain  enormous  masses  of 
psilomelane,  pyrolusite,  and  braunite;  during  the  process  of 
weathering  almost  all  the  silica  and  alumina  have  been  removed. 
Fermor  finds  no  evidence  that  the  alteration  has  been  caused  by 
sulphuric  acid,  but  holds  that  in  some  manner,  not  yet  fully 
understood,  it  has  been  effected  by  surface  waters. 

Many  of  the  deposits  extend  to  depths  far  below  the  water 
level  and  Fermor  believes  that  the  oxidation  may  be  of  very 
ancient  •  date,  perhaps  Archean.  In  some  ways  these  con- 
centrations by  surface  waters  recall  the  Lake  Superior  iron 
deposits. 

Origin. — The  manganese  ores  here  described  as  products  of 
weathering  and  rock  decay  are  in  the  main  similar  in  origin 
to  the  corresponding  deposits  of  iron  ore.  It  is  explained  on 
page  272  that  iron  and  manganese,  although  acting  in  a  similar 
manner,  are  usually  laid  down  separately  in  residual  and  sedimen- 
tary deposits  because  of  the  greater  solubility  of  the  manganese 
carbonate.  Where  sulphates  are  present  the  ferrous  salt  is  de- 
composed easily  by  oxygen,  while  manganese  sulphate  requires 

1  Literature  summarized  by  E.  C.  Harder,  Bull.  427,  U.  S.  Geol.  Survey, 
1910,  p.  183. 

2L.  L.  Fermor,  The  manganese  ore  deposits  of  India,  Mem.,  Geol. 
Survey  India,  vol.  37,  1909. 


ROCK  DECAY  AND  WEATHERING  345 

the  presence  of  calcium  carbonate  or  some  such  mineral.1  On 
the  whole  manganese  is  not  transported  far  from  its  original 
source  and  is  characterized  by  a  strong  tendency  to  segregation 
into  local  concretions  and  masses.  It  is  believed  that  in  the 
main  the  ordinary  surface  waters  effected  the  concentration  and 
that  the  metal  has  been  transformed  through  the  intermediate 
stage  of  carbonate. 

Production  and  Uses. — The  normal  domestic  output  of  man- 
ganese ores  containing  above  35  per  cent.  Mn  was  small.  Forced 
production  under  war  conditions  has  increased  the  output  to 
about  300,000  tons  (in  1918)  which  is  one-third  of  the  amount 
normally  needed.  Heavy  imports  come  from  Brazil  and  India. 

For  the  manufacture  of  spiegeleisen,  an  alloy  with  iron  con- 
taining less  than  20  per  cent.  Mn,  low  grades  of  manganiferous 
iron  ore  may  be  used,  but  for  other  purposes  the  ores  should 
contain  at  least  40  per  cent.  Mn,  less  than  12  per  cent.  SiOz,  and 
less  than  0.3  per  cent,  phosphorus. 

The  higher  grades  of  manganese  ores  are  used  extensively  for 
the  manufacture  of  ferromanganese  alloys,  which  are  employed 
for  many  purposes  in  the  iron-smelting  industry,  particularly  for 
hardening  steel.2  The  pure  manganese  dioxide  ores  also  find  an 
extensive  chemical  use,  for  the  generation  of  chlorine  and  for  the 
manufacture  of  cells  for  dry  electric  batteries. 

RESIDUAL  BARITE 

Barite  as  residual  material  and  nodular  concretions  is  not 
uncommon  in  the  residual  soils  of  Virginia  and  Georgia  and 
in  Washington  County,  Missouri.  In  Virginia  the  Cambro-Sil- 
urian  limestone,  according  to  T.  L.  Watson,  generally  contains 
a  notable  percentage  of  barium,  and  in  many  places  in  Georgia 
the  Weisner  sandstone,  of  the  .same  age,  also  carries  barium 
suphate.  In  Missouri  the  barite  is  concentrated  in  the  soil 
from  veins  in  the  Ordovician  Gasconade  limestone.  The  barium 
may  have  been  transported  as  the  carbonate,  which  is  slightly 
more  soluble  than  the  sulphate,  and  precipitated  by  water 
carrying  sulphate.  Much  of  the  barite  produced  in  the  United 
States  is  obtained  from  residual  clays  (p.  376). 

1  F.  P.  Dunnington,  Am.  Jour.  Sci.,  3d  ser.,  vol.  36,  1888,  p.  177. 

2  Mineral  Resources,  U.  S.  Geol.  Survey,  pt.  1,  1908,  p.  138,  and  in  later 


346  .MINERAL  DEPOSITS 


RESIDUAL  ZINC  ORE 

In  the  Appalachian  region,  in  western  Virginia  and  eastern 
Tennessee,  the  Cambro-Silurian  limestones  contain  in  places 
sulphides  of  lead  and  zinc  distributed  in  brecciated  and  crushed 
zones.  At  such  localities  the  deep  residual  soil  often  contains 
calamine  and  smithsonite,  the  hydrated  silicate  and  the  carbon- 
ate of  zinc,  with  some  cerussite  and  galena.  These  ores  occur 
next  to  the  limestone  at  the  bottom  of  the  clay  (Fig.  114),  not 
scattered  through  it  like  limonite  and  pyrolusite.1 


FIG.  114. — Section  in  open  cut  at  the  Bertha  zinc  mines,  Virginia,  show- 
ing'relations  of  the  residual  ore  to  the  limestone  chimneys  and  the  residual 
clay.  After  T.  L.  Watson. 

RESIDUAL  OCHERS2 

The  residual  ochers  are  impure  deep-red,  yellow,  or  brown 
pulverulent  materials  consisting  usually  of  predominant  limonite 
and  hematite  with  more  or  less  clay  and  are  generally  used  for 
pigments.  They  are  no  doubt  colloid  precipitations.  The  terms 
Indian  red,  sienna,  and  umber,  the  latter  two  for  the  darker 
yellowish-brown  and  brown  shades,  are  in  use.  Not  all  mineral 
pigments  are  natural  products,  for  roasted  pyrite,  siderite,  slates, 

1 W.  H.  Case,  The  Bertha  zinc  mines  at  Bertha,  Virginia,  Trans.,  Am. 
Inst.  Min.  Eng.,  vol.  22,  1894,  pp.  511-536. 

T.  L.  Watson,  Lead  and  zinc  deposits  of  Virginia,  Bull.  1,  Virginia 
Geol.  Survey,  1905. 

T.  L.  Watson,  Mineral  resources  of  Virginia,  1907. 
T.  L.  Watson,  Lead  and  zinc  deposits  of  the  Virginia-Tennessee  region, 
Trans.,  Am.  Inst.  Min.  Eng.,  vol.  36,  1906,  pp.  681-727. 

2E.  F.  Burchard  and  J.  M.  Hill,  Mineral  Resources,  U.  S.  Geol.  Survey, 
annual  publication,  "Mineral  Paints." 

G.  P.  Merrill,  Non-metallic  minerals,  1910,  pp.  104-111. 


ROCK  DECAY  AND  WEATHERING  347 

and  shales  are  also  used.1     The  southern  Clinton  iron  ores  are 
also  employed  for  these  purposes. 

The  residual  iron  ore  deposits  of  the  Southern  States  contain 
material  which  may  be  classed  and  is  used  as  ocher.  Especially 
interesting  are  the  Cartersville  deposits,2  in  Georgia.  These 
ochers  occur  only  in  the  Weisner  (Cambro-Silurian)  quartzite,  in 
the  lower  part  of  the  residual  zone  immediately  above  the 
yet  solid  rock,  and  also  in  shattered  zones  in  the  quartzite  itself. 
The  quartzite  contains  about  90  per  cent.  SiOa,  1 .5  per  cent.  FeS2, 
0.5  per  cent.  Fe203,  and  also  an  unusual  percentage  of  barium 
sulphate  (4.46  per  cent,  in  the  analysis  given  by  Watson). 
The  calculated  constituents  of  the  ocher  are  66  per  cent,  limonite, 
25  per  cent,  clay,  and  9  per  cent,  quartz;  a  little  hematite  is 
probably  also  present. 

Hayes  and  Watson  are  in  agreement  regarding  the  origin  of 
the  ocher,  considering  it  as  resulting  from  a  metasomatic  replace- 
ment of  the  cement  and  the  quartz  grains  of  the  quartzite  by 
limonite.  The  process  begins  by  the  permeation  of  the  grains 
by  dendritic  limonite.  This  direct  formation  of  the  ocher  is 
scarcely  probable,  but  more  likely  it  has  progressed  by  means  of 
an  intermediate  stage  of  siderite.  The  replacement  of  quartz 
by  iron  carbonate  is  a  well-known  phenomenon,  illustrated,  for 
instance,  in  the  Coeur  d'Alene  lead  deposits  of  Idaho. 

The  annual  domestic  production  of  natural  pigments  amounts 
to  about  57,000  tons.  The  mining  is  done  mainly  in  open  pits, 
and  the  material  is  crushed,  washed  in  a  log-washer,  and  allowed 
to  settle  in  tanks. 

RESIDUAL  PHOSPHATES 

As  described  more  fully  on  page  275,  many  sedimentary  beds 
contain  much  phosphate  of  calcium,  often  in  oolitic  or 
concretionary  form.  When  these  beds  are  exposed  to  surface 
waters  an  enrichment  usually  takes  place  by  solution  of  calcium 
carbonate,  provided  the  beds  are  permeable  to  the  circulating 

1B.  L.  Miller,  The  mineral  pigments  of  Pennsylvania,  Rept.  No.  4,  Topo- 
graphic and  Geologic  Survey  Commission  of  Pennsylvania,  Harrisburg,  1911. 
F.  T.  Agthe  and  J.  L.  Dynan,  Paint-ore  deposits  near  Lehigh  Gap,  Penn- 
sylvania, Bull.  430,  U.  S.  Geol.  Survey,  1909,  pp.  440-454. 

2C.  W.  Hayes,  Iron  ores  in  the  Cartersville  district,  Georgia,  Trans., 
Am.  Inst.  Min.  Eng.,  vol.  30,  1901,  pp.  403-419. 

T.  L.  Watson,  The  ocher  deposits  of  Georgia,  Bull.  13,  Georgia  Geol. 
Survey,  1906. 


348  MINERAL  DEPOSITS 

waters.  Many  important  phosphate  deposits — for  instance, 
those  of  Florida,  South  Carolina,  and  Tennessee — have  been 
thus  enriched. 

DEPOSITS  OF  HYDRATED  SILICATES  OF  NICKEL 

The  original  home  of  nickel,  cobalt,  and  chromium  is  in  the 
peridotitic  and  pyroxenic  rocks  and  in  the  serpentines  derived 
from  them,  although  traces  of  these  metals  are  also  frequently 
noted  in  analyses  of  other  basic  rocks.  The  primary  condition 
of  the  nickel  in  the  rocks  is  not  always  known;  probably  it 
occurs  both  as  silicate  and  as  sulphide,  the  latter  in  microscopic 
grains,  the  former  as  an  admixture  in  iron-magnesium  silicates. 
From  the  serpentines  and  peridotites  the  nickel  is  sometimes 
concentrated  in  commercially  important  quantities  by  processes 
of  weathering  and  the  ores  thus  formed  are  always  the  green 
hydrated  silicates  of  nickel.  Chromite,  which  always  occurs 
in  these  basic  rocks,  does  not  readily  yield  oxidized  minerals  in 
the  zone  of  weathering.  Sulphates  of  chromium  have  been 
observed  in  a  quicksilver  mine  in  California,  but  no  silicate 
analogous  to  garnierite  exists. 

Nickel  silicates  are  diverse  and  uncertain  in  composition. 
The  most  important  are  genthite,  H4Ni2Mg2(Si04)3.4H2O; 
connarite,  H4Ni2Si3Oio;  and  garnierite,  (Mg,Ni)Si03+nH20. 
According  to  an  analysis  by  A.  Liversidge  garnierite  contains 
38.35  per  cent.  SiO2;  32.52  per  cent.  NiO;  10.61  per  cent.  MgO; 
0.55  per  cent.  A1203  and  Fe2O3;  11.53  per  cent.  H2O  (at  red  heat) 
and  6.44  per  cent.  H20  (at  100°  C.). 

Such  deposits  are  superficial  and  the  oxidizing  surface  waters 
have  been  the  carrying  and  concentrating  agency.  The  ores 
rarely  extend  far  below  the  water-level  and  in  some  cases  are 
contained  in  the  residual  clays  of  the  completely  weathered 
rock.  These  nickel  ores  are  often  accompanied  by  cobalt  in  the 
form  of  separate  masses  of  asbolite,  a  rather  indefinite  mixture  of 
hydrous  oxides  of  manganese  and  cobalt. 

These  deposits  do  not  contain  sulphides,  and  copper  is  rarely 
present.  The  accompanying  minerals  are  quartz,  chalcedony, 
opal,  and  various  obscure  hydrous  magnesium  silicates,  some- 
times also  a  little  magnesite.  Nickel  ores  of  this  kind  are 
not  uncommon,  but  have  attained  commercial  importance  only 
in  New  Caledonia. 

The  nickel  mine  at  Riddles,  in  southern  Oregon,  has  been 


ROCK  DECAY  AND  WEATHERING  349 

described  by  several  authors.1  The  parent  rock  is  a  peridotite 
containing  0.10  per  cent,  of  NiO.  The  olivine  separated  from 
the  rock  contained  0.26  per  cent,  of  NiO  and  all  observers  agree 
that  the  nickel  ores  are  formed  from  this  silicate.  In  the  finest 
joints  of  the  rock  silica  and  nickel-magnesium  silicates  are 
deposited,  and  between  them  lies  the  oxidized  rock  converted  to 
a  limonite  with  some  clay  and  chromite. 

One  of  the  two  most  important  nickel-bearing  districts  of  the 
world  is  in  New  Caledonia.2  The  island  is  about  250  miles  long 
and  30  miles  wide;  one-third  of  the  area  is  underlain  by  post- 
Cretaceous  serpentine  and  peridotite.  The  lower  slopes  are 
covered  by  a  deep  mantle  of  decayed  rock  ("variegated  clay") 
which  really  is  an  iron  ore  containing,  in  per  cent.,  18  silica,  69 
ferric  oxide,  0.45  alumina,  1 .64  nickel  oxide  and  10  water.  The 
garnierite  deposits  are  found  at  elevations  of  from  400  to  2,500 
feet,  sometimes  on  fairly  steep  slopes,  or  in  the  saddles  of  ridges 
and  spurs.  Underneath  the  "variegated  clay"  at  depths  of 
from  20  to  75  feet  the  nickel  ores  occur  often  descending  into  the 
serpentine  along  fissures  and  accompanied  by  chalcedony  and 
opal.  There  are  many  small  deposits;  the  largest  contained 
only  600,000  tons.  The  ores  are  worked  by  open  cuts  and  care- 
fully graded  and  sorted.  Glasser  classifies  the  deposits  in  vein- 
like,  brecciated,  impregnations  and  earthy  masses.  In  the  latter 
there  is  much  dark  brown  "chocolate  ore"  in  which  the 
green  silicate  is  not  visible.  The  clayey  ore  averages,  in  per 
cent.,  23  water,  5-7  nickel  oxide,  10-12  ferric  oxide,  25  magnesia, 
40  silica,  no  lime,  1.1  chromic  oxide,  0.12  cobalt,  and  1.5  alumina. 
Most  of  the  ore  is  exported.  Some  of  it  is  sun  dried  and  bri- 
quetted  for  local  smelting  to  45  per  cent,  nickel  matte  with 
limestone  and  gypsum  flux. 

The  New  Caledonia  deposits  were  discovered  by  the  geologist 
Gamier  in  1864;  the  mines  were  opened  10  years  later,  and  the 
cheaply  mined  rich  ores  made  all  nickel  deposits  elsewhere  un- 
profitable. In  1906  the  maximum  output  of  144,000  metric  tons 

1  Diller  and  Clarke,  Bull.  60,  U.  S.  Geol.  Survey,  1890,  p.  21. 
G.  F.  Kay,  Bull.  315,  U.  S.  Geol.  Survey,  1907,  p.  120. 

2  E.  Glasser,  Rapport  sur  les  richesses  mine'rales  de  la  Nouvelle  Cal6donie, 
Ann.  des  Mines  (10),  vol.  5,  1904,  pp.  29-154,  503-701. 

G.  M.  Colvocoresses,  Eng.  and  Min.  Jour.,  Sept.  21  and  28,  1907. 
|  W.  G.  Miller,  Nickel  Deposits  of  the  World,  reprinted  from  Report  of 
Royal  Ontario  Nickel  Commission,  Toronto,  1917,  pp.  234-264. 


350  MINERAL  DEPOSITS 

was  reached.  Lately,  owing  to  the  active  competition  of  the 
Sudbury  mines  (p.  814),  the  output  has  been  materially  reduced. 
In  1916,  30,100  tons  of  ore  as  well  as  5,000  tons  of  nickel  matte 
were  exported  to  England  and  France.  A  small  quantity  of 
cobalt  ore,  a  black,  earthy  asbolite,  was  exported  for  a  number 
of  years  but  at  present  can  not  compete  with  the  ore  from  Cobalt, 
Ontario.  The  island  also  produces  much  chromite  (p.  794). 

BAUXITE  i 

Introduction. — Clay,  as  more  or  less  impure  kaolin,  is  the 
most  abundant  product  of  rock  decay,  but  although  it  carries 
39.8  per  cent,  alumina  its  use  as  a  source  of  metallic  aluminum 
has  not  been  found  possible.  Corundum  is  not  abundant  enough 
to  be  used  for  this  purpose.  Cryolite  (Na3AlFl6),  a  mineral 
obtained  from  pegmatitic  masses  occurring  in  Greenland,  was 
formerly  an  important  aluminum  ore  and  is  still  used,  in  smaller 
quantities,  in  the  electrolytic  processes  for  the  extraction  of 
aluminum. 

In  certain  places  the  weathered  zone,  however,  contains  the 
hydroxides  of  aluminum  and  of  these  bauxite  is  the  most  im- 
portant aluminum  ore.  There  are  three  aluminum  hydroxides: 
Diaspore,  A1203.H2O,  with  85  per  cent.  AhOs;  gibbsite  or 
hydrargillite,  A1203.3H2O,  with  65.4  per  cent.  A12O3;  and  bauxite, 
A12O3.2H2O,  with  74  per  cent.  A12O3. 

The  independence  of  bauxite  as  a  mineral  species  is,  however, 
questioned  and  many  authors  consider  it  a  hardened  and  in 
part  crystallized  hydrogel  of  indefinite  composition.  The 
Georgia  bauxite,  according  to  T.  L.  Watson,  corresponds  well 
to  gibbsite.  F.  Laur  states  that  the  French  bauxites  also 
agrees  with  the  formula  given  above  for  this  mineral.  Bauxite 
forms  compact,  earthy,  also  very  commonly  pisolitic  masses, 
the  individual  concretions  often  having  a  diameter  of  several 
centimeters.  It  is  gray,  cream-colored,  yellowish  or  brown  and 

1C.  W.  Hayes,  Bauxite,  its  occurrence,  geology,  etc.,  Sixteenth  Ann. 
Rept.,  U.  S.  Geol.  Survey,  pt.  3,  1895,  pp.  547-597. 

T.  L.  Watson,  Bull.  11,  Georgia  Geol.  Survey,  1904  (Bibliography). 

F.  W.  Clarke,  Geochemistry,  Bull.  616,  U.  S.  Geol.  Survey.  1916,  pp. 
493-501. 

W.  C.  Phalen,  Mineral  Resources,  U.  S.  Geol.  Survey,  Annual  pub- 
lication. 

J.  W.  Richards,  Mineral  Industry,  Annual  publication. 


ROCK  DECAY  AND  WEATHERING  351 

is  usually  admixed  with  silica  and  ferric  oxide.  Its  occurrence 
and  structure  lend  probability  to  the  view  that  it  has  originated 
as  a  colloid  precipitate.  The  bauxites  contain  in  places  crys- 
talline gibbsite  as  crusts  or  veinlets;  diaspore  has  been  identified 
more  rarely  and  quite  naturally  as  it  usually  formed  at  higher 
temperature  than  that  prevailing  in  residual  deposits. 

The  bauxites  always  contain  titanium,  averaging  as  much  as 
4  per  cent.  Ti02,  and  some  vanadium  but  in  this  they  merely 
share  the  peculiarities  of  residual  and  sedimentary  clays.  Some 
investigators  state  that  bauxite  contains  residual  rutile  while 
others  have  failed  to  find  any  titanium  mineral.  Most  probably 
the  titanic  dioxide  is  present  in  colloid  state. 

Little  or  no  hydroxide  of  aluminum  forms  in  ordinary  rock 
weathering.  Cameron  and  Bell1  state  that  during  an  examina- 
tion of  several  thousand  soils  from  all  parts  of  the  United  States, 
hydroxide  of  aluminum  was  observed  in  only  one  sample,  which 
came  from  southern  California.  Bauxite,  it  may  be  concluded, 
is  thus  rarely  formed  in  the  temperate  region. 

In  tropical  countries,  on  the  other  hand,  the  deep  residual 
soil  very  often  contains  aluminum  hydroxide.  This  has  Jaeen 
called  laterite  (later,  brick)  and  is  variously  defined.  We  may 
say  that  true  laterite  is  essentially  a  mixture  of  the  hydroxides 
of  iron  and  aluminum  with  more  or  less  free  silica,  but  there  are 
all  gradations  toward  an  ordinary  ferruginous  clay.  The  laterite 
may  be  derived  from  any  igneous  or  sedimentary  rock  but  ser- 
pentine and  limestone  are  specially  favorable.  The  iron  ore  from 
Mayari,  Cuba  (p.  334)  is  a  laterite  exceptionally  rich  in  iron. 
Many  so-called  laterites  are  not  true  residual  but  transported 
deposits.  Laterites  may  or  may  not  contain  bauxite  of  economic 
value;  they  have  been  described  from  many  lands  and  the 
literature  is  very  extensive.2 

*Bvll.  30,  Bureau  of  Soils,  U.  S.  Dept.  of  Agr.,  1905,  p.  28. 
2  A.    Streng,  Zeitschr.  deutsch.   Geol.   Gesell.,  vol.  39,   1887,    p.   621. 
(Germany). 

A.  Bauer,  Neues  Jabrb.,  Festband,  1907,  p.  33  and  1898,  pt.  2,  p.  192. 
(Seychelle  Islands). 

R.  D.  Oldham  and  T.  H.  Holland,  Records,  Geol.  Survey  India,  vol.  32, 
pt.  2,  1905,  pp.  175-184. 

L.  L.  Fermor,  The  manganese  deposits  of  India,  Mem.  Geol.  Survey  India, 
vol.  37,  1909,  pp.  370-380. 

'G.  C.  DuBois,  Min.  pet.  Mitt.,  vol.  22,  1903,  p.  1.     (Surinam). 

A.  Lacroix,  Nouv.  Arch.  Mus.  Hist.  Nat.  (Peris),  5th  ser.,  vol.  15,  1913 


352  MINERAL  DEPOSITS 

In  apparent  contradiction  to  this  many  of  the  worked  bauxite 
deposits  are  found  in  temperate  regions  such  as  Georgia,  Arkansas, 
France,  Hungary,  etc.,  but  this  is  explained  by  the  fact  that  these 
are  not  being  formed  at  the  present  time  but  are  of  Tertiary  age 
when  a  climate  like  that  of  Cuba  prevailed  in  large  parts  of  the 
temperate  zone. 

Origin. — The  desilication  of  clay  in  low  latitudes  has  been 
discussed  extensively.  The  action  of  nitric  acid,  supposedly 
derived  from  rain  during  tropical  thunderstorms,  has  been  sug- 
gested as  the  cause.  T.  H.  Holland1  has  mentioned  the  pos- 
sibility of  bacterial  action. 

Clay  is  decomposed  by  sulphuric  acid  and  by  sodium  hydroxide 
or  sodium  carbonate  and  at  some  places  aluminum  hydroxide  may 
have  originated  in  this  way.  W.  Maxwell2  has  demonstrated 
this  origin  for  some  of  the  soils  of  Hawaiian  volcanoes  and  it 
applies  also  to  a  deposit  of  alum  and  bauxite  on  the  upper 
Gila  River3  in  New  Mexico.  Nevertheless  it  is  clear  that  sul- 
phuric acid  does  not  always  produce  this  effect,  for  diaspore 
and  hydrargillite  occur  rarely  (Rosita  Hills,  Colorado;  Gold- 
field,  Nevada)  in  the  oxidized  portions  of  mineral  deposits  where 
the  sericitic  rocks  are  acted  upon  by  sulphuric  acid  solutions. 
Bauxite  also  has  rarely  been  observed.  In  the  oxidized  zone  the 
sulphuric  acid  transforms  sericite  into  kaolin,  which  is  frequently 
accompanied  by  more  or  less  alunite  (K20.3A12O3.4S03+6H20). 

These  suggestions  do  not  suffice  to  explain  the  formation  of 
the  lateritic  aluminum  hydroxides.  It  is  now  generally  con- 
ceded that  this  is  caused  simply  by  the  long  continued  action  of 
ordinary  groundwaters  under  special  conditions  of  moisture  and 
heat.  W.  J.  Mead4  has  shown  that  there  is  a  complete  grada- 
tion in  case  of  the  Arkansas  deposits  from  the  original  syenite 

p.  255,  reviewed  by  L.  L.  Fermor,  Geol.  Mag.,  1915,  pp.  28,  77,  123.  (French 
Guinea). 

J.  M.  VanBemmelen,  Zeitschr.  Anorg.  Chemie,  vol.  66,  1910,  p.  322 
(General  review). 

J.  Morrow  Campbell,  Laterite,  its  origin,  structure,  etc.,  Mining  Maga- 
zine (London)  Aug.-Nov.,  1917.  (Tropical  Africa.) 

1  T.  H.  Holland,  Geol  Mag.,  1903,  p.  59. 

2  W.  Maxwell,  Lavas  and  soils  of  the  Hawaiian  Islands,  1898. 
'  C.  W.  Hayes,  Bull.  315.  U.  S.  Geol.  Survey,  1906,  pp.  215-223. 
4  Econ.  Geol.,  vol.  10,  1915,  pp.  28-54. 

See  also  Leith  and  Mead,  Metamorphic  geology,  New  York,  1915. 
pp.  25-38. 


ROCK  DECAY  AND  WEATHERING 


353 


(Fig.  115)  that  the  pisolitic  structure  develops  in  place  and  that 
residual  syenite  boulders  are  surrounded  by  bauxitic  material. 
The  texture  of  the  syenite  is  sometimes  visible  in  the  pisolitic 
bauxite.  There  is  some  evidence  of  downward  leaching  of  the 
bauxite,  for  the  top  layer  is  usually  more  siliceous  than  the  lower 
parts  of  the  deposit.  J.  Morrow  Campbell  believes  that  bauxite 
only  forms  in  the  zone  of  percolation  close  to  the  fluctuating  water 
level  and  that  it  never  occurs  far  below  the  water. 


.SiOj  H,O 

FIG.  115. — Triangular  diagram  showing  the  gradation  from  syenite  to 
bauxite  in  terms  of  the  principal  chemical  constituents.  Each  triangle 
represents  an  analyzed  sample.  After  W.  J.  Mead.] 

Some  bauxites  occurring  as  veinlike  masses  in  limestone  of 
France1  and  Hungary2  have  been  explained  as  the  result  of  the 
action  of  ascending  waters  carrying  aluminum  sulphate  on 
limestone.  Such  a  mode  of  origin  is  admittedly  possible. 

The  sedimentary  bauxites  of  which  numerous  examples  may 
be  found  in  Georgia  and  Arkansas  in  the  Cretaceous  and  Tertiary 

1 F.  Laur,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  24,  1894,  p.  234. 
2  O.  Pauls,  Zeiischr.  prakt.  Geol,  vol.  21,  1913,  pp.  521-572. 


354  MINERAL  DEPOSITS 

beds  are  probably  deposits  swept  out  into  the  sea  by  the  normal 
processes  of  erosion  from  bauxite  rich  laterites  on  the  shore. 

Occurrences. — The  bauxite  deposits  of  commercial  importance 
are  of  several  different  types.  In  the  United  States  they  are 
confined  to  Arkansas  and  the  southern  Appalachian  States. 

In  Arkansas1  the  mineral  occurs  in  Pulaski  and  Saline  coun- 
ties as  superficial  beds  over  areas  of  various  sizes  up  to  20  acres. 
The  deposits  are  only  exceptionally  more  than  10  feet  in  depth. 
They  rest  on  nepheline  syenite  or  on  a  kaolinized  form  of  that 
rock;  the  lower  part  retains  traces  of  granitic  structure,  while 
the  upper  part  is  distinctly  pisolitic.  Tertiary  sands  and  clays 
in  places  cover  the  nepheline  syenite  and  the  bauxite. 

Other  deposits  of  importance,  described  by  Hayes2  and  also 
by  Watson,3  are  found  at  a  number  of  places  in  Georgia  and 
Alabama.  The  principal  occurrences  are  scattered  between 
Jacksonville,  Alabama  and  Cartersville,  Georgia,  along  a  belt 
about  60  miles  in  length,  one  of  the  typical  localities  being  at 
Rock  Run.  The  bauxite  occurs  as  pockets  and  irregular  masses 
or  curved  strata  of  various  colors,  with  clay  and  limonite,  in  the 
heavy  mantle  of  residual  clay  overlying  the  Knox  (Cambrian) 
dolomite,  but  sharply  separated  from  it.  The  ore  is  in  part 
pisolitic  and  is  mined  in  open  cuts,  at  some  places  to  a  depth 
of  50  feet  or  more.  .The  bottom  of  the  clay  masses  is  rarely 
exposed;  before  it  is  reached  the  pockets  of  bauxite  generally 
terminate  in  tapering  points.  Occasionally  associated  minerals 
are  gibbsite  (A1203.3H2O)  and  halloysite,  which  is  similar  to 
kaolin  in  composition  but  has  more  water. 

A  suggestive  fact  is  the  occurrence  of  the  deposits  at  or  about 
the  900-foot  contour,  which  coincides  with  the  elevation  of  a 
probable  Eocene  peneplain.  The  ores  were  thus  accumulated 
under  topographic  and  climatic  conditions  different  from  those 
which  prevail  to-day. 

Deposits  differing  considerably  from  those  already  described 
have  recently  been  found  in  Randolph  and  Wilkinson  County, 

1  C.  W.  Hayes,  Twenty-first  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  3,  1901, 
pp.  435-472. 

J.  C.  Branner,  Jour.  Geol.,  vol.  5,  1907,  pp.  263-289. 
2C.  W.  Hayes,  The  geological  relations  of  the  southern  Appalachian 
bauxite  deposits,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  24,  1895,  pp.  243-254. 

W.  J.  Mead,  op.  cit. 
8  T.  L.  Watson,  op.  cit. 


ROCK  DECAY  AND  WEATHERING 


355 


Georgia. l  They  occur  near  the  contact  of  the  flat-lying  sands  and 
clays  of  the  Tuscaloosa  (Lower  Cretaceous)  and  Claiborne 
(Tertiary)  formations.  The  ore  occurs  either  as  beds  resting 
directly  upon  Cretaceous  clay  or  disseminated  as  nodules  through 
it.  A  perfect  series  of  transition  to  clay  exists,  as  shown  by 
analyses.  Bauxite  beds  10  feet  in  thickness  have  been  observed; 
the  mineral  is  clayey,  dense,  or  pisolitic. 

Uses  and  Production. — The  annual  production  of  bauxite  in 
the  United  States  has  been  increasing  rapidly  and  in  1917  was 
569,000  long  tons,  most  of  which  was  mined  in  Arkansas.  The 
mines  in  France,  also  yield  an  increasing  amount,  about  300,000 
long  tons  in  1913.  New  deposits  are  being  opened  in  the 
British  and  'Dutch  Guianas.  The  bauxite  ores  contain  35  to 
57  per  cent.  A12O3,  a  greatly  varying  percentage  of  Fe203,  and 
up  to  30  per  cent.  Si02.  Ores  with  more  than  4  per  cent.  Fe2Oa 
are  not  utilized  at  present.  They  are  mined  in  open  cuts,  often 
necessitating  the  removal  of  heavy  overburden,  washed  to  re- 
move the  clay,  and  dried.  For  purposes  of  aluminum  smelting 
the  ores  must  be  of  high  grade  and  low  in  silica.  About  50,000 
short  tons  of  aluminum  are  now  produced  annually  in  the 
United  States;  exact  data  are  not  obtainable.  The  uses  of  the 
metal  and  its  alloys  are  steadily  increasing. 

Large  works  for  the  electric  smelting  of  aluminum  are  located 
at  Niagara  Falls  and  in  Tennessee.  Artificial  corundum  (alun- 
dum)  is  made  from  the  ore  by  the  electric  furnace.  Bricks  of 
bauxite  for  basic  non-corrosive  lining  of  furnaces  are  widely  used.2 
Aluminum  salts,  especially  alum,  are  also  manufactured  from 
bauxite.  An  addition  of  bauxite  promotes  the  rapid  setting  of 
cements. 

ANALYSES  OF  BAUXITE 


Si02 

TiO2 

A1203 

Fe20, 

H20 

Analyst 

Baux,  France  (pisolitic). 

4.8 

3.2 

55.4 

24.8 

10.8 

Deville. 

Jacksonville,  Ala  

21.08 

2.52 

48.92 

2.14 

23.41 

Hillebrand. 

Floyd  County,  Ga...  ... 

0.80 

3.52 

52.21 

13.50 

27.72 

Nichols. 

Pulaski  County,  Ark...  . 

2.00 

3.50 

62.05 

1.66 

30.31 

Wilkinson  County,  Ga.  . 

9.38 

2.76 

57.58 

0.96 

29.12 

E.  Everhart. 

1 0.  Veatch,  Butt.  18,  Georgia  Geol.  Survey,  1909,  pp.  430-447. 
2  Mineral  Resources,  U.  S.  Geol.  Survey,  1913. 


356  MINERAL  DEPOSITS 

Great  variations  are  often  shown  in  one  locality.  For  further 
analyses  see  G.  P.  Merrill,  Non-metallic  minerals,  1910,  p.  91. 
The  average  of  a  long  series  of  analyses  of  commercial  ore  from 
Georgia  tabulated  by  T.  L.  Watson1  gives:  SiO2,  4.274;  Ti02, 
3.791;  A1203,  58.622;  Fe203,  1.507;  and  H2O,  31.435;  total 
99.629.  This  corresponds  to  A1203.3H2O. 

1  Bull.  11,  Georgia  GeoL  Survey,  1904,  pp.  45-46. 


CHAPTER  XIX 

THE  HEMATITE  DEPOSITS   OF  THE  LAKE  SUPERIOR 
REGION 

General  Character,  Distribution. — The  iron-ores  mined  in  the 
Lake  Superior  region  in  Minnesota,  Michigan  and  Wisconsin 
amount  to  from  80  to  90  per  cent,  of  the  total  domestic  output 
and  in  1917  yielded  64,000,000  long  tons.  The  ore  is  mainly 
hematite  with  small  admixtures  of  limonite  and  magnetite.  It 
occurs  as  masses,  lenses,  or  flat  deposits  in  pre-Cambrian  sedi- 
mentary rocks  of  Algonkian  and  Archean  age.  The  deposits  are 
concentrated  by  the  oxidizing  and  silica-dissolving  effect  of  waters 
of  meteoric  origin,  in  original  sediments  called  "iron  formation" 
which  were  rich  in  carbonate  and  silicate  of  iron.  They  are 
products  of  pre-Cambrian  weathering  which,  probably  under  arid 
conditions,  reached  depths  not  approached  elsewhere.  Only  to  a 
small  degree  and  near  the  surface  does  this  ore  forming  activity 
of  the  waters  persist  at  the  present  time. 

We  owe  most  of  our  information  concerning  these  deposits  to 
the  work  of  C.  R.  Van  Hise,  C.  K.  Leith,  and  many  others  re- 
corded in  a  magnificent  series  of  monographs  of  the  United  States 
Geological  Survey.  These  and  other  papers  are  cited  below.1 

1  R.  D.  Irving  and  C.  R.  Van  Hise  (Penokee  district)  Man.  19,  U.  S. 
Geol.  Survey,  1892. 

C.  R.  Van  Hise  and  W.  S.  Bayley  (Marquette  district)  Mon.  28,  1897. 

J.  M.  Clements  and  H.  L.  Smyth  (Crystal  Falls  district)  Mon.  36,  1899. 

C.  K.  Leith  (Mesabi  district)  Mon.  43,  1903. 

J.  M.  Clements  (Vermilion  district)  Mon.  45,  1903. 

W.  S.  Bayley  (Menominee  district)  Mon.  46,  1904. . 

C.  R.  Van  Hise,  Iron  ore  deposits  of  the  Lake  Superior  region,  Twenty- 
first  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  3,  1901,  pp.  305-434. 

C.  R.  Van  Hise  and  C.  K.  Leith,  The  geology  of  the  Lake  Superior 
region,  Mon.  52,  1911. 

S.  Weidman,  The  Baraboo  iron-bearing  district,  Wisconsin,  Bull.  13, 
Wisconsin  Geol.  and  Nat.  Hist.  Survey,  1904. 

C.  K.  Leith,  A  summary  of  Lake  Superior  geology,  Trans.,  Am.  Inst. 
Min.  Eng.,  vol.  36,  1906,  pp.  101-153. 

C.  K.  Leith,  The  geology  of  the  Cuyuna  iron  range,  Minnesota,  Econ. 
357 


358  MINERAL  DEPOSITS 

There  are  seven  principal  districts  in  the  United  States  and 
three  or  four  in  Canada,  locally  called  ranges,  as  follows  (Fig. 
116): 

1.  The  Mesabi,  Vermilion,  and  Cuyuna  ranges  of  northern 
Minnesota. 

2.  The  Penokee-Gogebic,  Marquette,  Iron  River  and  Meno- 
minee  ranges,  mainly  in  northern  Michigan. 

3.  The  Baraboo  range  of  southern  Wisconsin. 

4.  The  Michipicoten,  Gunflint  Lake,  and  other  minor  districts 
in  Canada. 

Geology. — The  following  short  summary  is  in  part  taken  from 
the  resume's  by  Leith  and  Harder  cited  above.1 

The  principal  rocks  of  the  Lake  Superior  iron  ore  region  are 
divided  as  follows: 

Cambrian:   Potsdam  sandstone. 

Algonkian:  Keweenawan  series  (sediments,  basic  flows,  gabbro, 
Huronian  series: 

Upper  Huronian  (quartzite,  "iron  formation," 

and  slate). 
Middle  Huronian  (quartzite,  ''iron  formation," 

and  slate). 

Lower  Huronian  (quartzite,  conglomerate, 
dolomite,  slate,  "iron  formation,"  and 
intrusives).  ! 

Archean:  Laurentian  series  (granite,  gneiss,  and   porphyry). 
Keewatin  series  (greenstone,  amphibolite,  and  "iron 
formation"). 


Geol,  vol.  2,  1907,  p.  145;  The  iron  ores  of  Canada,  Econ.  Geol.,  vol.  3,  1908, 
pp.  276-291. 

E.  C.  Harder  and  A.  W.  Johnston,  Notes  on  the  geology  of  the  Cuyuna 
district,  Bull.  660,  U.  S.  Geol.  Survey,  1917,  pp.  1-26. 

J.  F.  Wolff  (Mesabi  range),  Eng.  and  Min.  Jour.,  July  17-Aug.  7,  1914; 
Trans.  Am.  Inst.  Min.  Eng.,  vol.  56,  1917,  pp.  142-169. 

E.  C.  Harder,  Mineral  Resources,  U.  S.  Geol.  Survey,  1908,  Summary. 

E.  F.  Burchard,  The  production  of  iron  ore,  etc.,  Mineral  Resources, 
Annual  publication. 

Carl  Zapffe,  The  Cuyuna  iron  ore  district,  Brainerd  Tribune,  Suppl., 
July  1,  1911. 

1  Classifications  differing  somewhat  from  that  here  given  have  been 
proposed  in  Canada  by  Coleman,  Miller  and  Knight,  but  for  present  pur- 
poses the  nomenclature  current  in  the  United  States  is  adhered  to.  See 
W.  G.  Miller  and  C.  W.  Knight,  Bull.  Geol.  Soc.  Am.,  vol.  26, 1915,  p.  87; 
Rept.  Ontario  Bur.  Mines,  vol.  22,  pt.  2,  1914. 


THE  HEMATITE  DEPOSITS 


359 


360  MINERAL  DEPOSITS 

Of  these  rocks  only  the  Upper  and  Middle  Huronian  and  the 
Keewatin  contain  deposits  of  hematite. 

The  Archean  or  basement  complex  consists  of  gneiss  and  gran- 
ite with  an  extensive  series  of  greenstones  (basalt,  gabbro,  am- 
phibolite),  which  are  largely  surface  lavas.  These  lavas  are  now 
regarded  as  the  oldest  formation  exposed;  the  character  of  the 
basement  upon  which  they  were  outpoured  is  unknown.  Above 
the  Keewatin  lavas  lie  sedimentary  rocks  of  the  iron  formation. 
The  gneisses  and  granites  are  in  part  certainly  intrusive  into  the 
Keewatin  series. 

In  the  Vermilion  and  Michipicoten  districts  the  productive 
formation  is  in  the  Keewatin  series. 

Unconformably  overlying  the  Archean  and  similarly  covered 
by  the  Cambrian  is  the  Algonkian,  which  in  its  complete  develop- 
ment consists  of  four  parts  separated  by  uncomformities.  The 
lower  three  divisions  are  collectively  referred  to  as  the  Huronian 
and  the  uppermost  as  the  Keweenawan.  The  principal  iron- 
bearing  formations  are  concentrated  in  the  Huronian,  but  the 
development  differs  materially  in  the  several  districts. 

In  the  Marquette  district  all  three  divisions  of  the  Huronian 
are  present.  The  lower  Marquette  series  consists  of  quartzite, 
dolomite,  and  slate  3,000  feet  in  maximum  thickness.  The 
middle  Marquette  series,  3,000  feet  in  maximum  thickness, 
includes  quartzite,  slate,  and  the  important  Negaunee  iron- 
bearing  formation.  The  upper  Marquette  series  includes  quart- 
zite, schist,  slates,  and  fraginental  basic  volcanic  rocks,  each 
member  accompanied  by  iron-bearing  formations. 

In  the  Crystal  Falls  and  Menominee  districts  similar  divisions 
appear. 

In  the  Penokee-Gogebic  district  the  Upper  and  Lower 
Huronian  series  are  present,  but  the  middle  series  appears  to 
be  lacking.  The  lower  division  consists  of  quartzite,  cherty  lime- 
stone, and  dolomite;  the  upper  part  includes  quartz  slates  and  the 
thick  Ironwood  "iron-bearing  formation"  and  may  aggregate 
about  13,000  feet  in  thickness. 

In  the  Mesabi  district  the  Lower  Huronian  consists  of  con- 
glomerates, graywackes,  and  slates  standing  vertically;  it  is 
intruded  by  the  granite  of  the  Giants  Range,  on  the  south  slopes 
of  which  the  iron  deposits  extend  from  east  to  west  for  a  distance 
of  100  miles.  The  Upper  Huronian  comprises  a  basal  quartzite, 
the  Biwabik  "iron-bearing  formation,"  and  the  overlying 


THE  HEMATITE  DEPOSITS  361 

Virginia  slate.  The  total  thickness  is  probably  over  2,000 
feet.  The  series  dips  gently  at  angles  of  5°  to  20°  and  is 
also  gently  cross  folded.  Intrusive  into  these  rocks  at  the  east 
end  of  the  district  are  Keweenawan  granite  and  basic  igneous 
rocks.  Near  these  intrusives  the  sedimentary  rocks  are  highly 
metamorphosed. 

Above  the  Huronian  rests  the  less  highly  metamorphosed 
Keweenawan  series  of  sandstones,  conglomerates,  and  igneous 
basic  flows;  the  thickness  is  estimated  to  be  over  35,000  feet. 
It  contains  no  iron  deposits. 

The  west  end  of  Lake  Superior  consists  of  an  eastward- 
pitching  synclinorium  of  Keweenawan  rocks.  The  next  underly- 
ing series,  the  Upper  Huronian,  takes  less  part  in  this  synclinal 
structure  and  borders  the  other  edge  of  the  Keweenawan  areas. 

We  have  thus  in  the  Lake  Superior  country  six  series,  consist- 
ing from  top  to  bottom  of  the  Keweenawan,  Upper,  Middle,  and 
Lower  Huronian,  Laurentian,  and  Keewatin,  all  but  the  last  two 
separated  by  unconformities.  Above  them  and  separated  by  a 
marked  unconformity  rests  the  Cambrian  Potsdam  sandstone. 

The  "Iron  Formations." — The  iron  ores  of  the  Lake  Superior 
region  are  believed  to  be  derived  by  concentration  by  means  of 
meteoric  waters  from  lean  "iron  formations"  containing  about 
25  per  cent.  iron.  The  ores  are  products  of  enrichment  of 
chemically  deposited  sediments,  such  as  siderite  and  hydrated 
iron  silicates,  for  the  most  part  interbedded  with  normal  clastic 
sediments,  such  as  slate  and  quartzite. 

The  iron  formations  range  from  a  few  feet  up  to  1,000  feet  in 
thickness  and  are  sedimentary  beds  consisting,  according  to  Leith, 
"mainly  of  chert,  or  quartz,  and  ferric  oxide  segregated  in  bands 
or  sheets,  or  irregularly  mingled.  Where  in  bands  with  the 
quartz  layers  colored  red  and  the  rock  highly  crystalline  it 
is  called  jasper.  Where  less  crystalline  and  either  in  bands  or 
irregularly  intermingled  the  rock  is  known  as  ferruginous  chert. 
The  silica  in  these  rocks  varies  from  32  to  80  per  cent.,  the 
ferric  oxide  from  31  to  66  per  cent.  Other  phases  of  the  iron 
formation,  subordinate  in  quantity,  are  (1)  ordinary  clay  slates, 
showing  every  possible  gradation  through  ferruginous  slates 
into  ferruginous  cherts;  (2)  paint  rocks,  oxidized  equivalents  of 
the  slates;  (3)  cherty  iron  carbonate  (siderite)  and  hydrous  fer- 
rous silicate  (greenalite) ;  (4)  the  iron  ores  themselves.  Almost 
the  entire  bulk  of  the  iron  formations  now  consists  of  iron  oxide 


362  MINERAL  DEPOSITS 

and  silica,  with  carbonates  and  alumina  present  in  subordinate 
quantity." 

Spurr  and  Leith  found  that  certain  rocks  of  the  Mesabi  district 
contained,  in  a  matrix  of  chert  and  iron  carbonate,  abundant 
round  granules  of  a  green  chloritic  substance  which  Leith  called 
greenalite;  its  composition  is  approximately  30  to  38  per  cent. 
SiO2,  8  to  34  per  cent.  Fe203,  25  to  47  per  cent.  FeO,  and  7  to  9 
per  cent.  H2O  (p.  263).  The  absence  of  potassium  shows  that 
the  mineral  is  not  glauconite.  The  greenalite  rocks  contain 
50  to  80  per  cent,  of  this  mineral,  which  is  soluble  in  acids. 
The  same  mineral  occurs  in  some  of  the  siderite  rocks  of  the 
more  easterly  districts. 

Regional  metamorphism  and,  to  a  greater  degree,  contact 
metamorphism,  caused  by  Keweenawan  intrusions  of  granites 
and  gabbros,  have,  in  places,  converted  the  siderite  and  the 
greenalite  rocks  to  magnetite-amphibole  schists  and  the  soft 
hematite  to  specularite;  this  is  especially  well  observed  in  the 
Marquette  and  the  Mesabi  ranges. 

The  Iron  Ores. — The  hematite  ores  are  derived  from  the  fer- 
ruginous cherts  by  a  process  of  concentration,  and  both  laterally 
and  in  depth  gradually  change  into  such  rocks.  The  ores  are 
admixed  with  enough  magnetite  to  affect  the  magnetic  needle 
and  render  possible  magnetic  surveys  of  the  fields.  The  hard 
blue  specular  ores  of  the  Marquette  range  contain  more  magnetite 
than  the  others  and  are  accompanied  by  contact-metamorphic 
jaspers  and  magnetite-amphibole  (griinerite)  rocks.  In  other 
ranges,  such  as  the  Mesabi,  Penokee,  and  Baraboo,  the  ore  is 
soft,  bluish,  red,  or  brown  in  color,  and  partly  hydrated.  A 
micaceous  or  foliated  development  of  the  iron  ore  is  not  common. 
The  average  analysis  of  Lake  Superior  ores  in  1909  is  as  follows: 

Per  cent. 

Moisture  (loss  at  100°  C.) 11.28 

Analysis  of  dried  ore : 

Iron 58.45 

Phosphorus 0.091 

Silica 7.67 

Alumina 2.23 

Manganese 0.71 

Lime 0.54 

Magnesia 0 . 55 

Sulphur 0.06 

Loss  by  ignition 4  12 


THE  HEMATITE  DEPOSITS 

This  corresponds  to  a  composition  as  follows : 

Hematite  (more  or  less  hydrated) 86.45 

Quartz 4.89 

Kaolin 5.25 

Chlorite 1.01 

Dolomite '. 0.81 

Apatite 0.48 

Miscellaneous ...  1.11 


100.00 

The  tenor  in  iron  of  the  shipped  ore  has  slowly  diminished 
during  recent  years;  in  1905  it  was  59.6  per  cent.  Fe. 

The  phosphorus  ranges  between  0.008  and  1.29,  the  bulk  of  the 
ore  being  of  Bessemer  grade — that  is,  containing  less  than  0.05 
per  cent,  phosphorus.  Small  parts  of  the  ore  shipped,  particu- 
larly from  the  Mesabi  and  Cuyuna  ranges  contain  as  much  as 
7  per  cent,  manganese.  The  sulphur  varied  from  0.003  to  1.87 
per  cent.,  but  it  averages  low.  Accessory,  more  or  less  rare  min- 
erals in  the  ore,  aside  from  quartz  or  chert,  are  apatite,  wavellite, 
adularia,  calcite,  dolomite,  siderite,  pyrite,  marcasite,  chal- 
copyrite,  tourmaline,  ottrelite,  chlorite,  garnet,  mica,  rhodo- 
chrosite,  barite,  gypsum,  analcite,  goethite,  and  turgite. 

The  ore  reserves  of  the  Mesabi  range  are  estimated  to  be 
1,385,000,000  tons;  those  of  the  whole  region  1,475,000,000  tons. 

The  total  yield  of  the  Lake  Superior  ores  from  1854  to  1916 
has  been  about  770,000,000  long  tons,  much  the  greater  propor- 
tion having  been  extracted  in  the  last  three  decades. 

Carbonate  ores  are  now  mined  in  the  Miehipicoten  district, 
Canada. 

Form  of  Ore  Bodies. — The  ore  forms  irregular,  often  very  large, 
but  as  a  rule  distinctly  bedded  or  banded  masses  in  the  "iron 
formations;"  in  places  it  is  entirely  embedded  in  them.  The 
shape  is  commonly  determined  by  impervious  basements  like 
clayey  dikes,  decomposed  amphibolitic  rocks,  or  folded  sedi- 
mentary beds  like  slate,  which  have  tended  to  guide  the  circula- 
tion of  surface  water  into  certain  channels;  the  ores  usually  occur 
in  pitching  troughs  caused  by  any  or  all  of  these  factors. 

In  some  ranges  like  the  Gogebic,  Marquette,  and  Iron  River  the 
strata  are  strongly  folded  and  may  dip  at  high  angles;  some  of 
the  ore-bodies  have  been  followed  to  a  depth  of  1,500  or  2,000  feet. 
Good  ore  is  mined  at  present  in  the  Newport  mine  in  the  Gogebic 
district  at  2,000  feet.  In  the  Mesabi  range  the  rocks  lie  horizon- 
tal; the  alteration  and  concentration  have  extended  over  a  wide 


364 


MINERAL  DEPOSITS 


area  and  few  of  the  mines  are  deeper  than  200  feet.  The  shal- 
low deposits  of  this  range  are  mined  on  an  enormous  scale  by 
steam-shovels.  The  annual  production,  which  reached  41,127,- 
323  long  tons  in  1917,  is  far  greater  than  that  of  other  districts. 
Marquette  Range. — The  mines  of  the  Marquette  range  are  near 
Negaunee  and  Republic,  south  and  southwest  of  Marquette. 
The  principal  "iron  formation,"  the  Negaunee,  is  in  the  Middle 
Huronian,  and  the  sedimentary  rocks  are  intruded  and  meta- 
morphosed by  basic  igneous  rocks.  Extensive  folding  has  taken 
place  and  the  strata  are  compressed  into  a  great  synclinal  basin. 


FIG.  117. — Longitudinal  section  of  the  Montreal   Mine,    Gogebic   Range, 
Michigan,  showing  dependence  of  bodies  of  oxidized  iron  ore  on  dikes.  \ 

The  ores  lie  at  the  base  of  the  Negaunee  formation,  where  the 
underlying  slates  have  been  folded  so  as  to  form  pitching  synclinal 
basins,  or  where  dikes  have  guided  the  concentrating  waters. 
In  part  they  occur  also  at  the  contact  of  the  iron  formation 
with  basic  intrusions — for  instance,  in  pitching  troughs  between 
igneous  masses  and  dikes  branching  from  them.  The  surfaces 
of  the  igneous  rocks  are  much  altered,  leached,  and  changed  to 
clayey  masses,  called  "soapstone"  and  "paint  rock." 

Menominee  Range. — The  iron-bearing  district  extends  from 
western  Michigan  into  Wisconsin,  the   principal    mines  being 


THE  HEMATITE  DEPOSITS 


365 


located  at  Iron  Mountain,  Norway,  and  Crystal  Falls.  The 
iron  formation  is  chiefly  in  the  Upper  Huronian  and  is  called 
the  Vulcan  formation;  it  is  overlain  by  Upper  Huronian  slate 
and  underlain  by  a  Lower  Huronian  dolomite.  Intricate 
folding  characterizes  the  structure  of  the  range,  the  ores  of  the 
different  areas  occurring  in  separate  local  basins.  The  deposits 
are  large  and  consist  of  soft  red  hematite,  considerably  hydrated 


FIG.  118. — Vertical  cross-section  of  the  Newport  Mine,  Gogebic  Range, 
Michigan,  showing  position  of  ore-bodies  above  dikes.  Data  from  H.  L. 
Smyth. 

in  places,  and  are  generally  found  in  pitching  synclinal  basins 
bottomed  and  capped  by  slate  layers. 

Penokee -Gogebic  Range. — This  range  is  in  northern  Michi- 
gan and  Wisconsin,  the  principal  mines  being  at  Hurley,  Iron- 
wood,  and  Bessemer.  The  ore  appears  in  the  Ironwood  forma- 
tion (Upper  Huronian),  which  is  overlain  by  slate  and  under- 


366  MINERAL  DEPOSITS 

lain  by  quartzite  and  black  slate.  The  dip  is  steep  and  the  sedi- 
ments are  in  part  metamorphosed  by  Keweenawan  gabbro;  for 
the  most  part  the  Ironwood  formation  is  ferruginous  chert.  The 
ores  are  concentrated  in  large  irregular  bodies  in  the  angles 
between  the  footwall  quartzite  or  black  slate  and  the  igneous 
dikes  (Figs.  117  and  118),  these  rocks  making  an  impervious 
trough,  toward  which  the  meteoric  waters  converged.  Most  of 
the  deposits  reach  depths  of  1,000  feet,  and  some  attain  2, 200  feet. 
Both  soft,  partly  hydrated  ore  and  hard  slaty  ore  occur. 

Cuyuna  Range. — The  Cuyuna  district  is  situated  near  Brain- 
erd  about  70  miles  southwest  of  the  Mesabi  mines.  It  extends 
for  65  miles  along  the  strike  of  the  rocks  in  a  northeast  direction. 
The  iron  ore  here  is  a,  partly  hydrated  hematite,  in  places  accom- 
panied by  an  unusual  amount  of  manganese  oxide  (up  to  30  per 
cent.  Mn).  It  is  contained  in  the  usual  iron  formation  of  fer- 


FIG.  119. — Generalized  cross-section  showing  relation  of  iron-bearing 
formation  to  associated  rocks  in  the  Mesabi  Range,  Minn.  After  J.  F. 
Wolff. 

ruginous  jasper  which  in.  depth  appears  to  change  to  cherty  iron 
carbonate.  The  enclosing  rocks  are  slates  of  various  kinds  com- 
pressed into  steep  folds,  the  details  of  which  are  difficult  to  trace 
owing  to  the  covering  glacial  drift.  The  ore  bodies  are  elongated 
following  the  strike  and  while  some  cease  at  shallow  depths  others 
have  so  far  been  followed  down  for  300  feet.  The  phosphorus 
ranges  from  0.1  to  0.5  per  cent.  The  Cuyuna  iron  ores  were  dis- 
covered by  means  of  the  magnetic  attraction  along  the  range, 
due  to  a  small  quantity  of  admixed  magnetite. 

Mesabi  Range. — In  northern  Minnesota  the  Mesabi  range 
extends  from  east  to  west  for  a  distance  of  75  to  100  miles  on  the 
south  slope  of  a  prominent  ridge  called  the  Giants  Range.  The 
principal  mines  are  situated  near  the  towns  of  Biwabik,  Eveleth, 
Virginia,  and  Hibbing.  The  Huronian  rocks  here  lie  at  gentler 
inclinations  than  elsewhere,  dipping  8°  to  10°  S.E.  so  that  the 
iron  formation  outcrops  in  a  general  northeast-southwest  belt 
(Fig.  119). 


THE  HEMATITE  DEPOSITS 


367 


The  Biwabik  iron  formation 
of  the  Upper  Huronian  con- 
tains the  deposits.  It  is  under- 
lain by  the  Pokegama  quartzite 
and  covered  by  the  thick  Vir- 
ginia slate,  a  chloritic  and 
aluminous  sedimentary  rock. 
Except  at  the  eastern  end  of  the 
range,  where  contact-metamor- 
phic  amphibole-magnetite  rocks 
have  developed,  the  iron  forma- 
tion is  composed  mainly  of  fer- 
ruginous chert.  The  iron  ores 
cover  large  irregular  areas  along 
the  outcrop  of  the  Biwabik  for- 
mation, but  descend  to  rela- 
tively slight  depths,  few  of  the 
mines  being  more  than  200  feet 
deep  (Fig.  120).  The  deposits 
are  most  abundant  at  the  syn- 
clines  of  the  transverse  folds  of 
the  formation.  They  are  bedded 
and  along  the  edges  change 
rather  abruptly  to  the  fer- 
ruginous chert,  from  which  they 
are  derived  by  leaching  of  the 
silica.  This  relationship  is 
clearly  indicated  by  the  slump- 
ing of  the  strata  near  the  edges 
of  the  ore  masses,  as  shown  in 
Fig.  121.  The  iron  formation 
is  locally  called  "taconite." 

The  rain  water  falling  on  the 
truncated  edges  of  the  beds  cir- 
culates toward  the  south,  its 
movement  being  controlled  by 
the' 'slight  synclinal  basins,  by 
impervious  layers  of  slate,  and 
by  fractures. 

The  secondary  concentration 
of  the  iron  ore  has  evidently 
taken  place  under  surface  con- 


m 


5 

S 


368 


MINERAL  DEPOSITS    • 


A -Tension.  Cracks   in  Iron -Formation  on  Axis  of  an  Anticline. 


-Ore  forming  by  alteration  of  Tacoflifc  along  Fissur»&  Bedding-Piano. 


0- Present  Condition    of    Average  Trough  Orebody. 


FIG,  121.- — Cross-section  showing  mode  of  development  and  slumping  of 
ore-body  at  Mesabi  Bange.    After  J.  F.  Wolff. 


THE  HEMATITE  DEPOSITS 


369 


ditions  since  the  remote  time  of  the  post-Keweenawan  folding, 
when  the  deposits  first  became  exposed;  it  has  also  taken  place 
below  as  well  as  above  the  present  water-level,  which  is  about 
75  feet  underneath  the  surface. 

Analyses  show  that  the  present  surface  water,  containing  about 
20  parts  per  million  of  Si02",  is  slowly  leaching  silica,  but  re- 
moves little  if  any  iron.  The  deposits  do  not  appear  to  continue 
underneath  the  edge  of  the  capping  Virginia  slate,  probably  be- 
cause of  the  ponding  of  the  water  below  that  impervious  forma- 
tion. The  amphibole-magnetite  rocks  in  the  eastern  part  of  the 

!"5  Original  Surface  Line 


Open  Pit 


13th  Level 

!rr:;:—:  i:^::r;- 

JSth  Level 

Greenstone 


FIG.  122. — Vertical  section  through,  the   Chandler  mine,    Vermilion 
range,  Minnesota.     After  J.  M.  Clements,  U.  S.  Geol.  Survey. 


district  are  more  stable  and  have  not  suffered  much  alteration  by 
oxidation. 

During  the  development  of  the  ore-bodies  erosion  has  continu- 
ally cut  down  the  iron  formation  and  this  truncation  has  been 
accompanied  by  slow  downward  and  lateral  migration  of  the 
iron.  Glacial  erosion  finally  removed  much  material. 

The  ore  is  a  soft  and  porous  hematite,  brown,  red,  or  blue  in 
color,  averaging  55  to  58  per  cent.  iron.  It  contains  little  mag- 
netite, but  some  turgite  and  goethite.  The  mineral  composition 
of  the  ore  in  1909  was  approximately  in  per  cent.:  hematite,  61.81; 


370 


MINERAL  DEPOSITS 


limonite,  25.95;  quartz,  4.10;  kaolin,  5.30;  manganese  dioxide, 
1.30;  miscellaneous,  1.54. 

Sulphur  is  low  and  phosphorus  varies  from  0.03  to  0.07  per 
cent.  There  is  considerable  more  phosphorus  in  the  ore  than  in 
the  ferruginous  chert;  the  greenalite  and  siderite  rocks  contain 
scarcely  any  phosphorus. 

Vermilion  Range. — Northeast  of  Mesabi,  near  the  Canadian 
boundary,  is  the  Vermilion  range,  the  principal  mines  being  near 
the  towns  of  Ely  and  Tower.  The  country  rock  is  mostly  the 
Keewatin  greenstone,  but  infolded  in  it  in  synclinal  basins  or 
troughs  is  the  Laurentian  iron  formation,  known  as  the  Soudan. 
The  ores  are  associated  with  ferruginous  jaspers  in  these  troughs 


FIG.  123. — Ferruginous  chert  with  greenalite  granules,  in  part  replaced 
by  ferric  oxide  (black).     Magnified  40  diameters.     After  C.  K.  Leith. 

and  generally  have  a  foot  wall  of  greenstone  (Fig.  122).  The 
ore  is  a  dense  and  hard  blue  or  red  hematite  which  contains  a 
little  chalcopyrite,  an  unusual  feature  in  this  region. 

Origin  of  Lake  Superior  Iron  Ores. — It  has  been  shown  by 
Van  Hise  and  Leith  and  their  associates  that  the  ferruginous 
cherts,  jaspers,  amphibolite-magnetite  schists,  and  iron  ores  of 
the  iron  formations  result  from  the  alteration  either  of  the 
cherty  iron  carbonate  or  of  the  greenalite.  The  small  amounts  of 
iron  carbonate  or  ferrous  silicate  now  found  in  the  formations 
represent  mere  remnants  left  unaltered  where  protected  by  other 
rocks.  The  steps  of  the  alteration  may  be  observed  and,  in 


THE  HEMATITE  DEPOSITS  371 

the  end  products,  the  structures  and  textures  of  the  original 
rock  are  often  remarkably  well  retained.  It  is  held  that  the  ores 
and  the  ferruginous  cherts  or  jaspers  on  one  hand  and  the  am- 
phibole  schists  on  the  other  hand  represent  alterations  from  the 
same  original  type.  The  source  of  the  ore  is  not,  as  a  rule,  in 
the  present  ferruginous  cherts,  but  it  was  developed  from  origi- 
nal lean  siderite  and  greenalite '  rocks.  It  is  held  that  in  the 
largest  deposits  ores  and  jaspers  may  have  developed  side  by  side, 
at  the  same  time,  from  such  original  minerals.  Iron  carbonate 
prevailed  in  the  Marquette,  Gogebic,  Vermilion,  and  Crystal 
Falls  districts;  greenalite  in  the  Mesabi  district  (Fig.  123). 

The  concentration  has  been  effected,  according  to  the  Lake 
Superior  geologists,  by  water  coming  more  or  less  directly  from 
the  surface,  especially  at  places  where  such  waters  converge 
owing  to  the  existence  of  impervious  underlying  formations,  such 
as  slate  or  "soapstone,"  that  form  pitching  troughs,  or  owing 
to  brecciation  and  fracturing  of  the  iron  formations. 

The  alteration  of  the  iron  formations,  resulting  in  the  concen- 
tration of  the  iron  ores  or  in  the  development  of  ferruginous 
cherts,  jaspers,  and  amphibolite  schists,  has  taken  place  in  dif- 
ferent geologic  periods  under  varying  conditions.  So  far  as  the 
alteration  has  proceeded  continuously  under  the  influence  of 
surface  waters,  without  interruption  by  igneous  activity  or 
orogenic  movements,  soft  ores  and  ferruginous  cherts  have  re- 
sulted. So  far  as  these  products  have  been  subjected  to  deep- 
seated  alteration  they  have  become  dehydrated  into  hard  red 
and  blue  specular  ores  and  brilliant  jaspers.  So  far  as  the  altera- 
tion of  the  original  iron  formations  has  taken  place  within  the 
sphere  of  influence  of  great  intrusive  masses,  when  waters  were 
heated  and  oxygen  not  abundant,  or  under  similar  conditions, 
developed  by  deep  submergence  or  by  orogenic  movement,  fer- 
rous silicates  and  magnetite  resulted,  as  shown  in  the  develop- 
ment of  the  grtinerite  schists. 

The  concentration  of  the  ores  was  far  advanced  before  Cam- 
brian time,  as  shown  by  the  fragments  of  ores  in  Cambrian  con- 
glomerates. Most  of  the  deposits  were  formed  between  the 
Keweenawan  and  the  Cambrian  deposition.  At  the  close  of 
pre-Cambrian  time  the  ores  were  largely  as  we  now  find  them, 
though  some  concentration  has  been  going  on  since.  During 
the  Cretaceous  period  the  region  of  the  Mesabi  range,  at  least, 
was  covered  by  the  sea. 


372  MINERAL  DEPOSITS 

Regarding  the  origin  of  the  cherty  iron  carbonates,  Van  Hisehas 
held  that  they  were  derived  largely  from  the  more  ancient  basic 
volcanic  rocks  of  the  Lake  Superior  region.  The  iron  was  leached 
by  underground  waters  and  carried  to  the  sea  as  carbonate, 
partly  also  as  sulphate  solution,  and  there  deposited  as  limonite, 
from  which  through  reduction  by  organic  matter  ferrous  car- 
bonate was  formed. 

Somewhat  different  views  have  lately  been  expressed  by  C.  K. 
Leith,1  who  sums  up  the  origin  of  the  ores  as  follows:  The  iron 
was  brought  to  the  surface  by  igneous  rocks  and  either  con- 
tributed directly  to  the  ocean  by  hot  magmatic  waters  or  later 
brought  there  by  surface  waters  from  weathered  rocks.  The 
iron-bearing  minerals  were  then  deposited  as  a  chemical  sediment 
in  a  conformable  succession  of  sedimentary  rocks  and  still  later, 
under  conditions  of  weathering,  were  locally  enriched  to  ore  by 
percolating  surface  waters.  "It  begins  also  to  appear  that  the 
iron,  copper,  nickel,  and  silver  ores  of  the  Lake  Superior  and  Lake 
Huron  districts  are  related  in  a  great  metallographic  province  in 
which  the  characteristics  and  distribution  of  the  different  ores 
are  initially  controlled  by  igneous  rocks.  As  first  deposited  the 
iron  formation  consisted  essentially  of  iron  carbonate  or  ferrous 
silicate  (greenalite)  with  some  ferric  oxide,  all  minutely  inter- 
layered  with  chert,  forming  the  ferruginous  chert.  When  these 
were  exposed  to  weathering  the  ferrous  compounds,  the  siderite 
and  greenalite,  oxidized  to  hematite  and  limonite,  essentially  in 
situ,  although  some  of  it  was  simultaneously  carried  and  rede- 
posited.  The  result  was  ferruginous  chert  or  jasper,  averaging 
less  than  30  per  cent,  of  iron.  The  concentration  of  the  iron  to 
50  per  cent,  and  over  has  been  accomplished  essentially  by  the 
leaching  of  silica  bands  from  the  ferruginous  chert  and  jasper. 
Infiltration  of  iron  has  been  on  a  smaller  and  more  variable 
scale.  The  leaching  of  the  silica  develops  pore  space  and  allows 
the  iron  layers  to  slump,  thereby  enriching  the  formation  suffi- 
ciently to  constitute  an  ore."  Only  a  small  part  of  the  volume  of 
the  iron  formations — less  than  2  per  cent. — has  been  altered  to  ore. 

Resume. — The  literature  of  the  Lake  Superior  iron  ores  is 
extensive  and  many  different  views  have  been  expressed.  J.  D. 
Whitney  regarded  the  ores  as  of  igneous  origin,  and  this  view  has 
also  been  advocated  by  N.  H.  Winchell.  T.  B.  Brooks  and  R. 
Pumpelly  at  one  time  considered  them  as  dehydrated  bog  iron 

1  C.  K.  Leith,  Iron  ores  of  Canada,  Econ.  Geol.,  vol.  3,  1908,  pp.  276-291. 


THE  HEMATITE  DEPOSITS  373 

ores,  and  this  view  has  lately  been  adopted  by  S.  Weidman  in  his 
description  of  the  Baraboo  ores  of  Wisconsin,  where  the  ores 
appear  to  grade  into  dolomites. 

The  views  of  Van  Hise  and  Leith  and  their  associates,  which 
appear  to  be  generally  accepted,  have  been  given  above  in  some 
detail  and  in  part  verbatim.  The  development  of  their  theory 
of  the  origin  of  the  iron  ores  has  been  gradual;  at  first  the  iron 
formations  were  considered  as  purely  sedimentary  and  the  re- 
crystallization  to  amphibole-magnetite  rocks  as  evidence  of 
regional  metamorphism;  later  the  effects  of  contact  metamor- 
phism  were  recognized,  and  finally  it  is  held  that  the  iron  of  the 
iron  formations  was  in  large  part  yielded  by  the  extensive  erup- 
tions accompanying  their  deposition.  Although  the  ferruginous 
cherts  are  still  thought  to  be  formed  by  the  oxidation  of  the 
siderite  and  greenalite  rocks,  which  now  form  a  small  part  of  the 
formations,  there  seems  to  be  a  tendency  to  regard  the  iron 
ores  as  mainly  formed  directly  by  the  solution  of  the  silica  in 
the  ferruginous  cherts.  The  history  of  any  one  group  of  these 
deposits  is  probably  even  more  complicated  than  would  appear 
from  the  descriptions.  In  the  Mesabi  range,  for  instance,  amphi- 
bole  and  adularia  occur  in  the  ore,  but  the  development  of  both  these 
minerals  is  incompatible  with  descending  and  oxidizing  waters. 

The  age  of  the  concentration  of  the  iron  deserves  emphasis. 
The  ores  were  formed  mainly  before  the  Cambrian,  as  indicated 
by  fragments  of  ore  in  the  Cambrian  conglomerate.  Indeed,  they 
were  in  part  developed  in  inter-Huronian  time,  even  in  early 
Huronian  time.  This  is  set  forth  in  the  publications  cited,  but 
is  not  generally  realized.  The  ores  are  not  the  product  of  the 
present  circulation  and  oxidation,  but  of  forces  acting  in  ancient 
periods  when  conditions  were  probably  widely  different  from 
those  of  to-day.  It  is  stated  that  the  concentration  has  also  pro- 
ceeded since  pre-Cambrian  time,  but  this  assertion  seems  far  from 
being  established,  even  for  the  surface  deposits  of  the  Mesabi  range. 

Weidman  (op.  tit.}  has  pointed  out  that  the  present  ground- 
waters  are  entirely  similar  in  composition  in  the  Paleozoic  rocks 
and  in  the  iron  formations  and  has  shown  that  they  do  not  now 
transport  or  dissolve  iron  or  notable  quantities  of  silica.  A.  C. 
Lane  has  shown  (p.  440)  that  the  depth  reached  by  the  potable 
surface  waters  is  limited  and  that  in  some  parts  of  the  iron  dis- 
tricts, as  well  as  in  the  copper  districts  of  the  Keweenawan,  they 
are  replaced,  at  depths  of  1,000  to  2,000  feet,  by  scant  and  appar- 


374  MINERAL  DEPOSITS 

ently  stagnant  water  rich  in  calcium  and  sodium  chlorides.  The 
present  ground-water  in  a  region  of  high  water-level  is  clearly 
unable  to  produce  the  extensive  oxidation  shown  by  the  iron  ores. 
Undoubtedly  special  conditions  of  circulation  existed  in  pre-Cam- 
brian  time  which  are  not  paralleled  to-day.  Oxidizing  waters  do 
not  penetrate  deeply  in  temperate  regions  of  high  water-level, 
and  even  where  they  reach  a  depth  of  a  few  hundred  feet  the 
product  is  a  limonite.  The  hematites  appear  to  result  from 
oxidation  only  in  arid  and  tropical  countries. 

The  only  times  at  which  large  bodies  of  rock  could  be  oxidized 
to  hematite  by  descending  waters  would  seem  to  be  during  epochs 
of  great  aridity,  when  the  water-level  was  exceptionally  low; 
possibly  just  such  conditions  prevailed  in  pre-Potsdam  time. 
Similar  extraordinary  deep  oxidation  of  pre-Cambrian  age  has 
been  described  by  L.  L.  Fermor  from  the  manganese  deposits  of 
India.1  It  may  be  pointed  out  that  deposits  of  hematite  were 
formed  during  this  period  in  the  Hartville  district  in  eastern 
Wyoming,2  and  finally  in  certain  recently  described  areas  in 
western  Arizona.3 

At  Hartville  lenses  of  hematite  occur  in  schist  along  a  lime- 
stone foot  wall  and  have  been  followed  to  a  depth  of  900  feet. 
Ball  shows  that  the  deposit  antedates  the  Guernsey  formation, 
the  lowest  Paleozoic  terrane  present,  and  believes  that  the  iron 
was  leached  by  descending  solutions  from  the  upper  part  of  the 
schist  and  deposited  in  its  lower  part  by  replacement. 

Another  question  of  possible  importance  relates  to  the  per- 
centage of  phosphorus  in  the  Lake  Superior  ores.  It  is  remark- 
ably low  for  sedimentary  deposits  in  the  origin  of  which  organic 
life  played  a  part.  It  is  still  more  remarkable  that  the  primary 
greenalite  rocks  at  Mesabi  are  almost  free  from  phosphorus. 

The  Baraboo  deposits  of  Wisconsin  are  peculiar  in  that  igneous 
rocks  are  there  entirely  absent,  and  in  that  the  hematite  grades 
into  the  overlying  dolomite;  the  argument  advanced  by  S. 
Weidman  in  favor  of  a  primary  deposition  of  the  ore  as  limonite 
or  hematite  is  not  without  strength. 

In  spite  of  the  great  amount  of  work  done  the  problem  of  the 
origin  of  the  Lake  Superior  hematites  still  possesses  some  puzzling 
features. 

1  Mem.,  Geol.  Survey  India,  vol.  37,  1909. 

2  S.  H.  Ball,  Bull.  315,  U.  S.  Geol.  Survey,  1907,  pp.  190-205. 

3  H.  Bancroft,  Bull.  451,  U.  S.  Geol.  Survey,  1911. 


CHAPTER  XX 

DEPOSITS  FORMED  BY  CONCENTRATION  OF  SUB- 
STANCES  CONTAINED  IN  THE  SURROUNDING 
ROCKS,  BY  MEANS  OF  CIRCULATING  WATERS 

GENERAL  STATEMENT 

The  water  which  sinks  through  the  soil  and  effects  the  weather- 
ing of  rocks  becomes  charged  with  small  amounts  of  carbonates 
of  calcium/  sodium,  magnesium,  potassium,  iron,  -and  other 
metals,  and  also  with  soluble  silica.  By  far  the  larger  part 
of  it,  after  a  short  journey  through  the  belt  of  oxidation,  re- 
turns to  the  surface  as  springs  and  seepage  and  is  carried  off  in 
the  watercourses  to  the  sea.  A  smaller  part  of  this  water  sinks 
into  the  ground  and  either  joins  the  active  circulation,  descending 
in  smaller  fractures  and  openings  to  ascend  on  the  larger  fissures 
and  other  waterways,  or  becomes  a  part  of  the  stagnant  or  al- 
most stagnant  and  gradually  diminishing  ground-water  of  deeper 
levels.  In  places  the  active  circulation  may  descend  to  depths 
of  10,000  feet.  In  comparison  with  the  depth  of  the  ground- 
water,  the  depth  of  oxidation  or  rock  decay  is  on  the  whole 
insignificant,  and  that  part  of  the  dissolved  substance  which 
is  carried  down  is  also  insignificant  in  comparison  with  the 
vast  amount  of  underlying  rocks,  so  that  we  cannot  expect 
that  the  material  added  from  the  zone  of  weathering  will  produce 
any  far-reaching  changes  in  the  composition  of  these  rocks. 

Nevertheless  dissolved  salts  are  carried  down  from  the  weath- 
ered belt  and  may  cause  deposits  in  open  cavities  or  may  form 
more  or  less  complex  replacements.  In  the  openings  silica  may 
be  deposited  as  chalcedony,  chert,  or  quartz;  calcium  carbonate 
may  fill  fissures  and  replace  silicates,  or  ferrous  carbonate  may  be 
substituted  for  limestone.  Chlorite,  kaolin,  and  sericite  may 
develop  in  igneous  rocks.  All  these  changes  are,  however, 
accompanied  by  renewed  solution,  and  it  is  a  debatable  question 
whether  the  solution  does  not  more  than  balance  deposition.  On 
the  other  hand,  the  water  returning  to  the  surface  after  a  jour- 
ney of  varying  length,  more  or  less  heavily  loaded  with  soluble 

375 


376  MINERAL  DEPOSITS 

salts  deposits  these  by  reason  of  decrease  of  temperature  or  by 
reaction  with  other  surface  waters  of  different  composition. 
Finally,  hydration  absorbs  much  water,  both  from  the  active  cir- 
culation and  from  the  more  stagnant  ground-water,  and  de- 
posits of  valuable  minerals  may  result  from  this  simple  process. 

In  a  rough  way  the  deposits  resulting  from  the  work  of  under- 
ground waters  of  meteoric  origin  may  be  divided  into  (1)  those 
formed  from  abundant  material  contained  in  the  surrounding 
rocks,  for  instance,  magnesite,  serpentine,  sulphur  (by  reduction 
of  gypsum),  and  certain  kinds  of  hematite;  and  (2)  those  formed 
by  the  deposition  of  rarer  substances  dissolved  by  the  water 
from  the  surrounding  rocks  or  from  rocks  that  lie  deeper.  In 
this  second  division  it  is  possible  to  indicate  with  great  confi- 
dence the  derivation  of  some  substances — e.g.,  barite  from  certain 
limestones,  and  copper  from  certain  basic  igneous  rocks;  but  the 
exact  derivation  of  some  other  substances  may  be  doubtful. 

Waters  of  atmospheric  origin  doubtless  have  the  power  to 
dissolve  many  of  the  rarer  metals  contained  in  rocks,  to  carry 
them  for  considerable  distances,  and  to  concentrate  them  in 
places  suitable  for  deposition;  but  unless  it  is  aided  by  higher 
temperatures  at  considerable  depths  below  the  surface  this  power 
is  probably  not  strong  enough  to  produce  important  deposits  of 
these  rarer  "metals. 

BARITE1 

Modes  of  Occurrence  and  Origin. — Barite,  the  sulphate 
of  barium,  also  known  as  barytes  or  heavy  spar,  contains  when 
pure  65.7  per  cent.  BaO  and  34.3  per  cent.  S03.  It  is  usually 
white  and  coarsely  crystalline  with  curved  cleavage  faces  but 
appears  also,  especially  in  residual  deposits,  with  granular, 
earthy  or  even  fibrous  texture.  Many  barites  contain  from  a 
fraction  to  several  per  cent,  of  strontium  sulphate;  the  material 
mined  is  often  quite  pure  except  for  small  amounts  of  silica, 
calcite,  gypsum,  kaolin  and  iron  hydroxide. 

Witherite,  the  barium  carbonate,  is  a  much  rarer  mineral  and 
is  found  in  barite  veins,  associated  with  galena.  It  occurs  rather 
abundantly  in  such  veins  in  Cumberland  and  Northumberland 

1  E.  F.  Burchard  and  W.  C.  Phalen,  Mineral  Resources,  U.  S.  Geol.  Sur- 
vey, Annual  publication. 

J.  M.  Hill,  Barytes  and  Strontium.  An  excellent  review  in  same  pub- 
lication for  1915,  pt.  2,  pp.  161-187. 

Mineral  Industry,  Annual  publication. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS  377 

in  England,  particularly  at  the  New  Brancepeth  colliery  where 
it  is  secondary  after  barite. 

Barite  is  not  a  mineral  of  igneous  origin,  nor  does  it  occur 
in  contact  metamorphic  deposits.  It  is  common,  however,  as 
lenses  and  veins  in  almost  all  kinds  of  rocks.  Most  abundantly, 
however,  it  occurs  in  sedimentary  rocks  and  is  probably  in  almost 
all  cases  leached  from  the  country  rock  by  circulating  waters. 

All  igneous  rocks  contain  at  least  a  trace  of  barium  oxide  but 
rarely  more  than  0.1  per  cent.  Leucite  and  analcite  rocks  from 
the  Wyoming-Montana  province  are  unusually  rich  in  this  metal, 
some  analyses  showing  from  0.5  to  1.2  per  cent,  of  BaO  probably 
present  in  the  feldspathoids  as  a  silicate.  Granites,  rhyolites, 
andesites,  and  basalts  are  poor  in  barium.  In  more  concentrated 
form  barium  is  often  present  in  sediments.  It  has  been  deter- 
mined in  some  limestones;  analyses  quoted  by  T.  L.  Watson  from 
the  Ordovician  of  Virginia  show  from  0.62  to  1.62  per  cent.  BaSO^1 
Limestones  of  the  same  age  from  Missouri  contain,  according  to 
Steel,  only  0.001  to  0.005  per  cent.  BaO.  Some  sandstones, 
like  the  Cambrian  Weisner  quartzite  of  Georgia  and  Alabama 
contain  barite;  it  has  also  been  found  in  shales  with  sedimentary 
manganese  ores  (p.  274).  Sea  water  contains  traces  of  barium 
and  strontium  and  many  natural  waters  particularly  salt  brines, 
hold  quite  a  little  barium  in  solution  as  chloride  or  carbonate. 
Many  cases  of  deposition  of  barite  by  natural  waters  are  men- 
tioned on  pp.  103-108. 

Barium  sulphate  is  soluble  in  water  to  the  extent  of  2.9  milli- 
grams per  liter;  it  is  somewhat  more  soluble,  probably  -  with 
decomposition,  in  waters  containing  alkaline  carbonates  and 
chlorides. 

Barite  is  a  common  gangue  mineral  in  many  ore  deposits  but 
is  here  scarcely  ever  of  economic  importance.  In  most  cases  its 
origin  is  to  be  sought  in  the  rocks  traversed  by  the  ascending 
solutions.  The  barite  deposits  worked  generally  contain  few 
other  minerals  and  are  found  in  sedimentary  rocks  of  all  ages 
as  veins  and  lenses  whose  width  in  places  may  be  from  10  to  50 
feet.  Sometimes  a  little  pyrite,  galena  and  sphalerite  is  asso- 

1  C.  W.  Dickson,  The  concentration  of  barium  in  limestone,  School  of 
Mines  Quart.,  vol.  23,  1902,  pp.  366-370.  This  author  fails  to  find  it  in 
many  limestones.  F.  W.  Clarke  does  not  mention  its  presence  in  single 
and  composite  analyses  of  limestone,  Geochemistry,  Butt.  616,  U.  S.  Geol. 
Survey,  1916,  p.  558. 


378  MINERAL  DEPOSITS 

ciated  with  the  barite.  The  source  of  the  mineral  is  undoubtedly 
in  the  surrounding  sediments  from  which  it  has  been  dissolved 
by  meteoric  waters. 

The  larger  part  of  the  barite  mined  in  the  United  States  is  a 
residual  mineral,  forming  concretions  in  clay  resulting  from  the 
decay  of  limestone.  It  is  sometimes  difficult  to  separate  the 
residual  and  the  strictly  epigenetic  deposits  for  it  appears  that 
the  same  solutions  which  formed  the  concretions  deposited  barite 
in  fracture  zones  in  the  underlying  rock. 

Deposits  in  the  United  States. — The  barite  deposits  now 
worked  are  mostly  contained  in  the  Paleozoic  limestones  in  the 
southern  Appalachian  and  the  central  States,  the  order  of  impor- 
tance being  Missouri,  Georgia,  Tennessee,  Kentucky  and  Virginia. 

In  Missouri  barite  often  accompanies  the  zinc  and  lead  deposits 
but  the  important  deposits  are  found  in  a  separate  area  in 
Washington  County,  in  southeastern  Missouri,1  not  far  from  the 
great  lead  mines  in  the  Bonneterre  (Cambrian)  dolomite  (p.  461). 
The  principal  deposits  are  found  in  the  shattered  and  dolomitized 
Gasconade  limestone  (Ordovician)  as  filling  of  irregular  veins 
and  other  open  cavities.  The  order  of  precipitation  is  given  by 
Steel  as  follows :  A  thin  coating  of  chalcedony  was  first  deposited  ; 
this  was  followed  by  deposition  of  a  little  galena;  and  this  in 
turn  was  succeeded  by  barite,  which  is  the  main  filling.  The 
series  of  events  was  closed  by  the  precipitation  of  marcasite, 
dolomitization,  and  the  formation  of  a  second  generation  of  barite. 
and  by  a  much  later  coating  of  ruby-red  sphalerite  on  the  older 
barite. 

In  Georgia  barite  occurs  in  the  Cartersville  district2  as  deposits 
from  solution  in  fractures  and  cavities  in  the  Weisner  quartzite 
in  intimate  association  with  yellow  ocher,  and  also  as  nodules 
embedded  in  residual  clays. 

The  barite  deposits  of  Virginia  have  been  described  by  T.  L. 
Watson,3  who  states  that  they  are  probably  caused  by  the  leaching 
of  limestones  by  meteoric  waters.  Deep  rock  decay  character- 

1  A.  A.  Steel,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  40,  1910,  pp.  85-117. 
Arthur  Winslow,  Missouri  Geol.  Survey,  vol.  7,  1894,  p.  678. 

E.  R.  Buckley,  Missouri  Bureau  of  Geology  and  Mines,  vol.  9,  pt.  1, 
1908. 

2  T.  L.  Watson  and  J.  S.  Grasty,  Barite  of  the  Appalachian  States,  Trans., 
Am.  Inst.  Min.  Eng.,  vol.  51,  1916,  pp.  514-559. 

3  Trans.,  Am.  Inst.  Mm.  Eng.,  vol.  38,  1907,  pp.  953-976. 
T.  L.  Watson  and  J.  S.  Grasty,  op.  cit. 


CONCENTRATIONS  FROM  SURROUNDING[ROCKS    379 

izes  the  whole  region.     The  barite  in  part  fills  fractures  and  in 
part  replaces  limestone.     It  occurs: 

1.  In    crystalline    Cambrian    or   pre-Cambrian   limestone  as 
irregular,  lenticular  lodes  or    pockets  replacing  the  limestone 
and  associated  with  calcite  and  chalcopyrite  (Fig.  124). 

2.  In  crystalline  schists  as  filling  of  fractures. 

3.  In    the    Shenandoah    (Cambro-Ordovician)    limestone    as 
filling  of  fractures  or  in  residual  soil. 

4.  In  Triassic  shales  and  limestone  as  filling  of  fissures  in  a 
crushed  zone. 


FIG.  124. — Section  of  the  Bennett  mine,   Virginia,  showing  occurrence  of 
barite  as  residual  and  as  replacement  deposit.    After  T.  L.  Watson. 


Large  and  pure  barite  veins  have  been  described  from  the  Ket- 
chikan  district1  and  near  Wrangell2  in  Alaska.  They  are  con- 
tained in  crystalline  limestone  and  in  schists. 

Foreign  Deposits. — Barite  deposits  are  common  in  all  countries. 
Bodies  of  exceptional  size  and  purity  are  found  in  central  Ger- 
many in  sedimentary  rocks  of  Permian  and  Triassic  age. 

1  T.  Chapin  and  G.  H.  Canfield,  Bull.  642,  TJ.  S.  Geol.  Survey,  1916. 

2  E.  F.  Burchard,  Bull.  592,  U.  S.  Geol.  Survey,  1914,  pp.  109-117. 


380  MINERAL  DEPOSITS 

Uses  and  Production. — Barite  is  used  extensively  as  a  pigment 
in  the  manufacture  of  mixed  paint  and  to  give  weight  to  paper. 
It  is  the  raw  material  for  other  barium  salts,  such  as  the  nitrate, 
which  is  used  in  pyrotechnics  for  green  fire.  For  most  of  the 
purposes  indicated  its  purity  and  white  color  are  essential.  The 
crude  material  is  crushed  and  treated  in  log  washers  and  jigs. 
After  grinding  the  pulp  is  classified  and  the  settled  cream-colored 
mud  is  finally  treated  with  sulphuric  acid  to  remove  the  staining 
ferric  hydrate.  The  domestic  production  in  1917  was  about 
207,000  tons,  which  came  from  a  great  number  of  small  operators 
in  Missouri,  Virginia,  Kentucky,  Georgia,  North  Carolina, 
and  Tennessee.  Before  the  European  war  began  from  16,000 
to  30,000  tons  of  barite  and  barium  salts  were  imported  annually, 
largely  from  England  and  Germany.  The  removal  of  competi- 
tion with  high-grade  European  barite  has  greatly  stimulated  the 
American  industry.  The  average  price  was  $5.66  per  ton. 

CELESTITE  AND   STRONTIANITE1 

Strontium  accompanies  barium  as  a  primary  constituent  of 
igneous  rocks  but  is  present  in  much  smaller  quantities.  As 
celestite  (SrS04)  and  strontianite  (SrCO3)  it  is  sometimes  found 
in  fissure  veins  of  hydrothermal  origin,  but  the  two  minerals  are 
much  more  commonly  found  as  veins,  nodules  and  layers  in  sedi- 
mentary rocks,  particularly  limestone.  In  the  latter  case  they 
are  undoubtedly  leached  by  cool  surface  waters  from  small  quan- 
tities in  the  sediments  and  deposited  in  convenient  places. 

Celestite  is  found  in  crystals  and  granular  masses  often  of 
bluish  color;  but  sometimes  it  is  dark  or  brownish.  It  usually, 
but  not  always,  contains  BaS04.  Strontianite  is  crystalline,  fine- 
grained, fibrous  or  nodular  and  has  white,  brownish  or  dark 
color.  It  has  often  been  mistaken  for  calcite,  and  always  con- 
tains a  few  per  cent,  of  CaC03. 

In  geods,  veins,  disseminations  and  replacements  celestite  is 
found  in  Paleozoic  dolomite  and  limestone  of  Michigan,2  New 
York3  and  Ohio.  A  cave  at  Put-in  Bay  is  said  to  have  yielded 

1 J.  M.  Hill,  Barytes  and  strontium,  Mineral  Resources,  U.  S.  Geol. 
Survey,  pt.  2,  1915,  pp.  161-187. 

2  E.  H.  Kraus  and  W.  F.  Hunt,  Am.  Jour.  Sci.,  4th  ser.,  vol.  21, 1906,  p.  237. 
W.  A.  Sherzer,  Am.  Jour.  Sd.,  3d  ser.,  vol.  50,  1895,  p.  246;  Rept., 
Michigan  Geol.  Survey,  vol.  7,  pt.  1,  1900,  p.  208. 

•  E.  H.  Kraus,  Am.  Jour.  Sci .  4th  ser.,  vol.  18,  1904,  p.  30;  vol.  19,  1905, 
p.  286. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS  381 

150  tons  of  celestite.  Deposits  in  the  limestone  quarries  of  north- 
western Ohio  and  southeastern  Michigan,  near  Toledo  are  said 
to  be  of  possible  economic  importance.  Strontianite  is  sub- 
ordinate. Celestite  also  occurs  in  Cretaceous  limestone  in  Texas1 
near  Austin.  Interesting  deposits  of  celestite  have  lately  been 
discovered  in  Tertiary  lake  beds  in  Arizona,2  near  Gila  Bend 
where  the  mineral  occurs  in  sandstone  and  shale  with  gypsum 
and  salt;  similar  beds  have  been  described  from  the  Awavatz 
mountains  near  the  southern  end  of  Death  Valley.  Strontianite 
deposits  in  Miocene  lake  beds  10  miles  north  of  Barstow,3  San 
Bernardino  County,  have  been  discussed  by  A.  Knopf  who  be- 
lieves that  the  beds,  veins  and  fibrous  concretions  at  this  place 
are  replacements  of  lacustrine  limestone. 

All  these  deposits  are  of  low  grade  but  it  is  planned  to 
utilize  them  at  the  present  time.  In  1916,  250  tons  of  strontium 
ore  was  mined  in  the  United  States,  the  first  production  for 
many  years.  In  1917  the  output  had  increased  to  4,035 
short  tons. 

The  principal  supply  of  strontium  was  for  many  years  derived 
from  the  Strontianite  veins  in  Cretaceous  marl  and  limestone  of 
Westphalia,  Germany;  the  mineral  is  here  accompanied  by  calcite 
and  a  little  pyrite.4 

The  strontium  used  in  the  United  States  before  the  war  came 
largely  from  England.  Important  celestite  deposits  are  found 
near  Bristol,  where  the  mineral  forms  lenses  and  veins  in  a 
Triassic  marl  and  in  the  underlying  rocks.6 

Celestite  is  also  concentrated,  like  barite,  during  the  formation 
of  sulphur  from  gypsum  and  as  noted  below  under  "Sulphur" 
it  occurs  in  considerable  quantities  in  the  sulphur  mines  of  Sicily. 
For  commercial  purposes  celestite  should  contain  at  least  95  per 
cent.  SrSO4. 

The  principal  use  of  strontium  is  in  sugar  refining,  in  the  so- 
called  Scheibler  process,  in  which  strontium  hydroxide  is  used 

1  First  report,  Geol.  Survey  Texas,  1889,  p.  125. 

F.  L.  Hess,  Eng.  and  Min.  Jour.,  July  17,  1909,  p.  117. 

2  W.  C.  Phalen,  Celestite  deposits  in  California  and  Arizona,  Bull.  540, 
U.  S.  Geol  Survey,  1914,  pp.  526-531. 

3  Strontianite  deposits  near  Barstow,  California,  Bull.  660,  U.  S.  Geol. 
Survey,  1918,  pp.  257-270. 

4  Getting,  Oesterr.  Zeitschr.  B.  u.  H,  Wesen,  vol.  37,  1889,  p.  113. 

8R.  L.  Sherlock,  Mem.,  Geol.  Survey  England,  Special  reports  on  mineral 
resources,  vol.  3,  1918,  pp.  48-61. 


382  MINERAL  DEPOSITS 

for  the  recovery  of  sugar  from  beet  sugar  molasses.  The  nitrate 
is  used  in  pyrotechnics  for  red  fire.  The  domestic  production 
has  been  stimulated  because  of  the  recent  embargo  on  exports 
from  Great  Britain.  The  price  of  British  celestite  was  about 
$12  per  ton  at  the  eastern  sea  board. 

SULPHUR1 

Modes  of  Occurrence. — Native  sulphur  may  be  formed  by 
various  reactions.  The  oxidation  of  pyrite  sometimes  results  in 
crusts  of  sulphur  coating  the  cavities  once  occupied  by  the  dis- 
solved crystals.  In  the  craters  of  volcanoes  where  sulphurous 
gases  ascend  on  crevices  sulphur  is  often  found,  as  the  result 
of  a  reaction  between  sulphur  dioxide  and  hydrogen  sulphide 
(H2S+2S02  =  H2SO4+2S),  or  more  probably  by  incomplete  oxi- 
dation of  hydrogen  sulphide  (2H2S+02  =  2H20+2S)  or  by  the 
by  the  reaction  3SO2+2H2O  =  2H2S04+S.  A  large  deposit  of 
this  kind  is  worked  at  the  Abosanobori  mine,  Hokkaido,  Japan, 
and  consists  of  clayey  beds  in  an  old  crater  lake.  Considerable 
quantities  are  exported  from  Japan  to  the  United  States.  It  has 
been  proposed  to  utilize  a  similar  deposit  in  the  crater  of  Popo- 
catepetl, Mexico;  other  deposits  are  found  in  the  volcanoes  of 
the  Chilean  and  Argentine  Andes. 

Much  more  commonly  sulphur  is  found  at  active  or  extinct 
hot  springs  in  the  tufas  or  other  adjoining  porous  rocks  like  vol- 
canic tuffs.  It  evidently  results  from  the  incomplete  oxidation 
of  H2S,  by  the  oxygen  or  by  bacterial  action.  Such  deposits  have 
been  observed  at  many  places  in  the  Western  States — for  in- 
stance, at  Cuprite,  Esmeralda  County,  Nevada;  at  the  Rabbit 
Hole  mines  in  Humboldt  County,  Nevada;2  at  Sulphur  Bank, 
California;  at  the  Cove  Creek  mine,  Beaver  County,  Utah;3  and 
at  Cody  and  Thermopolis,  in  Wyoming.4  The  three  last-named 
deposits  have  been  worked.  In  Wyoming  the  sulphur  in  part 

10.  Stutzer,  Die  wichtigsten  Lagerstatten  der  Nicht-Erze,  1911,  pp. 
185-263. 

W.  C.  Phalen,  Mineral  Resources,  U.  S.  Geol.  Survey,  Annual  publica- 
tion. 

S.  H.  Salisbury,  Mineral  Industry,  Annual  publication. 
2  G.  I.  Adams,  Bull.  225,  U.  S.  Geol.  Survey,  1904,  pp.  497-500. 
8  W.  T.  Lee,  Bull.  315,  U.  S.  Geol.  Survey,  1907,  pp.  485-489. 
4  E.  G.  Woodruff,  Bull.  340,  U.  S.  Geol.  Survey,  1908,  pp.  451-456. 
E.  G.  Woodruff,  Bull.  380,  U.  S.  Geol.  Survey,  1909,  pp.  373-380. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS    383 


replaces  the  limestoue  underlying  the  travertine  or  tufa.  All 
these  deposits  are  superficial,  and,  though  some  are  fairly  pro- 
ductive, they  play  no  great  part  in  the  world's  output. 

The  greater  part  of  the  native  sulphur  is  not  connected  with 
volcanic  processes  or  hot  springs  but  is  obtained  from  sedimen- 
tary beds,  in  close  association  with  gypsum  and  limestone; 
calcite,  aragonite,  barite,  celestite,  opal,  more  rarely  quartz, 
together  with  gaseous  and  solid  hydrocarbons,  are  found  with 
the  sulphur.  This  ^'associa- 
tion is  constant  and  char- 
acteristic and  recurs  in  al- 
most all  the  great  gypsum 
beds  of  the  world,  though 
the  sulphur  is  not  always 
present  in  quantities  of 
economic  importance.  As 
an  illustration  it  is  interest- 
ing to  scan  the  boring  records 
in  Louisiana,  contained  in 
the  bulletins  of  the  State 
Survey,  and  note  the  fre- 
quency with  which  sulphur 
accompanies  gypsum.  The 
sulphur  is  in  earthy  or 
resirious  masses  and  forms 
lenticular  beds,  veinlets,  and 
concretions  in  marl,  lime- 
stone, and  gypsum. 

Origin  of  Sulphur  Deposits  in  Gypsum.  —  Sulphur  is  undoubt- 
edly derived  from  gypsum  through  the  reducing  action  of  organic 
matter,  by  way  of  calcium  sulphide  and  hydrogen  sulphide. 
Regarding  the  details  of  the  transformation  the  views  are  not 
uniform;  it  is  certain  that  the  reaction  can  take  place  at  low 
temperature.  G.  Bischof,  in  the  middle  of  the  last  century,  first 
discussed  this  matter1  and  assumed  the  following  reactions: 

CaS04+2C  =  CaS+2CO2. 
CaS+C02+H20  =  CaCO3+H2S. 

=  H20+S. 


FIG.  125. — Banded  sulphur  rock 
irom  Sicily,  one-half  original  size. 
Black,  sulphur;  white,  calcite;  stippled, 
limestone.  After  0.  Stutzer. 


1  G.  Bischof,  Chemische  und  physikalische  Geologie,  vol.  2,   1851,  pp. 
144-164. 


384  MINERAL  DEPOSITS   ' 

The  objection  to  this  scheme  would  be  that  the  sulphur  is 
evidently  often  formed  at  depths  of  several  thousand  feet,  and 
that  .the  presence  of  much  oxygen  at  such  depths  would  be  im- 
probable; more  likely  the  hydrogen  sulphide  generated  from  the 
gypsum  reacts  upon  calcium  carbonate,  resulting  in  secondary 
gypsum  and  sulphur. 

The  deposits  of  Sicily  have  been  the  subject  of  extended  dis- 
cussion. A.  von  Lasaulx1  has  regarded  them  as  formed  in 
fresh-water  lakes  into  which  springs  containing  H2S  were  dis- 
charged. G.  Spezia2  has  advanced  a  similar  view,  believing, 
however,  that  the  hot  springs  deposited  the  sulphur  at  the 
bottom  of  a  sea  basin,  accounting  for  the  presence  of  celestite 
by  the  same  agency. 

Baldacci3  held  that  the  deposition  of  sulphur  took  place  in  a 
partially  evaporated  marine  basin,  in  or  near  which  numerous 
mud-volcanoes,  like  those  of  the  Apsheron  peninsula  in  the 
Caspian  Sea,  discharged  large  volumes  of  hydrocarbon  that 
effected  the  reduction  of  gypsum  to  calcium  sulphide. 

A  theory  of  the  purely  sedimentary  origin  of  the  sulphur 
deposits  of  Sicily  was  recently  advanced  by  0.  Stutzer.  The  well- 
defined  stratification  of  the  sulphur  beds,  with  occasional  cross- 
bedding,  the  occurrence  of  the  sulphur  in  limestone  and  its 
absence  in  the  overlying  gypsum,  and  finally  the  presence  of 
intercalated  clay  beds  which  would  prohibit  the  free  circulation 
of  water  are  cited  by  Stutzer  as  proofs  of  his  view.  Sedi- 
mentary sulphur  deposits  may  form,  according  to  him,  in  any 
closed  basin  in  which  hydrogen  sulphide  is  developed.  The  gas 
may  be  produced  by  decay  of  organisms,  or  by  reduction  of 
dissolved  calcium  sulphate  by  carbon,  or  by  hydrocarbons. 
The  oxidation  of  hydrogen  sulphide  is  effected  by  the  oxygen  of 
the  air  or  by  the  aid  of  bacteria.  In  organic  decay  many  bacteria 
reduce  sulphates  and  develop  hydrogen  sulphide.  Other  low 
organic  forms,  the  so-called  sulphur  bacteria,  oxidize  H2S  and 
accumulate  sulphur  in  their  cells  as  minute  particles.  The 
oxidation  of  this  sulphur  supports  the  life  of  the  organism,  the 
resulting  sulphuric  acid  being  converted  into  sulphates  by 

1  Neues  Jahrbuch,  1879,  pp.  490-517. 

2  G.  Spezia,  SulT  origine  del  solfo  nei  giacimenti  solfiferi  della  Sicilia, 
Torino,  1892.     Reviewed  in  Neues  Jahrbuch,  1893,  1,  p.  281. 

3  Descrizione    geol.    dell    Isola  di  Sicilia,   Mem.  descritt.  d.  Carta  geol. 
d'ltalia,  1,  1886. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS  385 

carbonates  which  are  absorbed  and  are  necessary  for  the  growth 
of  the  bacteria.  The  sulphur  bacteria  are  found  in  sulphur 
springs  and  in  the  mud  of  seas  and  lakes,  in  which  hydrogen 
sulphide  is  developed.  Stutzer  also  refers  to  the  fact  that  the 
water  of  closed  basins,  such  as  the  Black  Sea,  contains  H2S 
in  quantities  increasing  with  the  depth.  Stutzer's  conclusions 
are  supported  by  W.  F.  Hunt1  who  describes  the  bacterial 
action  in  detail. 

Interesting  as  these  views  are,  the  sedimentary  deposition 
of  sulphur  cannot  be  regarded  as  proved.  The  presence 
of  epigenetic  sulphur  throughout  large  masses  of  gypsum 
is  so  common  that  its  origin  through  the  direct  reduction 
of  gypsum,  by  way  of  hydrogen  sulphide  and  organic  matter, 
can  scarcely  be  doubted.  Conditions  in  the  Black  Sea  would 
seem  to  be  favorable  for  the  deposition  of  sulphur  and  yet  no 
sulphur  appears  to  have  been  brought  up  by  deep  dredgings  in 
that  basin.2 

Examples. — Sulphur  is  widely  distributed  in  the  Miocene  and 
Pliocene  of  the  Mediterranean  countries,  everywhere  accom- 
panied by  gypsum.  By  far  the  most  important  deposits  are 
in  Sicily,  which  for  years  has  supplied  the  bulk  of  the  world's 
production. 

The  sedimentary  rocks  of  Sicily,  in  part  marine,  in  part  land 
deposits,  consist  of  basal  clays  covered  by  diatomaceous  and 
radiolarian  shales.  Above  these  beds  the  sulphur-bearing  gyp- 
sum formation  extends  over  an  area  of  almost  800  square  kilo- 
meters. This  formation  is  about  300  feet  in  thickness;  gypsum, 
limestone,  salt,  clay,  and  sandstone  are  the  principal  rocks. 
There  are  three  or  four  beds  of  sulphur,  the  substance  ramifying 
through  the  bluish-gray  limestone.  Celestite  occurs  in  econom- 
ically important  quantities  and  with  sulphur,  gypsum,  calcite,  and 
more  rarely  barite  forms  beautiful  crystals  coating  the  walls  of 
cavities.  The  crude  ores  of  Sicily  contain  from  8  to  25  per  cent, 
sulphur. 

As  noted  above,  sulphur  is  common  in  the  Tertiary  and  Cre- 

1The  origin  of  the  sulphur  deposits  of  Sicily.  Econ.  Geol.t  vol.  ]0, 
1915,  pp.  543-579. 

1  Sir  John  Murray,  The  deposits  of  the  Black  Sea,  Scottish  Geog.  Mag., 
16,  1900,  pp.  673-702.  Stelzner  and  Bergeat,  Erzlagerstatten,  p.  470. 
A  comprehensive  review  by  Doss  is  given  in  the  Neues  Jahrbuch,  1900,  1, 
pp.  224-228. 


386 


MINERAL  DEPOSITS 


taceous  beds  underlying  the  Louisiana  and  Texas  coast.1  In 
1865  an  unusually  large  deposit  was  discovered  in  Calcasieu 
Parish  (230  miles  west  of  New  Orleans),  Louisiana,  at  a  depth  of 
443  feet  underneath  clay,  sand,  and  limestone  of  Tertiary  and 
Cretaceous  age.  The  borings  showed  a  thickness  of  100  feet  of 
almost  pure  sulphur,  underlain  by  a  great  thickness  of  sulphur- 
bearing  gypsum  (Fig.  126).  The  lateral  extent  is  sharply  de- 
fined as  a  circular  area,  half  a  mile  in  diameter.  It  is  now  evident 
that  these  deposits  have  been  formed  in  the  upper  part  of  one 
of  the  great  salt  domes  of  the  Gulf  coast  (Fig.  102,  see  also  p. 


FIG.    126. — Vertical  section   of   sulphur-bearing  bed   at    Calcasieu  parish, 
Louisiana.    After  Kirby  Thomas. 

309).  The  difficulties  of  sinking  a  shaft  through  the  quicksands 
for  a  long  time  prevented  the  utilization  of  this  deposit.  Later, 
however,  the  difficulties  were  overcome  by  the  invention  of  the 
Frasch  process.2  Through  bore  holes  superheated  water  is  forced 
down  to  the  sulphur,  which  is  thereby  melted;  hot  air  is  then 
supplied  under  pressure  to  aerate  the  molten  mass  and  facilitate 
its  ascent  by  water  pressure  to  the  surface 

1  J.  F.  Kemp,  Mineral  Industry,  1893,  p.  585. 

W.  C.  Phalen,  Mineral  Resources,  U.  S.  Geol.  Survey,  1907,  pt.  2,  p.  674. 
See  also  issue  for  1911,  pt.  2. 

2  H.  Frasch,  The  Mining  World,  December  14,  1907. 
W.  C.  Phalen,  Op.  tit. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS    387 

Similar  sulphur  beds  occur  at  several  places  along  the  Gulf 
Coast.  A  deposit  has  recently  been  opened  by  drilling  by  the 
Freeport  Sulphur  Company  near  the  mouth  of  the  Brazos  River, 
Texas,  and  is  now  in  successful  operation.  Here  the  sulphur 
beds  lie  under  760  feet  of  gravel,  sand  and  clay.  Below  this  are 
150  feet  of  sulphur  bearing  limestone,  gypsum  and  dolomite, 
containing  from  10  to  50  per  cent,  sulphur.  These  beds  are 
underlain  by  gypsum,  limestone,  sandstone  and  rock  salt.1  For 
some  reason  the  literature  on  Texas  and  Louisiana  sulphur 
deposits  is  extremely  scant. 

Production. — Until  recently  the  Sicilian  deposits,  with  an  an- 
nual output  of  about  450,000  metric  tons,  supplied  the  world's 
demand.  In  1901  the  Frasch  process  revolutionized  the  trade 
conditions  and  the  production  of  the  United  States  rose  at  once 
to  200,000  or  300,000  long  tons,  and  the  importation  from 
Sicily  fell  off  correspondingly.  The  interesting  trade  conditions 
developing  from  these  changes  and  the  struggles  of  the  Italian 
government  to  aid  the  distressed  Sicilian  operators  are  described 
in  the  articles  in  Mineral  Resources  cited  above. 

In  1915  the  Italian  production  was  364,  260  metric  tons  while 
that  of  the  United  States  was  in  excess  of  400,000  tons  with 
rapidly  diminishing  imports.  The  price  was  about  $17  per 
ton.  In  1917  the  sulphur  production  of  the  United  States  was 
probably  much  larger  though,  on  account  of  the  output  being 
divided  between  two  principal  producers,  the  figures  are  no 
longer  published  by  the  U.  S.  Geological  Survey.  Louisiana, 
Texas,  Wyoming  and  Nevada  are  the  producing  states. 

Uses. — The  manifold  industrial  uses  of  sulphur  need  not  be 
specified;  the  larger  part  is  used  for  the  manufacture  of  sulphuric 
acid,  for  bleaching  purposes  by  the  development  of  sulphur 
dioxide,  for  the  prevention  of  mildew  on  grapevines,  and  for  the 
manufacture  of  gunpowder,  matches,  etc. 

Sulphuric  Acid. — Just  before  the  war  the  production  of 
sulphuric  acid  in  the  United  States  was  about  3,500,000  short 
tons.  War  conditions  soon  created  an  enormous  demand  for 
sulphuric  acid,  mainly  for  explosives  and  the  price  has  risen 
rapidly.  In  1916  about  6,250,000  tons  of  acid  (50°  Baume) 
was  produced  and  for  1918  the  requirements  were  8,000,000 
tons.  Of  the  production  in  1916,  40  per  cent,  was  made  from 

1  W.  C.  Phalen,  quoting  Thomas  Kirby  in  Mineral  Resources,  IT.  S. 
Geol.  Survey,  1912,  p.  936. 


388  MINERAL  DEPOSITS 

Spanish  (Rio  Tinto)  pyrite,  6  per  cent,  from  Canadian  nyrite, 
13  per  cent,  from  domestic  pyrite,  marcasite  and  pyrrhotite, 
22  per  cent,  from  fumes  from  copper  and  zinc  smelters,  leaving 
about  19  per  cent,  which  had  to  be  supplied  from  native  sulphur1 
which  practically  did  not  appear  at  all  in  the  acid  production 
of  normal  times.  The  scarcity  of  shipping  has  reduced  the 
importations  from  Spain  so  that  great  efforts  have  been  made 
to  develop  our  native  supplies  of  pyrite  and  sulphur. 

It  appears  then  that  pyritic  ores  with  or  without  other  metals 
are  the  principal  source  of  sulphuric  acid.  Many  countries, 
particularly  Spain,  Norway,  Portugal,  France,  United  States,  Italy 
and  Germany  in  the  order  of  importance  stated,  produce  annually 
over  200,000  tons  of  pyrite. 

In  the  United  States  pyrite,  marcasite  and  pyrrhotite  with, 
respectively,  53.3  and  38.4  per  cent,  of  sulphur  are  the  prin- 
cipal sulphur  ores.  They  are  obtained : 

1.  From  pyrite  deposits  along  the   Appalachian  mountains 
from  Alabama  to  Vermont. 

2.  From  pyrite  deposits  in  California. 

3.  From  pyrrhotite  deposits  in  Virginia,  Tennessee  and  Maine, 

4.  From  marcasite  as  a  by-product  of  coal  mines  in  Illinois, 
Ohio,  Indiana  and  Pennsylvania. 

5.  From  marcasite  as  a  by-product  in  zinc-lead  mines  of  Wis- 
consin and  Illinois. 

The  domestic  production  of  pyrite  (including  marcasite  and 
pyrrhotite,  was  about  400,000  long  tons  of  which  the  larger  part 
came  from  Virginia  and  California.  Pyrite  was  also  imported 
from  Quebec  and  Ontario. 

The  "pyritic  deposits"  comprise  many  types  (p.  635),  but 
aside  from  the  minor  supplies  mentioned  under  4  and  5,  they 
are  mainly  products  of  high  or  intermediate  temperature  under 
intrusive  conditions  and  most  of  them  may  be  considered  as 
copper  deposits  of  very  low  grade.  Many  among  those  along 
the  Appalachian  belt  are  of  early  Paleozoic  age  and  more  or 
less  strongly  dynamo  metamorphosed.  We  may  mention  the 
pyrrhotite  deposits  of  Ducktown,  Tennessee,  and  of  the  "  Great 
Gossan  lead,"  Virginia,  further  the  pyritic  deposits  of  Louisa 
County,  Virginia,  which  form  long  lenses  in  a  Cambrian  sericite 
schist  and  northward  change  into  lead  and  zinc  deposits.  Other 

1  W.  Y.  Westervelt  and  A.  G.  White,  Bull  130,  Trans.,  Am.  Inst.  Min. 
Eng.,  October,  1917,  pp.  5-15. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS  389 

deposits  of  pre-Cambrian  age  are  found  in  St.  Lawrence  County, 
New  York.  The  ores  contain  from  30  to  50  per  cent,  of  sulphur 
with  up  to  10  per  cent,  of  insoluble.  Lead,  zinc,  antimony  and 
arsenic  are  objectionable  constituents.  The  residues  from 
sulphuric  acid  manufacture  are  often  used  as  copper  and  iron  ores. 

THE  MAGNESIAN  DEPOSITS 

The  magnesian  silicate  rocks  lend  themselves  easily  to  trans- 
formation and  yield  a  number  of  economically  valuable  products, 
among  them  serpentine,  magnesite,  meerschaum,  talc,  soapstone, 
and  asbestos.  All  of  these  result  from  the  action  of  water,  in 
most  cases  doubtless  of  atmospheric  origin,  on  peridotites, 
pyroxenites,  or  gabbros,  either  near  the  surface  or  with  the 
co-operation  of  stress  at  greater  depths.  Talc,  soapstone,  and 
asbestos  belong,  in  part,  to  the  latter  class. 

SERPENTINE1 

Serpentine  forms  by  simple  hydration  from  a  rock  consisting 
of  enstatite  and  olivine  according  to  the  following  equation : 

Mg2Si04+  MgSi03+2H2O  =  H4Mg3Si209. 

(Olivine)  (Enstatite)  (Serpentine) 

'  It  may  also  develop  from  olivine  alone,  with  the  removal  of 
some  magnesium  as  carbonate: 

2Mg2Si04+C02+2H20  =  H4Mg3Si209+MgC03. 

The  latter  equation  probably  represents  the  usual  process  of 
serpentinization  a  short  distance  below  the  surface.  Under 
oxidizing  conditions  serpentine  is  unstable,  though  of  course 
the  change  takes  place  very  slowly  and  erosion  may  work  far 
ahead  of  decomposition. 

Serpentine  is,  however,  also  formed  on  a  large  scale  at  greater 
^depths,  where  quantities  of  CO2  could  not  very  well  be  assumed 
for  the  reason  that  such  alteration  would  result  in  a  mixture  of 
serpentine  and  carbonates,  whereas  the  large  serpentine  masses 
rarely  contain  admixed  carbonates.  The  deep  canons  of  the 
Sierra  Nevada,  in  California,  show  clearly  that  the  serpentines 
of  this  range  are  not  superficial,  but  descend  to  the  depth  of 
several  thousand  feet.  The  modus  operandi  of  such^  exten- 

1  H.  Leitmeier  in  Doelter's  Mineralchemie,  vol.  1,  pt.  1,  1914,  'pp.  -385- 
428. 


390  MINERAL  DEPOSITS 

sive  hydration  is  not  fully  explained.  Some  have  held  that  it 
might  have  been  effected  by  ascending  waters,  shortly  after  the 
intrusions. 

Serpentine  is  generally  rich  in  iron,  for  the  original  rocks  are 
not  of  the  purity  indicated  by  the  equation  given  above;  the 
iron  is  present  both  as  silicate  and  magnetite,  and  also  in  the 
chromite  which  forms  a  characteristic  accessory.  Rock  that 
is  not  too  much  broken  by  joints  finds  fairly  extensive  use  as 
building  and  ornamental  stone.  For  the  latter  purpose  the  oily 
green  translucent  varieties  formed  in  crystalline  limestone  by 
serpentinization  of  the  contained  pyroxene  are  particularly  valued. 

MAGNESITE1 

Origin. — Magnesite  (MgCOs)  appears  in  two  modifications: 
(1)  As 'an  amorphous,  earthy,  hard  and  compact  mineral,  which 
probably  is  a  hardened  colloid  precipitate.  It  is  often  concre- 
tionary and  has  a  conchoidal  fracture  like  that  of  unglazed 
porcelain.  In  this  form  it  is  an  alteration  product  of  ser- 
pentine or  allied  magnesian  rocks  as  illustrated  by  the  equation- 
H4Mg3Si2O9+3CO3  =  3MgC03+2H2O+2SiO2.  (2)  As  a  crys: 
talline  mineral,  isomorphous  with  calcite  and  usually  holo- 
crystaUine  granular.  In  this  form  it  is  generally  a  replacement 
of  dolomite  produced  by  magnesian  solutions  in  connection  with 
intrusions.2 

1  Robert  Scheerer,  Der  magnesit,  Vienna  und  Leipzig,  1908,  p.  256. 

M.  Dittrich,  H.  Leitmeier,  K.  A.  Redlich  in  Doelter's  Handbuch  der 
Mineralchemie,  Dresden  and  Leipzig,  1912,  vol.  1,  pp.  212-267. 

F.  L.  Hess,  The  magnesite  deposits  of  California,  Bull.  335,  U.  S.  Geol. 
Survey,  1908. 

C.  G.  Yale  and  H.  S.  Gale,  Mineral  Resources,  TJ.  S.  Geol.  Survey, 
Annual  publication. 

S.  H.  Dolbear,  Mineral  Industry,  Annual  publication. 

2  Certain  minor  occurrences  are  of  interest:  Magnesite  of  the  amorphous 
type  is  found  as  sedimentary  beds  and  lenses  in  clays  of  Miocene  lake  beds 
near  Bissell,  San  Bernardino  County,  California.    See  H.  S.  Gale,  Bull.  540; 
U.  S.  Geol.  Survey,  1914,  p.  512. 

Crystalline  magnesite  occurs  in  many  crystalline  schists  of  the  Austrian 
Alps. 

Hydromagnesite  (3MgC03.Mg(OH)2  +  3H2O)  is  reported  from  Atlin, 
British  Columbia,  as  a  deposit  of  fine  white  powder  several  feet  deep  and 
appearing  like  a  spring  deposit.  In  connection  with  this  it  is  recalled  that 
H.  Leitmeier  found  that  a  magnesian  hydrocarbonate  was  deposited  by  the 
mineral  waters  of  Rohitch  in  Styria.  Zeitschr.  Kryst.  Min.,  vol.  47,  1909, 
p.  118. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS    391 

Occurrence. — The  amorphous  magnesite  is  not  uncommon 
in  areas  of  serpentine,  and  it  occurs  in  fissures  or  crush-zones  or 
irregular  masses,  often  mixed  with  more  or  less  serpentine  and 
some  opal  or  chalcedony.  It  is  often  very  pure  with  slight 
admixtures  of  iron,  alumina  and  lime;  a  few  per  cent,  of  free 
silica  are  often  present.  Magnesite  occurs  abundantly  but 
generally  in  small  deposits  in  the  California1  Coast  Ranges;  the 
best  deposits  are  near  Porterville,  Tulare  County.  Until 
recently  the  production  has  been  small  owing  to  distance  from 
the  eastern  market.  Similar  deposits  of  great  extent  producing 
annually  134,000  tons  are  worked  on  the  coast  of  Euboea,  in 
Greece.  Other  localities  are  found  at  Salem,  near  Madras, 
India,  in  the  Transvaal,  and  many  other  lands  where  serpen- 
tinoid  rocks  abound.  Magnesite  does  not  always  accompany 
serpentine,  however,  and  it  may  be  surmised  that  ascending 
springs  with  much  C02,  as  are  so  common  in  California,  may 
have  some  connection  with  its  genesis. 

The  largest  magnesite  deposits  in  the  world  are  at  Veitsch, 
in  Styria,2  Austria,  where  in  1914  about  200,000  metric  tons 
were  mined  in  open  quarries.  This  crystalline  magnesite  is  a 
replacement  of  dolomite  formed  under  the  influences  of  intru- 
sions of  porphyry  and  other  acidic  and  basic  rocks.  The 
Austrian  magnesite,  though  otherwise  pure,  contains  like  that 
from  Greece  a  few  per  cent,  of  iron  which  makes  it  desirable 
for  basic  linings  and  bricks.  In  1914,  110,000  tons  of  this 
material  were  imported  and  the  war  created  a  shortage  which 
stimulated  local  production  and  search.  In  1916  a  large  mag- 
nesite deposit  was  discovered  near  Chewelah,  Washington, 
which  bids  fair  to  supply  the  demand.  Like  the  Styrian  de- 
posits it  is  a  replacement  of  dolomite  of  Carboniferous  age,  near 
granite;  in  places  the  material  contains  a  few  per  cent,  of  silica, 
lime  and  iron.  Very  similar  are  also  the  deposits  found  some 
years  ago  in  the  Grenville  township,3  Quebec,  and  which  are 

1  F.  L.  Hess,  Op.  tit. 

H.  S.  Gale,  Bull.  540,  U.  S.  Geol.  Survey,  1914,  pp.  483-520. 

2  K.  A.  Redlich,  Die  Genese  der  Pinolitmagnesite,  Siderite  und  Ankerite 
der  Ostalpen,  Tsch.  m.  und  petr.  Mitt.,  vol.  26,  1907,  pp.  499-505. 

K.  A.  Redlich,  Genesis  der  kristallinen  Magnesite,  Zeitschr.  prakt. 
Geol,  vol.  21,  1913,  pp.  90-101. 

3  H.  J.  Roast,  The  development  of  Canadian  magnesite,  Trans.,  Canad. 
Min.  Inst.,  vol.  20,  1917,  pp.  237-255. 

M.  E.  Wilson,  Magnesite  deposits  of  Grenville  districts,  etc.  Mem. 
98,  Canada  Geol.  Survey,  1917. 


392  MINERAL  DEPOSITS 

now  being  worked.  At  this  locality  the  magnesite  contains 
several  per  cent,  of  lime  and  very  little  iron  which  to  some 
degree  has  made  difficult  its  use  for  refractories. 

Production  and  Use. — The  domestic  production  advanced 
sharply  in  1916  to  154,000  tons,  and  in  1917  reached  316,000 
tons.  The  price  was  about  $10  per  ton.  Magnesite  gives  oft" 
its  carbon  dioxide  at  800°  C.,  and  is,  therefore,  preferred  to 
calcite  in  the  production  of  this  gas.  After  calcining,  the  sub- 
stance is  used  for  the  manufacture  of  various  magnesium  salts, 
and  in  the  paper  and  sugar  industries.  It  is  employed  exten- 
sively with  magnesium  chloride  for  the  so-called  Sorel  cement, 
used  for  flooring,  etc.  Its  most  important  use  is  for  basic 
furnace  lining  in  the  Thomas  process.  Until  recently  the 
Styrian  magnesite  was  imported  mainly  for  this  purpose.  Mag- 
nesite for  bricks  should  contain  a  few  per  cent,  of  FeO  and  little 
CaO;  8  per  cent.  CaO,  being  the  allowable  limit. 

Magnesite  serves  also  as  an  ore  for  the  production  of  metallic 
magnesium,  which  on  account  of  its  low  specific  gravity  (1.74) 
is  now  used  in  alloys  with  aluminum  and  other  metals.  The 
reduction  is  effected  by  treating  the  chloride  made  from  mag- 
nesite in  an  electric  furnace.  The  best  ore  for  the  purpose  is 
naturally  the  carnallite  (KCl.MgCl2+6H2O)  from  the  Stassfurt 
salt  beds  (p.  312).  The  production  of  magnesium  in  United 
States  in  1917  was  116,000  pounds,  the  price  being  about  $2.00 
per  pound. 

MEERSCHAUM1 

Meerschaum  or  sepiolite  (H4Mg2Si3Oio,  containing  SiO2,  60.8 
per  cent.;  MgO,  27.1  per  cent.;  H^O,  12.1  per  cent.)  is  a  hydra  ted 
silicate  of  magnesia  of  tough,  compact  texture,  white  or  cream 
color,  and  smooth  feel.  As  is  well  known,  it  finds  a  rather  exten- 
sive use  in  the  manufacture  of  pipes  and  cigar  holders.  Its 
analysis  usually  shows  a  little  iron,  alumina,  and  lime.  It  is  prob- 
ably derived  from  serpentine  by  slow  hydration  and  is  in  most 
cases  a  colloid  precipitate.  The  principal  occurrence  is  in 
Asia  Minor  at  Eski-Shehr,  where  it  is  found  as  nodular  masses 
near  the  surface;  at  this  and  several  other  localities  in  Crimea 
and  Bosnia  serpentine  rocks  are  found  in  the  vicinity,  although 
the  material  itself  is  embedded  in  Quaternary  or  Tertiary  beds. 

1  G.  P.  Merrill,  Non-metallic  minerals,  1910,  pp.  218-221. 
C.  Doelter,  Mineralchemie,  vol.  2,  pt.  1,  1914,  pp.  374-383. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS  393 

A  different  occurrence  is  that  recently  discovered  in  New  Mexico,1 
on  the  upper  Gila  River,  where  the  substance  forms  veins  and 
balls  in  a  Paleozoic  cherty  limestone.  Here  it  is  probably 
derived  from  a  dolomitic  carbonate. 

TALC  AND  SOAPSTONE2 

General  Occurrence  and  Origin. — Talc  (HjMgaSi^^,  or 
3Mg0.4Si02.H2O;  65.5  per  cent.  SiO2,  31.7  per  cent.  MgO,  4.8 
per  cent.  H20),  is  a  hydrated  magnesium  silicate,  but  holds 
much  less  water  than  serpentine.  It  is  a  soft,  crystalline, 
foliated  or  compact  mineral  of  white,  gray,  or  pale-green  color 
and  a  greasy  feel.  The  more  compact,  as  well  as  some  impure 
varieties,  are  usually  called  soapstone;  they  may  contain  shreds 
of  chlorite  and  other  ferromagnesian  minerals,  like  enstatite  or 
amphibole.  Soapstone  is  easily  worked  and  is  of  great  resistance 
to  acids  and  high  temperatures. 

Talc  and  soapstone  are  products  of  the  hydration  of  magnesian 
rocks,  either  of  distinctly  igneous  origin,  like  gabbro,  pyroxenite, 
or  peridotite,  or  crystalline  schists  rich  in  such  minerals  as 
enstatite  and  tremolite  or  other  pyroxenes  and  amphiboles. 
These  schists  may  result  from  the  shearing  of  igneous  or  contact- 
metamorphic  rocks,  the  latter  derived  from  the  igneous  alteration 
of  limestone  and  dolomite.  The  purest  talc  deposits  are  asso- 
ciated with  crystalline  carbonate  rocks  containing  amphibole. 
In  general  serpentine  forms  from  olivine  and  talc  from  pyroxene 
and  amphibole,  but  this  rule  does  not  always  hold. 

Talc  often  contains  1  or  2  per  cent,  of  iron  and  aluminum,  as 
well  as  a  little  calcium;  according  to  the  analyses  given  by 
Merrill  (op.  tit.}  the  soapstones  contain,  in  addition  to  silica  and 
magnesia,  from  5  to  11  per  cent,  alumina,  7  to  13  per  cent, 
ferrous  oxide,  and  1  to  4  per  cent,  lime;  some  of  them  contain 
so  much  water  that  a  strong  admixture  of  serpentine  must  be 
assumed. 

The  formulas  show  that  talc  may  be  obtained  from  enstatite  or 
tremolite  by  the  addition  of  water  and  carbon  dioxide,  with  sepa- 
ration of  magnesium  or  calcium  carbonate,  which  is  probably 
carried  away  in  solution;  or,  in  case  of  deficiency  of  CO2,  the 

1  D.  B.  Sterrett,  Bull  340,  U.  S.  Geol.  Survey,  1908. 

2  J.  S.  Diller,  Mineral  Resources,  U.  S.  Geol.  Survey,  Annual  issues. 
G.  P.  Merrill,  op.  ctt.,  pp.  208-216. 

C.  Doelter,  op.  cit.,  pp.  356-374. 


394  MINERAL  DEPOSITS 

magnesia  may  combine  with  silica,  possibly  set  free  from  other 
minerals,  to  form  additional  talcose  material. 

The  exact  conditions  and  temperature  needed  for  the  formation 
of  talc  are  not  known,  but  it  seems  certain  that  dynamic  stress, 
together  with  a  limited  supply  of  water  not  over  rich  in  CO2,  is 
favorable  to  its  development;  it  also  undoubtedly  forms  from 
magnesian  minerals  by  the  aid  of  a  scant  supply  of  surface 
water  under  static  conditions.  It  is  also  known  that  talc  may 
develop  along  fissures  under  the  influence  of  ascending  hot  waters, 
whenever  magnesian  silicate  rocks  are  traversed. 

E.  Weinschenk/  in  his  description  of  the  talc  deposits  of 
the  Austrian  Alps,  holds  that  the  mineral  develops  by  replace- 
ment of  schist  composed  of  quartz,  chlorite,  chloritoid,  and 
graphite  along  its  contact  with  limestone  and  believes  this  trans- 
formation due  to  waters  following  the  irruption  of  large  igneous 
bodies. 

Occurrences. — The  crystalline  schists  of  all  countries  yield 
talc.  Some  occurrences  are  known  from  the  Pacific  coast,  but 
the  production  in  the  United  States  is  limited  exclusively  to 
the  belt  of  ancient  crystalline  rocks  which  forms  the  axis  of  the 
Appalachian  Mountain  system  from  Canada  to  Alabama. 

North  Carolina  is  rich  in  talc,  and  one  belt  of  Cambrian  marble 
along  the  Nantahala  Valley  and  Nottely  River2  yields  many 
lenses  as  much  as  200  feet  long  and  50  feet  thick.  The  mineral 
is  mined  in  open  cuts  and  by  shafts  and  tunnels. 

New  York  and  Vermont  easily  outrank  all  other  States  in 
the  production  of  talc.  The  output  of  New  York  comes  from 
a  small  district  about  12  miles  southeast  of  Gouverneur,3  which 
has  been  worked  for  many  years  by  underground  methods. 
One  mine  at  Talcville  has  attained  a  depth  of  550  feet.  The 
mineral  occurs  in  schistose  layers  of  enstatite  and  tremolite, 
gradually  merging  into  the  surrounding  crystalline  limestone. 
The  deposit  forms  a  persistent  layer  averaging  20  feet  in  width, 
within  the  enstatite-tremolite  rock. 

Virginia  yields  most  of  the  soapstone  produced  in  the  United 

1  Abhandl.  Bayer.  Akad.  d.  Wiss.,  vol.  21,  pt.  2,  1901,  p.  270. 

2  Arthur  Keith,  Bull.  213,  U.  S.  Geol.  Survey,  1903,  p.  443. 

J.  H.  Pratt,  North  Carolina  Geol.  Survey,  Economic  Paper  3,  1900, 
p.  99. 

J.  H.  Pratt,  Mineral  Resources,  U.  S.  Geol.  Survey,  1905,  p.  1361. 
*  C.  H.  Smyth,  Jr.,  School  of  Mines  Quarterly,  vol.  17,  1896,  pp.  333-341. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS    395 

States.  It  is  derived  from  a  belt  nearly  30  miles  long  and  less 
than  one  mile  wide.  According  to  T.  L.  Watson's  description1 
the  soapstone  occurs  as  sheets  or  dike-like  masses,  100  feet  or 
more  in  thickness,  conformably  interbedded  with  quartzitic 
schists,  but  is  probably  derived  from  an  igneous  rock. 

Production  and  Uses.- — The  rapidly  expanding  production 
of  talc  in  the  United  States  was  about  198,600  tons  in  1917. 
The  larger  part  was  sold  in  powdered  form.  The  value  of 
powdered  talc  is  about  $10  per  ton. 

Talc  is  used  as  a  filler  for  paper,  including  wall  paper;  also  for 
admixture  or  adulteration  of  pigment,  as  a  heat  insulator, 
lubricant,  polishing  powder  of  glass,  for  toilet  powders,  and  as  an 
absorbent  for  nitroglycerine.  The  compact  talc  or  soapstone  is 
used  for  fire-bricks,  laboratory  tables,  gas  burners,  crayons,  etc. 

Pyrophyllite.2 — Pyrophyllite  is  a  hydrous  silicate  of  alumina 
(H2Al3Si4Oi2),  containing  66.7  per  cent.  SiO2,  28.3  per  cent. 
A1203,  and  5.0  per  cent.  H2O.  In  composition  and  physical 
qualities  it  is  similar  to  talc,  though  it  does  not  command  as  high  a 
price  as  the  best  talc.  It  is  mined  in  Moore  and  Chatham 
counties,  North  Carolina,  where  it  occurs  in  thick  beds  asso- 
ciated with  slate. 

ASBESTOS3 

Amphibole  Asbestos. — The  asbestos  of  mineralogy  is  a  mono- 
clinic  amphibole  which  develops  in  seams  and  slips  in  normal 
amphibolitic  rocks,  especially  where  the  rocks  have  been  sub- 
jected to  pressure  and  movement.  Chemically  it  is  a  calcium- 
magnesium  metasilicate.  According  to  the  series  of  analyses 
given  by  Merrill  the  silica  varies  from  52  to  58  per  cent.,  the 
lime  from  12  to  16  per  cent.,  the  magnesia  from  20  to  30  per 
cent.  Other  constituents  are  alumina,  varying  from  1  to  6  per 
cent.,  and  ferrous  oxide,  usually  from  1  to  6  per  cent.,  though 
in  some  cases  considerably  higher.  Water  is  always  present,  the 
amount  generally  varying  between  2  and  5  per  cent.  Although 
contrary  to  the  views  of  some  authorities,  the  conclusion  can 

1  T.  L.  Watson,  Mineral  Resources  of  Virginia,  1907,  p.  '293. 

2  J.  H.  Pratt,  Op.  tit. 

3  G.  P.  Merrill,  Proc.  U.  S.  Nat.  Mus.,  vol.  18,  1895,  p.  181.    Bull.  Geol. 
Soc.  Am.,  vol.  16,  1905,  p.  113.     Non-metallic  minerals,  1910,  pp.  183-197. 

F.  Cirkel,  Chrysotile-asbestos,  Canada  Dept.  of  Mines,  Mines  Branch, 
1910;  316  pp. 

J.  S.  Diller,  Mineral  Resources,  U.  S.  Geol.  Survey,  Annual  publication. 


396  MINERAL  DEPOSITS 

hardly  be  avoided  that  the  water  is  an  essential  constituent  and 
that  the  mineral  is  really  a  hydrated  form  of  tremolite  or  actino- 
lite.  The  extinction  angle  appears,  however,  to  be  that  char- 
acteristic of  these  amphiboles,  or  about  18°.  No  experiments 
appear  to  have  been  made  as  to  the  temperatures  at  which  the 
water  is  driven  off.  The  normal  varieties  of  amphibole  also  hold 
a  little  water,  but  in  far  smaller  quantities  than  asbestos. 

Anthophyllite  (Mg,Fe)  SiO3,  and  crocidolite,  NaFeSi206.- 
FeSiO3,  a  dark  blue  sodium  amphibole,  also  yield  asbestiform 
varieties. 

Merrill  has  shown  that  the  fibers  are  polygonal  in  outline 
and  run  out  into  needle-like  points;  down  to  a  diameter  of  0.002 
or  0.001  millimeter  the  fibers  retain  their  uniform  diameter  and 
polygonal  outlines.  The  color  of  amphibole  asbestos  is  usually 
white  to  greenish  white.  Only  the  finer  kinds  are  utilized,  but 
even  these  are  less  valued  than  the  serpentine  asbestos.  They 
are  apt  to  be  less  flexible  and  somewhat  brittle. 

Most  of  the  small  quantity  of  asbestos  mined  in  the  United 
States  is  of  the  tremolite  or  actinolite  variety,  and  it  often 
occurs  in  limestones  which  have  been  partly  metamorphosed 
to  amphibolitic  rocks.  The  mineral  is  classed  as  slip-fiber 
or  cross-fiber,  according  to  the  position  of  the  fibers  in  the 
veinlets.  The  radial  or  divergent  structures  are  designated  as 
mass-fiber. 

There  are  many  occurrences,  mainly  in  pre-Cambrian  rocks 
along  the  Appalachian  Mountain  system,  from  Vermont  to  Ala- 
bama. One  of  the  most  important  localities  worked  is  at  Sail 
Mountain,  Georgia,  where,  according  to  Diller,  the  asbestos 
occurs  in  large  lenticular  masses  in  gneiss  and  is  believed  to  be 
an  altered  igneous  rock.  Almost  the  entire  domestic  production 
is  derived  from  Georgia. 

Serpentine  Asbestos  (Chrysotile). — Chrysotile  asbestos  is 
green  or  yellowish-green  and  is  easily  reduced  to  a  white  fluffy 
state.  The  fiber  is  short,  but  of  very  uniform  diameter  and 
great  divisibility  and  flexibility;  the  decomposing  effect  of 
hydrochloric  acid  also  distinguishes  it  from  amphibole  asbestos. 
In  composition  it  is  practically  identical  with  the  purer  kinds  of 
serpentine.  A  typical  analysis  of  the  Canadian  material  yielded 
per  cents,  as  follows:  42  Si02,  42  MgO,  14  H2O,  1  FeO,  and 
1.7  A12O3.  Fig.  127  shows  the  appearance  of  the  two  kinds  of 
asbestos. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS   397 

This  variety  is  found  as  veinlets,  rarely  over  6  inches  thick,  in 
serpentine  or  peridotite,  and  has  almost  always  a  cross-fiber — 
that  is,  the  silky  fibers  lie  perpendicularly  to  the  plane  of  the 
veinlet. 

The  pure  yellowish-green  serpentine  which  occurs  in  conta 
metamorphic  limestone  and  which  is  an  alteration  product  of 
diopside   sometimes  contains   chrysotile  of  exceptionally  high 


FIG.  127.- -Chrysotile    (a)    and    amphibole  (b)  asbestos.     Photograph  by 
J.  S.  Diller. 

grade.  A  deposit  of  such  aterial  is  now  worked  in  Arizona, 
n  or theast  of  G  lobe . l 

Chrysotile  veinlets  may  be  found  in  almost  any  serpentine 
area,  but  they  are  rarely  so  abundant  and  large  as  to  be  of 
economic  importance.  The  views  regarding  their  origin  differ. 

Dresser  shows  that  serpentinization  in  the  Canadian  deposits 
proceeded  along  irregular  cracks  in  the  peridotite,  and  the  chryso- 
tile veinlets  are  found  in  the  center  of  the  serpentinized  bands. 

1  J.  S.  Diller,  Mineral  Resources,  U.  S   Geol.  Survey,  1917,  pt.  2,  p.  197. 


398  MINERAL  DEPOSITS 

These  veinlets  were  interpreted  by  Pratt  and  Merrill  as  fillings 
of  contraction  cracks,  but  other  authors  are  probably  correct  in 
considering  them  the  result  of  a  recrystallization  of  the  serpen- 
tine, proceeding  inward  from  the  cracks. 

S.  Taber1  believes  that  all  cross-fiber  veins  are  formed  by  a 
process  of  lateral  secretion,  the  growing  veins  pushing  aside  the 
enclosing  walls.  Since,  however,  the  material  in  the  veins  is 
derived  from  the  serpentine  itself  it  is  not  apparent  why  there 
is  any  need  of  increase  of  volume. 

Until  about  1895  the  small  quantity  of  asbestos  used  in  the 
United  States  came  from  Italy.  Since  that  date  the  development 
of  the  asbestos  industry  in  Canada  has  been  extremely  rapid, 
and  the  Canadian  mines  now  supply  this  country.  The  Canadian 
deposits2  center  in  Asbestos  Hill  at  Thetford,  in  the  eastern  town- 
ships of  Quebec.  As  stated,  the  mineral  occurs  as  irregular 
veinlets  in  serpentine  and  peridotite.  These  rocks  are  in  places 
accompanied  by  somewhat  later  gabbro  and  granite  and  all  of 
them  are  intrusive  into  Ordovician  sediments.  The  mineral  is 
mined  in  open  pits,  one  of  which,  for  instance,  is  700  feet  long, 
200  feet  wide,  and  165  feet  in  greatest  depth.  A  small  percentage 
is  obtained  by  hand  cobbing,  but  the  larger  part — 30  to  60  per 
cent. — of  the  crude  material  quarried  is  crushed  and  screened, 
and  the  fibers  are  separated  by  air  currents.3  The  extraction 
of  fiber  of  the  milled  rock  is  from  6  to  10  per  cent. 

Of  late  years  the  Russian  chrysotile  from  the  Ural  Mountains 
and  the  deposits  in  southern  Rhodesia  as  well  as  the  crocidolite 
asbestos  from  Griqualand  West,  Cape  Colony,  are  becoming 
important.  The  large  deposits  of  crocidolite  occur  in  thin  layers 
interbedded  with  jaspers  and  iron  stones  of  the  Pretoria  series. 

In  the  United  States4  chrysotile  of  economic  importance  is 
worked  in  Vermont,  near  Casper,  Wyoming,  and  in  Arizona. 
Thus  far,  the  production  is  small. 

Uses. — "The  fundamental  property  of  asbestos,  upon  which 
its  use  depends,  is  its  flexible,  fibrous  structure,  but  coupled  with 
this  are  the  scarcely  less  important  qualities  of  incombustibility 

1  Bull.  120,  Am.  Inst.  Min.  Eng.,  Nov.,  1916,  pp.  1973-1998. 

2  J.  A.  Dresser,  Econ.  Geol.,  vol.  4,  1909,  pp.  130-140. 

J.  A.  Dresser,  Preliminary  report  on  the  serpentine,  etc.,  of  southern 
Quebec,  Mem.  22,  Canada  Geol.  Survey,  1913,  p.  103. 

3W.  J.  Woolsey,  Jour.  Can.   Min.  Inst.,  vol.   13,   1910,  pp.  408-413. 
4  J.  S.  Diller,  Bull.  470,  U.  S.  Geol.  Survey,  1910,  pp.  506-524. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS    399 

and  slow  conduction  of  heat  and  electricity  when  the  mass  is 
fiberized  and  porous."  The  spinning  and  weaving  of  fire-proof 
cloth  form  an  important  part  of  the  asbestos  industry  carried  on 
in  the  United  States  with  Canadian  raw  material.  The  highest 
grade  of  the  crude  mineral  is  expensive,  costing  $275  to  $350  per 
ton;  the  fines  cost  $25  to  $125  per  ton,  while  the  lowest  grade — a 
mixture  of  serpentine  and  asbestos — is  sold  at  less  than  $1  per 
ton.  Amphibole  asbestos  is  much  cheaper,  costing  about  $18 
per  ton.  Crocidolite  is  more  easily  fusible  but  is  more  resistant 
than  chrysotile  to  acids  and  sea  water.  The  London  price  is 
about  $125  per  ton.1 

The  Canadian  production  in  1917  was  144,185  tons;  almost 
the  whole  production  was  exported  to  the  United  States.  In  the 
same  year  the  output  in  the  United  States  was  1,683  tons. 

ORES  OF  COPPER,  LEAD,  VANADIUM,  AND  URANIUM  IN  SAND- 
STONE AND  SHALE 

General  Features. — Ores  of  copper,  lead,  vanadium,  and  ura- 
nium are  often  found  disseminated  in  sandstones  and  shales  far 
from  igneous  rocks.  The  sedimentary  strata  containing  the 
ores  are  usually  parts  of  thick  series  of  terrigenous  or  shallow- 
water  beds,  commonly  of  reddish  color.  The  ores  are  of  low 
tenor  and  can  be  utilized  only  in  exceptional  cases.  Never- 
theless this  class  of  deposits  presents  many  interesting  features. 

The  ore  minerals  are  chalcocite,  galena,  roscoelite  (a  vana- 
dium mica),  various  copper  and  lead  vanadates,  carnotite  (a 
vanadate  of  uranium),  etc.  Bornite,  chalcopyrite,  and  pyrite 
are  less  common.  The  ores  frequently  carry  small  amounts 
of  silver,  nickel,  cobalt,  molybdenum,  and  selenium.  Gangue 
minerals  occur  sparingly  and  are  usually  confined  to  a  little 
barite,  calcite,  and  gypsum.  The  outcrops  are  likely  to  be  bril- 
liantly colored  by  malachite  and  azurite.  While  the  deposits  are 
confined  to  certain  formations  or  members,  they  do  not  continu- 
ously follow  a  particular  horizon  and  give  no  evidence  of  being 
of  sedimentary  origin.  They  often  appear  in  fractured  and  brec- 
ciated  beds  or  in  strata  rich  in  carbonaceous  matter  and  plant 
remains.  More  rarely  the  ores  follow  distinct  fissures  in  the 
sedimentary  rocks.  They  do  not  seem  to  have  any  genetic 
relation  with  thermal  springs.  The  copper,  lead,  and  vanadium 

*P.  A.  Wagner,  South  African  Journal  of  Industries,  Nov.,  1917. 


400  MINERAL  DEPOSITS 

deposits  form  three  groups  in  this  class,  but  each  group  is  likely 
to  contain  more  or  less  of  the  other  metals.  There  is  no  reason 
why  the  deposits  should  be  confined  to  any  particular  geological 
age,  but  as  a  matter  of  fact  almost  all  of  them  are  in  the  upper 
Carboniferous,  Permian,  Triassic,  or  Jurassic. 

Origin. — In  considering  the  class  as  a  whole  it  appears  that 
igneous  agencies  had  no  part  in  the  genesis.  The  ores  are 
assuredly  epigenetic  and  their  universal  appearance  in  land  or 
shallow-water  beds  is  significant.  In  all  probability  these  ores 
have  been  concentrated  by  meteoric  waters  which  leached 
the  small  quantities  of  metals  disseminated  in  the  strata.  The 
sediments  were  rapidly  accumulated,  under  arid  conditions,  from 
adjacent  land  areas  and  the  metals  were  probably  carried  down 
in  fine  detritus  and  in  solutions  from  older  ore  deposits  in  these 
continental  areas. 

The  waters  which  concentrated  the  ores  are  believed  to  have 
been  mainly  sodium  chloride  and  calcium  sulphate  solutions 
containing  sulphates  and  perhaps  chlorides  of  copper  and  lead. 
The  mineral  association  and  geological  features  indicate  deposi- 
tion at  low  temperature,  probably  well  below  100°  C.,  and  at 
shallow  depths  but  below  the  zone  of  direct  oxidation.  Very 
likely  these  ores  have  been  forming  continuously  since  the  estab- 
lishment of  active  water  circulation  in  the  beds;  in  favorable 
places  below  the  surface  concentration  may  now  be  in  progress. 

COPPER  AND  LEAD  DEPOSITS  IN  SANDSTONE 

European  Occurrences.1— The  European  occurrences  are  con- 
fined to  the  Permian  and  the  Triassic,  both,  generally  speaking, 
ages  of  arid  climate  and  saline  deposits. 

The  Russian  Permian,  extending  far  west  from  the  Urals,  con- 
sists in  its  lower  division  of  sandstones,  marls  (in  part  marine), 
and  conglomerates.  The  sandstones  are  rich  in  vegetable  re- 
mains. Copper  ores  are  found  over  wide  areas,  but  have  not 
been  worked  extensively  of  late.  The  average  tenor  is  said  to 
be  0.9  per  cent,  metallic  copper.  The  chalcocite  ores  replace 
plant  remains  and  tree  trunks  or  form  the  cement  of  the  sand- 
stones. The  minerals  mentioned  from  this  locality  are  (besides 

1  For  an  excellent  review  of  European  localities,  as  well  as  complete  index 
of  literature,  in  part  difficultly  accessible,  see  Stelzner  and  Bergeat,  Die 
Erzlagerstatten,  1904,  pp.  388-439. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS  401 

secondary  malachite  and  azurite)  chalcocite,  chalcopyrite,  barite, 
vanadinite,  and  volborthite  (vanadate  of  copper  and  calcium.) 

Recently  much  interest  has  been  taken  in  the  copper  deposits 
of  the  Khirgiz  Steppes,1  between  the  Urals  and  the  Altai,  in  the 
Karkaralinsk  and  Akmolinsk  districts.  Very  rich  copper  ores, 
consisting  of  malachite,  azurite,  and  bornite,  have  been  found 
here  in  sandstones  reported  to  be  of  Paleozoic  age.  At  Nankat, 
west  of  Kokand  in  Turkestan,  deposits  of  metallic  copper  have 
been  discovered  in  sandstones  and  gypsiferous  marls  of  Tertiary 
age;  fossil  wood  and  chalcocite  are  also  found.2 

The  lower  Permian  (Rothliegende)  of  Bohemia,3  along  the 
south  slope  of  the  Rieserigebirge,  contains  similar  ores. 

Over  a  large  part  of  western  Europe  the  Triassic  is  copper- 
bearing,  and  together  with  the  copper  more  or  less  lead  is  found. 

In  England,  at  Alderley  Edge  and  Mottram  St.  Andrews,4 
south  of  Manchester,  copper  ores  have  been  mined.  They  occur 
in  the  cement  of  Triassic  sandstones  and  conglomerates  and 
consist  of  copper  carbonates,  galena,  pyromorphite,  and  vana- 
dinite; also  some  barite,  manganese,  and  cobalt.  The  ores  are 
said  to  contain  at  most  1.4  per  cent,  copper.  The  mineral  mott- 
ramite,  a  vanadate  of  copper  and  lead,  was  discovered  at  this 
place  and  vanadium  was  extracted  from  the  ores. 

In  Germany  the  Triassic  is  divided  into  three  parts — the  lower 
Variegated  Sandstone  ("Buntsandstein"),  the  middle  Shell 
Limestone  (Muschelkalk) ;  and  the  upper  marls  and  sandstones 
(Keuper);  of  these  the  lower  and  upper  divisions  contain  lead 
and  copper  ores. 

In  Bavaria  the  Keuper  contains  galena  and  chalcopyrite  in 
certain  gypsiferous  beds,  and  these  minerals  are  associated  with 
a  little  zinc  blende  and  barite. 

In  Wurttemberg  galena  with  barite  and  some  oxidized  copper 
ores  is  generally  distributed  in  the  Corbula  bed  of  the  lower, 
gypsiferous  Keuper.  In  the  Palatinate,  near  Freihung,  the 
littoral  characteristics  of  the  formation  are  plainly  indicated  and 
there  is  an  abundance  of  fossil  wood;  at  two  horizons  the  sand- 
stones contain  galena  and  cerussite  and  were  formerly  worked. 

1  A.  Addiassewich,  A  journey  to  central  Asia,   Trans.,  Inst.  Min.  and 
Met.,  vol.  17,  1907-1908,  pp.  498-522. 

2  R.  Beck,  Lehre  von  den  Erzlagerstatten,  1909,  vol.  2,  p.  172. 

3  F.  Gurich,  Zeitschr.  prakt.  Geol,  1893,  pp.  370-371. 

4  Phillips  and  Louis,  Ore  deposits,  1896,  pp.  266-269. 


402  MINERAL  DEPOSITS 

In  the  "  Buntsandstein "  in  Prussia  and  Lorraine,  near  Saar- 
louis  and  other  places,  a  formation  known  as  the  Voltzia  sand- 
stone is  particularly  rich  in  lead  and  copper  ores,  which  at  times 
have  been  mined.  The  bed  contains  abundant  plant  remains. 
The  minerals  are  cerussite,  galena,  chalcocite  (?),  and  copper 
carbonates. 

The  best-known  deposits  of  the  European  Triassic  are  those 
of  Commern  and  Mechernich,  not  far  from  Aix-la-Chapelle,  in 
Prussia.  Lead  ores  have  been  mined  here  for  several  hundred 
years,  but  it  is  reported  that  the  mines  may  soon  be  closed.  The 
ores  are  of  low  grade  and  are  mined  in  open  cuts  by  removing 
about  130  feet  of  overburden.  In  1903  the  ores  averaged  1.5 
per  cent.  lead.  The  ore  minerals  are  galena  and  cerussite,  with 
a  little  chalcopyrite  and  barite,  the  latter  filling  veins  and  veinlets 
in  the  sandstone.  Small  amounts  of  silver,  nickel,  and  cobalt 
are  present.  The  thickness  of  the  ore-bearing  sandstone 
is  about  20  meters.  The  general  occurrence  of  the  galena  in 
so-called  "Knoten"  or  knotty  concretions  is  very  remarkable. 
They  often  enclose  several  sand  grains  and  some  of  them  are 
bounded  by  the  crystal  faces  of  the  galena.  The  epigenetic 
character  of  the  ore  is  beyond  doubt. 

American  Occurrences.1 — On  the  North  American  continent 
copper  ores  are  widely  distributed  in  the  "Red  Beds"  of  the 
Southwest,  in  Texas,  Oklahoma,  New  Mexico,  Arizona,  Colorado, 

1  E.  T.  Dumble,  First  Ann.  Rept.,  Geol.  Survey  Texas,  1889,  p.  186. 

E.  J.  Schmitz,  Copper  ores  in  the  Permian  of  Texas,  Trans.,  Am.  Inst. 
Min.  Eng.,  vol.  26,  1896,  pp.  1051-1052. 

S.  F.  Emmons,  Copper  in  the  Red  Beds,  Bull.  260,  U.  S.  Geol.  Survey, 
1905,  pp.  221-232. 

W.  H.  Emmons,  The  Cashin  mine,  Bull.  285,  U.  S.  Geol.  Survey,  1906, 
pp.  125-128. 

E.  P.  Jennings,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  34,  1904,  p.  839. 

H.  W.  Turner,  Trans.,  Am.  Inst. Min.  Eng.,  vol.  33,  1903,  p.  678. 

W.  Lindgren,  L.  C.  Graton,  and  C.  H.  Gordon,  The  ore  deposits  of 
New  Mexico,  Prof.  Paper  68,  U.  S.  Geol.  Survey,  1910. 

H.  S.  Gale  (Idaho),  Bull.  430,  U.  S.  Geol.  Survey,  1909,  pp.  112-121. 

W.  Lindgren  (Colorado),  Bull.  340,  U.  S.  Geol.  Survey,  1907,  pp. 
170-174. 

W.  A.  Tarr  (Oklahoma),  Econ.  Geol,  vol.  5,  1910,  pp.  221-226. 

A.  E.  Fath  (Oklahoma),  Econ.  Geol.,  vol.  10,  1915,  pp.  140-150. 

L.  M.  Richard  (Texas),  Econ.  Geol,  vol.  10,  1915,  pp.  634-650. 

A.  F.  Rogers,  Origin  of  copper  ores  of  the  "Red  Bed"  type,  Econ.  Geol, 
vol.  11,  1916,  pp.  366-380. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS    403 

Wyoming,  Utah,  and  Idaho  and,  though  conspicuous  by  green 
and  blue  colors,  rarely  prove  of  economic  importance. 

The  ore  occurs  in  arkose  sandstone,  conglomerate,  or  clay  shale 
and  is  usually  associated  with  plant  remains  and  fossil  wood. 
These  strata  were  accumulated  in  shallow  seas  or  as  subaerial 
deposits  by  a  process  of  rapid  degradation  of  adjacent  land  areas 
of  the  Rocky  Mountain  region,  and  they  have  been  referred  to  the 
upper  Carboniferous,  Permian,  Triassic,  and  Jurassic. 

In  Texas  the  copper-bearing  beds  appear  over  large  areas  in 
Permian  sandstones  and  shales.  They  lie  at  several  horizons, 
in  strata  rich  in  plant  remains;  covellite,  chalcocite,  chalcopyrite, 
and  pyrite  are  the  minerals  mentioned.  The  area  is  said  to  ex- 
tend from  33°  to  34°  in  latitude  and  from  98°  to  100°  in  longitude. 


FIG.  128. — Chalcocite  nodules  replacing  fossil  wood  and  carbonaceous  shale 
of  "Red  Beds,"  Red  Gulch,  Colorado.     Natural  size. 

In  Oklahoma  nodules  of  chalcocite  are  found  in  red  shales  and 
sandstones  of  the  same  age.  Fossil  wood  is  often  converted  to 
chalcocite,  sometimes  with  a  shell  of  chalcopyrite;  unusually 
high  silver  assays  of  31  ounces  per  ton  and  traces  of  gold  are 
reported. 

In  Colorado  these  ores  have  been  recorded  at  several  places, 
notably  at  Red  Gulch,  Fremont  County,  where  Lindgren  ob- 
served nodules  of  chalcocite  with  barite  in  black  carbonaceous 
shale;  sections  (Fig.  128)  show  that  the  copper  sulphide  actually 
replaces  the  coal  and  shale.  The  horizon  is  probably  the  upper- 
most Carboniferous.  In  northeastern  Arizona,  according  to  Greg- 
ory,1 small  quantities  of  oxidized  copper  ores  are  frequently  seen 

1  H.  E.  Gregory,  Prof.  Paper  93,  U.  S.  Geol.  Survey,  1917,  p    140 
J.  M.  Hill,  Bull.  540,  U.  S.  Geol.  Survey,  1913,  p.  163. 


404  MINERAL  DEPOSITS 

in  the  La  Plata  sandstone.  North  of  the  Colorado  River  they 
appear  in  the  Carboniferous  of  the  Kaibab  Plateau.  S.  F. 
Emmons  believed  that  the  copper  in  the  oxidized  ore  and  chal- 
cocite  in  the  Aubrey  limestone  near  Grandview,  Arizona,  was 
leached  from  the  "Red  Beds"  and  carried  down  into  the 
limestone. 

In  southwestern  Colorado  copper,  often  accompanied  by 
vanadium  ores,  is  widely  distributed  in  the  Jurassic  La  Plata 
sandstone.  W.  H.  Emmons  has  described  the  Cashin  vein  in 
this  formation  near  Placerville.  The  ores  are  here  argentiferous 
chalcocite,  covellite,  and  bornite,  with  some  calcite.  No  igneous 
rocks  are  present  and  Emmons  believes  that  the  ores  were  leached 
from  the  " Red  Beds."  There  is  an  active  circulation  of  water  in 
the  formation,  and  springs  with  salt,  sulphates,  and  hydrogen 
sulphide  abound.  A  production  of  about  300,000  ounces  of 
silver  and  700,000  pounds  of  copper  is  recorded  from  this  mine. 

The  greatest  development  of  the  copper-bearing  sandstones  is 
in  New  Mexico;  considerable  production  from  picked  ore  has 
been  achieved  at  the  Nacimiento  deposits,  in  the  northern  part 
of  the  State,  where  the  "Red  Beds"  rest  on  pre-Cambrian  gra- 
nitic rocks  which  contain  much  older  copper  deposits.  The  beds 
have  been  referred  to  the  Triassic  on  the  evidence  of  fossil  plants. 
According  to  Schrader1  most  of  the  copper  ores  occur  in  the  basal 
beds  and  are  confined  within  a  thickness  of  25  feet  in  a  reddish- 
white  sandstone  rich  in  fossil  wood,  which  is  largely  chalcocitized. 
A  tree  trunk  60  feet  long  with  a  basal  diameter  of  2%  feet  is  men- 
tioned, which  was  almost  wholly  converted  to  copper  glance. 
Besides  malachite,  azurite,  and  chrysocolla,  there  is  some  barite 
and,  at  one  place,  cerussite.  The  low-grade  ores  have  not  been 
utilized. 

According  to  the  same  geologist  the  copper-bearing  beds  of 
the  Zuni  Mountains,  in  northeastern  New  Mexico,2  lie  at  the 
base  of  the  "Red  Beds,"  resting  on  pre-Cambrian  gneisses  which 
contain  older  copper  veins.  The  sandstones,  shales,  and  marls 
for  30  to  60  feet  just  above  the  base  of  the  beds  contain  oxidized 
ores  and  chalcocite  replacing  wood. 

Graton  describes  in  detail  the  ores  from  the  Tecolote  district, 
San  Miguel  County,  which  are  partly  in  the  "Red  Beds"  of  the 
upper  Carboniferous  (Abo  formation),  and  partly  at  a  higher 

1  F.  C.  Schrader,  Prof.  Paper  68,  U.  S.  Geol.  Survey,  pp.  141-149. 
*  Idem,  p.  134. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS  405 

horizon,  perhaps  in  the  Dakota  sandstone.  The  calcareous 
cement  of  the  arkose  is  replaced  by  chalcocite,  bornite,  chal- 
copyrite,  and  pyrite,  the  replacement  extending  into  the  feld- 
spar grains. 

In  the  Oscura  Range,  also  in  New  Mexico,  red  sandstones, 
probably  Carboniferous  "Red  Beds,"  contain  chalcocite,  bornite, 
and  chalcopyrite,  in  part  as  replacement  of  fossil  wood.  Turner 
mentions  the  occurrence  of  plant  remains,  said  to  have  been 
identified  as  the  Triassic  Podozamites  crassifolia,  the  same  cycad 
which  is  characteristic  of  the  deposits  at  Abiquiu  first  studied 
by  Newberry. 

Graton  believes,  contrary  to  Turner,  that  the  copper  ores  have 
been  introduced  into  the  strata  by  waters  ascending  along  a 
number  of  dislocations  in  the  sandstone. 

Finally,  H.  S.  Gale  describes  copper  ores  from  southern  Idaho 
which  occur  in  the  Ankareh  maroon  shales  and  sandstones  of 
the  Triassic  or  Carboniferous  (equivalent  to  the  Permo-Carbonif- 
erous  of  the  Fortieth  Parallel  Survey).  (See  Fig.  10.)  A  thick 
limestone  (including  the  Meekoceras  beds)  underlying  these 
shales  is  believed  by  some  geologists  to  be  Triassic. 

There  are  then  at  least  two  main  cupriferous  formations  in  the 
Southwest — (1)  the  upper  Carboniferous  "Red  Beds,"  equivalent 
to  the  Permo-Carboniferous,  or  the  Abo  formation;  (2)  the 
undoubtedly  Jurassic  La  Plata  sandstone. 

The  silver  deposits  in  the  supposedly  Triassic  sandstones  of 
Silver  Reef,1  in  southern  Utah  (Harrisburg  district),  which 
created  a  boom  about  1880,  are  now  worked  only  on  a  small 
scale.  The  ores  were  silver  chloride  above  the  water  level  and 
native  silver  and  argentite  in  depth;  copper  was  also  present,  and 
selenium  is  reported.  Plant  remains  were  abundant.  A  sec- 
ondary concentration  from  a  primary  argentiferous  chalcocite 
is  the  probable  genesis. 

In  Nova  Scotia,  Cumberland  County,  chalcocite  nodules, 
with  remains  of  pyrite,  and  also  chalcocitized  wood,  are  found 
in  the  Permian  sandstone. 

1  C.  M.  Rolker,  The  silver  sandstone  district  of  Utah,  Trans.  Am.  Inst. 
Min.  Eng.,  vol.  9,  1881,  pp.  21-33. 

J.  P.  Rothwell,  The  silver  sandstone  formation  of  Silver  Reef,  Eng.  and 
Min.  Jour.,  vol.  29,  1880,  pp.  25,  48,  79. 

J.  S.  Newberry,  Report  on  the  property  of  the  Stormont  Silver  Mining 
Company,  Eng.  and  Min.  Jour.,  vol.  30,  1880,  p.  269;  vol.  31,  1881,  pp.  4-5. 

J.  F.  Kemp,  Ore  deposits  of  the  United  States,  1900,  p.  334. 


406  MINERAL  DEPOSITS 

South  America. — The  well-known  and  long-worked  copper 
deposits  of  Coro-Coro,1  in  Bolivia,  are  contained  in  a  series  of 
sandstones,  believed  to  be  of  Permian  age.  There  are  several 
cupriferous  beds  with  disseminated  native  copper,  in  places  den- 
dritic, and  much  gypsum,  also  some  native  silver,  domeykite, 
and  chalcocite.  The  copper-bearing  beds  are  much  lighter  in 
color  than  the  prevailing  deep-red  sandstones. 

According  to  Steinmann  the  strata  are  of  Cretaceous  age  and 
the  copper  was  introduced  by  hot  waters  derived  from  an  intru- 
sion of  diorite.  Nevertheless  the  descriptions  suggest  strongly 
that  the  deposits  belong  in  a  different  class. 

Africa. — Sufficient  information  is  not  at  hand  to  decide  whether 
the  recently  opened  Katanga  ores2  of  southeastern  Belgian 
Kongo,  near  Rhodesia,  belong  to  the  class  of  deposits  described 
in  this  chapter.  Large  masses  of  high-grade  oxidized  copper  ores 
are  contained  in  sandstones,  shale,  and  limestone,  probably  of 
Paleozoic  age.  The  ores  are  of  high  grade  (8  to  12  per  cent, 
copper)  and  are  said  to  contain  a  little  gold  and  silver;  some 
manganese,  cobalt,  and  nickel  are  present.  Barite  and  quartz 
appear  as  gangue  minerals. 

Genesis  of  Sedimentary  Copper  Ores. — The  epigenetic  char- 
acter of  the  copper  deposits  in  sandstone  is  proved  beyond  rea- 
sonable doubt.  The  replacement  of  coal,  carbonaceous  shale, 
and  calcareous  or  kaolinic  sandstone  cement  by  chalcocite  is 
also  proved.  The  gangue  minerals  are  few  and  quartz  is  con- 
spicuously absent.  Barite  in  small  amounts  is  rather  common. 
Irregularity  in  dissemination  is  typical,  though  the  ores  often 
follow  certain  horizons  rather  persistently.  The  entire  independ- 
ence of  the  occurrence  of  igneous  rocks  is  marked. 

The  occurrences  are  mainly  on  the  flanks  of  older  continental 
areas  containing  pre-Cambrian  copper  deposits;  the  sandstones 
were  rapidly  laid  down  as  arkoses,  indicating  a  long  epoch  of  rock 
decay,  the  products  of  which  were  swept  away  during  a  following 
arid  epoch.  Considering  the  evidence  as  a  whole  the  sedimen- 

1  Older  literature:  See  Stelzner  and  Bergeat,  Die  Erzlagerstatten,  vol.  1, 
1904,  p.  419. 

G.  Steinmann,  Rosenbusch  Festschrift,  1906,  pp.  335-368. 
F.  C.  Lincoln,  Min.  and  Sd.  Press,  Sept.  29,  1917. 
Lester  W.  Strauss,  Min.  Mag.,  vol.  7,  1912,  p.  207. 

2  S.  H.  Ball  and  M.  K.  Shaler,  Mining  conditions  in  the  Belgian  Congo, 
Trans.,  Am.  Inst.  Min.  Eng.,  vol.  41,  1911,  pp.  189-219;  also  Econ.  Geol, 
vol.  9,  1914,  pp.  617-632,  with  literature. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS    407 

tary  deposits  must  have  contained  finely  divided  copper  ores, 
in  part  from  solutions  derived  from  the  land  area,  in  part  as 
cupriferous  detritus.  When  atmospheric  waters  charged  with 
salt  and  gypsum  searched  these  beds  they  must  have  taken  this 
copper  into  solution  and  concentrated  it  at  certain  horizons  when 
reducing  substances  like  coaly  vegetable  matter  were  available. 
In  most  cases  the  solution  probably  contained  the  copper  as 
sulphate,  though  where  much  salt  was  present  it  might  well  have 
been  transformed  into  chloride. 

Genesis  by  ascending  thermal  solutions  of  meteoric  origin  is 
a  possible  cause  of  some  deposits. 

Graton,  Fath  and  Rogers  have  noted  pyrite,  bornite  and  chal- 
copyrite  in  the  chalcocite  nodules  and  the  latter  two  authors  have 
shown  that  the  chalcocite  replaces  earlier  pyrite  or  marcasite 
(Fig.  128).  No  doubt  this  is  true  in  places  but  this  view  may 
not  be  universally  applicable.  Rogers1  believes  that  the  fossil 
wood  was  successively  replaced  by  hematite,  pyrite,  bornite  and 
chalcocite,  a  rather  improbable  series  of  events,  considering  that 
the  wood  structure  is  preserved  even  in  the  fourth  and  last  re- 
placement. Graton  finds  sharp  cubes  of  pyrite  in  the  chalcocite 
and  no  evidence  of  replacement  of  pyrite. 

In  the  precipitation  the  most  important  chemical  reactions 
were  those  between  the  hydrocarbons  of  plant  remains  and  the 
calcareous  cement  or  the  kaolin  in  the  sandstone  on  one  hand 
and  the  cupriferous  solutions  on  the  other  hand.  How  the 
metallic  copper  in  these  ores  was  precipitated  is  not  known. 
The  Bolivian  occurrences  show  distinct  bleaching  of  the  reddish 
sandstone  around  the  copper  aggregates,  from  which  it  may  be 
inferred  that  the  solutions  were  reducing  in  character. 

VANADIUM  AND  URANIUM  ORES  IN  SANDSTONES' 

Composition. — Many  of  the  copper  deposits  described  above 
carry  some  vanadium  as  vanadinite  or  volborthite.  Lately 
vanadium  with  some  uranium  and  a  trace  of  radium  has  been 

1  Op.  tit. 

2  G.  P.  Merrill,  Non-metallic  minerals,  1904,  pp.  299-320. 

W.  F.  Hillebrand  and  F.  L.  Ransome,  Carnotite,  etc.,  in  western 
Colorado,  Am.  Jour.  Sci.,  4th  ser.,  vol.  10,  1900,  pp.  120-144.  Bull.  262, 
U.  S.  Geol.  Survey,  1905,  pp.  9-13. 

H.  Fleck  and  W.  G.  Haldane,  Rept.  State  Bureau  of  Mines,  Colorado, 
1907,  pp.  47-115. 


408  MINERAL  DEPOSITS 

shown  to  be  common  in  certain  Jurassic  sandstones  in  Colorado 
and  Utah.  The  deposits  in  western  Colorado  are  now  worked 
and  a  reduction  plant  is  located  at  Vanadium,  near  Placerville. 

A  number  of  unusual  minerals  are  contained  in  these  deposits. 
One  of  the  most  conspicuous  is  carnotite  ^UsOs.V^Os.I^O.SHaO) 
a  crystalline  potassium-uranium  vanadate,  first  named  by  Fuchs 
and  Cumenge,  which  forms  a  bright  yellow  powder  occurring  in 
seams  and  on  fossil  wood.  An  analysis  by  W.  F.  Hillebrand  gave : 

Per  cent.  Per  cent. 

UO,.... 54.89        CuO 0.15 

VZO8 18.49         MoO3 0.18 

CaO 3.34        H20 4.54 

BaO 0.90        CO- 0.56 

K20 6.52        Insoluble 7.10 

PbO 0.13 

In  calcio-carnotite  potassium  is  replaced  by  calcium. 

Associated  with  carnotite  are  a  number  of  other  obscure 
vanadium  'minerals,  which  appear  as  earthy  black,  brown  and 
red  coatings  or  fissure  fillings.  They  are  crystalline  and  highly 
hydrous  vanadates.  Metahewettite1  (CaO.SV^Os.OH^O)  is  a  dark 
red  calcium  vanadate  containing,  in  per  cent.,  70V203,  7.25CaO 
and  21.30H20. 

Pintadoite  is  another  mineral  of  similar  composition  but  with 
only  42.4  per  cent.  V2O5;  it  forms  green  efflorescences  and  occurs 
in  Utah.2  Uvanite,  a  brownish-yellow  uranium  vanadate  (2U03.- 

J.  M.  Boutwell,  Bull.  260,  U.  S.  Geol.  Survey,  1905,  p.  205. 

H.    S.    Gale  (Carnotite  in  Colorado),  Bull.  340,  U.  S.  Geol.  Survey, 
1908;  Idem,  Bull.  315,  1906,  pp.  110-117. 

F.  L.  Hess,  Vanadium  deposits  in  Colorado,  Utah,  and  New  Mexico, 
Bull  530,  U.  S.  Geol.  Survey,  1912. 

Idem,  Mineral  Resources,  Annual  issue,  1912,  pp.  1003-1036. 

F.  L.  Hess,  A  hypothesis  for  the  origin  of  the  carnotites,  Econ.  Geol., 
vol.  9,  1914,  pp.  675-688. 

R.  B.  Moore  and  K.  L.  Kitthil,  A  preliminary  report  on  uranium,  radium 
and  vanadium,  Bull.  70,  U.  S.  Bureau  of  Mines,  1913. 

i  K.  L.  Kitthil  and  John  A.  Davis,  Mining  and  concentration  of  carnotite 
ore,  Bull  103,  U.  S.  Bureau  of  Mines,  1917. 

Parsons,  Moore,  Lind  and  Schaefer,  Extraction  and  recovery  of  radium, 
uranium  and  vanadium,  etc.,  Bull.  104,  U.  S.  Bureau  of  Mines,  1915. 

1  W.  F.  Hillebrand,  H.  E.  Merwin  and  F.  E.  Wright,  Proc.,  Am.  Philos. 
Soc.,  vol.  53,  1914,  pp.  31-54. 

2  F.  L.  Hess  and  W.  T.  Schaller,  Jour.,  Washington  Acad.  Sci.,  vol.  4, 
pp.  576-579. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS  409 

3V205.15H20)  with  39.60  per  cent.  UO3  and  37.70  per  cent.  V208 
is  mined  in  Emery  County,  Utah. 

Volborthite  and  calcio-volborthite  both  vanadates  of  copper,  and 
a  uranium  sulphate  have  also  been  identified. 

Roscoelite,  a  dark  green  vanadium  mica  is  abundant  as  veins 
and  as  replacements  in  the  cement  of  some  sandstones  (Fig.  129) 
at  Placerville,  Colorado.  About  two-thirds  of  the  aluminum  is 
replaced  by  vanadium  so  that  it  contains  from  20  to  29  per  cent. 
V2O3.  Some  sandstones  contain  as  much  as  20  per  cent,  of 
roscoelite.1 


Fia.  129. — Vanadium  ores  in  sandstone.  White  areas,  quartz;  shaded 
areas,  roscoelite,  partly  radial.  Magnified  25  diameters.  After  F.  L.  Hess, 
U.  S.  Geol.  Survey. 

Chromium  is  also  present  in  these  remarkable  ores,  probably 
as  mariposite,  or  chromium  mica.  Barium,  copper,  lead,  molyb- 
denum and  arsenic  are  contained  in  the  ores  as  shown  by  the 
analysis  of  carnotite.  Molybdenum  is  quite  abundant  in  some 
places  and  appears  to  occur  as  a  soluble  sulphate — the  dark  blue 
ilsemannite.  A  similar  occurrence  of  the  latter  mineral  is  re- 
ported from  South  Africa.  Native  selenium  has  been  identified 
by  Hillebrand,  associated  with  metahewettite.  Some  of  these 
minerals  are  quite  certainly  secondary  products. 

1  Roscoelite  is  also  known  from  some  gold-quartz  veins  and  is  often 
intimately  associated  with  native  gold.  Mariposite  is  a  characteristic 
mineral  of  many  gold  quartz  veins. 


410 


MINERAL  DEPOSITS 


The  carnotite,  which  is  the  most  abundant  uranium  mineral, 
contains  a  small  trace  of  radium1  which  is  recovered.  Gypsum 
is  about  the  only  gangue  mineral  associated  with  the  ores. 

The  ores  are  not  rich.  The  carnotite  ores  contain  about  1.5 
to  3  per  cent.  U608  and  3  to  5  per  cent.  V203.  Concentration 
has  been  attempted  in  some  cases. 

The  roscoelite  ore  at  Placerville  contains  about  3.50  per  cent. 
V203  and  0.05  per  cent.  U03. 

Occurrence. — The  ores  are  found  in  the  Plateau  province  of 
horizontal  or  gently  inclined  strata  in  southwestern  Colorado  and 
eastern  Utah.  The  best  known  localities  are  at  Placerville,  Col- 


Apparent 
nconformity 
Seam 


.  Coarse .  .'sandstone.' 


j?  Feet 


FIG.  130. — Sketch  of  vanadium-bearing  sandstone  at  mine  of  Primes 
Chemical  Company,  on  the  east  side  of  Bear  Creek,  Newmire,  Colo.  After 
F.  L.  Hess,  U.  S.  Geol.  Survey. 


orado,  and  in  the  LaSal,  Paradox  and  Sindbad  valleys  somewhat 
farther  west. 

The  ores  are  mainly  confined  to  the  McElmo  and  LaPlata 
formations  of  white,  often  cross  bedded  Jurassic  sandstone  which 
frequently  contain  much  transported  partly  carbonized  wood. 
They  follow  certain  horizons  or  appear  in  fissures  of  flat  veins 
or  in  brecciated  zones  (Fig.  130)  and  are  often  associated  with 
the  fossil  wood. 

Some  observers  have  thought  the  ores  merely  superficial  but 
it  now  seems  certain  that  they  may  be  found  in  depth.  At 
Placerville  the  workings  are  said  to  have  penetrated  2,000  feet 
under  ground  in  horizontal  direction. 

1  At  the  rate  of  one  gram  of  radium  in  3, 000  kilograms  of  metallic  uranium. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS    411 

Not  similar  to  those  deposits,  yet  perhaps  of  a  similar  origin, 
are  the  recently  discovered  important  vanadium  deposits'  at 
Minasragra,1  near  Quisque,  Province  of  Pasco,  Peru,  described 
by  D.  F.  Hewett  and  W.  F.  Hillebrand.  The  vanadium  sulphide, . 
patronite,  occurs  here  on  a  large  scale  as  greenish-black  masses 
associated  with  a  hydrocarbon  and  a  peculiar  nickel-bearing  py- 
rite.  An  analysis  of  the  patronite  gave  58.79  per  cent,  sulphur, 
19.53  per  cent,  vanadium,  0.18  per  cent,  molybdenum,  1.87  per 
cent,  nickel,  and  3.47  per  cent,  carbon.  The  deposit  is  said  to  be 
a  vein  with  much  bitumen  and  clay  in  gently  dipping  Cretaceous 
strata.  Igneous  rocks  are  present  in  abundance. 

Genesis. — Traces  of  vanadium  are  found,  according  to  Hille- 
brand, in  most  igneous  rocks,  and  some  varieties  of  augites  carry 
as  much  as  2  per  cent,  of  the  metal.  Titanic  iron  ores  usually 
contain  a  fraction  of  a  percent.  Smaller  quantities  occur  as 
oxidation  products  in  many  ore  deposits,  mostly  as  vanadinite 
or  descloizite.  Vanadium  tends  to  concentrate  in  clays  and 
shales;  it  is  also  concentrated  in  coal;  the  ashes  of  many 
varieties  are  rich  in  vanadium.2  A  coal  from  the  copper  deposits 
in  sandstone  of  Red  Gulch,  Colorado,  contained,  according  to 
Hillebrand,3  0.18  per  cent,  vanadium.  The  processes  of  weather- 
ing and  vegetation  appear  to  be  favorable  to  its  concentration; 
and,  to  some  extent,  uranium  shares  in  this  behavior. 

F.  L.  Hess  believes  that  adjoining  land  areas  may  have  con- 
tained uranium-  and  vanadium-bearing  veins  at  a  certain  level 
which  would  account  for  the  ores  being  practically  confined  to  one 
stratigraphic  horizon,  but  such  an  assumption  is  scarcely  sup- 
ported by  any  facts.  At  present  the  localization  of  the  ores  can- 
not be  satisfactorily  explained.  The  deposits  are  probably 
products  of  concentration,  by  meteoric  waters,  of  small  quantities 
of  the  metals  distributed  in  h'ttoral  beds  or  in  land  deposits  and 
derived  from  older  deposits  of  some  kind  in  ancient  land  areas  of 
igneous  or  metamorphic  rocks. 

Production  and  Use. — In  1915,  47,000  tons  of  ore  were  mined 
in  Colorado  containing,  according  to  U.  S.  Geological  Survey, 

1  D.  F.  Hewett,  Vanadium  deposits  of  Peru,  Trans.,  Am.  Inst.  Min.  Eng., 
vol.  40,  1909,  pp.  274-299. 

W.  F.  Hillebrand,  The  vanadium  sulphide,  patronite,  Jour.,  Am.  Chem. 
Soc.,  vol.  29,  1907. 

W.  F.  Hillebrand,  Am.  Jour.  Sci.,  4th  ser.,  vol.  24,  1907,  p.  141. 

2  F.  W.  Clarke,  Geochemistry,  Butt.  616,  U.  S.  Geol.  Survey,  1916,  p.  705. 
» Bull.  340,  U.  S.  Geol.  Survey,  1908,  p.  172. 


412  MINERAL  DEPOSITS 

19.9  tons  of  metallic  uranium,  627  tons  of  vanadium  (mainly 
from  roscoelite  ores)  and  6.1  grams  of  radium,  the  value  of  the 
metals  being  about  $700,000.  Some  years  ago  most  of  the 
uranium  ores  were  exported  and  radium  extracted  abroad. 
In  1914,  the  ores  mined  contained  87.2  tons  of  uranium  and  22.3 
grams  of  radium.  The  pure  vanadium  ores  of  Placerville 
are  roasted  with  sodium  chloride,  the  resulting  sodium  vanadate 
extracted  with  water  and  precipitated  with  ferrous  sulphate 
as  iron  vanadate  which  is  shipped  east  for  reduction  to  ferro- 
vanadium.  About  1  per  cent,  vanadium  added  to  steel 
increases  its  toughness  and  resistance  to  torsion  and  high  tem- 
perature. It  is,  however,  less  essential  to  the  steel  industry 
than  tungsten,  and  the  principal  supply  is  obtained  from  the 
Peruvian  patronite  mine.  The  value  of  ferro-vanadium  alloy 
is  about  $1,000  per  ton.  Minor  amounts  of  vanadium  salts 
are  used  as  mordants  for  dyeing  and  cloth  printing  and  for  other 
chemical  purposes.  Various  ore  deposits  yield  small  quantities 
of  vanadium  ores  such  as  vanadinite  and  descloizite.  In  the  car- 
notite  ores  there  is  difficulty  in  separating  uranium  from  vana- 
dium and  only  a  small  price  is  paid  for  the  latter. 

Uranium  salts  have  a  limited  use  for  a  yellowish-green  glass 
and  for  pottery;  also  as  a  mordant  in  dyeing.  Ferro-uranium 
is  at  present  not  used  in  the  stc-el  industry.  Radium  is  separated 
from  uranium  by  a  complicated  process  explained  in  Bull.  104, 
U.  S.  Bureau  of  Mines.  It  is  produced  as  a  chloride  or  bro- 
mide and  its  principal  use  is  in  medical  science,,  various  diseases 
yielding  more  or  less  to  its  emanations.  It  is  said  that  it  can 
be  extracted  at  a  cost  of  $37  per  milligram. 

Radium  in  corresponding  quantities  is  also  contained  in  urani- 
nite  (crystalline)  and  pitchblende  (amorphous),  both  essentially 
UOz.UOs  with  80  + per  cent,  uranium  oxides.1  It  is  obtained 
in  small  quantities  from  gold-pyrite  veins  of  Gilpin  County,  Cali- 
fornia, from  cobalt  arsenide  veins  of  Joachimsthal,  Bohemia, 
and  from  tin  veins  of  Cornwall,  all  of  which  are  deposits  formed 
at  higher  temperatures.  Various  uranium  minerals  also  occur 
in  pegmatite  dikes. 

!£.  S.  Bastin  and  J.  M.  Hill,  Prof.  Paper  94,  U.  S.  Geol.  Survey,  1917, 
pp.  121-128. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS  413 


THE  COPPER-BEARING  SHALES  OF  MANSFELD' 

It  seems  proper  to  consider  at  this  place  the  celebrated  cu- 
priferous shale  (Kupferschiefer)  of  Mansfeld,  in  central  Germany, 
for,  though  not  identical  with  the  deposits  described  in  this 
chapter,  it  presents  most  interesting  analogies  to  them. 

A  flourishing  mining  industry  is  still  based  on  the  Kupfer- 
schiefer, the  annual  production  being  approximately  700,000 
metric  tons  of  ore  containing  between  2  and  3  per  cent,  of  copper. 

The  first  stratum  deposited  in  the  subsiding  basin  of  the  upper 
Permian  in  central  Germany  was  a  marine  conglomerate  of  slight 
thickness.  ''Above  it  extends  like  a  black  shroud  the  thin 
bed  of  cupriferous  shale,  one  of  the  most  remarkable  products 


Prospecting 
.  Shaft 

0  /^n^v  * 


FIG.  131. — Section  of  a  part  of  the  copper-bearing  shale  bed  at  Mansfeld, 
Germany.     After  Schroder. 

of  the  geologic  ages.  Characterized  by  its  fauna  as  a  shallow-sea 
deposit,  full  of  plant  remains  carried  in  from  adjacent  coasts, 
the  formation  bears  the  stamp  of  an  organic  mud  deposit  admixed 
with  organic  precipitates.2 

Above  the  cupriferous  shale,  which  is  less  than  1  meter  thick, 
extends  a  marine  limestone  (Zechstein),  6  to  10  meters  thick, 
and  above  that  lie  the  great  gypsum  and  salt  masses  of  the  upper- 
most Permian.  Folding  and  faulting  have  since  affected  the 
beds,  and  the  mining  now  follows  the  inclined  strata  to  a  depth 
of  500  meters.  The  cupriferous  bed  averages  50  centimeters  in 
thickness,  but  only  the  lower  20  to  30  centimeters  is  utilized  as 
ore  (Fig.  131). 

1  Best  description  with  literature  in  Stelzner  and  Bergeat,  Die  Erzlager- 
statten,  vol.  1,  1904,  pp.  391-417. 

2  F.  Beyschlag,  in  Deutschlands  Kalibergbau,  1907,  p.  4. 


414  MINERAL  DEPOSITS 

The  ores  are  sulphides,  in  minute  distribution  through  the 
shale,  giving  it  a  bronzy  appearance.  Chalcopyrite  predominates, 
but  there  are  also  bornite,  pyrite,  chalcocite,  and  rarely  galena 
and  tetrahedrite.  Small  quantities  of  nickel,  cobalt,  selenium, 
vanadium,  and  molybdenum  have  been  recognized;  there  is 
also  about  0.015  per  cent,  of  silver.  Zinc  is  present,  and  in 
the  upper  part  of  the  bed,  not  mined,  there  is  more  zinc  than 
copper.  An  average  analysis  is  as  follows:1 

AVERAGE  ANALYSIS  OF  CUPRIFEROUS  SHALE 
Dr.  Haase,  Analyst 

Per  cent.  Per  cent. 

SiO2 33.15  Ag 0.014 

A12O3 17.3  Ni 0.018 

CaO 10.4  S 2.31 

MgO 1.0  CO2 9.24 

Fe 2.6  H20 1.7 

Zn 1.276  Bitumen 9.06 

Cu 2.75 

There  is  about  3  per  cent.  K2O  and  1  per  cent.  Na20;  lead 
amounts  to  1.50  per  cent.,  manganese  to  about  0.25  per  cent. 

There  is  practically  no  gangue,  except  veinlets  of  gypsum  and 
barite.  The  bed  is  cut  by  faults,  along  which  in  places  occur 
barite,  anhydrite,  calcite,  niccolite,  pyrite,  and  chalcopyrite, 
and  near  these  breaks  (called  "Riicken")  the  metal  content  is 
subject  to  enrichment,  impoverishment,  or  removal  upward  in 
adjacent  beds.  Bergeat  declares  that  these  changes  take  place 
on  secondary  fissures  and  cracks. 

There  has  been  much  controversy  about  the  Mansfeld  deposits. 
The  majority  of  geologists  regard  them  as  sedimentary  and 
syngenetic:  von  Groddeck,  Stelzner,  Freiesleben,  and  von  Cotta 
held  this  view,  and  it  is  shared  by  Bergeat.  Posepny  and  Beck 
believe  them  epigenetic  and  think  that  the  metals  were  probably 
introduced  in  the  shale  from  the  Rucken. 

The  Kupferschiefer  is  certainly  not  an  ordinary  marine 
deposit  precipitated  from  the  sea  water.2  It  was  laid  down  in  a 
shallow  sea  which  was  full  of  decaying  vegetable  and  animal 
remains  and  into  which  were  probably  discharged  cupriferous 

1  Stelzner  and  Bergeat,  Die  Erzlagerstatten,  vol.  1,  1904,  p.  396. 

2  Sea  water  contains  a  trace  of  copper,  as  shown  by  Dieulafait  (Ann. 
chim.  phys.,  5th  ser.,  vol.  18,  1879,  p.  359;  C.  R.,  90,  p.  1573;  96,  p.  70; 
101,  p.  1297)  and  others,  but  the  amount  present  seems  utterly  insufficient 
to  account  for  the  Mansfeld  deposits. 


CONCENTRATIONS  FROM  SURRO  UNDING  ROCKS    415 

waters  from  the  surrounding  littoral,  most  likely  sulphate  solu- 
tions derived  from  the  eruptives  and  the  ore  deposits  of  the  early 
Permian  epochs.  No  one  can  read  the  description  of  the  great 
uniformity  of  distribution  without  being  impressed  with  the  very 
strong  arguments  for  a  syngenetic  origin. 

The  characteristic  presence  of  nickel,  cobalt,  vanadium,  and 
selenium  recalls  the  epigenetic  deposits  in  sandstone  so  abundant 
around  the  shores  of  the  Permian  sea,  in  Bohemia  and  Russia, 
for  instance.  The  Mansfeld  basin  was  simply,  then,  the  final 
collecting  place  of  the  solutions  derived  from  adjacent  desert 
shores. 

COPPER  SULPHIDE  VEINS  IN  BASIC  LAVAS 

General  Features. — All  basic  lavas  contain  copper,  but  in 
many  cases  conditions  were  evidently  unfavorable  for  the  con- 
centration of  copper  immediately  after  the  eruption,  and  the 
rocks  retained  their  copper  until  later  opportunities  for  ore  forma- 
tion were  offered.  The  existence  of  vast  masses  of  such  basic 
lavas  near  the  surface,  without  any  indication  of  copper  con- 
centration (e.g.,  the  Columbia  River  lava  or  the  basalts  of  the 
Hawaiian  volcanoes),  shows  plainly  that  the  ordinary  surface 
waters  at  slight  depth  are  not  competent  to  dissolve  and  concen- 
trate accessory  metals  contained  in  these  rocks.  A  depth  of 
perhaps  a  few  thousand  feet  seems  to  be  necessary,  under  the 
most  favorable  conditions,  for  waters  of  meteoric  origin  to  ex- 
tract the  copper;  though  it  is,  of  course,  possible  that  such  waters, 
when  ascending  in  suitable  channels,  may  deposit  the  dissolved 
copper  at  higher  horizons.  In  some  of  the  veins  here  discussed 
epidote  is  present,  but  more  frequently  it  is  absent,  and  the  veins 
then  assume  the  well-known  type  of  chalcopyrite  in  a  quartz- 
calcite-siderite  gangue.  Such  veins,  deposited  by  ascending 
surface  waters  of  the  deeper  circulation,  may  not  be  easy  to 
distinguish  from  those  whose  development  is  a  phase  of  intrusive 
after-effects.  Nor  can  it  be  denied  that  in  these  veins  may  be 
concentrated  some  gold  and  silver  from  the  igneous  rock;  in 
general,  however,  they  will  be  found  much  poorer  in  gold  and 
silver  than  deposits  connected  with  the  intrusive  processes. 

jWhether  native  copper,  bornite,  or  chalcopyrite  will  form 
seems  to  be  dependent  upon  the  quantity  of  sulphur  which  the 
lavas  were  able  to  retain  at  their  eruption. 


416  MINERAL  DEPOSITS 

The  Nikolai  Greenstone. — The  copper  deposits  in  the  Nikolai 
greenstones  of  the  Copper  River  region,  described  by  F.  C. 
Schrader,  W.  C.  Mendenhall,  A.  C.  Spencer,  and  lately  again  by 
F.  H.  Moffit,1  are  of  special  interest.  Flows  of  Triassic  or  Car- 
boniferous' basalts  about  4,000  feet  in  thickness  are  covered  by 
2,000  feet  of  Triassic  limestone,  which  in  turn  is  overlain  by  a 
thick  series  of  Jurassic  strata.  The  latter  are  cut  by  monzonitic 
intrusives,  which  are  accompanied  by  a  different  kind  of  metalli- 
zation characterized  by  prominent  gold  deposits. 

The  Nikolai  greenstones  are  amygdaloid  flows  of  basalt;  the 
amygdules  contain  scarcely  any  zeolites,  but  are  filled  with 
chlorite,  chalcedony,  and  quartz  and  carry  no  copper.  Copper 
sulphides  are  extremely  common  in  the  flows,  but  occur  in  slips, 
brecciated  zones,  and  faults.  The  minerals  are  chalcopyrite, 
pyrite,  and  bornite,  with  calcite  and  a  little  quartz;  there  is  some 
epidote,  not  always  present. 

One  of  the  fissure  zones  extends  up  into  the  Triassic  limestone 
above  the  greenstone.  In  the  latter  a  little  bornite  and  chalco- 
cite  appears  and  the  zone  cuts  across  an  older  series  of  quartz- 
epidote  veins  carrying  the  same  two  sulphides  with  a  little  native 
copper.  In  the  limestone  the  fissure  zone  develops  into  the 
remarkable  and  valuable  deposit  worked  in  the  Bonanza  mine. 
It  is  an  almost  solid  body  of  massive  chalcocite  with  conchoidal 
fracture,  traced  for  400  feet  and  with  a  greatest  width  of  25  feet; 
its  depth  is  apparently  limited.  There  are  no  gangue  minerals 
and  the  limestone  adjoining  the  chalcocite  is  not  altered.  No 
intrusive  rocks  are  present. 

It  is  probable  that  the  ores  characteristic  of  the  Nikolai  green- 
stones are  derived  from  the  rock  itself.  The  copper  deposits 
seemed  to  be  formed  mainly  after  the  Triassic  limestone  had 
been  laid  down,  and  it  is  likely  that  meteoric  waters  did  the  work. 
The  waters  must  have  descended  through  limestones  and  shales 
in  which  they  would  have  acquired  chlorides,  sulphates,  car- 
bonates, carbon  dioxide,  and  hydrogen  sulphide,  and  they  would 
therefore  be  competent  to  dissolve  copper  from  the  greenstones 
which  they  traversed.  The  chalcocite  alters  superficially  to 
covellite  and  copper  carbonates;  there  is  no  evidence  that  it  has 
replaced  pyrite  and  it  may  have  been  deposited  JD  its  present 
form. 

1  F.  H.  Moffit  and  S.  R.  Capps,  Bull.  448,  U.  S.  Geol.  Survey,  1911. 
F.  H.  Moffit,    Bull  662,  U.  S.  Geol.  Survey.  1917.  Dp.  155-182. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS  417 

C.  F.  Tolman1  has  recently  found  remnants  and  structures 
indicating  bornite  and  believes  that  the  chalcocite  is  secondary 
and  mainly  derived  from  bornite.  The  final  word  about  this 
interesting  deposit  has  not  been  said. 

COPPER  SULPHIDE  VEINS  IN  INTRUSIVE  BASIC  ROCKS 

Veins  containing  pyrite  and  chalcopyrite,  occasionally  with 
other  sulphides,  in  a  gangue  of  quartz,  calcite,  dolomite,  and 
siderite,  more  commonly  quartz  alone,  occur  abundantly  in 
intrusive  basic  rocks,  such  as  diabase  or  gabbro.  Here,  however, 
the  distinction  between  the  results  effected  by  water  of  atmos- 
pheric origin  and  those  effected  by  magmatic  solutions  becomes 
increasingly  difficult. 

F.  E.  Wright2  has  pointed  out  the  fact  that  the  intrusive 
Keweenawan  gabbro  of  Mount  Bohemia  contains  veins  with 
chalcopyrite,  bornite,  chalcocite,  calcite,  and  quartz,  while  in  the 
surface  lavas  of  the  same  series  native  copper  is  the  principal 
ore  mineral.  This  seems  an  excellent  illustration  of  the  reten- 
tion of  volatile  sulphur  by  intrusives,  contrasted  to  its  escape 
from  the  extrusive  flows.  The  origin  of  the  water  which  was  the 
vehicle  of  deposition  in  these  veins  may  remain  an  open  question. 

Along  the  foot-hills  of  the  Sierra  Nevada  of  California  extends 
a  belt  of  andesitic  rocks  of  Jurassic  age  collectively  called  "green- 
stones." They  consist  of  massive  and  schistose  rocks,  including 
surface  flows,  tuffs,  and  intrusions  mixed.  Within  this  belt, 
for  instance  in  Yuba  and  Nevada  counties,  short  and  irregular 
quartz  veins  with  pyrite  and  chalcopyrite  are  common.  Proba- 
bly these  veins  derived  their  copper  from  the  greenstones,  and 
undoubtedly  they  were  formed  at  a  time  when  the  rocks  now 
exposed  were  covered  by  several  thousand  feet  of  overlying  and 
now  eroded  igneous  rocks. 

Other  deposits,  such  as  those  at  the  Dairy  Farm  in  Placer 
County  and  at  Campo  Seco  in  Calaveras  County,  are,  according 
to  A.  Knopf,3  replacement  deposits  along  shear  zones  in  the  same 
belt  of  amphibolites  and  other  greenstones.  The  minerals  are 
pyrite  and  chalcopyrite,  with  a  trifle  of  galena  and  zinc  blende, 

1  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  54,  1917,  pp.  402-441. 

2  F.  E.  Wright,  The  intrusive  rocks  of  Mount  Bohemia,  Michigan,  Seventh 
Ann.  Rept.,  Geol.  Survey  Michigan,  1908. 

3  A.  Knopf,  Notes  on  the  foot-hill  copper  belt  of  the  Sierra   Nevada, 
Bull.  17,  California  Univ.  Dept.  Geology,  vol.  4,  1906,  pp.  411-421. 


418  MINERAL  DEPOSITS 

associated  with  quartz,  calcite,  epidote,  chalcedony,  and  some- 
times zeolites.  The  sulphides  contain  a  little  silver  and  a  trace  of 
gold.  Similar  are  the  so-called  "iron  belts"  of  pyrite  and 
chalcopyrite  in  the  Ophir  mining  district  of  gold-silver  quartz 
veins  in  Placer  County.1  They  are  contained  in  amphibolite, 
but  do  not  always  extend  parallel  to  the  schistosity;  the  width 
varies  from  a  few  feet  to  400  feet,  the  length  is  in  places  half  a  mile. 
No  shear  planes  are  visible  along  these  zones.  They  enrich  the 
distinctly  later  gold-quartz  veins  which  cross  them.  The  sul- 
phides are  associated  with  the  amphibole,  epidote,  feldspar,  and 
quartz  of  the  amphibolites  and  are  often  intergrown  with  mag- 
netite or  ilmenite.  In  the  paper  cited  they  were  interpreted 
as  concentrations  of  copper  from  the  surrounding  rocks,  formed 
by  chemical  action  during  the  progress  of  the  dynamic  meta- 
morphism  which  produced  amphibolites  from  primary  diabasic 
rocks. 

Other  and  much  larger  copper  deposits  are  found  in  the  same 
region,  on  the  north  in  Shasta  County  and  on  the  south  in  Cala- 
veras  County,  but  at  both  places  the  evidence  points  clearly  to  an 
origin  by  solutions  derived  directly  from  the  magma. 

In  the  Encampment  district,  Wyoming,  A.  C.  Spencer2 
studied  deposits  of  primary  chalcopyrite  enriched  by  chalco- 
citization,  and  probably  of  pre-Cambrian  age.  The  metalliza- 
tion is  localized  in  shattered  zones  in  quartzite,  or  between 
quartzite  and  schist,  close  to  intrusive  gabbro  or  diorite  which 
contains  copper  (p.  9),  sometimes  visible  as  chalcopyrite. 

.Spencer  gives  several  good  structural  reasons  indicating  that 
the  depositing  waters  were  ascending  and  believes  that  the  copper 
was  leached  from  the  cupriferous  gabbro.  The  minerals  present 
do  not  indicate  especially  high  temperatures.  Although  the 
deposits  were  formed  at  considerable  depth,  as  shown  by  the 
flexing  of  the  schist  bands,  the  quartzite  was  decidedly  in  its  zone 
of  fracture. 

OTHER  VEINS  DEPOSITED  BY  WATERS  OF  THE  UPPER 
CIRCULATION 

In  the  preceding  pages  it  has  often  been  pointed  out  that  the 
competency  of  the  circulation  of  certain  kinds  of  atmospheric 

1  W.  Lindgren,  Fourteenth  Ann.  Rept.,  U.  S.  Geol.  Survey,   1895,  pp. 
262-264. 

2  The   copper  deposits  of  the   Encampment  district,   Wyoming,   Prof. 
Paper  25,  U.  S.  Geol.  Survey,  1904. 


CONCENTRATIONS  FROM  SURROUNDING  ROCKS    419 

waters  to  form  many  mineral  deposits  cannot  be  questioned 
and  that  it  may  be  difficult  or  impossible  to  determine  the  origin 
of  certain  occurrences. 

Nevertheless,  the  fact  stands  firm  that  surface  waters  of  the 
ordinary  type,  even  in  slightly  heated  ascending  currents,  do 
not  form  mineral  deposits  even  in  localities  where  the  condi- 
tions are  such  that  they  might  be  expected  to  do  so,  as  in  the 
Alps,  for  instance.  There  are,  however,  other  localities,  particu- 
larly in  the  region  of  the  saline  Paleozoic  and  Mesozoic  beds 
of  central  Germany,  where  such  deposition  appears  to  have  taken 
place.  Veins  of  this  origin  are  likely  to  contain  an  abundant 
gangue  of  calcite,  dolomite,  or  barite;  with  some  quartz  and  a 
scant  amount  of  sulphides.1 

The  sweeping  generalizations  of  F.  Hornung  and  his  interpre- 
tation of  all  the  mineral  veins  of  the  Harz  Mountains  as  being 
formed  by  inter- Permian  brines  cannot  be  accepted,  but  it  is  not 
improbable  that  he  is  correct  in  believing  that  many  barite  and 
hematite  veins  have  had  this  origin.2  In  connection  with  this 
K.  Ochsenius3  showed  that  solutions  containing  2.59  per  cent. 
NaCl,  3.16  per  cent.  MgCl2,  and  1.85  per  cent.  MgSO4  decom- 
pose chalcopyrite  and  chalcocite  at  room  temperature.  This 
action  is  slow  and  is  noticeable  only  after  several  years.  Galena 
was  not  dissolved. 

Similar  examples  of  ore  deposition  by  saline  waters  also  exist 
in  the  western  part  of  the  United  States;  one,  the  Cashin  mine 
of  Colorado,  is  mentioned  above  (p.  404).  The  prevailing 
influence  of  igneous  intrusions  on  ore  deposition  is,  however,  so 
strong  that  it  is  difficult  to  establish  the  proofs  of  the  less  con- 
spicuous deposition  by  purely  meteoric  water. 

That  the  ordinary  surface  waters  are  in  most  cases  quite  incom- 
petent to  effect  concentration  is  plainly  shown  by  the  lack  of 
important  mineralization  in  fissures  and  joints  cutting  the  rocks 
of  mining  districts.  In  the  Globe  district,  Arizona,  for  instance, 
the  Paleozoic  rocks  are  intersected  by  a  network  of  dislocations 
which  would  offer  excellent  paths  for  these  waters;  and  yet  the 
important  deposits  are  in  no  way  connected  with  these  fractures. 

1  P.   Krusch,   Ueber  die  Zusammensetzung  der   Westfalischen  Spalten- 
wasser,  Zeitschr.  prakt.  Geol,  vol.  12,  1904,  p.  252. 

2  F.  Hornung,  Ursprang  und  Alter  des  Schwerspates  und  der  Erze  im 
Harze,  Zeitschr.  Deutsch.  geol.  Gesell.,  vol.  57,  1905,  pp.  291-360. 

3  Idem,  p.  567. 


420  MINERAL  DEPOSITS 

Similarly  "cross  courses"  often  fault  the  gold-quartz  veins  of 
California  and  yet  they  are,  as  a  rule,  absolutely  barren,  often 
open  fissures.  Similar  post-mineral  fissures  traverse  lead  and 
zinc  veins  in  the  Coeur  d'  Alene  district,  Idaho,  but  generally 
show  no  trace  of  mineralization. 

The  Cordilleran  region  contains  many  great  ranges  of  pre-Cam- 
brian  rocks  capped  in  places  by  Paleozoic  and  Mesozoic  strata. 
Among  them  may  be  mentioned  the  Front  Ranges  of  the  Rocky 
Mountains  in  Colorado,  the  Wind  River  Range  in  Wyoming,  and 
the  Mission  Range  in  Montana.  Uplift,  folding,  and  faulting 
have  in  each  of  these  ranges  intensified  the  circulation  of  me- 
teoric waters,  but  in  spite  of  this  the  ranges  are  remarkably 
poor  in  mineral  deposits,  which  appear  only  in  the  vicinity  of 
later  intrusives.  These  relations  show  very  plainly  the  slight 
concentrating  power  of  ordinary  cool  surface  waters  and  even 
of  the  waters  of  atmospheric  origin  that  have  become  a  part  of 
the  deeper  circulation. 


CHAPTER  XXI 

DEPOSITS  RESULTING  FROM  REGIONAL  METAMOR- 
PHISM 

Rocks  subjected  to  stress  at  moderate  depths  within  the  zone" 
of  fracture  may  rupture  in  closely  spaced  breaks,  producing  the 
appearance  of  a  schistose  structure.  In  such  rocks  no  great 
chemical  changes  would  occur,  except  perhaps  by  subsequent 
deposition  along  the  tight  fissures.  At  greater  depth  deformation 
may  take  place  by  granulation  and  recrystallization,  accompanied 
by  chemical  changes  that  are  effected  by  the  aid  of  the  scant 
rock  moisture.  Uralite  and  chlorite  may  form  from  pyroxene, 
the  soda-lime  feldspars  may  recrystallize  to  zoisite  and  albite, 
the  quartz  crystals  may  be  crushed  and  elongated,  new  mica, 
particularly  muscovite,  may  develop;  also  crystals  of  aluminum 
garnet.  The  chemical  composition  of  the  rock  will,  however, 
change  but  little;  although  the  various  transformations  involve 
transportation  of  substance,  this  movement  is  not  free,  but  is 
limited  and  hindered  in  all  directions. 

Under  these  circumstances  it  is  improbable  that  processes  of 
concentration  could  have  much  opportunity  to  assert  themselves; 
the  minute  quantities  of  useful  metals  contained  in  the  original 
rock  could  not  easily  assemble  to  form  larger  masses. 

In  amphibolite  schist  small  grains  of  chalcopyrite,  often 
intergrown  with  pyrrhotite  and  magnetite,  appear  to  be  more 
common  than  in  the  primary  igneous  rock  from  which  the  schist 
was  derived.  If  even  the  slightest  and  slowest  circulation  of 
water  was  established  during  the  deformation,  some  concentra- 
tion of  chalcopyrite  could  well  take  place,  as  it  does  in  fissures 
traversing  similar  rocks. 

When  the  deformation  takes  place  at  higher  temperatures  a 
number  of  minerals  are  developed  which  are  similar  or  identical 
with  those  of  contact  metamorphism.  It  is  often  difficult,  indeed, 
to  draw  the  line  between  regional  and  igneous  metamorphism, 
especially  in  intensely  metamorphic  regions  where  intrusive 
masses  are  abundant.  There  is  reason  to  believe  that  at  tem- 
peratures of,  say,  several  hundred  degrees  some  of  the  rocks,  par- 

421 


422  MINERAL  DEPOSITS 

ticularly  limestones,  become  permeable  to  the  gaseous  emanations 
of  water  and  metallic  compounds  yielded  by  intrusive  masses, 
and  thus  an  opportunity  is  afforded  for  the  introduction  of  new 
substances  which  in  places  may  become  concentrated  into  ore 
deposits.  To  such  a  permeation  in  the  deep  zone  of  anamor- 
phism many  of  the  most  enigmatic  ore  deposits  of  the  crystalline 
schists  may  owe  their  origin.  These  deposits  would  then  differ 
in  some  respects  from  the  ordinary  contact-metamorphic  ores, 
which  have,  as  a  rule,  developed  only  close  to  intrusive  contacts, 
in  most  cases  also  actually  within  the  zone  of  fracture. 

Dissemination  of  sulphides  is  a  phenomenon  often  encountered 
in  almost  any  area  of  crystalline  schists.  In  the  majority  of 
occurrences  pyrite,  pyrrhotite,  and  chalcopyrite  are  prominent; 
the  sulphides  of  lead  and  zinc  are  far  less  common.  Such  dis- 
seminations are  also  particularly  connected  with  amphibolitic  or 
chloritic  rocks.  As  indicated  above,  these  ore  minerals  may  have 
various  modes  of  origin.  In  the  first  place  the  dissemination 
may  be  caused  by  mineralization  along  both  sides  of  a  fissure, 
parallel  with  the  schistosity — that  is,  by  the  formation  of  a 
"bedded  vein."  Such  mineralization  is  later  than  metamor- 
phism,  and  the  metamorphic  minerals  will  probably  be  found  to 
be  altered — sericitized,  carbonatized,  or  more  rarely  silicified. 

If,  on  the  other  hand,  the  sulphide  minerals  were  contained 
in  the  rock  previous  to  metamorphism,  or  if  they  were  devel- 
oped during  that  process,  they  will  be  found  intergrown  with  the 
metamorphic  minerals,  such  as  amphibole,  epidote,  chlorite, 
garnet,  and  albite,  and  are  usually  accompanied  by  some  mag- 
netite or  ilmenite. 

Larger  pyritic  masses  of  this  kind  are,  in  most  cases,  probably 
original  products  of  magmatic  concentration;  or  they  may  be 
old  fissure  veins  or  replacement  veins  which  have  been  rendered 
unrecognizable  by  deformation;  or,  finally,  they  may  be  of  con- 
tact-metamorphic origin. 

Sparser  disseminations,  often  following  certain  lines  along  the 
strike  of  the  schist,  are  often  called  "fahlbands"  (the  German 
"fahl"  meaning  rusty  brown  and  referring  to  the  oxidized  out- 
crops). Such  fahlbands,  first  noted  in  Kongsberg,  Norway,1 
where  they  enrich  the  silver  veins,  may  be  several  miles  long  and 
vary  in  thickness  between  a  fraction  of  a  foot  and  several  hundred 

1  C.  A.  Miinster,  ref.  in  Zeitschr.  prakt.  Geol,  1896,  p.  93. 
J.  H.  L.  Vogt,  idem,  1899,  pp.  177-181. 


DEPOSITS  FROM  REGIONAL  METAMORPHISM    423 

feet.  The  enclosing  rocks  vary  from  gneiss  to  mica  schist, 
diorite,  and  amphibolite.  The  ore  minerals  are  pyrite,  pyrrho- 
tite,  zinc  blende,  chalcopyrite,  molybdenite,  and  sometimes  cobalt 
minerals.  They  are  often  intergrown  with  amphibole  or  garnet. 
The  fahlbands  are  rarely  of.  economic  importance,  but  many  of 
them  characteristically  enrich  intersecting  veins,  causing  native 
silver  and  gold  as  well  as  cobalt  and  nickel,  ores  to  appear  at 
the  intersections.  This  is  probably  only  a  special  case  of  the 
general  law  that  veins  are  enriched  where  they  cut  across  belts 
of  pyritic  impregnation.  Fahlbands  rich  in  cobaltite,  with 
pyrite,  chalcopyrite,  pyrrhotite,  and  molybdenite,  were  worked 
at  Skutterud  and  Snarum,  in  the  Modum  parish,  Norway,  from 
1776  to  1899.  For  a  long  time  these  deposits  were  among  the 
principal  sources  of  cobalt  oxide,  which  is  used  to  impart  a  deep 
blue  color  to  glass  and  porcelain.  According  to  the  older  litera- 
ture quoted  by  Stelzner  and  Bergeat1  the  fahlbands  at  Skutterud 
lie  between  gneiss  or  quartz  schist  and  amphibolite.  Other 
minerals  mentioned  are  malacolite,  antophyllite,  and  rarely 
graphite  and  tourmaline.  The  ores  were  poor,  containing,  even 
when  sorted,  less  than  1  per  cent,  cobalt.  A  parallel  belt  at 
Snarum  is  said  to  be  enclosed  in  amphibolite  and  contains  more 
copper. 

The  fahlbands  have  been  variously  interpreted.  At  a  time 
when  the  crystalline  schists  were  generally  considered  as  altered 
sediments,  they  were  held  to  be  sedimentary  deposits.  Ball  and 
Kjerulf,2  in  1880,  held  them  to  be  impregnations  related  to 
gabbro  intrusions.  Vogt  considered  the  gray  gneiss  of  Kongs- 
berg  as  a  pressed  granite  and  held  that  it  had  been  impregnated 
with  sulphides  at  the  same  time  as  the  surrounding  schists. 

That  the  dissemination  of  sulphides  in  its  present  form  is 
dependent  upon  dynamo-chemical  metamorphism  is  clearly 
shown  by  the  minerals  with  which  the  sulphides  are  now  inter- 
grown.  Sulphide  emanations  from  intrusive  magmas  at  a  con- 
siderable distance  from  their  source  do  not  usually  crystallize 
with  amphibole,  pyroxene,  and  garnet,  but  rather  with  calcite, 
sericite,  and  quartz  as  gangue  minerals.  Still,  the  recrystalliza- 
tion  under  pressure  does  not  necessarily  explain  the  ultimate 
origin  of  the  minerals  and  it  is  probably  hopeless  to  speculate  on 
this  subject  until  the  metamorphic  series  at  the  location  of  typical 

1  Die  Erzlagerstatten,  1,  1904,  pp.  269-271. 

2  Die  Geologie  des  siidlichen  und  mittleren  Norwegens,  1880. 


424  MINERAL  DEPOSITS 

fahlbands  has  been  more  carefully  examined  as  to  the  original 
character  of  its  rocks. 

Somewhat  similar  fahlbands  in  amphibolite  and  gneiss  are 
reported  in  the  older  literature  from  Schladming,  in  Styria, 
where  they  enrich  intersecting  cobalt-nickel  veins,  and  from  Les 
Challanches,  in  France,  where  similar  relations  exist.1  Recent 
descriptions  from  both  places  show  that  the  so-called  fahlbands 
are  in  reality  narrow  veins  accompanied  by  alteration  of  the 
wall  rocks.2 

1  Stelzner  and  Bergeat,  Die  Erzlagerstatten,  1,  1904,  pp.  268-269. 

2C.  Schmidt  and  J.  H.  Verloop  (Schladming),  Zeitschr.  prakt.  Geol,  vol. 
17,  1909,  pp.  271-276. 

T.  A.  Rickard  (Challanches),  Trans.  Am.  Inst.  Min.  Eng.,  vol.  24,  1894, 
pp.  689-705. 


CHAPTER  XXII 

DEPOSITS  OF  NATIVE  COPPER  WITH  ZEOLITES  IN 
BASIC  LAVAS 

GENERAL  STATEMENT 

Native  copper,  chalcocite,  bornite,  much  more  rarely  chalco- 
pyrite,  and  their  products  of  oxidation  are  often  found  in  flows 
of  basic  lavas,  particularly  in  basalts,  associated  with  minerals  of 
the  zeolite  group,  such  as  analcite,  natrolite,  stilbite,  chabazite, 
and  laumontite,  and  the  minerals  prehnite  and  datolite;  together 
with  these  calcite,  quartz,  chalcedony,  chlorite,  epidote,  and 
adularia  may  be  present,  sometimes  in  predominating  quantity. 
These  gangue  minerals,  together  with  the  copper  minerals,  fill 
vacuoles  or  blowholes  in  the  basic  rocks  or  replace  the  rock. 
Pyrite  and  sulphides  of  metals  other  than  copper  rarely  occur. 

These  deposits  have  been  formed  near  the  surface  under  condi- 
tions which  will  be  discussed  in  a  following  paragraph.  The 
mineral  association  does  not  indicate  a  deep-seated  origin. 

Instances  of  native  copper  occurring  in  this  manner  are  plenti- 
ful, though  the  occurrences  are  not  always  of  economic  impor- 
tance. Among  the  numerous  localities  the  following  may  be 
mentioned:  The  Faeroer,1  north  of  Scotland;  Sterling,2  in  Scot- 
land; Oberstein  a.  d.  Nahe,  Germany;  Sao  Paulo,3  Brazil;  the 
Kristiania  region,4  Norway;  the  Triassic  "traps"  of  New 
Jersey5  and  Connecticut;  New  Guinea;6  the  Transbaikalian  prov- 
inces7 on  the  Dochida  River;  the  Bay  of  Fundy,8  Nova  Scotia. 

1  F.  Cornu,  Zeitschr.  prakt.  Geol,  vol.  15,  1907,  p.  321. 

2  Carl  Hintze,  Handbuch  der  Mineralogie,  1898. 

3E.  Hussak,  Centralblatt  f.  Min.,  1906,  p.  333.  (No  zeolites;  copper 
between  the  peripheral  covering  of  the  amygdules,  consisting  of  an  iron 
silicate  and  the  filling  of  chalcedony.) 

4  J.  H.  L.  Vogt,  Zeitschr.  prakt.  Geol,  vol.  7,  1899,  p.  12. 

8Volney  Lewis,  Ann.  Rept.,  Geol.  Survey  New  Jersey,  1907,  pp.  157 
and  165. 

•  R.  Beck,  Lehre  von  den  Erzlagerstatten,  I,  1909,  p.  345. 

7  Idem,  p.  346;  also  Zeitschr.  prakt.  Geol,  vol.  9,  1901,  p.  391.     (With 
opal,  chalcedony,  calcite,  epidote,  and  prehnite.) 

8  R.  W.  Ells,  Copper  in  the  provinces  of  Nova  Scotia,  New  Brunswick, 
and  Quebec,  Min.  Res.  Can.,  Geol.  Survey  Canada,  1904,  58  pp. 

425 


426  MINERAL  DEPOSITS 

The  first  six  of  these  occurrences  have  not  been  worked  as  copper 
deposits.  The  last  four  are  of  some  economic  importance. 

A  pre-Cambrian  series  of  basaltic  rocks  in  the  Lake  Superior 
region  contains  the  most  prominent  example  of  this  class  of 
deposits  in  the  world,  which  will  be  described  in  more  detail 
below. 

In  eastern  Oregon,1  about  20  miles  east  of  Baker  City,  and 
along  the  Snake  River  canon,  opposite  the  Seven  Devils  Moun- 
tains in  Idaho,  are  extensive  areas  covered  by  a  basaltic  amygda- 
loid flow  of  Jurassic  or  Triassic  age.  This  rock  contains  native 
copper  and  chalcocite,  sparsely  disseminated  or  along  obscure 
fracture  zones,  in  association  with  epidote,  chlorite,  calcite, 
and  zeolites.  The  ores  are  of  low  grade  and  have  not  yet  been 
worked  with  profit. 

Another  occurrence  of  interest  in  the  White  River  region  in 
Alaska  has  recently  been  described  by  Adolph  Knopf.2  The  ba- 
saltic amygdaloids,  with  tuffs  and  breccias,  are  interbedded  with 
sediments  of  Carboniferous  age  and  have  probably  been  erupted 
in  part  under  submarine  conditions.  Placer  copper  is  common  in 
the  creeks,  and  some  large  masses  have  been  found.  The  copper 
minerals  are  chalcocite,  chalcopyrite,  and  native  metal  in 
stringers  and  seams,  with  prehnite,  laumontite,  thomsonite,  and 
calcite;  also  native  copper  with  zeolites  filling  blowholes  in 
reddish,  highly  amygdaloid  lava. 

These  statements  will  serve  to  show  that  the  zeolitic  copper 
deposits  in  basaltic  lavas  represent  a  type  of  world-wide  distribu- 
tion; the  same  processes  of  concentration  are  evidently  applicable 
to  all  cases. 

ORIGIN  OF  THE  ZEOLITIC  COPPER  ORES 

Probable  Source  of  Copper. — Basic  igneous  rocks  such  as  gab- 
bro,  diabase,  basalt,  some  andesites,  and  basaltic  flows  designated 
melaphyres  or  amygdaloids  probably  always  contain  copper, 
in  some  cases  as  much  as  0.1  or  0.2  per  cent.,  more  commonly 
about  0.02  per  cent,  of  the  metal  (p.  8).  According  to  Volney 
Lewis  and  F.  F.  Grout,  the  copper  is  present  as  a  silicate,  pos- 
sibly in  part  as  a  sulphide,  such  as  bornite  or  chalcocite. 

It  is  likely  that  the  copper  is  present  mainly  as  a  silicate 

1  W.  Landgren,  The  gold  belt  of  the  Blue  Mountains  of  Oregon,  Twenty- 
second  Ann.  RepL,  IT.  S.  Geol.  Survey,  pt.  2,  1901,  pp.  551-776. 
*  Ec<m.  Geol,  vol.  5,  1910,  pp.  247-256. 


DEPOSITS  OF  NATIVE  COPPER  427 

in  effusive  rocks,  while  in  intrusive  rocks  a  part  of  the  copper  is 
held  as  a  sulphide.  Sulphur  compounds  escape  in  large  quantities 
from  basalts  during  eruption.  In  the  intrusive  rocks  this  sulphur 
is  retained. 

The  minute  quantity  of  copper  in  these  rocks  may  become 
concentrated  to  valuable  deposits  in  various  ways.  This  can 
be  effected  at  a  certain  depth  below  the  surface  by  circulating 
waters  of  atmospheric  origin,  or  by  ascending  currents  of  thermal 
waters  of  deep-seated  origin,  or  during  regional  metamorphism  in 
the  zone  of  combined  fracture  and  flow.  It  is  believed  that  any 
of  these  processes  may  result  in  copper  deposits,  few  of  which, 
however,  will  be  extensive  or  valuable. 

The  concentration  of  the  copper  stands  in  intimate  connection 
with  the  development  of  zeolites,  and  it  will  first  be  necessary  to 
discuss  this  process. 

The  Occurrence  of  Zeolites  and  the  Process  of  Zeolitization. — 
The  zeolites  are  mainly  aluminum-calcium  silicates  with  8  to  15 
per  cent,  of  water  of  hydration.  Sodium  or  sometimes  potas- 
sium may  replace  part  of  the  calcium,  and  in  some  zeolites 
barium  or  strontium  is  present.  Magnesium  does  not  usually 
enter  into  their  composition,  but  appears  in  the  associated 
chloritic  minerals. 

Analcite,  a  sodium-aluminum  silicate  containing  8  per  cent. 
H2O,  is  also  considered  to  belong  to  the  zeolites.  Prehnite, 
H2Ca2Al2Si30i2,  containing  4.37  per  cent.  H2O,  and  datolite, 
H2Ca2B2Si2Oio,  with  5.63  per  cent.  H2O,  do  not  strictly  belong 
to  the  zeolites,  but  are  commonly  associated  with  them.  The 
most  common  zeolites  are  natrolite,  desmine,  chabazite,  apo- 
phyllite,  thomsonite,  and  laumonite. 

The  zeolites  can  be  easily  produced  by  synthesis  at  tempera- 
tures of  100°  to  500°  C.  Some  of  them,  like  apophyllite,  are 
soluble  in  water  with  or  without  CO2  or  Na2CO3  at  180°  to 
189°  C.  at  a  pressure  of  10  to  12  atmospheres,  crystallizing  again 
after  cooling.  Chabazite  was  recrystallized  in  a  closed  tube  by 
Doelter  at  150°  C.,  also  in  fluid  CO2  at  30°  C.  Datolite  and 
prehnite  have  not  been  produced  by  synthesis. 

The  different  modes  of  occurrence  of  zeolites  may  be  classified 
as  follows:1 

1.  Filling  amygdules  and  veins  in  flows  of  basic  lavas.  This  is 
the  most  common  occurrence. 

1  References  generally  from  Hintze's  "Mineralogy." 


428  •     MINERAL  DEPOSITS 

2.  Filling  miarolitic  cavities    in  granite  and  here   probably 
formed  shortly  after  the  consolidation  of  the  rock. 

3.  In  pegmatite  dikes  as  products  of  the  last  epoch  of  crystal- 
lization.1 

4.  As  veins  or  coatings  of  joint  planes  in  granite  or  gneiss  or 
various  volcanic  rocks;  here  associated  with  calcite  and  some- 
times with  amethystine  quartz,  occasionally  with  albite. 

5.  In   contact-metamorphic   mineral   deposits    in    limestone. 

6.  In  the  so-called  Alpine  type  of  veins,  common  in  Switzer- 
land, Tyrol,  and  the  French  Alps.     With  quartz,  adularia,  and 
many  rare  and  well-crystallized  minerals. 

7.  In   mineral  veins,  associated  with   sulphides.     Very  rare 
and  mainly  as  last  products  of  crystallization.     Andreasberg,  Ger- 
many; Kongsberg,  Norway;  Guanajuato,  Mexico;  Arqueros  and 
Rodaito,  Chile. 

8.  As  products  of  deposition  of  hot  springs  at  their  orifices, 
as  at  Plombieres,  Bourbonne-les-Bains  and  Luxeuil  in  France; 
at  Oran,  Algeria;  at  Hunter  and  Boulder  Hot  Springs,  Montana. 

9.  In    deep-sea    deposits.     Phillipsite    has    frequently    been 
found  in  the  mud  brought  up  by  the  dredges. 

Undoubtedly  zeolites  may  form  at  low  temperatures,  as  shown 
by  the  mentioned  occurrence  of  phillipsite.  The  ranges  of 
stability  of  the  various  zeolites  may  differ  considerably.  C. 
Doelter2  believes  that  the  limits  for  the  development  of  analcite 
lie  between  180°  and  440°  C.;  for  natrolite  he  thinks  they  are 
considerably  lower,  say  from  0°  up  to  180°  C.  Experimentally 
the  latter  mineral  has  been  obtained  at  a  temperature  as  low  as 
90°  C.  It  is  probable  that  datolite,  prehnite,  and  adularia  do 
not  develop  at  temperatures  much  lower  than  100°  C. 

In  many  occurrences  it  can  be  shown  that  the  zeolites  formed 
as  the  last  phase  of  the  consolidation  of  a  magma;  their  mode 
of  appearance  in  pegmatite  dikes  and  close  to  igneous  contacts 
points  plainly  to  this  origin. 

Furthermore,  they  seem  to  require  stagnant,  quiet  conditions, 
such  as  prevailed  in  cooling  bodies  or  in  rocks  impregnated  with 
warm  water,  as  in  the  Roman  brickwork  at  Plombieres.  Their 
general  absence  from  mineral  veins  shows  that  swiftly  moving  or 
ascending  water  is  distinctly  unfavorable  for  their  development. 

1  W.  C.  Brogger,  Zeitschr.  Kryst.  Min.,  Bd.  16. 

2  C.   Doelter,   Minerogenese  und  Stabilitatsfelder  der    Minerale,    Tsch. 
M.  und.  P.  Mitt.,  vol.  25,  1906,  pp.  79-112. 


DEPOSITS  OF  NATIVE  COPPER  429 

The  accepted  authorities  are  more  or  less  vague  in  their  state- 
ments as  to  the  formation  of  zeolites,  especially  in  amygdaloid 
rocks.  The  most  common  statement  is  that  these  minerals  are 
deposited  by  percolating  waters.  Van  Hise  considers  them  to  be 
formed  in  the  zone  of  cementation  by  descending  surface  waters, 
and  also  by  similar  waters  percolating  through  lava  flows  and 
extracting  the  material  for  the  zeolitization  from  the  rock  itself. : 

Zeolites  are  manifestly  unstable  in  the  zone  of  weathering  and 
must  have  been  formed  at  some  depth.  Of  late  years  the  opinion 
has  been  gaining  ground2  that  zeolitization,  in  basic  volcanic 
rocks,  is  distinctly  connected  with  the  cooling  processes  and  in 
fact  should  be  regarded  as  an  after-effect  of  volcanism,  their 
deposition  taking  place  in  the  still  hot  rocks. 

That  zeolitization  is  far  from  being  simply  an  effect  of  the 
leaching  by  surface  waters  is  shown  by  the  absence  of  the  zeo- 
lites from  large  areas  of  basic  flows,  many  of  them  full  of  vacuoles 
or  blowholes.  Few  occurrences  have  been  recorded  from  the 
Hawaiian  flows,  which  are  apparently  well  suited  for  their  de- 
position, nor  from  the  extensive  flows  of  the  Columbia  lava  in 
Oregon  and  Washington. 3  There  are,  therefore,  certain  conditions 
— not  yet  fully  elucidated — which  are  necessary  for  the  deposi- 
tion of  zeolites.  It  is  probable  that  their  development  would  be 
greatly  furthered  if  the  eruption  of  the  effusive  rock  took  place 
under  water;  the  sea  water  would  cool  the  surface  of  the  flow 
and  a  slow  downward  movement  would  be  caused  in  the  porous 
rock.  Besides,  these  conditions  would  give  rise  to  a  systejn,  cool 
at  one  end,  hot  at  the  other,  in  which  circulation  competent  to 
effect  concentration  would  be  initiated. 

One  of  the  most  convincing  proofs  that  zeolitization  follows 
closely  upon  eruption  has  been  given  by  Knopf,4  who  describes 
an  occurrence  in  the  White  River  region  of  Alaska  where  a  sheet 
of  amygdaloid  rock  containing  copper  is  covered  by  a  coarse 

'  C.  R.  Van  Hise,  Mon.  47,  U.  S.  Geol.  Survey,  1904,  pp.  333 
and  633. 

2  J.  Volney  Lewis,  Ann.  Rept.,  State  Geol.  New  Jersey,  1907,  p.  167. 
Alfred  Barker,  The  natural  history  of  igneous  rocks,  1909,  p.  308. 
Adolph  Knopf,  Econ.  Geol,  vol.  5,  1910,  pp.  247-256. 

C.  N.  Fenner,  The  Watchung  basalt  and  the  paragenesis  of  its  zeolites, 
Annals.,  New  York  Acad.  Sci.,  vol.  20,  pt.  2,  1910,  pp.  97-187. 

3  According  to  F.  C.  Calkins  (oral  communication)  zeolites  were  found  at 
one  locality  in  these  lavas  in  the  John  Day  region. 

4  Adolph  Knopf,  op.  cit.,  p.  251. 


430  MINERAL  DEPOSITS 

pyroclastic  bed,  the  breccias  of  which  include  fragments  of  the 
cupriferous  amygdaloid,  proving  that  the  filling  of  the  vacuoles 
took  place  during  the  interval  between  successive  extrusions  of 
lava.  In  places — for  instance,  where  the  cupriferous  zeolit  s 
occur  in  fissures — there  was  probably  a  longer  interval,  but  all 
the  infilling  was  probably  accomplished  before  the  rock  had  cooled. 
C.  N.  Fenner  has  recently  investigated  the  zeolites  of  certain 
Triassic  basalts  of  New  Jersey,  which  cover  land  sediments  and 
old  playas  or  shallow  desert  basins  of  the  same  age,  and  finds 
that  the  zeolitization  took  place  mainly  where  the  basalt  flows 
covered  the  shallow  lakes;  he  concludes  that  the  circulation 
originated  from  the  waters  of  these  lakes.  The  general  process, 
he  says,  was  that  of  a  slow  cooling  of  the  igneous  rock,  through 
which  aqueous  solutions  were  percolating.  Material  for  solution 
was  contributed  by  the  basalt  and  by  the  previously  evolved  subli- 
mates. The  character  of  the  minerals  changed  during  the  cool- 
ing. Pyrite  and  chalcopyrite  are  among  the  metallic  minerals; 
native  copper  is  not  mentioned,  but  occurs  at  many  places  in 
these  Triassic  flows.  Three  periods  of  crystallization  are  distin- 
guished. Beginning  with  the  oldest  they  are  as  follows: 

1.  Boric  acid  period (a)  Albite,  quartz,  garnet,  amphibole,  specularite, 

sulphides. 

(6)  Datolite,  prehnite,  pectolite,  amphibole,  specu- 
larite, sulphides. 

2.  Zeolite  period Andesine,  chabazite,  heulandite,  stilbite,  natro- 

lite,    laumontite,    apophyllite,    amphibole, 
chlorite,  specularite,  sulphides. 

3.  Calcite  period Thaumasite,     calcite,     gypsum,     amphibole, 

chlorite,  specularite,  sulphides. 

This  combination  is  of  special  interest,  as  it  shows  a  peculiar 
combination  of  high-temperature  minerals  like  garnet  and  amphi- 
bole with  the  zeolitic  deposits.  Extensive  replacements  were 
noted,  similar  to  processes  described  long  ago  by  Pumpelly,  from 
observations  in  the  Lake  Superior  copper  mines.  Minerals 
stable  under  new  conditions  replace  those  formed  in  older  crys- 
tallizations. Datolite,  prehnite,  pectolite,  chabazite,  stilbite, 
natrolite,  apophyllite,  and  calcite — all  replace  the  older  albite. 
Quartz  is  replaced  by  calcite  and  various  zeolites.  Datolite  is 
replaced  by  zeolites. 

Knopf1  has  justly  stated  that  "any  theory  accounting  satis- 
Knopf,  op.  cit.,  p.  253. 


DEPOSITS  OF  NATIVE  COPPER  431 

factorily  for  the  zeolites  will  also  account  for  the  copper." 
There  is  surely  just  as  much  of  a  concentrating  process  involved 
in  obtaining  fluorine  for  apophyllite,  boron  for  datolite,  or  barium 
for  harmotome  as  there  is  in  producing  an  ore  with  2  per  cent, 
copper  from  an  amygdaloid  containing  0.02  per  cent,  of  the  metal. 
Following  Lane  (p.  438),  I  believe  that  the  water  of  seas  or 
lakes,  mingling  with  the  exhalations  from  the  magma,  decom- 
posed the  copper  silicate  contained  in  the  pyroxenes,  and  that 
the  resulting  chlorides  of  iron  and  copper  were  decomposed  by 
silicates  or  carbonates  of  calcium,  with  the  formation  of  native 
copper,  ferric  oxide,  and  calcium  chloride. 

THE  LAKE  SUPERIOR  COPPER  DEPOSITS 

In  the  following  list  of  references  on  the  copper  deposits  of  the 
Lake  Superior  region  only  the  more  important  works  of  the 
extensive  literature  are  mentioned. 

H.  Credner,  Neues  Jahrb.,  1869,  pp.  1-14. 

R.  Pumpelly,  Geol.  Survey  Michigan,  vol.  1,  pt.  2,  1873. 

R.  Pumpelly,  The  metasomatic  development  of  the  copper-bearing  rocks 
of  Lake  Superior,  Proc.,  Am.  Acad.  Arts  and  Sci.,  vol.  13,  1877-1878,  p.  253. 

C.  Rominger,  Geol.  Survey  Michigan,  vol.  5,  1895. 

R.  D.  Irving,  The  copper-bearing  rocks  of  Lake  Superior,  Mon.  5 
U.  S.  Geol.  Survey,  1883. 

H.  L.  Smyth,  Theory  of  origin  of  the  copper  ores  of  the  Lake  Superior 
district,  Science,  vol.  3,  1896,  p.  251. 

M.  E.  Wadsworth,  Origin  and  mode  of  occurrence  of  the  Lake  Superior 
copper  deposits,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  27,  1898,  pp.  669-696. 

L.  Hubbard,  Keweenaw  Point,  Geol.  Survey  Michigan,  vol.  6,  pt.  2, 
1898. 

A.  C.  Lane,  Geological  report  on  Isle  Royale,  Geol.  Survey  Michigan, 
vol.  6,  pt.  1,  1898. 

A.  C.  Lane,  The  theory  of  copper  deposition,  Ann.  RepL,  State  Geol. 
Michigan,  1903. 

A.  C.  Lane,  Native-  copper  deposits,  Bull.  13,  Canadian  Min.  Inst., 
February,  1911,  pp.  81-87. 

A.  C.  Lane,  The  Keweenaw  series  of  Michigan,  Mich.  Geol.  and  Biol. 
Survey,  Lansing,  1911. 

T.  A.  Rickard,  The  copper  mines  of  Lake  Superior,  1905,  p.  164.  (Ex- 
cellent brief  description  of  geology  and  technology.) 

L.  C.  Graton,  Mineral  Resources,  U.  S.  Geol.  Survey,  Annual  publication, 
1906-1907. 

B.  S.  Butler,  idem,  1908-1917. 

C.  R.  Van  Hise  and  C.  K.  Leith,  Mon.  52,  U.  S.  Geol.  Survey,  1911, 
pp.  578-592. 


432 


MINERAL  DEPOSITS 


General  Occurrence. — The  great 
deposits  of  native  copper  in  the 
Keweenawan  volcanic  flows  and 
conglomerates  of  the  pre-Cambrian 
in  Michigan  form  one  of  the  prin- 
cipal items  of  the  copper  wealth 
of  the  United  States.  These  de- 
posits are  mainly  concentrated  in 
Houghton  County,  on  the  Ke- 
weenaw  peninsula  of  northwest- 
ern Michigan,  on  the  southern 
shore  of  Lake  Superior  (Fig.  116). 
The  copper  belt  continues,  how- 
ever, in  a  northeasterly  direction 
to  the  point  of  the  peninsula  and 
southward  into  Ontonagon  County 
and  into  northern  Wisconsin  and 
eastern  Minnesota,  having  a  total 
length  of  about  300  miles.  The 
cupriferous  formation  is  found 
also  on  Isle  Royale  and  at  Mich- 
ipicoten  in  Ontario,  Canada,  op- 
posite Keweenaw  Point.  The 
present  productive  belt  extends 
in  a  northeasterly  direction  from 
the  Lake  mine,  43  miles  southwest 
of  Houghton,  the  center  of  the  in- 
dustry, to  the  Cliff  mine,  22  miles 
northeast  of  that  place. 

The  so-called  Keweenawan 
series,  the  uppermost  part  of  the 
Algonkian,  unconformably  covers 
the  Huronian,  which  in  turn  rests 
discordantly  on  the  Archean  green- 
stones and  gneisses. 

The  Keweenawan  series  forms 
a  huge  synclinorium,  bounding  and 
underlying  the  western  part  of 
Lake  Superior.  The  upper  part 
of  the  series  consists  of  10,000 
or  12,000  feet  of  red  arkose  sand- 


DEPOSITS  OF  NATIVE  COPPER 


433 


stones  and  shales;  the  lower  part  consists  of  a  vast  accumula- 
tion of  basaltic  lavas  perhaps  25,000  feet  in  total  thickness. 
On  the  Keweenaw  peninsula  the  whole  series  strikes  northeast, 


parallel  to  the  direction  of  the  peninsula,  and  dips  northwest 
from  30°  to  75°.  The  sandstones  follow  the  northwest  coast. 
The  rapidly  alternating  series  of  compact  diabases  or  basalts 


434  MINERAL  DEPOSITS 

(traps)  and  amygdaloid  beds  occupies  the  central  belt.  On  the 
southeast  the  Keweenawan  series  is  cut  off  by  a  long  fault  line 
parallel  to  the  strike,  and  the  southeast  coast  of  the  peninsula 
is  underlain  by  horizontal  non-productive  Cambrian  sandstones. 
Embedded  in  the  volcanic  flows  are  a  few  strata  of  sandstone, 
shale,  or  conglomerate.  Quartzose  porphyries  (felsites)  erupted 
in  the  same  epoch  enter  conspicuously  into  the  composition  of 
these  conglomerates  (Figs.  132  and  133). 

Copper  in  visible  grains  is  widely  distributed  through  the 
amygdaloid  rocks,  but  is  of  economic  importance  in  only  a  few 
places.  The  deposits  which  now  yield  the  great  bulk  of  the  pro- 
duction are  beds  of  amygdaloid  rocks  of  great  persistency.  Much 
copper  is  also  mined  from  a  bed  of  volcanic  conglomerate,  called 
the  Calumet  conglomerate.  Veins  cutting  across  the  strike  of 
the  beds  were  mined  in  the  early  years,  but  are  now  of  little  im- 
portance. The  mining  operations  were  begun  about  1846. 

The  Calumet  Conglomerate. — This  bed  is  worked  mainly  in 
the  Calumet  &  Hecla  and  Tamarack  mines;  in  the  former  mine  it 
is  opened  for  a  distance  of  nearly  2  miles  along  the  strike  by  10 
incline  shafts  and  one  vertical  shaft,  with  an  aggregate  of  about 
200  miles  of  workings.  The  Red  Jacket  vertical  shaft  is  4,900 
feet  deep  and  reaches  to  the  fifty-seventh  level  of  the  incline.  The 
Tamarack  mine,  in  which  the  continuation  of  the  shoot  in  depth 
has  been  found,  reaches  the  conglomerate  by  four  vertical  shafts 
from  3,409  to  5,309  feet  in  depth,  the  latter  being  one  of  the  deep- 
est shafts  in  the  world.  Only  parts  of  the  conglomerate  are  of 
profitable  grade,  the  ore-shoot  trending  north  on  the  bed.  In 
depth  it  appears  leaner  and  less  regular  in  tenor.  Along  the 
surface  only  parts  of  the  bed  contain  commercial  ore;  it  is  poor 
both  northeast  and  southwest  of  the  Calumet  shoot.  The  tenor 
of  the  ore  handled  has  decreased  from  4  per  cent,  of  copper  in 
former  years  to  1.5  per  cent,  in  1916.  The  ore  itself  is  undoubt- 
edly becoming  leaner,  but  the  apparent  decrease  is  to  a  consid- 
erable extent  due  to  improvements  in  mining  and  milling  that 
allow  the  handling  of  lower  grades  of  ore. 

The  conglomerate  is  10  to  25  feet  thick,  dips  36°  to  39°  north- 
west, and  forms  a  compact  reddish-brown  rock  easily  breaking 
across  the  pebbles.  The  copper  occurs  mainly  as  small  particles 
in  the  cement  between  the  cobbles,  which  are  well  rounded  and 
consist  of  quartz  porphyry  with  some  basic  igneous  rocks.  The 
hanging  wall  is  a  dark  fine-grained  diabase,  the  footwall  a  thin 


DEPOSITS  OF  NATIVE  COPPER 


435 


layer  of  sandstone.     About  one-third  of  the  copper  production  of 
Michigan  is  obtained  from  this  conglomerate. 

The  Amygdaloids. — The  amygdaloid  copper-bearing  beds, 
which  occur  at  seven  principal  horizons,  are  named  the  Baltic, 
Kearsarge,  Pewabic,  Osceola,  Isle  Royale,  Atlantic,  and  Winona 
amygdaloids  and  are  worked  by  a  dozen  large  mines.  These 
beds  are  vesicular  basalts,  usually  brownish  in  color,  with  earthy 
fracture  and  filled  with  amygdules  of  calcite,  epidote,  and  zeo- 
lites (Fig.  134).  The  copper  occurs  in  these,  but  also  replaces 
the  rock  itself  Some  native  silver,  in  places  intergrown  with 


FIG.  134. — Amygdaloid  basalt,  Houghton,  Michigan.  Black  areas,  native 
copper;  larger  areas  represent  fillings  of  blow  holes,  with  calcite  at  right 
margin.  Smaller  black  areas  represent  replacement  of  igneous  rock  by 
copper.  Magnified  15  diameters. 

copper,  occurs  in  the  amygdaloids;  scarcely  any  is  found  in  the 
conglomerate.  The  flows  are  naturally  more  vesicular  in  the 
upper  part  than  in  the  bottom  part.  Both  width  and  distribu- 
tion of  copper  are  irregular.  The  Osceola  bed  is  worked  by  the 
Calumet  &  Hecla,  Tamarack,  and  Osceola  mines;  in  the  Osceola 
it  is  developed  to  a  depth  of  4,500  feet  on  the  incline.  In  the 
Calumet  &  Hecla  mine  this  amygdaloid  is  30  to  35  feet  thick, 
but  the  mineralization  is  mainly  confined  to  a  strip  8  or  10  feet 
thick  along  the  hanging  wall  and  also  a  streak  along  the  footwall. 
In  the  Baltic  mine  the  amygdaloid  bed  is  from  15  to  80  feet  in 


436  MINERAL  DEPOSITS 

stopiug  width;  this  deposit  has  produced  much  coarse  copper  and 
contains  veinlets  of  chalcocite,  bornite,  and  copper  arsenides. 
The  Kearsarge  amygdaloid  is  worked  continuously  for  a  distance 
of  12  miles. 

The  amygdaloid  ores  yield  an  average  recovery  of  0.88  per 
cent,  copper,  actually  varying  from  0.5  to  1.6  per  cent.  In 
1916,  716,640  ounces  of  silver  was  obtained,  in  small  part  as  nug- 
gets or  "  pickings,"  but  mainly  from  the  electrolytic  refining  process, 
to  which  a  part  of  the  copper  produced  was  subjected.  The 
largest  lump  of  silver  on  record  from  the  district  weighed  12 
pounds. and  was  found  in  the  Mass  mine,  in  the  southern  part 
of  the  district.1  The  average  recovery  of  silver  per  ton  of  ore  is 
0.33  ounce  per  ton. 

The  Veins. — A  third  mode  of  occurrence  of  the  copper  ore  is  as 
veins  following  fracture  zones,  in  the  northern  part  of  the  pen- 
insula and  in  Ontonagon  County  to  the  south.  During  the  early 
years  of  the  industry  these  veins  yielded  much  copper,  but  are 
at  present  of  little  importance.  Most  of  the  veins  cross  the 
bedding  and  stand  at  a  steep  angle,  though  in  Ontonagon  County 
many  strike  veins  are  also  found.  In  places  they  are  also  par- 
allel to  the  dip  of  the  strata.  Some  of  them  could  be  followed 
by  the  drift  for  a  distance  of  2,000  or  3,000  feet.  According  to 
Credner,  Pumpelly,  and  Irving  many  of  them  were  wide,  though 
they  averaged  only  3  feet. 

In  part  these  veins  were  formed  by  filling,  but  they  were 
chiefly  the  result  of  metasomatic  replacement.  Much  of  the 
native  copper  was  coarse;  some  masses  of  unusual  size  were  found, 
the  most  famous  being  that  encountered  in  the  Minnesota  vein 
in  1880.  The  mass  weighed  500  tons,  was  46  feet  long,  18.6 
feet  wide,  and  8.5  feet  thick.1  At  the  Cliff  mine  many  masses 
which  weighed  from  40  to  100  tons  were  discovered.  From  the 
vein  the  copper  seems  to  have  had  a  tendency  to  extend  into  the 
various  amygdaloid  flows.  Most  of  the  veins  became  impover- 
ished at  a  depth  of  a  few  hundred  feet. 

The  amygdaloid  beds  are  cut  by  many  minor  cross  fractures 
and  slip  faults,  but  according  to  the  accounts  these  contain  little 
or  no  copper. 

Mineral  Association. — In  all  three  modes  of  occurrence  the 
mineral  association  is  the  same.  It  consists  of  native  copper, 
quartz,  calcite,  chlorite,  epidote,  datolite  and  prehnite,  with  a 

1  T.  A.  Rickard,  op.  tit'. 


DEPOSITS  OF  NATIVE  COPPER  437 

number  of  zeolites.  There  is  always  some  ferric  oxide,  in  places 
staining  the  ore  deep  red  or  brown.  Chalcocite  and  some  rare 
arsenides  of  copper  are  entirely  subordinate.  The  following 
table,  adapted  by  Lane  from  Pumpelly,  gives  the  general  para- 
genesis.  The  more  common  minerals  are  printed  in  heavy  type. 

Early.  Late. 

Laumontite 
Quartz 

Delessite  and  chlorite 
Epidote 
Prehnite 


Calcite  _J^L_  _ white  —  —  colorless 

Copper 

Silver 

Datolite 

Analcite 

Orthoclase 

Apophyllite 

The  stages  of  alteration  and  filling  in  the  amygdaloid  rock  are 
indicated  as  follows  by  Pumpelly:  (1)  Decomposition  of  the 
ferromagnesian  silicate  and  deposition  of  iron-rich  chlorite  (del- 
essite).  (2)  Individualization  of  the  non-alkaline  silicates  (lau- 
montite,  prehnite,  and  epidote).  (3)  Deposition  of  quartz. 
(4)  Introduction  of  native  copper,  with  replacement  of  prehnite 
by  delessite.  (5)  Appearance  of  the  alkaline  silicates  (analcite, 
apophyllite,  adularia),  representing  the  decomposition  of  labra- 
dorite  in  the  original  rock.  Many  interesting  replacements 
have  taken  place  in  the  rock  itself:  Prehnite  is  pseudomorphic 
after  labradorite  and  many  amygdaloids  are  largely  prehnitized. 
This  prehnite  may  in  turn  be  replaced  by  adularia  and  the  latter 
may  change  into  epidote  and  quartz.  Sericite  is  absent. 
Needles  of  actinolite  are  sometimes  seen  in  the  amygdules. 
Datolite  is  present  in  flinty,  massive  and  crystallized  form.1 

Origin. — The  ore  deposits  are  of  considerable  antiquity  and 
it  is  probable  that  the  present  mine  waters  have  little  to  do  with 

1  An  unusual  occurrence  of  copper  is  in  the  Nonesuch  sandstone  of 
Ontonagon  County  where  it  forms  replacement  of  the  cement.  Irving, 
as  well  as  Van  Hise  and  Leith,  states  that  the  copper  contains  cores  of  mag- 
netite. K.  Nishio  found  that  the  "magnetite"  consisted  of  a  black  hydro- 
carbon. Econ.  Geol,  vol.  14,  No.  3,  1919. 


438  MINERAL  DEPOSITS 

the  origin,  though  they  may  have  effected  slight  changes  and 
local  concentration.  U.  S.  Grant1  has  shown  that  the  deposits 
were  in  existence  when  the  Cambrian  strata  along  the  great  fault 
in  the  southeastern  part  of  the  peninsula  sank  to  the  level  of  the 
Keweenawan  series;  it  is  indeed  most  likely  that  they  were 
formed  before  the  deposition  of  this  Cambrian  sandstone.  The 
continental  Quaternary  ice  sheet  doubtless  swept  away  the 
altered  upper  part  of  the  beds,  so  that  the  native  copper  now 
outcrops  almost  at  the  surface. 

The  association  of  minerals  is  entirely  different  from  that 
found  in  ordinary  fissure  veins,  in  which,  we  have  reason  to 
believe,  the  deposition  was  effected  by  ascending  thermal  solu- 
tions. The  so-called  "  Alpine  veins  "  (p.  631)  offer  some  analogies ; 
likewise  the  veins  of  Andreasberg  and  Kongsberg  (p.  623). 
There  is  also  some  resemblance  to  propylitization,  but  in  that 
process  zeolites  rarely  form. 

Evidence  has  already  been  adduced  that  all  the  fresh  diabasic 
and  basaltic  rocks  of  the  series  contain  copper,  probably  as  a 
silicate,  and  throughout  the  vast  extent  of  the  Keweenawan  the 
amygdaloids  show  traces  of  the  metal  itself. 

Van  Hise2says: 

There  is  scarcely  a  locality  in  the  Lake  Superior  region  where  the 
Keweenawan  basic  lavas  occur  in  which  small  amounts  of  copper  are 
not  found.  Almost  every  porous  amygdaloid  shows  flakes  of  it. 
*  *  *  To  me,  the  almost  universal  association  of  small  quantities  of 
copper  with  the  Keweenawan  lavas  is  the  most  conclusive  evidence 
that  these  lavas  are  the  source  of  the  metal. 

Pumpelly  suggested  that  the  presence  of  ferric  oxide  and  epi- 
dote  (in  which  the  iron  is  in  the  ferric  state)  indicated  a  reduc- 
tion of  copper  salts  (sulphate  and  carbonate)  by  the  ferrous 
minerals  abundantly  present  in  the  rock. 

Lane3  has  recently  proposed  a  modification  of  this  view,  based 
on  some  valuable  experiments  undertaken  by  G.  Ferriekes. 
After  the  submarine  effusion  of  the  lavas,  sea  water  penetrated 

1  Bull  6,  Wisconsin  Geol.  and  Nat.  Hist.  Surv.,  1901. 

2  C.  R.  Van  Hise,  Mon.  47,  U.  S.  Geol.  Survey,  1904,  p.  1103. 

3  A.  C.  Lane,  Salt  water  in  the  Lake  mines,  Proc.,  Lake  Superior  Min 
Inst.,  vol.  12,  1906. 

A.  C.  Lane,  Native  copper  deposits.  Jour.,  Canadian  Min.  Inst.,  vol.  14, 
1911,  pp.  316-325. 


DEPOSITS  OF  NATIVE  COPPER  439 

the  beds,  decomposing  the  silicates  and  converting  a  part  of  the 
iron  and  all  of  the  copper  to  chlorides.  The  reduction  of  the 
cuprous  chloride  was  effected  by  calcium  salts,  with  the  formation 
of  ferric  oxide  and  calcium  chloride.  The  process  may  have 
persisted  during  the  slow  cooling  until  fissures  and  joints  had 
formed  in  the  beds,  and  this  would  explain  the  deposition  on 
such  fractures. 

The  experiments  of  Fernekes1  have  shown  that  metallic  copper 
is  precipitated,  together  with  ferric  oxide,  from  a  mixture  of 
ferrous  and  cuprous  chlorides,  in  a  tube  one  end  of  which  is  heated 
to  200°  to  280°  C.,  while  the  other  end  is  cooled.  The  precipita- 
tion takes  place,  however,  only  in  the  presence  of  a  substance  or 
mineral  which  neutralizes  the  hydrochloric  acid,  hydrolyzed 
from  FeCl3.  Calcium  carbonate,  datolite,  and  prehnite  were 
found  to  have  this  neutralizing  property.  No  results  were 
obtained  with  laumontite  and  labradorite.  The  equations  are: 

2FeCl2+2CuCl2  =  Cii2Cl2+2FeCl3. 
2FeCl2+2CuCl  =  2Cu+2FeCl3. 

The  presence  of  silver  is  explained  by  the  solubility  of  the 
chloride  of  that  metal  in  strong  salt  solutions.  Lane  expresses 
the  above  equation  schematically  as  follows : 

2FeCl2+2CuCl+3CaSi03  =  2Cu+Fe2O3+3Si02+3CaCl2. 

Owing  to  the  strong  dehydrating  power  of  chloride  solutions, 
ferric  oxide  will  be  deposited  instead  of  limonite.  At  an  earlier 
date  H.  N.  Stokes2  had  ascertained  that  hornblende  and  siderite 
precipitate  native  copper  from  sulphate  solution  at  200°  C.,  under 
conditions  similar  to  those  in  Fernekes's  experiments. 

The  boron  and  fluorine  in  datolite  and  apophyllite  were  prob- 
ably also  concentrated  from  the  amygdaloids.  Much  of  the 
carbon  dioxide  and  chlorine  may  well  have  been  contributed 
by  the  volcanic  flow  itself.  Copper  and  silver  form  an  alloy  at 
540°  C.3  As  the  two  metals  exist  in  close  contact  in  the  Lake 
Superior  deposits  the  conclusion  is  justified  that  these  deposits 
were  formed  at  a  lower  temperature. 

Whitney,  Pumpelly,  and  Wadsworth,  have  advocated  a  theory 
of  deposition  by  descending  surface  waters.  Pumpelly  assumed 

1  G.  Fernekes,  Earn.  Geol,  vol.  2,  1907,  pp.  580-584. 

2  Econ.  Geol,  vol.  1.  1906,  p.  648. 

3  F.  E.  Wright,  Science,  vol.  25,  1907,  p.  389 


440  MINERAL  DEPOSITS 

that  the  copper  sulphide  present  in  the  beds  was  first  oxidized 
to  sulphate  and  carbonate  and  subsequently  reduced,  but,  as 
has  been  shown,  this  hypothesis  is  not  necessary.  Van  Rise 
and  H.  L.  Smyth  believed  the  deposits  to  be  caused  by  ascend- 
ing thermal  waters,  but  the  whole  character  of  the  mineralization 
is  directly  opposed  to  such  a  view. 

Mine  Waters.1 — The  present  condition  of  the  underground 
waters  in  the  copper  region  is  most  interesting.  Lane  has  shown 
that  the  water  in  the  upper  levels  is  soft  and  potable  and  has 
the  normal  composition  of  surface  waters.  It  decreases  in 
quantity  as  depth  is  gained  and  ceases  at  a  depth  of  1,000  to 
1,500  feet  below  the  surface. 

ANALYSIS  OF  NORMAL  SURFACE  WATER  FROM  MICHIGAN  COPPER  DISTRICT 
(Parts  per  million) 

Ca 19  SiO2 10 

Mg 4  CO3 40 

Na 2.3  SO4 6 

Cl 3.5  (Al,Fe)2O, 1.5 


As  depth  is  attained  in  the  mines  the  quantity  of  chlorine  and 
calcium  increases  very  materially,  and  at  the  same  tune  the 
mine  water  is  less  abundant.  Finally,  at  a  depth  of  3,000 
to  5,000  feet,  the  mine  waters  are  almost  entirely  absent;  they 
constitute  feeble  drips  here  and  there  and,  of  course,  may  collect 
in  small  quantities  in  the  sumps.  They  are  extremely  strong 
solutions  of  calcium  chloride  with  bromine  and  many  other 
substances  in  small  quantities. 

Other  samples  of  these  waters  contain  an  appreciable  amount 
of  zinc  and  some  strontium. 

In  the  most  concentrated  waters  99  per  cent,  of  the  salts  con- 
sist of  the  chlorides  of  calcium  and  sodium,  and  three-fourths  of 
the  remainder  is  sodium  bromide. 

Lane  points  out  that  waters  of  this  composition  are  not  un- 
known in  other  deep  sedimentary  series  and  suggests  that  they 
may  be  "connate"  waters — that  is,  residual  waters  from  those 
deposited  with  the  sediments  and  derived  from  the  Keweena- 
wan  pre-Cambrian  sea.  The  strong  percentage  of  bromine  is 

1  A.  C.  Lane,  Salt  water  in  the  Lake  Mines,  Proc.,  Lake  Superior  Min. 
Inst ,  vol.  12,  1906,  pp.  154-163. 


DEPOSITS  OF  NATIVE  COPPER  441 

additional  evidence  that  we  have  here  really  to  deal  with  a 
residual  sea  water,  a  remnant  of  that  which  long  ago  was  active 
in  forming  these  deposits.  Van  Hise  and  Leith  believe  that  the 
deep  mine  water  may  represent  the  residuum  of  the  ore  forming 
solutions  but  do  not  consider  it  as  residual  sea  water. 


ANALYSIS   OF   MINE    WATER,    QUINCY    MINE 

of  No.  6  shaft.    G 
4,000  feet' 

(Grarr.s  per  liter) 


From  drippings  on  55th  level  north  of  No.  6  shaft.    G.  Fernekes,  analyst.    Depth  about 
4,000  feet* 


Cl 

176  027 

SiO 

020 

Br 

2  200 

(Fe  Al)  O 

010 

Ca  

Na  
K  
SO... 

86.478 
15.188 
.411 
.110 

Mn  
Cu  

NI  : 

Me... 

.004 
.016 
.006 
.020 

280.490 

Total  solids  determined  280.500. 
Traces  of  boron  and  strontium.     No  barium,  lithium,  or  carbon  dioxide. 

Rock  Alteration. — The  ores  have  often  a  yellowish  green  color 
from  disseminated  epidote;  in  places  they  are  bleached  and 
contain  much  calcite  and  chlorite.  Van  Hise  and  Leith2  have 
published  several  analyses  of  such  rocks  and  Lane  has  examined 
bleached  pebbles  in  the  Calumet  conglomerate.  These  analyses 
show  that  the  alteration  lacks  uniformity;  they  do  not  indicate 
the  influence  of  normal  weathering  nor  are  the  changes  similar 
to  those  of  hydrothermal  alteration  of  wall  rocks  of  veins.  The 
silica  has  decreased  in  some  rocks;  in  others,  there  is  strong  en- 
richment of  lime  or  soda  according  to  the  stage  of  mineral  para- 
genesis  reached.  There  is  no  concentration  of  potash.  The 
rock  is  moderately  hydrated  but  there  is  no  kaolin  present.  The 
results  are  perhaps  most  similar  to  the  widespread  "propylitic" 
alteration  of  igneous  rocks  in  hydrothermal  areas. 

Mining  and  Smelting  Operations. — In  the  copper  mines  of 
Lake  Superior  mining  operations  are  conducted  on  a  large  scale. 
The  total  amount  of  copper  ore  (locally  called  "rock")  hoisted 
in  Michigan  per  annum  is  about  12,000,000  tons.  This  is  crushed 
coarse  with  steam  stamps,  each  one  having  a  daily  capacity  of 

1  At  the  Franklin  mine  the  limit  between  the  upper  potable  waters  and 
the  salt  waters  is  about  1,300  feet  below  the  surface,  or  200  feet  below  sea 
level.     In  general  the  chloride  waters  appear  about  sea  level. 

2  Mon  52,  U.  S.  Geol.  Survey,  1911,  p.  583. 


442  MINERAL  DEPOSITS 

500  to  700  tons;  wet  concentration  is  used  with  jigs,  tables,  etc., 
the  resulting  concentrates  (locally  called  "mineral")  amounting 
in  1916  to  about  200,000  tons,  of  an  average  copper  content  of 
66  per  cent. 

This  concentrate  of  native  copper  is  smelted  and  refined  by  a 
single  operation  in  reverberatory  furnaces,  the  smelting  works 
being  located  in  the  district  and  at  Buffalo.  A  small  part  of  the 
copper  is  electrolytically  refined  in  order  to  eliminate  the  small 
amount  of  arsenic  contained.  A  demand  for  copper  containing 
arsenic  that  has  recently  arisen  has  resulted  in  a  decrease  of  the 
quantity  refined  by  the  electrolytic  process.  The  annual  copper 
production  of  Michigan  increased  steadily  to  1905  when  it  reached 
230,000,000  pounds.  Since  then  the  changes  have  not  been  great, 
except  that  re-working  of  tailings  has  lately  increased  the  output. 
It  was  roundly  256,000,000  pounds  in  1917.  The  reserves  of 
amygdaloid  copper-bearing  rock  are  of  great  extent. 

THE  COPPER  DEPOSIT  OF  MONTE  CATINI 

The  celebrated  copper  deposit  of  Monte  Catini,  on  the  western 
coast  of  Italy,  near  Livorno  (Leghorn)  and  the  ancient  Etruscan 
city  of  Volterra,  has  been  described  by  many  authors.  A.  Ber- 
geat  has  given  an  excellent  review  of  this  literature,  in  connection 
with  his  own  observations.1  Another  detailed  description  is 
given  by  L.  de  Launay.2  The  mines  have  .been  worked  to  a 
depth  of  850  feet. 

Irregular  laccolithic  stocks  of  diabase,  with  some  gabbro, 
break  through  Eocene  marly  limestones  and  siliceous  shales;  near 
the  contacts  the  igneous  rock  is  in  part  glassy,  so  that  the  in- 
trusions clearly  took  place  near  the  surface.  The  ore  occurs  ex- 
clusively in  the  diabase,  particularly  in  its  lower  part  at  or  near 
the  contact,  but  also  reaches  the  surface.  In  the  ore-body  the 
diabase  is  crushed  to  reddish  clayey  masses  seamed  with  zeolites 
and  calcite.  The  ores  contain  native  copper  in  crevices  and 
druses,  with  calcite,  prehnite,  datolite,  analcite,  and  laumontite; 
also  sulphides,  especially  chalcocite,  bornite,  and  chalcopyrite, 
sometimes  massive,  but  partly  in  large  and  small  rounded  con- 

1  A.  W.  Stelzner  and  A.  Bergeat,  Die  Erzlagerstatten,  vol.  2,  1906,  pp. 
835-842. 

2  Mefcallogenie  de  1'Italie,  Corn-pie  rendu,  10th  Internat.  Geol.  Congress, 
vol.  1,  1907,  pp.  603-621. 


DEPOSITS  OF  NATIVE  COPPER  443 

cretions  surrounded  by  clayey,  crushed  rock  and  consisting  of 
the  several  sulphides  in  concentric  intergrowth.  The  tenor  and 
distribution  of  the  ores  are  very  irregular. 

The  whole  aspect  of  this  unique  deposit  seems  to  indicate  that 
the  copper  was  concentrated  from  the  diabase  shortly  after  its 
consolidation  and  the  crushing  which  followed.  It  is  more  dif- 
ficult to  point  to  the  source  of  the  concentrating  waters,  but  it  is 
probably  safe  to  say  that  the  present  ground  waters  have  had 
nothing  to  do  with  the  formation  of  the  ore. 

There  are  remarkable  similarities  between  the  mineral  asso- 
ciation at  Monte  Catini  and  that  of  the  amygdaloid  flows  of 
the  Lake  Superior  region,  and  the  processes  of  concentration  may, 
in  the  main,  have  been  identical. 

NATIVE  COPPER  WITH  EPIDOTE  IN  BASIC  LAVAS 
(CATOCTIN  TYPE) 

In  some  copper  deposits  contained  in  basic  lavas  the  zeolites 
are  absent  and  the  mineral  association  is  mainly  native  cop- 
per, epidote,  quartz,  and  calcite.  Such  occurrences,  which  are 
of  slight  economic  importance,  have  been  found  in  the  Appa- 
lachian region  in  Virginia  and  Pennsylvania.1 

The  rocks  are  basaltic  flows  of  pre-Cambrian  age,  in  part 
amygdaloid,  in  part  schistose.  They  contain,  in  irregular  frac- 
tures and  along  shear  zones,  abundant  epidote,  native  copper, 
calcite,  and  chlorite;  in  places  chalcopyrite  and  bornite  occur  in 
the  gangue  or  in  the  rock  itself.  Weed  named  this  group  of  ores 
the  "Catoctin  type"  and  suggested  that  it  owed  its  origin  to  infil- 
tration from  the  present  surface.  This  seems  improbable  ;jnore 
likely  the  copper  was  extracted  from  the  basic  flows  shortly  after 
their  eruption  and  consolidation.  The  derivation  of  the  waters 
is  uncertain;  at  any  rate  they  were  not  ascending  thermal  waters 
rich  in  carbon  dioxide,  for  under  such  influences  epidote  could 
hardly  be  expected  to  form. 

1  W.  H.  Weed,  Types  of  copper  deposits  in  the  southern  United  States, 
Trans.  Am.  Inst.  Min.  Eng.,  vol.  30,  1900,  pp.  449-504. 

W.  C.  Phalen,  Copper  deposits  near  Luray,  Virginia,  Bull.  285,  U.  S. 
Geol.  Survey,  1906,  pp.  140-143. 

G.  W.  Stose,  Copper  deposits  of  South  Mountain,  Pennsylvania,  Bull. 
430,  U.  S.  Geol.  Survey,  1909,  pp.  122-131. 


CHAPTER  XXIII 

LEAD  AND  ZINC  DEPOSITS  IN  SEDIMENTARY  ROCKS ; 
ORIGIN  INDEPENDENT  OF  IGNEOUS  ACTIVITY 

Characteristic  Features. — The  lead  and  zinc  deposits  which 
form  the  subject  of  this  chapter  represent  a  type  of  world- wide 
distribution  and,  in  spite  of  local  variations,  of  remarkably  con- 
stant characteristics.  They  appear  to  be  entirely  independent 
of  igneous  rocks  and  occur  in  limestones,  dolomites,  cherts 
(derived  from  limestone),  or  calcareous  shales.  In  the  United 
States  this  type  is  represented  by  the  ores  in  the  limestone  of 
the  Mississippi  Valley;  the  largest  deposits  are  in  Missouri. 

The  mineral  composition  is  simple,  and  the  ore  minerals  few. 
Galena  and  zinc  blende  are  essential  constituents,  with  their 
train  of  oxidized  minerals  near  the  surface1  (sulphates,  carbonates, 
and  silicates) ;  there  is  more  or  less  pyrite,  almost  always  marca- 
site,  occasionally  a  little  chalcopyrite.  Gold,  antimony,  arsenic, 
and  molybdenum  are  conspicuously  absent ;  in  some  districts  the 
galena  contains  a  little  silver,  but  on  the  whole  the  deposits  are 
non-argentiferous.  Cadmium  is  often  contained  in  the  zinc 
blende,  which  is  mainly  red,  light  brown,  or  yellow  and  carries 
little  iron.  Cadmium  sulphide,  greenockite,  occurs  as  a  second- 
ary mineral.  Nickel  and  cobalt  are  often  present  in  small 
quantities.  Among  the  gangue  minerals  dolomite  is  the  most 
characteristic;  quartz  in  crystals  is  not  common,  but  a  secondary 
chert  with  bitumen  is  typical  of  many  districts;  barite  is  found, 
but  is  not  characteristic. 

The  ores  lie  in  zones  of  local  brecciation  or  in  crevices  (gash 
veins)  or  joints  which  have  been  enlarged  by  solution.  Less 
commonly  they  occupy  fault  fissures;  sometimes  they  are  purely 
metasomatic,  the  minerals  occurring  disseminated  in  limestone 

1  The  oxidation  of  lead  and  zinc  sulphides  is  treated  in  Chapter  XXXI. 
The  principal  oxidized  zinc  minerals  are  smithsonite  and  calamine,  while 
hydroziucite  and  willemite  are  rarer;  goslarite,  the  soluble  sulphate,  is 
frequently  found  as  efflorescences  but  not  in  quantities  sufficient  to  be 
regarded  as  an  important  ore  mineral. 

444 


LEAD  AND  ZINC  DEPOSITS  445 

or  dolomite  and  closely  following  certain  sedimentary  horizons. 
Even  in  this  case  they  are  not  spread  over  irregular  areas,  but 
tend  to  follow  certain  lines  in  the  plane  of  stratification  (so- 
called  "runs").  In  regions  of  slightly  disturbed  strata  many 
observers  have  noted  the  tendency  of  the  ore  to  follow  pitching 
troughs.  The  ores  usually  lie  within  a  few  hundred  feet  of 
the  surface  and  are  oxidized  in  the  vicinity  of  the  water  level. 
Frequently  they  are  found  below  impervious  shale  beds. 

Origin. — Simple  as  the  deposits  of  this  type  are,  the  views  as  to 
their  origin  are  still  divergent.  The  earliest  interpretation  of 
them  as  marine  deposits  is  generally  abandoned;  it  is  recognized 
that  even  if  the  metals  are  derived  from  primary  ocean  sediments 
the  finely  divided  sulphides  must  have  been  concentrated  and 
redeposited.  Their  epigenetic  nature  is  clear.  Some  geologists 
hold  the  ores  to  be  deposited  by  ascending  waters;  others  see  in 
them  the  work  of  descending  surface  waters. 

In  either  case  American  geologists  generally  believe  that  atmos- 
pheric waters  have  effected  the  concentration  of  the  lead  and  zinc 
from  sedimentary  Paleozoic  rocks  and  that  igneous  agencies 
have  had  nothing  to  do  with  the  deposition.  This  opinion  is 
not  unanimous  for  a  number  of  investigators  have  suggested  an 
origin  by  thermal  waters  ascending  from  great  depths.  Some  of 
these  consider  that  the  metals  were  extracted  by  the  hot  waters 
from  the  underlying  pre-Cambrian  rocks,  while  others  believe 
they  can  see  a  relationship  of  the  deposits  with  deep  intrusions 
and  magmatic  sources.  Beyschlag,  Krusch  and  Vogt  in  their 
recently  published  handbook  on  ore  deposits  uphold  the  theory 
of  origin  by  thermal  waters. 

No  one  can,  however,  deny  that  galena  and  sphalerite  are  of 
widespread  occurrence  in  many  limestones  and  dolomites  far 
from  regions  of  deep  fissuring  and  igneous  action.1  Before  ap- 
pealing to  igneous  agencies  it  will  be  advisable  to  examine  into 
the  competency  of  waters  of  atmospheric  origin  to  effect  the  con- 
centration of  these  metals. 

1  Cfr.  for  instance  the  repeated  finds  of  galena  and  other  minerals  in 
deep  borings  on  the  Gulf  Coast.  See  A.  C.  Veatch  and  G.  D.  Harris, 
Bull.  7,  Louisiana  Geol.  Survey,  1908,  p.  25;  also,  G.  D.  Harris,  Bull.  429, 
U.  S.  Geol.  Survey,  1910,  p.  45. 

Gilbert  Van  Ingen  has  pointed  out  the  frequent  occurrence  of  grains  of 
galena  and  sphalerite  in  fossils  where  perhaps  decaying  organic  matter 
might  have  brought  about  precipitation,  Bull.  Geol.  Soc.  Am.,  vol.  26, 
1915,  p.  85. 


446  MINERAL  DEPOSITS 

In  the  first  place,  the  mineral  association  indicates  a  shallow 
deposition  at  temperatures  and  pressures  not  very  different 
from  those  prevailing  at  the  surface.  The  deposits  contain 
no  substances  carried  by  thermal  waters  of  volcanic  origin, 
and  no  primary  silicate  minerals.  The  marcasite  suggests 
strongly  deposition  near  the  surface.  Barite  would  be  easily 
concentrated  from  the  limestones.  Fluorite  is  rare  in  these 
deposits. 

Regarding  nickel  and  cobalt,  it  has  already  been  pointed  out 
that  minerals  of  these  metals  are  not  uncommon  in  sedimentary 
strata,  as  is  shown,  for  example,  by  their  occurrence  with  the 
marine  siderites  and  limonitic  oolites,  or  by  the  occasional 
discovery  of  millerite  in  limestone.  This  granted,  it  remains 
to  account  for  the  two  principal  metals,  lead  and  zinc.  The  ma- 
jority of  geologists  who  have  studied  these  deposits  believe  that 
the  lead  and  zinc  originally  were  contained  as  silicates  or  sul- 
phides in  the  older  crystalline  rocks  from  which'the  limestones 
and  other  sedimentary  rocks  were  derived.  A  number  of  analy- 
ses have  been  made  particularly  by  J.  D.  Robertson  (p.  10), 
which  indicated  the  presence  of  copper,  lead  and  zinc.  Still 
more  convincing  is  the  analysis  by  George  Steiger  of  a  compos- 
ite sample  of  329  igneous  rocks  which  have  been  analyzed  in 
the  laboratory  of  the  U.  S.  Geological  Survey.  This  gave  in 
per  cent.  0.00513  zinc,  0.00075  lead,  0.00932  copper,  0.00515 
nickel  and  0.00048  arsenic. 1  Another  series  of  analyses  of  igneous 
rocks  from  England  by  A.  M.  Finlayson2  gave  an  average  of 
0.0032  per  cent,  lead  and  0.0016  per  cent.  zinc. 

Sedimentary  rocks  contain  apparently  less  of  metals  than 
igneous  rocks.  According  to  J.  B.  Weems  and  J.  D.  Robertson 
the  Cambrian  and  Ordovician  limestones  of  Missouri  average  in 
per  cent.  0.00425  zinc,  0.00096  lead,  and  0.00126  copper.3  Con- 
sidered in  conjunction  with  the  composite  analyses  of  silts  from 
the  Mississippi  River  delta  (p.  252)  these  figures  in  part  support 
the  opinion  referred  to  regarding  decreasing  metal  content  in 
successive  sedimentations. 

No  analyses  of  shales  are  included  among  those  given  above. 
They  can  not  be  safely  excluded,  however,  and  it  is  probable 
that  they  will  average  higher  in  metal  content  than  the  limestones 

1  E.  C.  Siebenthal,  Bull  606,  U.  S.  Geol.  Survey,  1915,  p.  67. 

2  Geol.  Soc.  London,  Quart.  Jour.,  vol.  66,  1916,  p.  301. 
8  E.  C.  Siebenthal,  op.  tit.,  pp.  79. 


LEAD  AND  ZINC  DEPOSITS  447 

as  indeed  suggested  by  partial  analyses  quoted  by  E.  R.  Buckley1 
and  G.  H.  Cox.2 

It  is  assumed  by  Siebenthal  that  the  metal  contents  of 
the  igneous  rocks  is  gradually  dissipated  in  successive  sedi- 
mentations. This  may  be  offset,  however,  by  the  fact  that  lime- 
stones are  far  more  easily  leached  by  waters  than  the  crystalline 
rocks. 

In  the  publication  referred  to  Siebenthal  has  compiled  all 
available  analyses  of  foreign  and  domestic  waters  and  has  shown 
that  zinc  particularly,  but  also  copper  and  lead,  is  contained  in 
many  samples  of  the  deeper  circulation  of  meteoric  waters.  Out 
of  392  waters  from  Kentucky  analyzed  by  A.  M.  Peters  89  con- 
tained zinc;  of  these  waters  36  also  contained  H2S  or  Na2S. 
Most  of  these  waters  were  obtained  from  Silurian  or  Ordovician 
formations.  Similar,  though  less  extensive  data  are  shown  from 
Missouri  waters.  The  zinc  is  carried  by  sulphureted  salt  waters 
and  by  alkaline-earthy  carbonate  waters,  the  latter  usually 
containing  H2S  or  CO2  or  both.  That  acid  waters  derived  from 
pyritic  shales  also  contain  zinc,  copper,  lead  and  nickel  is  shown 
by  the  analyses  given  on  pages  55  and  58,  and  these  certainly 
demonstrate  that  the  metals  may  be  extracted  from  sedimentary 
silt  deposits.  Siebenthal  finally  found  that  reservoir  deposits 
from  fifteen  deep  wells  of  alkaline  or  saline  type  in  Missouri, 
Kansas  and  Oklahoma  contained  much  iron  sulphide  as  well  as 
zinc,  lead  and  copper,  all  of  them  probably  also  present  as  sul- 
phides. The  dried  deposits  contained  a  maximum  of  0.6  per 
cent,  zinc,  0.2  per  cent,  lead  and  0.1  per  cent,  copper.  Zinc  was 
present  in  thirteen  samples,  lead  in  eleven  and  copper  in  nine. 
The  waters  themselves  commonly  yielded  a  trace  of  zinc,  the 
greatest  amount  found  being  0.6  part  per  million. 

According  to  these  investigations3  which  represent  the  most  de- 
tailed evidence  offered  by  those  who  advocate  an  origin  from 
meteoric  waters  of  the  deposits  under  discussion,  the  zinc  and 
lead  existed  as  finely  disseminated  sulphides  in  the  older  Paleo- 
zoic limestone.  Waters  containing  carbon  dioxide  decomposed 
the  sulphides  with  the  formation  of  bicarbonates  and  hydrogen 
sulphide.  In  the  presence  of  carbon  dioxide,  H2S  is  not  an  effect- 
ive precipitating  agent,  but  when  the  moving  solutions  become 

1  Missouri  Bur.  Geology  and  Mines,  vol.  9,  1909,  p.  221. 

2  Earn.  Geol,  vol.  6,  1916,  p.  587. 

3  E.  C.  Siebenthal,  op.  tit.,  pp.  42-66. 


448  MINERAL  DEPOSITS 

stagnant  in  places  suitable  for  deposition,  CC>2  would  escape  and 
the  remaining  H2S  precipitated  the  metals  as  sulphides. 

The  chlorides  of  lead  and  zinc  are  far  more  soluble  than  the 
bicarbonates  and  strong  brines  of  sodium  chloride  are  undoubt- 
edly effective  in  the  transportation  of  the  metals.  R.  C.  Wells 
found  that  weak  salt  solutions  decomposed  but  little  zinc  sul- 
phide.1 Stronger  solutions  might  be  more  active.  The  theory 
explained  is  then  based  on  the  leaching  of  lead  and  zinc  occur- 
ring as  minutely  disseminated  sulphides  in  limestone  and  shale. 

Moresnet.2 — The  Moresnet  district  in  Belgium,  Luxembourg, 
and  Prussia,  is  situated  in  a  region  of  folded  Devonian  and  Car- 
boniferous limestones  and  slates  cut  by  several  large  faults  and 
covered  unconformably  by  Cretaceous  beds.  In  the  main  the 
ore  follows  these  dislocations,  in  part  as  filled  veins,  in  part  as 
large  replacement  deposits  in  limestone  at  the  slate  contacts  or 
at  the  intersection  of  faults.  Dolomitization  of  the  limestone 
is  often  mentioned.  The  ore  contains  zinc  blende,  galena,  iron 
sulphides,  and  calcite,  and  the  galena  and  zinc  blende  are  often 
intimately  intergrown.  Nickel  is  occasionally  present.  Masses 
of  calamine  appeared  near  the  surface  and  extended  to  depths 
of  160  feet;  in  some  cases,  notably  at  Vieille  Montagne,  they 
were  of  enormous  size  and  reached  a  depth  of  330  feet;  the  sul- 
phides appeared  at  depths  of  170  to  330  feet,  much  below  the 
water  level.3  Galena  is  in  general  the  oldest,  pyrite  or  marcasite 
the  youngest  of  the  minerals;  concentric  intergrowths,  of  wurt- 
zite,  zinc  blende,  and  galena  (schalenblende)  are  not  uncommon. 

The  quantity  of  ore  is  said  to  diminish  in  depth,  and  large 
amounts  of  water  are  found.  A  considerable  part  of  the  world's 
production  of  zinc  has  been  obtained  from  these  deposits. 

Silesia.4 — Silesia,  a  province  of  Prussia,  remains  one  of  the 
world's  most  important  zinc-producing  regions.  The  ore  occurs 
in  Triassic  sandstone  and  limestone,  which  lie  in  flat  syn- 

1  Bull.  606,  U.  S.  Geol.  Survey,  1915,  p.  58. 

2  Ch.  Timmerhans,  Les  gttes  m6talliferes  de  la  region  de  Moresnet,  Liege, 
1905,  p.  28. 

F.  Klockmann,  Die  Erzlagerstatten  der  Gegend  von  Aachen,  Berlin,  1910. 
See  also  text -books  of  Stelzner  and  Bergeat  and  R.  Beck. 

3  The  oxidation  of  this  deposit  may  be  of  pre-Cretaceous  age. 

4  G.  Gurich,  Zur  Genesis  der  oberschlesischen  Erzlagerstatten,   Zetischr. 
prakt.  Geol,  1903,  pp.  202-205. 

A.  Sachs,  Die  Bildung  der  schlesischen  Erzlagerstatten,  Centralblattf.  Min., 
1904,  pp.  40-49;  Zeitschr.  Deutsch.  geol.  Gesell.,  vol.  56,  1904,  pp.  269-272 
See  also  text-books  of  Stelzner  and  Bergeat  and  R.  Beck. 


LEAD  AND  ZINC  DEPOSITS 


449 


clines  (Fig.  135).  In  the  lower  part  of  the  "  Muschelkalk " 
extensive  dolomitization  has  taken  place,  mainly  along  fissures, 
and  the  dolomite  is  underlain  by  an  impermeable  "  Sohlenstein  " 
or  clay  rock.  The  ground-water  circulation,  dolomitization, 
and  mineralization  are  all  apparently  closely  connected.  The 
replacement  ore  occurs  along  two  horizons — the  lower  in  a  bed 
of  dolomite  several  meters  thick,  carrying  galena,  zinc  blende, 
and  abundant  marcasite,  and  the  upper  in  a  bed  of  smithsonite 
(zinc  carbonate)  of  considerable  thickness.  The  smithsonite 
and  calamine  are  considered  products  of  oxidation.  The  zinc 
blende  is  in  part  intergrown  with  wurtzite.  The  galena  contains 
a  little  silver  (0.02  to  0.03  per  cent.) ;  manganese  as  psilomelane 
is  sometimes  present.  The  ores  are  said  to  contain  on  the  average 
about  17  per  cent,  zinc  and  5  per  cent.  lead.  The  marcasite 
contains  a  little  arsenic  and  a  trace  of  nickel.  The  succession 
is  marcasite  (oldest),  zinc  blende  and  galena. x 


p.st. 


FIG.  135. — Section  through  the  synclines  of  Tarnowitz  and  Beuthen, 
Silesia.  P.  St.,  Carboniferous;  B,  Triassic  sandstone;  s,  Triassic  limestone; 
Do,  dolomite;  b,  galena  deposits;  z,  zinc  blende  deposits;  o,  oxidized  zinc 
ores;  e,  limonite;  T,  Tertiary  beds;  Dt,  Quaternary  beds.  After  Gurich. 

There  has  been  much  discussion  concerning  the  origin  of  these 
ores.  Beyschlag  and  Michael2  have  shown  that  some  ore-bearing 
fissures  descend  into  the  Carboniferous  and  hence  believe  that 
ascending  waters  did  the  work;  others,  like  Sachs,  believe  that 
the  ores  resulted  from  descending  waters  and  that  organic 
matter  caused  the  precipitation. 

Alpine  Trias. — The  Alpine  Trias  in  Austria  contains  a  number 
of  deposits  of  this  type.  At  Bleiberg,  in  Carinthia,  the  ores 

1  E.  Schulz,  Geol.  Rundschau,  vol.  4,  1913,  pp.  126-136 

2  Beyschlag,  Zeitschr.  prakt.  Geol,  1902,  p.  143. 

Michael,   Zeitschr.  Deutsch.  geol.  Gesell.,  voL  56,  1904,  Protocol,   pp. 
127-139. 


450  MINERAL  DEPOSITS 

occupy  filled  flats  and  gash  veins;  they  consist  of  light-colored 
zinc  blende  and  marcasite,  with  calcite  and  barite  gangue,  and  a 
little  anhydrite  and  fluorite,  but  no  quartz.  No  silver,  antimony, 
copper,  or  arsenic  is  present. 

At  Raibl,  made  famous  by  Posepny 's  investigations,1  the 
ores  form  fillings  and  replacements  along  three  dislocations. 
The  minerals  are  sphalerite,  occasionally  with  wurtzite,  and 
galena,  with  a  little  marcasite  and  chalcopyrite,  and  their  deposi- 
tion was  accompanied  by  extensive  dolomitization.  Posepny 
describes  stalactites  of  galena,  pyrite,  and  zinc  blende,  but  such 
occurrences  are  exceptional. 

Other  European  Localities.- — The  great  deposits  of  Santander, 
Spain,  are  contained  in  Carboniferous  limestone  and  are  said  to 
be  replacements  connected  with  dislocations.  The  light-yellow 
zinc  blende  from  these  deposits  is  famous.  Some  cinnabar  is 
present. 

At  Monteponi,2  Sardinia,  large  "stocks"  of  galena  with  zinc 
blende  and  pyrite  are  contained  in  Paleozoic  limestones. 
There  is  much  dolomitization,  and  a  little  quartz  and  barite  also 
occur.  Cinnabar  is  reported  and  the  ores  contain  silver  in  part. 
Igneous  rocks  are  represented  only  by  a  diabase. 

The  genetic  relations  of  both  of  these  deposits  are  as  yet 
uncertain. 

The  Lead -Zinc  Ores  of  the  Mississippi  Valley. — One  of  the  most 
remarkable  metallogenetic  provinces  characterized  by  lead  and 
zinc  ores  extends  over  the  valley  of  the  Mississippi  in  the  generally 
flat-lying  limestones  of  the  Paleozoic,  ranging  from  the  Ordovi- 
cian  to  the  lower  Carboniferous  (Mississippian)  inclusive.  These 
ores  are  found  in  Arkansas,  Missouri,  Oklahoma,  Kansas,  Illinois, 
Wisconsin,  and  Iowa  and  reach  eastward  as  far  as  western 
Virginia  and  Tennessee.  The  ores  are  mined  on  a  large  scale  in 
comparatively  few  regions.  Small  deposits  of  lead  and  zinc  are 
widely  spread  and  are  even  found  in  Pennsylvania,  New^York 
and  Ontario.  Igneous  rocks  are  absent.  There  are,  however,  a 
few  small  deposits  in  southern  Arkansas,  Kentucky,  and  southern 

1  F.  Posepny,  Jahrb.  K.  k.  geol.  Reichsanstalt,  vol.  23,  1873,  pp.  315-420. 
The  genesis  of  ore  deposits,  Trans.  Am.  Inst.  Min.  Eng.,  vol.  23,  1894, 

pp.  197-369. 

An  elaborate  series  of  illustrations  of  this  deposit  was  recently  published 
by  the  Department  of  Agriculture  of  Austria. 

2  R.  Beck,  Lehre  von  den  Erzlagerstatten,  vol.  2,  1909,  p.  257. 


LEAD  AND  ZINC  DEPOSITS  '  451 

Illinois  in  which  gold,  silver,  antimony,  or  fluorite  is  present  and 
which  appear  to  be  genetically  related  to  local  intrusions  of 
igneous  rocks.  The  main  characteristics  of  the  predominating 
type  are  sufficiently  described  in  the  introduction  to  this  chap- 
ter. In  details  they  differ  considerably. 

In  point  of  production1  the  deposits  in  Missouri  easily  pre- 
dominate. The  zinc-mining  industry  centers  in  the  southwestern 
part  of  that  State,  about  Joplin,  and  in  1917  yielded  132,730 
short  tons  of  spelter,  Jof  a  value  of  about  $27,000,000,  making  about 
20  per  cent,  of  the  production  of  the  United  States.  The  lead 
mining  in  the  southeastern  part  of  the  State  in  the  same  year 
produced  204,545  short  tons  of  lead,  to  which  should  be  added 
29,611  tons  from  the  Joplin  region,  making  a  total  value  of 
$40,000,000.  This  is  37  per  cent,  of  the  lead  production  of  the 
United  States.  The  ore  mined  is  generally  referred  to  as  "  dirt; " 
the  concentrates  are  spoken  of  as  "ore."  The  total  quantity  of 
crude  ore  raised  annually  in  Missouri  is  now  about  19,000,000 
tons,  consequently  it  is  of  low  grade.  Practically  all  of  it  is 
treated  in  concentrating  works,  to  yield  high-grade  material 
suitable  for  the  reduction  plants. 

Southwestern  Missouri.2 — The  Joplin  region  includes,  outside 
of  Missouri,  adjacent  parts  of  Kansas]and  Oklahoma.  The  prin- 
cipal camps  are  at  Aurora,  Granby,  Webb  City,  Alba,  Neck, 
Joplin,  Galena,  Badger,  Quapaw,  and  Miami.  In  the  early  days 
lead  was  the  only  metal  won,  but  since  1870  zinc  ores  have  been 
mined  and  now  predominate  entirely.  From  the  districts  in 
Missouri,  near  Joplin,  the  quantity  of  lead  recovered  is  about 
one-fourth  as  much  as  zinc.  The  yield  of  lead  and  zinc  con- 
centrates from  the  crude  ore  averages,  according  to  Siebenthal, 
about  3.7  per  cent,  corresponding  to  1.9  per  cent.  zinc.  The 

1  C.  E.  Siebenthal  and  J.  P.  Dunlop,  in  Mines  Report  of  Missouri,  Mineral 
Resources,  U.  S.  Geol.  Survey.     Annual  publication. 

2  W.  P.  Jenney,  Trans.  Am.  Inst.  Min.  Eng.,  vol.  22,  1894. 
A.  Winslow,  Missouri  Geol.  Survey,  vols.  6  and  7.  1895. 

E.  Haworth,  Relations  between  the  Ozark  uplift  and  ore  deposits, 
Bull.,  Geol.  Soc.  Am.,  vol.  11,  1900,  pp.  231-240. 

H.  F.  Bain  (with  C.  R.  Van  Hise),  Preliminary  report  on  the  lead  and 
zinc  deposits  of  the  Ozark  region,  Twenty-second  Ann.  Rept.  U.  S.  Geol. 
Survey,  pt.  2,  1901. 

W.  S.  T.  Smith  and  C.  E.  Siebenthal,  U.  S.  Geol  Atlas,  Folici  148  (Joplin). 

E.  R.  Buckley  and  H.  A.  Buehler,  The  geology  of  the  Granby  area, 
Missouri  Bur.  Geol.  and  Mines,  vol.  4,  1909. 


MINERAL  DEPOSITS 

concentrates,  averaging  58  per  cent,  zinc, 
are  smelted  in  Kansas  and  Oklahoma. 

The  districts  are  situated  on  the  flanks 
of  the  Ozark  uplift  (Fig.  136).  The  ore 
occurs  in  the  Boone  formation  and  in 
rocks  of  Kinderhook  age,  both  belonging 
to  the  lower  Carboniferous  (Mississippian) . 
The  beds  form  a  very  flat  anticline  pitch- 
ing gradually  northwest  and  are  displaced 
slightly  by  the  Seneca  fault  in  Missouri  and 
Oklahoma  as  well  as  by  the  Miami  fault  in 
Oklahoma  and  Kansas.  All  three  structural 
features  appear  to  be  of  importance  in  the 
ore  deposition.  The  Boone  formation  con- 
tains much  light-colored  chert,  especially  in 
the  Grand  Falls  chert  member,  which  con- 
tains the  so-called  "sheet  ground"  deposits. 
The  surface  of  the  Boone  contained  numerous 
sink-holes  and  caves,  perhaps  also  drainage 
channels,  and  over  this  "Karst"  topography 
were  deposited  the  sandstones  and  shales, 
in  part  carbonaceous,  of  the  Coal  Measures 
(Pennsylvanian) ;  there  is,  then,  an  uncon- 
formity by  erosion.  Post-Carboniferous 
erosion  has  now  removed  much  of  these 
rocks,  but  near  Joplin  the  Pennsylvanian 
shale  still  remains  in  many  of  the  old  de- 
pressions (Fig.  137). 

The  succession  of  the  ore  minerals  is 
given  by  Smith  and  Siebenthal  as  follows: 
Dolomite  (oldest),  chalcopyrite,  galena, 
sphalerite,  galena,  chalcopyrite,  marcasite, 
pyrite,  calcite,  barite,  and  marcasite,  the 
whole  series  of  course  being  seldom  found 
in  one  locality.  All  the  minerals  are  fre- 
quently well  crystallized.  There  is  general 
agreement  among  the  investigators  that 
the  mineralization  began  by  dolomitization, 
and  Bain  sees  in  it  a  result  of  the  more 
rapid  diffusion  of  the  magnesia  in  the  ore- 
forming  solutions  than  of  the  zinc.  The 


LEAD  AND  ZINC  DEPOSITS 


453 


sphalerite  occurs  as  crystals  and  grains  in  the  secondary  chert 
which  forms  the  gangue  of  the  ore,  the  primary  chert  containing 
no  metasomatic  sphalerite  (Fig.  138).  This  secondary  chert 
largely  made  up  of  cryptocrystalline  or  microcrystalline  quartz, 
contains  much  organic  matter  with  minutely  disseminated  sul- 
phides,1 and  is  darker  than  the  primary  chert  which  antedated 


Cherokee  formation: 
Shale,  sandstone,  and 
thin  coal  beds. 

— Unconformity.  — 

Carterville  formation : 
Shale  and  sandstone. 

— Unconformity. — 

Short  Creek  oolite  member. 

Boone    formation : 
Limestone  with 
chert  beds. 


Irand  Falls  chert  member. 


FIG.  137. — Generalized    section  for  the  Joplin  district,   Missouri.     After 
Smith  and  Siebenthal,  U.  S.  Geol.  Survey. 


ore  deposition.  The  dark  chert  is  probably  in  part  a  replace- 
ment of  limestone,  in  part,  where  cementing  breccias,  a  silicified 
mud. 

1  Cox,  Dean  and  Gottschalb,  Studies  on  the  origin  of  Missouri  cherts 
and  zinc  ores,  Bull.,  School  of  Mines  and  Met.,  Nov.,  1916. 


454 


MINERAL  DEPOSITS 


A  composite  sample  of  the  zinc  concentrate  representing  3,800 
lots  has  the  composition  given  below.1 

ANALYSIS  OF  CONCENTRATED  ZINC  BLENDE  FROM  THE  JOPLIN  REGION 


Zinc 

Cadmium 

Lead 

Iron 

Manganese 

Copper 


58.260 
0.304 
0.700 
2.230 
0.010 
0.049 


Sulphur 30.720 

Calcium  carbonate ....     1 . 880 
Magnesium  carbonate.     0.850 

Barium  sulphate 0 . 820 

Silica...  .     3.950 


>.773 


FIG.  138. — Thin  section  of  "black  chert"  showing  matrix  of  fine-grained 
quartz  with  grains  of  zinc  blende  (shaded)  and  crystals  of  dolomite.  Note 
quartz  crystals  developing  in  dolomite.  Magnified  53  diameters.  After 
Smith  and  Siebenthal,  U.  S.  Geol.  Survey. 


The  galena  contains  only  a  trace  of  silver. 

The  ores  are  found  as  irregular  deposits  in  the  "broken  ground " 
near  the  surface  and  as  a  flat  "blanket  deposit"  or  "sheet 
ground"  in  a  chert  member  of  the  Boone  formation  at  depths 
of  150  to  300  feet.  Below  this  horizon  there  are,  as  yet,  un worked 

1  W.  G.  Waring,  The  zinc  ores  of  the  Joplin  district,  Trans.,  Am.  Inst. 
Min.  Eng.,  vol.  57.  1918,  pp.  657-670.  Waring  has  also  found  thallium, 
indium,  gallium  and  germanium  in  the  flue  dust  and  in  the  zinc  metal. 


LEAD  AND  ZINC  DEPOSITS 


455 


deposits  of  disseminated  ore  of  doubtful  value.  The  ore  occurs 
mainly  as  fillings  of  cavities,  the  fillings  of  distinct  veins  or  crevices 
being  subordinate.  The  ore  minerals  with  secondary  chert  fill 
spaces  of  brecciation  or  solution  cavities  along  the  stratification, 
perhaps  also  spaces  of  discission  in  limestone  caused  by  stretch- 
ing and  adjustments. 

In  the  "broken  ground,"  which  extends  for  100  or  150  feet 
below  the  surface,  the  ores  occur  in  clayey  chert  breccias  in  old 
sink  holes  filled  with  Pennsylvanian  sediments,  or  along  the  out- 
side of  such  sink  holes,  forming  "circles"  where  the  slipping 
and  settling  provided  open  ground  (Fig.  139).  In  these  occur- 


"Circle"'with 
Broken  Ground 


Pennsylvaniai!  Shale 


FIG.  139. — Diagram  of  zinc-lead  deposits  at  Joplin  showing  "broken 
ground"  around  "circle"  near  surface  and  "sheet  ground"  deposit  in 
Grand  Falls  chert  member  below.  Black  areas  represent  ore.  Scale  100 
feet  to  one  inch. 


rences  the  galena  predominates,  partly  because  of  solution  and 
oxidation  of  zinc  blende,  and  partly  because  the  deposition  of 
galena  prevailed  at  these  upper  levels,  below  the  shale.  Large 
masses  of  galena  are  found  here,  in  contrast  to  the  conditions 
in  the  sheet  ground. 

Both  at  Joplin  and  at  Aurora  (Fig.  140),  as  well  as  in  camps 
in  Oklahoma,  the  "runs"  are  also  a  characteristic  form  of  the 
upper  deposits;  these  sometimes  extend  for  1  or  2  miles,  following 


456 


MINERAL  DEPOSITS 


the  same  horizon  at  depths  less  than  150  feet — usually  much  less. 
At  Granby  the  width  of  the  run  is  rarely  more  than  50  to  150 
feet.  Each  run  has  usually  several  "openings"  (brecciated 
ground  filled  with  ore),  each  opening  being  rarely  more  than 
5  or  6  feet  thick.  These  runs  appear  to  be  solution  cavities 
controlled  by  joints  in  the  rocks. 

While  in  places  the  brecciation  and  mineralization  continue 
down  to  the  blanket  deposits  of  the  sheet  ground,  the  latter 
extends  in  the  main  independently  of  the  old  pre-Pennsylvanian 
surface.  In  this  sheet  ground,  which  is  from  6  to  15  feet  thick, 
the  galena  and  zinc  blende  occur  in  dark  chert,  filling  brecciated 

L 


r*    '^ 

i          v~- 

SSSi, 

f 

-< 

sa^Sf-^^ 
Sec.6 

U8ec.5 

Sec.  4 

*3 

m&*& 

**3&&" 

Sec.7 

Sec.8 

8ec.9 

~4 

a 

FIG.  140. — Plan  showing  shafts  and  workings  along  run  of  galena 
ore,  north  of  Aurora,  Missouri.     After  Arthur  Win&low. 

old  chert,  and  in  elongated,  narrow  solution  cavities  due  to  dis- 
solved streaks  of  limestone  in  the  prevailing  chert.  The  sheet 
ground  is  the  most  important  source  of  ore  (Fig.  141). 

The  newly  discovered  deposits  in  Oklahoma1  at  Miami  and 
Quapaw  have  assumed  great  importance.  The  ore  bodies  form 
"runs"  which  in  the  main  extend  N.E.  approximately  parallel 
to  the  Miami  fault.  The  ore  which  is  richer  than  at  Joplin 
occurs  at  a  depth  of  200  feet  or  less  in  Mississippian  rocks  under- 
neath a  rather  thick  cover  of  Pennsylvanian  shales.  The  mine 
water  appears  to  form  part  of  an  artesian  circulation  and  contains 
much  H2S.  Bitumen  is  in  part  so  abundant  as  to  become  an 
objectionable  constituent.  The  whole  occurrence  appears  to 
support  the  theory  of  origin  by  ascending  waters. 

1  E.  C.  Siebenthal,  Bull  340,  U.  S.  Geol.  Survey,  1908,  pp.  187-228. 

Maps  by  H.  A.   Buehler  in  "War  minerals  of  the  Joplin  district," 
Am.  Inst.  Mtn.  Eng.,  Joplin-Miami  meeting,  Oct.,  1917. 


LEAD  AND  ZINC  DEPOSITS 


457 


Arkansas. — In  northern  Arkansas,1  a  short  distance  southeast 
from  the  Joplin  region,  the  zinc  blende,  generally  without  galena, 
occurs  in  fissures  or  crevices,  in  fault  breccias,  and  in  solution 
breccias,  accompanied  by  secondary  chert  or  dolomite,  sometimes 


FIG.  141. — Sketch  illustrating  the  occurrence  of  galena  and  sphalerite  in 
cavities  in  the  sheet  ground,  Joplin,  Mo.  After  C.  E.  Siebenthal,  U.  S. 
Geol.  Survey. 

also  by  crystallized  quartz;  the  ores  are  found  in  Ordovician 

limestone   and  also  in   the   Boone    (Mississippian)   formation. 

Upper  Mississippi  Valley.'2' — The  districts  of  the  upper  valley 

1  G.  I.  Adams,  Pro/.  Paper  24,  U.  S.  Geol.  Survey,  1904. 
.1.  C.  Branner,  Arkansas  Geol.  Survey,  vol.  5,  1900. 

2  J.  D.  Whitney,  Geology  of  Wisconsin,  vol.  1,  1862. 

T.  C.  Chamberlin,  Geology  of  Wisconsin,  vol.  4,  1882. 
W.  P.  Jenney,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  22,  1894,  pp.  208-209 
C.  R.  Van  Hise,  Some  principles  controlling  the  deposition  of  ores, 
Trans.,  Am.  Inst.  Min.  Eng..  vol.  30,  1901. 

H.  F.  Bain,  Bull.  294,  U.  S.  Geol.  Survey,  1906. 

U.  S.  Grant,  Bull.  14,  Wisconsin  Geol.  Survey,  1906. 

G.  H.  Cox,  Econ.  Geol.,  vol.  6,  1911,  pp.  427-448;  582-603. 

H.  C.  George,  Bull.  132,  Am.  Inst.  Min.  Eng.,  1917,  pp.  2045-2074. 


458 


MINERAL  DEPOSITS 


lie  in  Wisconsin,  Iowa,  and  Illinois.  The  most  important  dis- 
tricts are  in  Wisconsin  and  yielded  in  1917  about  4,100  short 
tons  of  lead  and  59,700  tons  of  zinc,  with  a  total  value  of  about 
$13,000,000.  The  ore  deposits  are  found  in  Ordovician  strata  of 
almost  horizontal  position.  The  following  formations  are 
recognized : 

Feet 

160 

240 

55 

100 

Lower  magnesian  limestone 350 

Below  the  magnesian  limestone  is  700  feet  of  the  Cambrian 
Potsdam  sandstone.     The  so-called  "oil  rock,"  a  thin  bed  of 


Cincinnati  or  Maquoketa  shale . . 

Galena  dolomite 

Platteville  limestone  (Trenton) . . 
St.  Peter  sandstone . . . 


FIG.  142. — Section  showing  occurrence  of  lead  and  zinc  in  vertical 
crevices,  flats,  and  pitches;  also  of  disseminated  ores  in  the  rocks,  gd, 
Galena  dolomite;  tk,  Trenton  limestone;  of,  upper  flat;  uf,  lower  flat;  k,  con- 
necting flats,  pitches,  and  verticals.  After  T.  C.  Chamberlin. 

bituminous  shale,  is  found  at  the  base  of  the  Galena  formation 
or  at  the  top  of  the  Trenton.  The  rocks  dip  gently  southwest 
and  are  flexed  into  very  shallow  troughs. 

The  ores  are  confined  to  the  Galena  limestone  and  the  upper 
part  of  the  Platteville  limestone;  the  minerals  consist  of  mar- 
casite,  sphalerite,  and  galena,  deposited  in  the  order  noted.  The 
gangue  is  crystallized  calcite,  rarely  barite.  Cadmium  is  absent, 
but  a  trace  of  silver  is  found.  The  abundance  of  marcasite 
causes  metallurgical  difficulties  and  necessitates  treatment  of  the 
concentrates  in  electrostatic  or  magnetic  separators,  in  the 
latter  case  preceded  by  partial  roasting.  The  ores  occur  as 


LEAD  AND  ZINC  DEPOSITS  459 

fillings  of  open  spaces,  vertical  crevices,  or  "gash  veins"  con- 
nected with  "pitches"  or  "flats,"  all  probably  due  to  solution 
along  joint  planes  (Fig.  142).  Stalactites  of  sulphides  are  some- 
times found  indicating  that  the  spaces  were  not  always  filled 
by  solutions.  In  part  there  are  also  flat  bodies  of  disseminated 
ores.  The  galena  predominates  near  the  surface,  probably 
largely  because  the  zinc  blende  has  been  dissolved  as  sulphate 
and  transformed  to  silicate  or  carbonate  in  the  lower  levels 
(p.  455).  In  depth  zinc  blende  with  a  little  galena  is  the  prin- 
cipal ore.  Mining  operations  extend  to  a  depth  of  at  most 
200  feet.  The  distribution  of  the  oil  shale,  according  to  Bain, 
seems  to  coincide  with  the  extent  of  the  deposits.  Cox,  however, 
holds  that  the  metal  was  derived  from  the  overlying  Maquoketa 
shale  and  carried  down  to  be  concentrated  in  the  Galena  limestone. 

Virginia  and  Tennessee.'* — In  western  Virginia,  and  near 
Knoxville,  Tennessee,  lead  and  zinc  ores  occur  in  the  Cambro- 
Ordovician  (Shenandoah)  limestones,  mostly  where  the  rocks  are 
faulted  or  brecciated  or  where  they  carry  much  organic  matter. 
The  gangue  consists  of  calcite,  dolomite,  and  rarely  barite.  There 
is  little  quartz  or  pyrite  and  no  definite  order  of  crystallization. 

Southeastern  Missouri.2 — In  eastern  Missouri  not  far  from  the 
Mississippi  River  and  south  of  St.  Louis  lead  mining  has  been 
carried  on  more  or  less  extensively  since  the  early  part  of  the 
eighteenth  century,  but  in  the  last  ten  years  the  industry  has 
assumed  very  large  proportions.  In  1917  the  yield  of  lead 
from  this  region  was  204,545  short  tons,  worth  more  than 
$35,000,000.  The  crude  ores,  which  yield  on  the  average  5.5 
per  cent.  of.  lead  concentrates,  are  treated  at  the  rate  of  20,000 
tons  per  day  in  local  concentrating  works  and  a  part  of  the  galena 

1  T.  L.  Watson,  Lead  and  zinc  deposits  of  Virginia,  Geol.  Survey  Virginia, 
vol.  57,  1905. 

Frank  L.  Xason,  Characteristics  of  zinc  deposits  in  North  America, 
Trans.  Am.  Inst.  Min.  Eng.,  vol.  57,  1918,  pp.  830-855. 

H.  A.  Coy  and  H.  B.  Henegar,  Mining  methods  of  the  American  Zinc 
Co.  of  Tenn.,  idem,  vol.  58,  1918,  pp.  36-47. 

2  A.  Winslow,  Missouri  Geol.  Survey,  vols.  6  and  7,  1894. 
A.  Winslow,  Bull.  132,  U.  S.  Geol.  Survey,  1896. 

C.  R.  Keyes,  Missouri  Geol.  Survey,  vol.  9,  1896. 

E.  R.  Buckley,  Geology  of  the  disseminated  lead  deposits,  Missouri 
Bur.  Geol.  and  Mines,  vol.  9,  pts.  1  and  2,  1909. 

A.  P.  Watt,  Concentration  practice  in  southeastern  Missouri,  Trans., 
Am.  Inst,  Min.  Eng.,  vol.  57,  1918,  pp.  322-419 


460 


MINERAL  DEPOSITS 


is  smelted  in  the  district.     Practically  no  zinc  is  contained  in  the 
ore. 

The  geological  position  of  the  deposits  is  in  the  Cambrian  and 
therefore  lower  than  those  of  the  other  Mississippi  Valley  ores. 
On  an  irregular  surface  of  pre-Cambrian  granite  and  porphyry 
rests  the  basal  La  Motte  sandstone,  about  200  feet  thick  (Fig. 
143).  Above  this  lies  the  arenaceous  dolomite  of  the  Bonne- 
terre  formation,  often  chloritic,  with  beds  of  shale  having  in 
all  a  thickness  of  300  to  400  feet.  Covering  the  Bonneterre 
are  the  Davis,  Derby,  Doe  Run,  and  Potosi  formations,  which 


soo' 


Surface  <>f  Ground 


Uunucterre    Formation 


Horizontal  and  Vertical  Scale 


FIG.  143. — Vertical  section  showing  workings  in  mine  No.  4,  Federal 
Lead  Company,  southeastern  Missouri.  Horizontal  bodies  of  disseminated 
ore,  following  bedding  of  shaly  dolomite  of  Bonneterre  formation.  After 
E.  R.  Buckley. 

are  mainly  dolomites  and  shales  and  all  of  which  belong  to  the 
Upper  Cambrian. 

The  principal  ore  horizon  is  in  the  lower  part  of  the  Bonne- 
terre dolomite,  though  some  galena  occurs  throughout  that  for- 
mation. A  second,  less  important  ore  horizon  is  in  the  Potosi 
dolomite,  where  the  galena  is  accompanied  by  barite.  The 
strata  are  horizontal  or  have  very  gentle  dip. 

The  ore  minerals  are  mainly  galena  accompanied  by  calcite,  a 
little  pyrite,  and  sometimes  chalcopyrite.  In  places — for  in- 
stance, at  Mine  La  Motte  and  Fredericktown — the  ores  contain 


LEAD  AND  ZINC  DEPOSITS 


461 


nickel  and  cobalt,  as  linnseite  (Co,Ni)sS4;  some  of  the  ores  have 
been  worked  for  these  metals.  Watt  quotes  a  representative 
analysis  of  the  crude  ore  of  the  southeastern  district. 

'ANALYSIS  OF  DISSEMINATED  ORE-FROM  SOUTHEASTERN  MISSOURI 


Pb 

Cu '. 

Zn 

S 

Si02 

Fe203 


4.32 
0.03 
0.50 
0.97 
4.83 
6.64 


A1203 1.16 

CaO.... 30.80 

MgO 17.96 

CO2 32.79 


100.00 


Silver  0.12  oz.  per  ton.     Trace  Ni,  CO,  Mn. 


Watt  states  that  the  silver  follows  the  zinc.  Concentrates 
of  zinc  blende  contain  up  to  10  ounces  of  silver  per  ton. 

The  ores  are  often  called  disseminated,  for  the  galena  usually 
occurs  as  grain  or  crystals  disseminated  in  the  greenish-gray 
dolomite  (Fig.  144);  sometimes 
these  crystals  are  several  centi- 
meters in  diameter.  According 
to  Buckley  the  ores  of  the  lower 
part  of  the  Bonneterre  occur  as 
follows : 

1.  As  horizontal  sheets  along 
bedding  planes,   generally  along 
the  upper  side  of  thin  shale  beds. 

2.  Disseminated  in  dolomite. 

3.  Filling  or  lining  joints. 

4.  In  cavities  or  vugs. 

The  galena  is  persistently  asso- 
ciated with  dark  dolomite  and 
black  shale. 

The  ores  are  mined  from  ver- 
tical shafts,  100  to  550  feet  deep. 
The  ore  does  not  extend  in  all  directions  like  a  coal  bed,  but  the 
flat  shoots  or  "runs"  follow  rather  persistently  one  or  two  direc- 
tions, undoubtedly  controlled  by  joints  and  small  faults.  Some 
of  these  rune  have  been  followed  for  miles  and  may  be  several 
hundred  feet  wide;  some  of  the  mine  workings  in  the  Bonne- 
terre district  are  100  feet  high. 

Genesis  of  the  Mississippi  Valley  Deposits. — An  unusually 
extensive  literature,  full  of  controversy  and  divergent  views, 
covers  the  question  of  the  genesis  of  these  ores.  A  majority  of 


FIG.  144. — Crystals  of  galena 
developing  in  shaly  dolomite. 
Black,  galena;  shaded  and  stip- 
pled, shaly  dolomite;  white, 
quartz.  Magnified  about  10  diam- 
eters. After  E.  R.  Buckley. 


462  MINERAL  DEPOSITS 

the  authors  agree  that  the  source  of  the  ores  was  in  the  Paleozoic 
sedimentary  beds,  also  that  the  deposition  was  effected  by  atmos- 
pheric waters,  and  finally  that  the  metals  were  in  solution  mainly 
as  sulphates.  Summaries  of  the  various  views  are  found  in  the 
text-books  of  Kemp  and  Hies  and  in  the  reports  of  A.  Winslow 
and  E.  R.  Buckley.  At.  present  there  are  two  strongly  contrast- 
ing opinions  regarding  the  Missouri  deposits.  The  descensionists 
are  represented  by  Whitney,  Chamberlin,  Blake,  Robertson, 
Winslow,  Buckley,  and  Buehler  and  the  ascensionists  by  Jenney, 
Nason,  Van  Hise,  Bain,  W.  S.  T.  Smith,  and  Siebenthal.  From 
the  latter  we  may  separate  Jenney  and  Nason,  who  see  in  the 
ore  deposits  the  result  of  fissuring  extending  into  the  underlying 
pre-Cambrian  rocks,  through  which  thermal  waters  ascended. 
Buckley  and  Buehler  hold  that  the  source  of  the  lead  and  zinc 
was  in  the  Pennsylvanian  sediments,  which,  however,  contain 
no  important  deposits  and  only  in  places  small  amounts  of  galena 
and  zinc  blende.  The  finely  distributed  sulphides  were  dissolved 
as  sulphates  and  carried  downward  in  acid  solutions  which  finally 
mingled  with  neutral  or  alkaline  solutions  from  the  unoxidized 
parts  of  Pennsylvanian  sediments.  These  mingled  waters  de- 
posited galena  and  zinc  blende  in  the  sink  holes  and  drainage 
channels  of  the  underlying  Mississippian  limestone  and  chert. 
Unlike  Winslow,  Buckley  and  Buehler  do  not  believe  that  the 
deposition  was  effected  at  the  time  of  deposition  of  the  Pennsyl- 
vanian shale,  but  later,  after  the  erosion  of  a  part  of  those  beds. 
The  obvious  difficulty  in  their  theory  appears  to  be  that  it  re- 
quires the  waters  to  have  descended  through  an  impervious  shale 
cover. 

A  similar  theory  is  advocated  by  Buckley  for  the  lead  deposits 
of  southeastern  Missouri.  The  pre-Cambrian  rocks  are  held  to 
be  the  original  source  of  the  metals.  The  water  flowing 
into  the  Cambrian  sea  contained  lead,  which  was  deposited 
with  the  Bonneterre  dolomite  as  small  particles.  After  the 
subsequent  formations  were  laid  down  the  concentration  of  lead 
by  surface  waters  began.  Finally  the  Pennsylvanian  shales 
were  laid  down  over  this  area  and  from  them  the  greatest  amounts 
of  metals  were  derived.  The  solutions  were  thus  in  the  main 
descending,  though  in  part  they  may  have  ascended  in  artesian 
circulation  through  the  La  Motte  sandstone.  Buckley  states, 
indeed,  that  even  at  present  there  are  strong  indications  of  arte- 
sian conditions  in  the  mines.  On  the  whole  the  ore  deposition  is 


LEAD  AND  ZINC  DEPOSITS  463 

post-Pennsylvanian.  Here  again  the  impermeable  character  of 
the  Pennsylvanian  may  be  advanced  as  an  argument  against 
Buckley's  view,  as  well  as  the  improbability  of  a  strong  descend- 
ing flow  through  the  great  thickness  of  Cambrian,  Ordovician, 
and  Mississippian  beds.  A  satisfactory  explanation  of  the 
southeastern  Missouri  deposits  is  as  yet  lacking. 

On  the  other  hand,  Van  Hise,  Bain,  Tangier  Smith,  and 
Siebenthal,  who  have  studied  the  Joplin  district,  believe  that 
the  source  of  the  ores  is  in  the  various  formations  below  the 
Pennsylvanian,  particularly  in  the  Cambro-Ordovician,  and  that 
atmospheric  waters  penetrating  these  rocks  were  carried  up 
against  the  impervious  beds  of  the  Pennsylvanian  and  here  de- 
posited in  the  pre-Pennsylvanian  breccias  and  sink-holes.  Smith 
and  Siebenthal  hold  that  the  ores  were  formed  much  later  than 
the  Pennsylvanian,  after  the  Ozark  uplift  (Fig.  1 36)  had  established 
an  artesian  circulation.  The  surface  waters  entered  the  older 
Paleozoic  outcrops  to  the  south  and  east  of  the  Joplin  region. 
After  following  these  beds  they  passed  upward  through  the 
jointed  and  brecciated  Mississippian  limestone  until  they  reached 
the  vicinity  of  the  impermeable  shales.  There  is,  indeed,  in  the 
deep  wells  of  Joplin  good  evidence  of  the  existence  of  artesian 
pressure.  Siebenthal's  recent  contribution  in  which  very  strong 
arguments  are  advanced  in  favor  of  the  artesian  theory  has 
already  been  reviewed  in  the  general  part  of  this  chapter. 

For  the  deposits  of  Wisconsin  and  Illinois,  Van  Hise  and  Bain 
assume  that  the  metals  were  minutely  disseminated  as  sulphides 
through  the  Galena  dolomite  and  concentrated,  probably  in 
late  Tertiary  or  post-Tertiary  time,  by  the  action  of  surface 
waters  descending  in  shallow  troughs  through  the  fractured 
and  slightly  inclined  Galena  limestone,  and  that  the  reduction 
was  effected  by  the  organic  matter  of  the  oil  rock.  In  the 
Galena  limestone  the  solutions  were  practically  confined  between 
two  beds  of  almost  impermeable  'shale. 

According  to  Cox  the  metals  were  derived  from  the  overlying 
Maquoketa  shale,  in  which  he  finds  some  evidence  of  the  presence 
of  sulphides,  particularly  sphalerite.  He  points  out,  with  good 
reason,  that  the  shales  are  better  suited  as  receptacles  for  metallic 
ores  than  the  limestones.  The  latter  are  deposited  in  deeper 
water,  while  the  shales  are  shore  formations  in  which  the  metallic 
substances  derived  from  adjacent  continents  would  most  easily 
be  deposited  as  detritus  or  precipitated  from  solutions. 


464  MINERAL  DEPOSITS 

We  find  here  the  same  conflict  of  opinion,  as  in  the  case  of  the 
Missouri  deposits  and  those  of  Silesia,  between  the  ascensionists 
and  the  descensionists.  The  problem  is  not  yet  solved,  but 
looking  beyond  these  controversies,  we  cannot  deny  that  in 
many  countries  transition  types  appear  which  seem  to  connect 
these  apparently  distinct  non-igneous  deposits  with  deposits  of 
igneous  affiliations. 

L.  V.  Pirsson1  in  1915  expressed  the  view  that  the  zinc-lead 
deposits  of  the  Mississippi  valley  type  might  well  result  from 
"the  quiet  upward  movement  of  volatile  magmatic  material" 
thus  ranging  himself  with  W.  P.  Jenney  and  F.  L.  Nason.  It  will 
be  incumbent  upon  the  supporters  of  this  theory  to  controvert 
the  strong  arguments  offered  by  Siebenthal  in  favor  of  leaching 
of  limestones  by  ascending  saline  meteoric  waters. 

Origin  of  certain  ore  deposits,  Econ.  Geol,  vol.  10,  1915,  pp.  180-186. 


CHAPTER  XXIV 

METALLIFEROUS  DEPOSITS  FORMED  NEAR  THE  SUR- 
FACE BY  ASCENDING  THERMAL  WATERS  AND  IN 
GENETIC  CONNECTION  WITH  IGNEOUS  ROCKS 

CHARACTER  AND  ORIGIN 

General  Features. — The  deposits  at  the  orifices  of  hot  ascend- 
ing springs  have  been  described  in  Chapter  VII.  It  has  been 
shown  that  they  consist  of  opal,  chalcedony,  quartz,  calcite, 
aragonite,  barite,  and  fluorite,  with  a  number  of  other  gangue 
minerals,  and  that  they  also  contain  in  places  metallic  gold  and 
certain  sulphides,  such  as  cinnabar,  stibnite,  and  pyrite,  but  not 
the  other'  common  ore  minerals  such  as  chalcopyrite,  galena,  zinc 
blende,  and  arsenopyrite.  The  ore  deposits  described  in  the 
present  chapter  present  some  striking  analogies  to  those  products 
of  the  hot  springs. 

In  regions  of  comparatively  recent  volcanic  activity  where 
the  measure  of  erosion  since  the  eruptions  ceased  is  in  hundreds 
rather  than  in  thousands  of  feet  we  find  a  group  of  important 
ore  deposits,  most  commonly  in  the  form  of  fissure  veins.  They 
generally  occur  in  igneous  flow  rocks1  and  also  'cut  the  un- 
derlying or  adjacent  formations.  They  constitute  the  source 
of  a  large  part  of  the  world's  production  of  gold,  silver,  and 
quicksilver,  and  they  contain  the  spectacular  bonanzas  of  the 
Cordilleran  region,  of  which  examples  are  found  at  Tuscarora, 
Virginia  City,  Goldfield,  Cripple  Creek,  Pachuca,  Guanajuato, 
and  many  other  districts.  Following  the  Tertiary  outbursts  of 
effusive  rocks,  these  deposits  accompany  the  "circle  of  fire" 

1  We  are  accustomed  to  consider  as  intrusive  rocks  those  which  have 
congealed  with  granular  texture  far  below  the  surface.  Intrusions  are, 
however,  not  confined  to  any  particular  depth  or  texture.  Intrusive  bodies 
may  be  found  in  any  series  of  rocks  even  near  the  surface  and  may  then  have 
fine-grained,  trachytic  or  even  glassy  texture.  The  distinction  between 
flows  and  intrusions  may  in  such  cases  become  difficult  and,  as  shown  in 
case  of  the  Tonopah,  Waihi  and  other  districts,  the  relations  may  have 
far  reaching  bearing  upon  the  richness  and  continuation  of  the  deposits  con- 
tained in  such  a  series  of  rocks. 

465 


466  MINERAL  DEPOSITS 

that  encompasses  the  Pacific  Ocean.  We  find  them  in  Japan, 
in  the  East  Indian  Islands,  and  in  New  Zealand.  They  are 
characteristically  developed  in  that  classical  mining  region  of 
the  Old  World,  in  Hungary  and  Transylvania,  where  one  of  the 
elements — tellurium — which  so  often  accompanies  them  was 
first  found. 

Though  most  of  these  ore-deposits  are  found  in  the  Tertiary 
flow  rocks  they  are  not  confined  to  rocks  of  this  period.  There 
is  good  reason  to  believe  that  veins  are  developing  now  in  some 
regions  of  recent  volcanism,  and  also  that  similar  veins  have 
been  formed  during  pre-Tertiary  outbreaks,  although  erosion 
has  removed  most  of  the  older  representatives  of  this  type. 
These  deposits  have  certain  well-marked  characteristics  which 
are  partly  of  a  mechanical,  partly  of  a  chemical  origin. 

Because  the  fissuring  of  the  rocks  took  place  near  the  surface, 
under  slight  load,  open  cavities  were  abundant,  and  filling, 
crustification,  and  comb  structure  are  conspicuous.  The  walls 
are  likely  to  be  irregular,  and  the  vein  matter  is  often  "frozen" 
to  the  walls.  Splitting,  chambering,  and  brecciation  are  features 
of  the  veins.  While  metasomatic  processes  have  been  active 
in  the  surrounding  rocks,  the  ore  is  usually  confined  to  the  open 
fissures.  Short  and  irregular  veins  are  more  frequent  than  the 
regularly  developed  conjugated  fractures  resulting  from  strong 
compressive  stress.  Divergent  systems  of  fractures  or  several 
parallel  systems  with  little  apparent  relationship  are  thought 
to  be  due  to  the  gravitative  settling  of  volcanic  piles. 

Banding  caused  by  crustification  is  common,  as  illustrated  in 
Figs.  160,  166,  172,  176  and  180.  It  is  much  more  delicate 
and  frequent  than  in  deposits  formed  at  greater  depth  and 
higher  temperature. 

The  occurrence  of  the  ore  in  "stock works,"  or  in  pipes,  or 
below  impervious  beds  is  often  observed.  In  superimposed  lava 
flows  of  different  kinds,  some  are  usually  better  adapted  to  the 
deposition  of  ore  than  others  and  this  difference  may  result  in  the 
development  of  ore-shoots  which  are  approximately  horizontal. 

Among  the  metals  contained  in  these  deposits  gold  and  silver 
are  by  far  the  most  important.  Base  metals  are  present,  plenti- 
fully enough  in  places,  but  the  mines  are  rarely  worked  for  these. 
Large  bodies  of  galena  and  zinc  blende  occur  in  some  places, 
but  it  is  decidedly  rare  to  find  important  copper  deposits.  The 
"pyritic"  deposits  are  not  represented;  they  are  confined  to  the 


DEPOSITS  FORMED  NEAR  THE  SURFACE      467 

deeper  zones  or  to  those  of  higher  temperatures.  Arsenic  and 
antimony,  bismuth,  tellurium,  and  selenium  are  common  but 
are  rarely  of  economic  importance;  quicksilver  is  present  in  some 
deposits  and  indeed  the  typical  quicksilver  deposits  belong  to 
this  class.  Cobalt  and  nickel,  tungsten,  and  molybdenum  are 
not  unknown,  but  are  entirely  subordinate.  Their  home  is  in  the 
deeper  deposits. 

The  pure  gold  deposits  are  relatively  scarce.  Those  carrying 
silver  only  are  common  in  certain  regions,  like  Mexico.  The 
usual  metals  are  gold  and  silver  occurring  together  in  varying 
proportions. 

Among  the  ore  minerals  native  gold  should  be  mentioned  first. 
It  contains  silver,  as  a  rule,  and  is  of  pale  yellow  color;  a  propor- 
tion sometimes  occurring  is  ounce  for  ounce  when  the  mineral 
is  of  very  pale  grayish-yellow  color  (electrum).  Deep  yellow 
gold  is  not  unknown,  however.  The  gold  is  often  present  in 
very  fine  mechanical  distribution,  being  sometimes  so  closely 
intergrown  with  ore  minerals  and  gangue  that  no  colors  can  be 
obtained  by  panning.  When  derived  by  oxidation  of  tellurides 
it  is  of  dull  brown  color  and  is  difficult  to  recognize  even  in  rich 
specimens.  The  whole  series  of  tellurides  is  present.  As  the 
gold  generally  occurs  in  minute  particles  rich  placers  below  the 
croppings  of  these  deposits  are  rather  unusual. 

Native  silver  is  ordinarily  a  product  of  oxidation.  The 
primary  and  most  abundant  silver  mineral  is  argentite;  com- 
plex silver  sulphantimonides  and  sulpharsenides  are  also  charac- 
teristic ;  it  is  often  difficult  to  say  which  are  secondary  and  which 
are  primary.  Among  them  are  proustite,  pyrargyrite,  miargy- 
rite,  stephanite,  polybasite,  tetrahedrite,  and  more  rarely 
enargite. 

Stibnite  is  plentiful  in  deposits  of  certain  types.  Among  the 
base  minerals  pyrite  is  always  present,  but  in  small  quantity 
and  fine  distribution;  marcasite,  a  mineral  typical  of  deposition 
near  the  surface,  is  not  unusual;  often  it  is  secondary.  There 
are  also  galena,  zinc  blende,  chalcopyrite,  and  sometimes  ala- 
bandite;  rarely  arsenopyrite ;  never  pyrrhotite  or  magnetite. 

Of  gangue  minerals  quartz  is  the  most  abundant,  and  crystals 
of  it  are  plentiful  but  rarely  large;  an  amethyst  color  is  often 
noticeable.  The  quartz  aggregates  are  not  glassy  or  milky  but 
usually  fine-grained  (hornstone)  and  often  chalcedonic,  with 
banding  a-nd  rapidly  changing  grain.  Chalcedony  and  opal 


468  MINERAL  DEPOSITS 

are  usually  later  than  the  quartz.  Calcite,  dolomite,  barite, 
and  fluorite  are  locally  the  dominant  gangue  minerals,  while 
siderite  is  rare.  Manganese  minerals  like  rhodochrosite  and 
sometimes  rhodonite  are  typical  of  certain  groups.  Kaolin 
accompanies  the  veins,  sometimes  in  large  amounts,,  but  is 
probably  in  most  cases  a  product  of  secondary  changes  by 
descending  waters.  Sericite  and  chlorite  appear  in  the  altered 
country  rock.  Zeolites  are  present  in  some  deposits,  but  are 
certainly  of  exceptional  occurrence. 

One  of  the  most  widespread  and  characteristic  gangue  minerals 
and  the  most  difficult  to  explain  is  adularia  (or  valencianite).1 
Discovered  by  Breithaupt  in  specimens  from  the  Valenciana 
mine  at  Guanajuato,  this  mineral  has  since  been  found  in  numer- 
ous other  places,  mainly  in  the  Cordilleran  region,  as  part  of  the 
filling,  and  as  a  metasomatic  product  in  the  country  rock. 
Among  the  places  where  this  feldspar  plays  an  important  part 
may  be  mentioned  Silver  City  (Idaho),  Tuscarora,  Tonopah, 
and  Rawhide  (Nevada),  Gold  Road  (Arizona),  Republic  (Wash- 
ington), and  Cripple  Creek  and  Creede  (Colorado).  It  does  not 
occur  at  Goldfield,  Nevada,  where  solutions  of  acid  reaction 
appear  to  have  deposited  the  ore.  The  orthoclase  mineral  is 
usually  a  pure  potassium  feldspar,  although  varieties  with  several 
per  cent,  of  sodium  have  been  found  at  Waihi,  New  Zealand, 
and  in  the  Gold  Spring  district,  Utah.2  Sometimes  the  adularia 
replaces  orthoclase,  biotite,  and  other  rock  minerals  (Fig.  145); 
it  is  also  found  in  the  form  of  well-developed  crystals  of  prism 
and  dome  intergrown  with  vein  quartz  (Fig.  146).  The  cross- 

1  A.  Breithaupt,  Ueber  die  Felsite  und  einige  neue  Specien  ihres  Gesch- 
lechts,  Schweigg.  Jour.,  Bd.  60,  p.  322,  1830. 

W.  Lindgren,  Orthoclase  as  a  gangue  mineral  in  fissure  veins,  Am. 
Jour.  Sci.,  4th  ser.,  vol.  5,  1898,  p.  418. 

W.  Lindgren,  Twentieth  Ann.  Rept.  U.  S.  Geol.  Survey,  pt.  3,  1900,  p.  167. 

J.  E.  Spurr,  Prof.  Paper  42,  U.  S.  Geol.  Survey,  1905,  p.  86. 

W.  Lindgren  and  F.  L.  Ransome,  Prof.  Paper  54,  U.  S.  Geol.  Survey, 
1906,  p.  187. 

A.  F.  Rogers,  Orthoclase-bearing  veins  from  Rawhide,  Nevada,  Econ. 
Geol,  vol.  6,  1911,  p.  790. 

F.  C.  Schrader,  Mineral  deposits  of  the  Cerbat  Range,  Black  Mountains 
and  Grand  Wash  Cliffs,  Mohave  County,  Arizona,  Bull.  397,  U.  S.  Geol. 
Survey,  1909. 

F.  C.  Schrader,  A  reconnaissance  of  the  Jarbidge  district,  Nevada. 
Bull.  497,  U.  S.  Geol.  Survey,  1912. 

2  B.  S.  Butler,  U.  S.  Geol.  Survey,  oral  communication. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      469 

sections  of  the  adularia  crystals  are  usually  of  rhombic  shape. 
The  mineral  also  occurs  abundantly  in  some  veins  that  had 
originally  a  calcite  gangue,  now  replaced  by  an  intimate  inter- 
growth  of  adularia  and  quartz. 

The  high-temperature  minerals,  such  as  augite,  amphibole, 
olivine,  biotite,  tourmaline,  topaz,  garnet,  magnetite,  ilmenite, 
and  chromite,  are  conspicuously  absent. 

Successive  Phases  of  Mineralization. — Veins  formed  near  the 
surface  in  volcanic  regions  are  sometimes  subject  to  peculiar 
changes,  which  are  rarely  observed  in  deposits  of  more  deep- 


^^V^^^^^^B^^— 

.FiG.  1 45.—  Adularia  (Ad)  replacing  soda-lime  feldspar  (An)  in  andesito 
from  Tonopah,  Nevada.  Magnified  17  diameters.  After  J.  E.  Spurr,  U.  S. 
Geol.  Survey. 

seated  origin.  An  earlier  gangue  mineral,  such  as  calcite  or 
barite,  may  be  wholly  wiped  out  and  replaced  by  a  new  gangue 
of  quartz  and  adularia.  This  alteration  has  nothing  to  do  with 
surface  waters  though  the  latter  may  sometimes  produce  a  simi- 
lar cellular  or  lamellar  structure;  it  is  plainly  caused  by  a  change 
in  the  composition  of  the  ascending  currents.  Indications 
of  this  process  may  be  seen  even  where  it  has  not  been  carried 
to  completion.  In  many  veins  at  Cripple  Creek  deposition  be- 
gan by  the  growth  of  slender  crystals  of  celestite  from  the  walls, 


470 


MINERAL  DEPOSITS 


and  these  crystals  were  subsequently  replaced  by  quartz,  in  which 
the  pseudomorphs  are  now  embedded.  In  the  Trade  Dollar 
vein  at  Silver  City,  Idaho,  the  filling  consists  of  quartz  and  adu- 
laria,  but  casts  of  barite  or  calcite  covered  with  minute  crystals  of 
adularia  indicate  that  here  also  there  was  a  preliminary  carbon- 
ate or  sulphate  stage. 

In  many  instances  the  vein  was  completely  filled  by  calcite, 
each  grain  separated  by  a  slender  partition  of  quartz;  at  the 
beginning  of  the  second  stage  this  calcite  was  dissolved,  leaving 
a  skeleton  of  thin  silica  walls;  secondary  quartz  and  often  also 


FIG.  146. — Intergrowth  of  quartz  (q)  and  adularia  (a),  Fraction  vein, 
Tonopah,  Nevada.  Magnified  38  diameters.  After  J.  E.  Spurr,  U.  S. 
Geol.  Survey. 

adularia  were  deposited  upon  these  walls,  giving  them  more 
strength,  but  the  ore  remains  a  delicate  aggregate  of  "hackly" 
or  lamellar  quartz,  such  as'is  exceedingly  characteristic  of  some 
mining  district.  At  De  Lamar,  Idaho,  this  ore  consists  only  of 
quartz  (Figs.  147  and  148).  In  the  veins  at  Gold  Road,  Arizona, 
and  many  other  veins  in  the  same  district,  the  original  gangue 
material  consisted  of  calcite  and  fluorite  and  the  "pseudo- 
morphic"  ore  consists  of  quartz  and  large  amounts  of  adularia. 
Similar  ore  may  be  seen  in  the  Mount  Baldy  district,  southern 


DEPOSITS  FORMED  NEAR  THE  SURFACE      471 

Utah,   at  Jarbidge,   Nevada   (Fig.   149),   and    at    many    other 
places.     This    important    development    of    adularia,  involving 


FIG.  147. — Lamellar  quartz,  replacing  calcite  gangue,  De  Lamar,  Idaho. 
One-half  natural  size. 


FIG.  148. — Section  of  lamellar  ore,  De  Lamar,  Idaho.     Natural  size. 

transportation  of  alumina  by  siliceous  solutions,  remains  without 
full  explanation.  The  composition  of  the  ore  may  be  similar 
to  that  of  a  pegmatite  dike,  but  the  structure  is  wholly  different. 


472 


MINERAL  DEPOSITS 


There  is  reason  to  believe  that  this  ''pseudomorphism"  is 
accompanied  by  a  change  in  the  metal  content  of  the  vein.  At 
least  it  seems  as  if  the  original  filling  of  barite,  calcite,  and 
fluorite  carried  more  silver  and  as  if  the  silicification  and  feld- 
spathization  was  accompanied  by  a  concentration  of  the  gold. 
Similar  processes  may  be  traced  in  some  quartz  veins  of  the 
Republic  district,  Washington.  Here  quartz  with  some  adularia 
replaces  a  slender  acicular  or  thin  tabular  mineral,  probably 
calcite,  developed  parallel  to  c  and  r,  which  seems  to  have  been 
deposited  only  along  the  walls  of  the  vein. 

Zeolitic  Replacement. — Zeolites  are  foreign  to  veins  of  the 
deeper  zones  ;'in  the'veins  formed  near  the  surface  they_are  occa- 


FIG.  149. — Thin  section  of  lamellar  quartz  and  adularia,  pseudomorphic 
after  calcite,  Jarbidge,  Nevada.  Magnified  12  diameters.  After  F.  C. 
Schroder,  U.  S.  Geol.  Survey. 

sionally  found,  but  they  are  rare.  At  a  few  places  zeolites 
are  reported  in  the  altered  country  rock  (Tonopah,  the  Comstock, 
and  Waihi).  At  Guanajuato  zeolites  are  found  in  the  filling  of 
the  veins,  but  here  they  always  belong  to  the  latest  phases  of 
vein  formation.  Apophyllite,  laumontite,  and  stilbite  are  the 
species  reported.  Few  of  these  occurrences  in  the  vein  filling 
have  been  carefully  studied.  In  the  Southern  Republic  mine 
at  Republic,  Washington,  laumontite,  associated  with  calcite, 
occurs  on  a  fairly  large  scale.1  At  this  place  the  ordinary  fine- 
1  W.  Lindgren,  Trans.,  Canadian  Min.  Inst.,  vol.  15,  1912,  pp.  187-191. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      473 

grained  banded  quartz  filling  had  evidently  been  dissolved  and 
the  laumontite-calcite  aggregate  was  deposited  in  its  place. 
The  ore  in  this  zeolitic  zone  or  shoot  contains  mainly  silver, 
whereas  elsewhere  in  the  mine  gold  predominates  in  the  quartz 
gangue.  It  seems  to  be  worth  while  to  draw  attention  to  this 
occurrence  with  a  view  to  ascertaining  whether  the  development 
of  zeolites  is  not  favorable  to  silver  enrichment.  Many  facts 
noted  in  veins  of  other  classes,  like  those  of  Kongsberg  and 
Andreasberg,  point  in  this  direction. 

Primary  Ore  Shoots,  Oxidation,  and  Sulphide  Enrichment. — 
Rich  oxidized  ores  are  often  encountered  in  these  deposits  at 
the  surface  and  down  to  the  water-level.  Whether  the  primary 
ore  is  greatly  enriched  in  this  zone  depends  really  more  on  the 
texture  and  composition  of  the  ore  than  on  its  original  tenor. 
In  veins  of  hard  fine-grained  quartz  oxidation  often  fails  to  pro- 
duce an  ore  of  higher  grade.  There  are  many  districts  in  which 
the  oxidized  ores  are  little,  if  any,  richer  than  those  below  the 
oxidized  zone.  Such  conditions  exist,  for  instance,  at  Cripple 
Creek  and  at  Tonopah. 

The  largest  and  richest  masses  of  ore  are  often  found  just 
below  the  oxidized  zone  and  in  general  contain  sulphides,  sulph- 
antimonides,  and  sulpharsenides.  It  will  suffice  to  call  atten- 
tion to  the  great  silver  bonanzas  of  Guanajuato  and  Pachuca; 
to  the  Comstock,  where  in  one  month  silver-gold  ores  valued 
at  $6,000,000  were  extracted;  to  Tonopah,  Nevada,  where  in 
three  months  ore  yielding  over  $3,000,000  was  extracted;  to  the 
Caledonian  mine  at  Thames,  New  Zealand,  which  in  one  year 
from  a  small  ore-shoot  produced  $6,000,000  in  gold;  to  Cripple 
Creek,  Colorado,  where  in  one  year  from  a  small  area  but  a 
considerable  number  of  mines  $18,000,000  in  gold  was  pro- 
duced; to  Goldfield,  Nevada,  where  during  a  recent  year  over 
$10,000,000  in  gold  was  recovered  from  one  property,  the 
ore  averaging  $38.50  per  ton,  and  where,  of  the  phenomenally 
rich  ore  shipped  in  1907,  one  carload  of  47  tons  yielded  $600,000 
in  gold. 

At  the  same  time  it  is  well  to  emphasize  the  fact  that  most  of 
these  high  yields  proved  ephemeral.  The  bonanzas  were  ex- 
tracted, poorer  ore  was  found  in  depth,  and  the  mine  was  aban- 
doned or  continued  in  feeble  existence. 

These  great  bonanzas  are  due  in  part  to  primary  deposition — 
in  large  degree  probably  to  the  later,  reconcentrating  phases  of 


474  MINERAL  DEPOSITS 

primary  deposition;  and  in  part  to  sulphide  concentration  by 
descending  waters  charged  with  precious  metals  from  the  upper 
parts  of  the  veins.  It  should  not  be  overlooked  that  ore-shoots 
of  primary  origin  are  common  enough.  Take,  for  instance, 
the  Cripple  Creek  gold  ores,  in  which  evidence  of  enrichment 
is  conspicuously  lacking;  these  primary  shoots  are  usually  of  a 
markedly  irregular  form;  many  of  the  smaller  ore-bodies  are 
likely  to  follow  intersections  of  fissures.  In  depth  the  rich  shoots 
show  a  tendency  to  contract,  to  feather  out,  or  simply  to  become 
impoverished.  In  many  cases  zinc  blende  in  a  quartz  gangue 
appears  in  depth.  On  the  other  hand  sulphide  enrichment  is 
conspicuously  marked,  especially  in  silver  veins.  In  gold-bearing 
veins  the  enrichment  in  gold  is  likely  to  be  localized  in  the  lower 
part  of  the  zone  of  oxidation.  The  secondary  silver  minerals 
are  native  silver,  argentite,  ruby  silver,  stephanite,  and  polyba- 
site — that  is,  the  same  minerals  (except  the  native  silver)  as  occur 
in  primary  ore.  As  a  consequence  it  is  often  exceedingly  difficult 
to  distinguish  primary  ore  and  enriched  ore,  and  when  in  addition 
to  this  we  recognize  that  the  later  effects  of  primary  mineraliza- 
tion may  sometimes  closely  simulate  the  products  of  descending 
surface  waters,  the  difficulties  of  correct  interpretation  will  be 
fully  realized.  The  distinction  is  made  easier  when  the  secondary 
sulphides  form  a  distinct  zone  immediately  below  the  oxidized 
part  of  the  lode. 

The  conditions  for  the  deposition  of  gold  and  silver  are  appar- 
ently much  more  favorable  near  the  surface  than  at  greater 
depths;  on  the  other  hand,  deposition  took  place  rapidly  and 
the  gold  and  silver  contents  of  the  solutions  were  doubtless  ex- 
hausted before  they  reached  the  actual  surface. 

Types  of  Deposits. — The  merging  of  the  various  types  makes 
it  difficult  to  establish  a  rigid  classification. 

One  type,  namely,  the  zeolitic  copper  deposits  in  amygdaloid 
rocks,  has  been  left  out  of  consideration  at  this  time,  for  it  really 
represents  a  mineralization  of  the  lava  derived  from  its  own  body. 
To  gain  a  general  orientation  the  deposits  here  described  are 
classified  as  follows: 

1.  Cinnabar  Deposits. — Cinnabar,  marcasite,  stibnite,  hydro- 
carbons, quartz,  opal,  calcite. 

2.  Stibnite  Deposits. — Stibnite,  pyrite,  and  some  other   sul- 
phides; also  quartz% 

3.  Base  Metal  Deposits. — Chalcopyrite,  galena,   zinc  blende, 


DEPOSITS  FORMED  NEAR  THE  SURFACE       475 

tetrahedrite,  in  an  abundant  gangue  of  quartz,  carbonate,  or 
barite.     Principal  values  usually  in  gold  and  silver. 

4.  Gold  Deposits. — Native   gold,    alloyed   with    silver.     Sub- 
ordinate argentite,  ruby  silver,  etc.     Quartz. 

5.  Argerdite-Gold    Deposits. — Argentite,    ruby    silver,     tetra- 
hedrite, etc.;  native  gold,  quartz,  calcite. 

6.  Argentite   Deposits. — Argentite,    ruby    silver,  tetrahedrite, 
etc.;  quartz  or  calcite,  barite,  and  fluorite. 

7.  Gold  Telluride  Deposits. — Gold  tellurides,  quartz,  or  quartz 
and  fluorite. 

8.  Gold    Telluride    Deposits    with    Alunite. — Gold     tellurides, 
pyrite,  alunite,  kaolin. 

9.  Gold  Selenide  Deposits. — Gold  selenides,  pyrite,  quartz,  calcite. 

Older  Representatives  of  this  Class. — The  types  just  enum- 
erated almost  wholly  represent  veins  or  similar  deposits  in  Terti- 
ary lavas  of  the  Cordilleran  or  Pacific  or  Hungarian  regions,  but 
a  close  examination  will  easily  discover  examples  of  similar  de- 
posits of  a  greater  geological  age.     Beck1  described  relatively 
unimportant  deposits  in  the  Mesozoic  melaphyres  and  quartz 
porphyries  at  Imsbach,  in  the  German  Palatinate,  that  carry 
chalcopyrite,  galena,  and  tetrahedrite  in  a  gangue  of  calcite,  ba- 
rite, and  rhodochrosite  and  are  probably  ancient  representatives 
of  this  class.     The  celebrated  veins  of  Freiberg,  or  at  least  three 
types  of  them,  namely,  the  "noble  quartz  formation,"  the  "noble 
carbonate  formation"  and  especially  the  "barytic  lead  forma- 
tion," should  be  mentioned  in  this  connection.     There  seems  to 
be  good  evidence  that  these  are  Carboniferous  representatives  of 
veins  formed  near  the  surface,  although  the  lavas  in  which  they 
probably  reached  the  surface  are  now  eroded.     Their  structure' 
and  composition  point  clearly  to  a  shallow  deposition,  and  were 
the  physiographic  conditions  in  the  Erzgebirge  fully  analyzed 
the  results  would  probably  confirm  this  view.     The  "barytic  lead 
formation,"   for  instance,   carries  barite,   fluorite,   quartz,  and 
dense  quartz  as  gangue  minerals  with  beautiful  crustification, 
while  the  ore  minerals  are  argentiferous  galena,  tetrahedrite, 
bournonite,  and  chalcopyrite. 

Another  occurrence  that  might  well  be  cited  comprises  the 
insignificant  veins  in  the  Triassic  diabase  flows  at  Bergen  Hill, 
New  Jersey,  which  contain  pyrite  and  galena  in  a  gangue  quartz 
and  adularia,  with  secondary  zeolites. 

1  R.  Beck,  Lehre  von  den  Erzlagerstatten,  vol.  1,  1909,  p.  334. 


i 


476  MINERAL  DEPOSITS 

Genesis. — In  the  preceding  pages  attention  has  been  called  to 
the  strong  evidence  connecting  the  class  of  deposits  here  dis- 
cussed with  igneous  action  and  pointing  to  ascending  hot  waters 
as  the  agents  of  deposition.  The  best  proof  that  the  ores  were 
not  formed  by  the  ordinary  circulation  of  surface  waters  is  the 
fact  that  deposition  has  not  proceeded  uniformly,  but  that  the 
vein-forming  epochs  were  of  brief  duration  and  followed  closely 
after  each  considerable  eruption.  Evidence  of  this  relation  is 
available  from  many  important  districts.  At  Tonopah  the  prin- 
cipal mineralization  followed  the  eruption  of  the  earlier  andesite 
and  the  veins  are  truncated  by  the  flow  of  the  later  andesite 
and  the  later  rhyolite.  At  Jarbidge,  Nevada,  the  veins  are  con- 
tained in  the  early  rhyolite,  while  the  later  rhyolite  is  barren. 
At  Waihi,  New  Zealand,  the  rich  veins  are  sharply  truncated  by 
erosion  and  capped  by  a  rhyolite  of  later  age. 

The  occurrence  of  these  deposits  in  lavas  really  counts  for  but 
little;  there  are  vast  areas  of  lava  flows  absolutely  barren  of 
mineral  deposits.  On  the  other  hand,  several  of  the  Hungarian 
authors  have  pointed  out  the  fact  that  the  veins  are  confined 
mainly  to  the  vicinity  of  volcanic  necks  or  centers  of  eruption. 
Exactly  the  same  conclusions  have  been  reached  in  the  United 
States.  This  feature  serves  to  connect  the  veins  formed  near 
the  surface  with  those  of  greater  depths.  The  deposits  in  the 
surface  lavas  are,  then,  simply  the  tops  of  veins,  the  roots  of 
which  are  to  be  found  in  the  intrusive  masses  of  the  depths. 
No  matter  whence  all  the  water  or  part  of  the  water  came,  the 
deposition  of  the  substance  of  the  veins — their  valuable  content 
• — appears  to  be  a  phenomenon  connected  with  intrusive  activity 
and  not  merely  dependent  upon  the  heat  furnished  by  the  lava 
flows  to  circulating  surface  waters.  The  metals,  as  well  as  the 
sulphur,  carbon  dioxide,  and  fluorine,  were  in  all  probability 
derived  from  intrusive  underlying  masses. 

Proof  of  Depth  below  Surface. — Physiography  furnishes  the 
data  on  the  original  surface  during  deposition.  We  may  be  able 
to  trace  the  old  surface  of  the  volcanic  slope  or  plateau  and 
ascertain  the  relation  of  the  outcrops  to  the  uppermost  flow,  or 
in  dissected  volcanic  piles  it  may  be  possible  to  reconstruct 
approximately  the  old  surface  of  the  volcanic  cone.  Of  this 
latter  possibility  Cripple  Creek  is  an  instance  (Fig.  167);  the 
present  surface  was  probably  less  than  1,500  feet  below  the  origi- 


DEPOSITS  FORMED  NEAR  THE  SURFACE      477 

nal  surface  of  the  volcanic  cone.  Ransome  estimates  that  at 
Goldfield,  Nevada,  the  surface  has  been  degraded  but  a  few 
hundred  feet  below  the  original  contours  of  the  flows.  A  fine 
example  showing  the  connection  of  deposits  formed  near  the 
surface  with  those  of  more  deep-seated  type  is  offered  by  the 
San  Juan  region,  in  Colorado,  where  erosion  has  not  only  inter- 
sected the  flows  but  laid  bare  the  intrusive  masses  forced  into 
them — all  within  a  vertical  interval  of  6,000  feet. 

Proof  of  Temperature. — The  similarity  to  hot-spring  deposits 
is  least  marked  in  deep-seated  veins,  but  becomes  striking  in 
the  veins  here  under  consideration.  The  fine-grained  chalcedonic 
and  banded  quartz  of  spring  deposits  (Fig.  5,  p.  101)  is  entirely 
similar  to  the  often  delicate  and  beautifully  banded  and  crustified 
portions  of  these  veins.  The  evidence  indicates  deposition  by 
waters  that  held  in  solution  large  quantities  of  substances  not 
easily  soluble — that  is,  by  hot  waters  which  at  the  surface 
could  not  have  had  a  temperature  of  more  than  100°  C.  The 
minerals  present  are  those  which  we  have  reason  to  believe  were 
developed  at  a  temperature  less  than  200°  C.  What  the  actual 
temperatures  were  in  each  case  is  of  course  scarcely  possible  to 
ascertain. 

The  hot  springs  are  volcanic  "after  effects"  and  usually  ascend 
through  the  cooled  lavas.  In  some  cases  the  waters  rise 
through  bodies  of  hot  rocks  and  then  the  pressure  may  become 
so  high  that  the  solutions  issue  at  the  surface  as  gases  and 
form  "fumaroles"  and  "soffioni"  which  sometimes,  at  their 
orifices,  have  a  temperature  of  as  much  as  200°  C.  In  these 
rarer  instances  the  high  temperature  deposits,  marked  for  in- 
stance by  tourmaline  or  cassiterite,  may  develop  close  to  the 
surface. 

Relation  to  Other  Veins. — The  question  naturally  arises  as  to 
the  character  of  these  veins  in  depth.  Do  they  actually  change 
to  assume  the  aspect  of  the  veins  of  the  deeper  zones?  The 
evidence,  scant  as  it  is,  indicates  that  this  is  probably  true.  In 
regions  of  deeply  eroded  volcanic  flows,  like  the  San  Juan  country 
in  Colorado,  the  veins  in  the  lower  exposures  show  an  approach 
to  the  types  of  deep-seated  origin.  During  the  long  ascent  there 
was  no  doubt  a  progressive  change  in  the  nature  of  the  depositing 
waters;  some  of  their  constituents  were  deposited  and  others 
were  acquired  from  the  rocks  they  traversed. 


478  MINERAL  DEPOSITS 

METASOMATIC  PROCESSES 

Extent  of  Alteration. — At  considerable  depths  the  ore-forming 
solutions  move  in  the  paths  prescribed  by  fissuring  and  breccia- 
tion;  they  rarely  penetrate  great  masses  of  rocks.  Near  the 
surface,  especially  in  the  great  volcanic  piles,  different  conditions 
prevail.  There  are  here  thick  beds  of  tuffs  and  agglomerates 
with  great  porosity,  and  the  stresses  may  irregularly  shatter 
large  volumes  of  rocks.  The  solutions  and  gases — of  meteoric 
or  telluric  origin — move  far  more  freely  and  alteration  is  effected 
on  the  largest  scale.  Here,  too,  we  find  most  emphasized  the 
peculiar  effects  of  the  mingling  of  ascending  and  descending 
solutions. 

Any  one  who  has  visited  an  active  or  recently  extinct 
volcano  has  undoubtedly  observed  the  areas  of  discolored  red- 
dish, brown,  and  yellow  rocks  which  indicate  alteration. 
Erosion  of  older  volcanoes  discloses  similar  zones  of  alteration 
on  a  large  scale  and  exposes  metalliferous  deposits  formed  in 
their  interior. 

Types  of  Alteration. — Gases  given  off  by  the  ascending  lavas 
penetrate  the  volcanic  cones.  They  are  admixed  with  water 
vapor,  which  may  or  may  not  be  of  intratelluric  origin,  and,  near 
the  point  of  issue,  oxidation  and  interaction  produce  compounds 
like  sulphur  dioxide,  sulphuric  acid,  hydrochloric  acid,  and 
sulphur.  Of  these  reagents  carbon  dioxide  and  sulphuric  acid 
are  most  effective  in  rock  alteration.  The  volcanic  rocks  are 
converted  to  kaolin.  Alunite,  jarosite,  and  other  sulphates  are 
often  mixed  with  these  minerals. 

These  masses  of  altered  rocks,  which  are  formed  on  the  slopes 
of  volcanoes,  scarcely  ever  carry  valuable  ores,  probably  because 
the  metallic  load  of  ascending  waters  is  usually  deposited  before 
the  cool  surface  zone  is  reached. 

Hot  springs  begin  to  issue  after  the  explosive  igneous  action 
has  declined  and  the  rocks  cooled  so  that  the  fumarolic  action 
is  supplanted  by  rising  aqueous  solutions.  These  waters  contain 
no  strong  acids,  but  probably  mainly  carbon  dioxide,  silica/and 
hydrogen  sulphide,  and  are  of  alkaline  rather  than  acid  reaction. 
Some  of  these  waters  move  slowly,  percolating  through  great  mas- 
ses of  rocks;  others  move  rapidly  in  prescribed  channels  and  effect 
extensive  changes  in  the  immediately  adjoining  rock. 

One  of  the  most  common  types  o"f  alteration  is  that  resulting  in 


DEPOSITS  FORMED  NEAR  THE  SURFACE      479 

the  "propylitic  facies;"  it  affects  mainly  andesites  and  basalts, 
more  rarely  rhyolites,  often  spreading  over  wide  areas  in  mineral- 
ized districts.  Its  mineralogical  characteristics  consist  in  the 
abundant  development  of  chlorite  and  pyrite,  sometimes  also 
epidote;  in  places  it  is  accompanied  by  the  development  of  carbon- 
ates and  a  little  sericite.  The  rock  assumes  a  dull  green  color. 
The  chemical  changes  consist  of  a  moderate  leaching  of  both 
potassium  and  sodium;  the  silica  is  usually  decreased,  as  are 
also  calcium  and  magnesium,  except  when  carbonates  of  these 
metals  are  formed.  The  composition  of  the  rock  changes 
but  Little  and  the  additions  consist  only  of  sulphur  and  some 
water  of  hydration. 

Still  another  type  of  alteration,  seen  mostly  in  siliceous  rocks 
like  rhyolite,  but  also  in  other  kinds,  consist  in  a  general  silicifica- 
tion  of  the  groundmass  and  phenocrysts,  with  aureoles  of  quartz 
developing  around  quartz  phenocrysts.  More  rarely  sericite 
develops  in  abundance  and  the  effusive  rocks  are  converted  over 
large  areas  to  a  mixture  of  quartz  and  sericite,  with  more  or  less 
pyritization. 

Near  the  veins  the  alteration  is  usually  most  intense,  although 
here,  too,  simply  chloritized  rock  may  often  adjoin  the  fissure 
filling.  In  sericitization  sodium  is  almost  entirely  carried  away 
and  potassium  is  accumulated  in  a  marked  degree  in  sericite  and 
adularia;  the  latter  mineral  has  a  wide  distribution,  both  in  the 
altered  country  rock  and  in  the  fissure  filling.  Unless  carbonates 
are  formed,  calcium  and,  to  a  less  degree,  magnesium  are  carried 
away;  much  pyrite  is  introduced  which  usually  contains  at  least 
traces  of  precious  metals.  The  percentage  of  silica  is  reduced. 
Close  to  the  vein,  silicification  often  assumes  the  ascendancy  and 
a  quartzose  mass  of  silica,  adularia,  and  sericite,  with  more  or  less 
sulphides,  develops  and  may  form  part  of  the  ore.  In  rare  cases 
hydrargillite  and  zeolites  may  appear  in  the  altered  rock.  Rutile 
appears  to  be  the  only  stable  titanium  mineral.  Manganese, 
titanium,  and  phosphorus  are  partly  removed  from  the  rock. 

Nearer  the  surface  another  potassium-aluminum  mineral — • 
alunite — appears  in  considerable  quantities.  This  hydrous  sul- 
phate is  characteristic  of  large  altered  areas  in  volcanic  regions,1 
but,  being  inconspicuous,  is  easily  overlooked.  That  it  often 
occurs  together  with  pyrite  and  sericite  is  clearly  proved,  and 
its  development  in  this  phase  is  probably  confined  to  the  zone 

'  B.  S.  Butler  and  H.  S.  Gale,  Alunite,  Bull.  511,  TJ.  S.  Geol.  Survey,  1912. 


480  MINERAL  DEPOSITS 

where  the  descending  waters  carrying  free  sulphuric  acid  meet 
the  ascending  currents  of  alkaline  waters.  It  appears  to 
belong  to  a  distinctly  higher  horizon  than  the  sericite  and 
adularia.  In  some  alunites  the  potassium  is  in  part  replaced 
by  sodium. 

In  eroded  and  mineralized  volcanic  regions  there  is  finally 
another  type  of  alteration,  the  effects  of  which  were  super- 
imposed upon  the  earlier  changes  and  tend  to  confuse  the  true 
history.  As  soon  as  the  mineralized  rocks  become  exposed  to 
the  air  oxidation  begins  and  sulphuric  acid  is  generated  by  the 
action  of  oxygen  on  sulphides.  This  sulphuric  acid  descends 
with  the  surface  water  and  converts  the  sericitized  rocks  into 
kaolin  mixed  with  alunite  and  other  oxidation  products.  Where 
waters  exceptionally  rich  in  sulphuric  acid  have  acted  on  the  rocks 
almost  everything  but  quartz  is  carried  away  and  the  final  result 
is  a  loose  quartz  aggregate.  Descending  still  farther  these 
sulphuric  acid  solutions  may  lose  their  oxygen,  and,  under 
certain  circumstances,  secondary  sulphides,  with  alunite  and 
sericite,  may  again  develop. 

This  brief  sketch  indicates  how  complicated  the  series  of  reac- 
tions may  be  and  how  the  same  minerals  may  form  at  different 
steps  of  the  process. 

It  is  assumed  in  the  above  discussion  that  ascending  alkaline 
waters  do  not  form  kaolin.  This  is  undoubtedly  true  in  general, 
but  it  is  possible  that  kaolin  may  be  formed  in  places  by  such 
waters  close  to  the  surface.  The  processes  of  alteration  by  hot 
ascending  waters  seem  to  result  in  minerals  of  only  moderate 
hydration;  zeolites,  kaolin,  and  other  strongly  hydrated  minerals 
are  conspicuously  absent.  The  zeolites  appear  to  require  quies- 
cent, stagnant  conditions,  such  as  do  not  exist  close  to  strong 
ascending  currents. 

Metasomatic  Processes  at  Thames  and  Waihi. — Extensive  pro- 
pylitization  has  been  described  by  several  authors  from  observa- 
tions in  the  Hauraki  Peninsula,  in  the  northern  island  of  New 
Zealand,1  where  rich  gold-bearing  veins  appear  in  volcanic  rocks 
like  pyroxene  andesite  or  dacite.  The  extreme  phase  close  to  the 
veins  is  a  grayish-white  rock,  but  a  more  widespread  type  is  a 
chloritized  andesite  which  corresponds  to  the  propylitic  facies 
as  defined  on  a  previous  page.  In  this  second  type  the  ferric 

1  A.  M.  Finlayson,  Problems  in  the  geology  of  the  Hauraki  gold  fields, 
Econ.  Geol,  vol.  4,  1909,  pp.  632-645. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      481 

minerals  are  chloritized,  the  pyroxene  often  passing  first  through 
a  uralitic  stage,  while  the  plagioclase  remains  comparatively 
fresh,  but  contains  some  calcite  and  sericite. 

Finlayson  has  presented  two  extremely  valuable  series  of 
analyses,  which  are  given  below  in  full.  They  represent  rocks 
from  Thames  and  Waihi,  the  two  most  important  fields  in  the 
peninsula.  The  first  column  in  each  table  gives  the  composition 
of  the  fresh  rock,  the  second  that  of  the  propylitic  or  chloritic 
facies,  and  the  rest  are  analyses  of  the  more  altered  forms  in 
which  sericite  and  adularia  are  the  predominating  metasomatic 
products.  The  chemical  changes  during  propylitization  are  not 
great. 

ANALYSES  OF  FRESH  AND  ALTERED  ROCKS  IN  THE  THAMES  DISTRICT 


SiO2  

57.42 

52.69 

57.99 

55.38 

58.98 

Ti02  

0.68 

0.53 

0.51 

0.24 

0.11 

AlA  

17.61 

18.33 

17.59 

15.63 

11.21 

FeA  

2.34 

2.32 

1.56 

1.88 

1.45 

FeO  

3.77 

2.98 

2.37 

2.95 

2.42 

MnO 

0.43 

0.25 

0.21 

0.23 

0.11 

MgO  

2.19 

3.09 

2.01 

1.88 

1.43 

CaO  

5.69 

7.87 

5.45 

6.01 

8.11 

Na2O  

3.22 

2.62 

1.98 

0.83 

0.61 

K20  

1.94 

0.98 

1.65 

3.28 

3.93 

H2O-  

0.85 

0.73 

1.56 

2.41 

2.54 

H2O+  

2.62 

3.71 

1.89 

1.92 

1.15 

CO2  

0.95 

3.59 

3.89 

4.58 

4.69 

0.31 

0.42 

0.35 

0.11 

0.06 

FeS2  

1.42 

2.35 

3.13 

Total  

100.02 

100.11 

100.43 

99.68 

99.93 

1.  Fresh  hornblende  andesite,  Thames. 

2.  Chloritized  hornblende  andesite,  Halcyon  mine. 

3.  Altered  andesite,  14  feet  from  Ophir  vein, 

Halcyon  mine. 

4.  Altered  andesite,  5  feet  from  Ophir  vein, 

Halcyon  mine. 

5.  Altered   andesite,   adjoining  Ophir  vein,    I 

Halcyon  mine.  J 


Sericite-py  rite  -  carbonate 
rock  from  the  386-foot 
level. 


482  MINERAL  DEPOSITS 

ANALYSES  OF  FRESH  AND  ALTERED  ROCKS  AT  THE  WAIHI  MINE 


2 

3 

4 

5 

6 

1. 

SiO2  '.  63.45 

58.39 

61.78 

69.35 

76.61 

85.65 

TiO2  0.75 

0.68 

0.69 

0.43 

0.28 

tr. 

A12O3  !   lu.26 

16.51 

14.89 

11.66 

8.31 

1.35 

Fe2O3  '  2.28 

2.46 

2.08 

1.53 

1.08 

0.43 

FeO  3.01 

2.98 

2.51 

1.66 

0.59 

0.21 

MnO  i     0.36 

0.32 

0.28 

0.11 

0.11 

0.12 

MgO  1.29 

1.66 

1.08 

0.46 

0.51 

0.31 

CaO  3.44 

4.08 

3.16 

2.09 

3.61 

2.56 

Na2O  2.21 

2.08 

2.18 

1.06 

0.29 

0.28 

K20  1.78 

2.89 

3.68 

3.31 

1.98 

1.41 

H2O-  1.10 

2.41 

1.89 

1.61 

0.43 

0.24 

H2O  +  2.90 

2.87 

3.05 

2.12 

1.08 

1.33 

CO2  1.08 

1.56 

2.01 

2.24 

1.87 

2.04 

P2O5  0.29 

0.31 

0.30 

0.26 

0.11 

tr. 

FeS2.. 

0.65 

1.88 

3.59 

4.69 

Total  99.20 

99.20 

100.23 

99.77 

100.45 

100.62 

1.  Fresh  hornblende  dacite,  Waihi. 

2.  Chloritized  hornblende  dacite,  45  feet  from  Empire  vein. 

3.  Altered  dacite,  30  feet  from  Empire  vein.  ) 

4.  Altered  dacite.  15  feet  from  Empire  vein. 

„  .   .       „       .          .  >  850-foot  level. 

5.  Altered  dacite,  adjoining  Empire  vein. 

6.  Replacement  ore,  Empire  vein. 

Propylitization  involves  a  distinct  hydration,  caused  by  the  de- 
velopment of  chlorite.  Where  carbonates  are  formed,  magnesia 
and  lime,  especially  the  latter,  are  somewhat  increased.  The  per- 
centages of  alkali  metals  decreases,  but  only  in  moderate  degree. 
If  sericite  has  formed,  the  potassium  may  be  somewhat  higher 
in  the  altered  rock.  Within  the  influence  of  the  vein-form- 
ing solutions  the  normal  alteration  to  sericite  and  adularia 
asserts  itself.  The  two  excellent  series  of  analyses  quoted 
above  show  a  slightly  differing  trend.  At  Thames  the  altered 
rocks  contain  10  or  11  per  cent,  of  carbonates,  while  at  Waihi 
the  carbonates  form  only  one-half  of  that  amount.  As  in  the 
California  gold-quartz  veins,  this  development  of  carbonates 
results  at  Thames  in  the  fixing  of  calcium,  while  magnesium 
shows  slight  changes.  At  Waihi  there  is  little  change  in  calcium, 
while  the  magnesium  has  been  somewhat  reduced.  In  both 


DEPOSITS  FORMED  NEAR  THE  SURFACE      483 

places  there  is  strong  leaching  of  sodium  and  progressive  accumu- 
lation of  potassium,  except  that  at  Waihi  the  potassium  finally 
diminishes  in  the  highly  quartzose  vein  material.  Iron  in  ferric 
and  ferrous  state  is  converted  to  pyrite,  but  the  total  iron  is 
not  much  increased.  At  Thames,  where  carbonates  are  abundant, 
the  silica  tends  to  decrease;  at  Waihi  the  opposite  is  true.  In 
both  places  there  is  an  apparent  decrease  in  alumina,  and  also  a 
remarkable  and  unmistakable  leaching  of  titanium,  phosphorus, 
and  manganese,  as  has  also  been  noted  by  Spurr  at  Tonopah. 

Mineralogically  the  alteration  near  the  vein  results  in  sericite, 
calcite,  siderite,  pyrite,  quartz,  and  adularia,  the  last  mineral 
in  places  forming  pseudomorphs  after  soda-lime  feldspars,  while 
it  also  occurs  in  small  fissures.  The  adularia  (valencianite) 
from  Waihi  was  analyzed  by  Finlayson  and  found  to  contain 
11.25  per  cent.  K2O  and  4.11  per  cent.  Na2O,  while  the  material 
from  Silver  City,  Idaho,  and  Tonopah,  Nevada,  previously  ex- 
amined yielded  only  a  very  small  quantity  of  Na2O. 

Stilbite  and  laumontite  have  been  identified  in  the  altered 
rocks  of  Waihi,1  and  analyses  4  (Waihi)  and  5  (Thames)  suggest 
the  possibility  of  their  presence. 

Finlayson  does  not  accept  Spurr's  view  that  the  vein-forming 
waters,  filtered  through  rock  masses,  caused  propylitization,  but 
thinks  that  this  alteration  is  due  to  solutions  or  gases  rich  in 
CO2,  which  permeated  the  rocks  immediately  after  solidification; 
the  sericite-pyrite  carbonate  rock  along  the  veins,  according  to 
Finlayson,  is  caused  by  ascending  solutions  of  a  different  class. 

Metasomatic  Processes  at  Tonopah. — During  the  alteration  of 
the  trachyte  near  the  veins  at  Tonopah,  Nevada,  2biotite  and  horn- 
blende, have  usually  been  completely  destroyed;  their  outlines 
are  marked  by  aggregates  of  sericite,  quartz,  pyrite,  and  siderite, 
the  latter  two  often  crystallizing  together.  The  primary  an- 
desine-oligoclase  has  changed  to  quartz  and  sericite  or  to  adularia; 
the  latter  two  are  not  often  associated  in  the  same  specimens. 
The  microlitic  groundmass  is  largely  altered  to  fine-grained 
quartz  with  fibers  of  sericite;  pyrite  and  siderite  are  disseminated. 
Apatite  and  zircon  are  residual  minerals.  Kaolin,  when  present, 
is  believed  to  result  from  the  alteration  of  sericite  by  descending 
solutions. 

1  P.  G.  Morgan,  Trans.,  Aust.  Inst.  Min.  Eng.,  vol.  8,  1902,  p.  186. 

2  J.  E.  Spurr,  Prof.  Paper  42,  U.  S.  Geol.  Survey,  1905;  Econ.  Geol.,  vol. 
10,  1915,  pp.  713-769. 


484 


MINERAL  DEPOSITS 


At  a  distance  from  the  larger  veins  a  propylitic  type  of  altera- 
tion appears,  in  which  calcite  and  chlorite,  together  with  pyrite 
and  siderite,  are  the  important  minerals.  The  feldspars  are 
altered  to  calcite  with  a  little  quartz;  epidote  is  not  abundant. 
There  are  transitions  between  the  propylitic  and  the  sericitic 
alteration,  and  according  to  Spurr  they  were  produced  by  the 
same  waters.  Near  the  veins  these  waters  introduced  silica, 
potassium,  and  metallic  sulphides;  as  they  penetrated  farther 


ANALYSES  OF  FRESH  AND  ALTERED  ROCKS, 
TONOPAH,  NEVADA 


-  - 

1 

2 

3 

4 

5 

SiO2 

67  69 

55  60 

72  98 

73  50 

91  40 

TiO2.. 

0  72 

0  44 

0  47 

0  07 

A1203  
Fe2O3  

17.67 
2  43 

16.70 

2  23 

" 

14.66 
1  01 

14.13 
1  51 

4.31 
0.77 

FeO  

0  80 

3  51 

0  16 

0  26 

0.11 

MgO  
CaO 

0.88 
0  45 

2.60 
4  27 

0.33 
0  18 

0.21 
0  12 

0.18 
none 

BaO 

0  12 

0  19 

0  02 

Na2O 

2  54 

4  08 

none 

0  24 

0  06 

K2O  
H2O-.    . 

5.11 

3.17 

0  88 

6.03 
0  97 

5.11 
1  07 

1.68 
0  46 

H,0  +  
C02  

P2OB 

3.06 
2.76 
0  28 

2.95 
none 
0  16 

2.81 
none 
0  09 

0.98 
none 
0  04 

97.57 

99.98 

99.87 

'99.71 

2100.08 

1.  Partial    analysis    of    relatively    fresh    "Mizpah"    trachyte.     Booth, 
Garrett  and  Blair,  analyst. 

2.  Altered   andesite,    Siebert   shaft.     Propylitic    alteration    to    quartz, 
calcite,  chlorite,  and  sericite.     George  Steiger,  analyst. 

3.  Altered   trachyte,    Mizpah  mine.     No   original   minerals   remaining. 
Sericitic  alteration.     George  Steiger,  analyst. 

4.  Altered  trachyte,  Mizpah  Hill.     Typical  alteration  to  adularia  with 
a  little  sericite.     George  Steiger,  analyst. 

5.  Ore  material  of   Mizpah  vein.     Dense   quartzose  rock  mixed  with 
kaolin-like  material.     Silicified  trachyte.     George  Steiger,  analyst. 

1  Also  0. 17  SO3  and  0.03  S. 

2  Also  0.12  ZrO2  and  0.06  MnO. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      485 

from  these  channels  their  metal  contents  were  exhausted,  while 
silica  and  potassium  were  still  introduced;  finally  only  carbon 
dioxide  and  hydrogen  sulphide  were  left  in  the  cooling  waters, 
which,  therefore,  had  little  to  precipitate  and  small  power  of 
abstracting.  The  wall  rock  acted  as  a  screen  for  the  traversing 
solutions. 

As  noted  above,  these  views  are  not  entirely  accepted  by 
Finlayson. 

The  most  prominent  features  of  the  alteration,  as  shown  by 
analyses,  are  the  almost  complete  removal,  adjacent  to  the  veins, 
of  ferrous  iron,  calcium,  magnesium,  and  sodium  and  the  partial 
removal  of  ferric  oxide.  Even  the  resistant  apatite  and  rutile 
seem  to  have  been  dissolved  to  some  degree,  as  shown  by  the 
relations  of  phosphorus  and  titanium.  On  the  other  hand, 
there  is  a  decided  increase  of  silica,  and  the  potassium  has  in- 
creased. There  is  a  moderate  hydration,  but  no  carbonates 
appear. 

A  later  mineralization,  which  affected  the  later  andesite  at 
Tonopah,  is  materially  different;  the  waters  by  which  it  was 
effected  appear  to  have  contained  practically  no  gold  and  silver. 
The  course  of  the  alteration1  involves  no  silicification  and  practic- 
ally no  change  in  calcium.  The  sodium  is,  as  before,  almost  wholly 
removed,  and  likewise  a  large  part  of  the  potassium.  Carbonates 
are  present  in  abundance,  with  pyrite,  and  some  zeolite  is  prob- 
ably present,  possibly  also  some  talc  and  hydrargillite. 

The  Development  of  Kaolin. — It  has  been  stated  above  that 
kaolin  in  the  altered  rocks  of  mineral  deposits  results  mainly 
from  the  leaching  by  surface  waters  containing  free  sulphuric 
acid,  and  that  this  mode  of  alteration  is  frequently  superimposed 
upon  the  products  of  chloritic  and  sericitic  alteration  by  ascend- 
ing waters.  The  sulphuric  acid  attacks  and  removes  all  calcium, 
magnesium,  sodium,  and  potassium;  and  the  final  result  is  a  mix- 
ture of  kaolin  and  quartz.  Below  the  influence  of  free  oxygen  sul- 
phides may  be  deposited  with  the  kaolin;  pyrite,  more  frequently 
marcasite  in  arborescent  forms,  chalcocite,  covellite  and  rich 
silver  ores  like  argentite  and  stephanite  may  also  develop  (see 
discussion  of  sulphide  enrichment,  Chapter  31).  In  places — 
for  instance,  at  De  Lamar,  Idaho — this  kaolin  may  contain  much 
gold  in  extremely  finely  divided  state,  undoubtedly  concentrated 
by  secondary  reactions. 

1  Prof.  Paper  42,  U.  S.  Geol.  Survey,  1905,  p.  241. 


486  MINERAL  DEPOSITS 

Metasomatic  Processes  at  Silverton,  Colorado. — The  process  of 
kaolinization  is  well  described  in  Ransome's  report  on  the 
Silverton  district,  Colorado.1  Crystallized  kaolinite — a  rare 
occurrence — was  found  in  the  National  Belle  mine,  but  is  here, 
too,  later  than  the  ore. 

The  normal  alteration  at  Silverton  is  of  propylitic  aspect, 
changing  near  the  veins  to  sericitic  facies.  In  a  series  of  rocks 
occurring  in  the  Silver  Lake  basin  the  andesite  breccia  150  feet 
from  the  vein  is  only  slightly  altered  by  the  destruction  of  the 
dark  silicates  and  by  the  beginning  of  replacement  of  feldspars 
by  sericite  (perhaps  with  some  kaolin)  and  calcite.  At  100  feet 
from  the  vein  the  quantity  of  chlorite  and  calcite  increases. 
Fifty  feet  away  from  the  vein  the  breccia  structure  is  still  visible, 
but  the  rock  is  wholly  re-crystallized  to  quartz,  chlorite,  sericite, 
calcite,  and  rutile,  with  residual  apatite.  Two  feet  from  the 
vein  there  is  less  chlorite,  and  the  rock  consists  mainly  of  sericite 
and  quartz,  with  some  grains  of  galena.  Close  to  the  wall  there 
is  but  little  chlorite,  and  considerable  pyrite  has  been  introduced. 
This  general  process  corresponds  fairly  closely  to  that  at  Tonopah. 

In  the  same  region,  at  Red  Mountain,  the  alteration  of  the 
rocks  is  carried  to  its  ultimate  conclusion.  The  mine  waters 
show  the  presence  of  free  sulphuric  acid  and  alumina  and  the 
white  kaolinized  rock  at  the  surface  shows  the  following  com- 
position calculated  from  the  analysis. 

COMPOSITION  OF   ALTERED    ROCK  AT    RED  MOUNTAIN,  NEAR   NATIONAL 
BELLE  MINE 

Quartz GO .  9 

Kaolinite 26 . 3 

Pyrite 5.6 

Diaspore 3.8 

Sericite 0.6 

Apatite 0.6 

Rutile 0.6 

98.4 

A  still  more  advanced  silicification  is  shown  by  the  following 
analysis  of  an  altered  rock  at  the  White  Cloud  Mine  in  the  same 
region,  from  which  the  mineral  composition  of  the  rock  may  be 
calculated  as  follows: 

1  F.  L.  Ransome,  Economic  geology  of  the  Silverton  quadrangle,  Bull. 
182,  U.  S.  Geol.  Survey,^1901. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      487 

Quartz , 78.5 

Kaolin  minerals 16.0 

Pyrite... 3.4 

Rutile 0.6 

Sulphates  (Fe,  Ca,  Ba) 1.3 


99  8 

This  extreme  mode  of  alteration  by  sulphuric  acid  solutions 
results  in  the  almost  total  elimination  of  calcium,  magnesium, 
alkali  metals,  and  carbon  dioxide;  its  operation  is  similar  to  that 
of  weathering  by  oxygenated  waters,  although  in  that  process,  of 
course,  pyrite  cannot  form.  It  differs  radically  from  the  serici- 
tization  and  carbonatization  described  above. 

Summary. — Summing  up  we  may  say  that  sericite  and  car- 
bonates mark  the  alteration  by  ascending  metalliferous  solutions 
at  intermediate  and  moderate  depths;  that  nearer  the  surface 
adularia  appears  as  an  important  metasomatic  mineral  in  addi- 
tion to  sericite;  and  that  still  nearer  the  surface  or  under  the  influ- 
ence of  descending  sulphuric  acid  solutions  we  find  kaolin,  or, 
wher.e  sulphuric  acid  is  present  in  abundance,  alunite. 

QUICKSILVER  DEPOSITS 

The  Ores  and  Their  General  Occurrence. — The  principal 
quicksilver  ore  is  cinnabar  (HgS),  which  contains  86.2  per  cent, 
mercury.  A  black  modification  of  this  mineral,  called  meta- 
cinnabar,  rarely  occurs  in  large  amounts  and  is  probably,  a  sec- 
ondary sulphide  deposited  by  descending  waters.1  Native 
quicksilver,  silver  and  gold  amalgam,  calomel  (HgCl),  quicksilver 
oxide  (montroydite),  and  several  oxychlorides  are  evidently 
secondary  minerals,  developing  from  the  sulphide  (p.  892). 
Primary  but  rare  minerals  are  the  black  telluride  of  quicksilver, 
coloradoite;  the  selenide,  tiemannite;  the  sulphoselenide,  onofrite; 
and  other  still  rarer  combinations  of  the  selenides  of  copper,  lead, 
and  quicksilver.  Mercurial  tetrahedrite  is  not  uncommon,  and 
some  varieties  contain  as  much  as  17  per  cent,  quicksilver, 
although  the  percentage  is  usually  much  smaller.  In  smaller 
quantities  this  metal  is  also  sometimes  present  in  other  minerals, 
for  instance,  in  the  native  silver  of  Kongsberg,  Norway,  and  in  the 
dyscrasite  of  the  silver-bearing  veins  of  Cobalt,  Ontario  (p.  626). 
The  occurrence  of  quicksilver  minerals  is  by  no  means  confined 
1  E.  T.  Allen  and  J.  L.  Crenshaw,  The  sulphides  of  zinc,  cadmium  and 
mercury,  Am.  Jour.  Sci.,  4th  ser.,  vol.  34,  1912,  pp.  367-383. 


488  MINERAL  DEPOSITS 

to  any  certain  kind  of  deposits  or  to  any  given  age  or  epoch  of 
metallization.  However,  such  minerals  are  not  known  to  occur 
in  deposits  of  distinctly  igneous  origin,  nor  in  pegmatite  dikes, 
nor  in  veins  of  the  deepest  zone.  High  temperature  is  evidently 
unfavorable  for  their  development.  The  most  noteworthy 
occurrence  is  that  of  coloradoite  in  the  gold  telluride  veins  of 
western  Australia,  which  contain,  among  other  minerals,  mag- 
netite and  tourmaline,  indicating  deposition  at  fairly  high  tem- 
perature. In  gold-bearing  quartz  veins  of  the  ordinary  type, 
believed  to  have  been  formed  at  a  considerable  depth,  but  at  con- 
siderably lower  temperature  and  pressure  than  pegmatite  dikes, 
cinnabar  is  not  an  uncommon  mineral.  It  occurs  in  several 
of  these  veins  in  California,1  as  well  as  in  the  similar  veins  of 
central  Idaho,  and  is  frequently  found  in  the  placers  derived  from 
the  erosion  of  these  veins,  as  at  Stanley  Basin  and  Warren, 
Idaho.  In  northeastern  Oregon  the  gold-quartz  veins  contain 
mercurial  tetrahedrite,  as  well  as  secondary  cinnabar  formed 
from  that  mineral.2  In  the  placers  below  the  veins  of  Susan- 
ville,  in  the  same  region,  pebbles  showing  masses  of  cinnabar  in- 
closed in  massive  white  vein  quartz  have  been  found.  One  often 
finds  apparently  reliable  statements  that  during  the  process  of 
amalgamation  and  refining  of  the  gold  from  such  deposits  more 
quicksilver  was  recovered  than  was  added  for  metallurgical 
purposes.  In  small  quantities  cinnabar  occurs  in  the  lead  and 
zinc  deposits  of  Monteponi,  in  Italy,  and  at  Santander,  in  Spain. 
Many  occurrences  of  mercurial  tetrahedrite  in  Europe  and  South 
America  have  been  described. 

In  few  of  these  deposits  are  the  mercurial  minerals  abundant 
enough  to  constitute  an  ore,  and  in  the  majority  of  the  deposits 
formed  at  a  considerable  depth  the  metal  is  apparently  entirely 
absent.  The  commercial  quicksilver  ores  are  practically  con- 
fined to  a  small  and  well-defined  group  of  deposits,  which  will 
be  described  in  the  following  pages  and  which  are  of  particular 
interest  because  their  genesis  can  be  fairly  accurately  ascertained. 

A  scant  association  of  ore  minerals  characterizes  these  deposits. 
Besides  cinnabar  and  metacinnabar,  as  well  as  a  few  minerals 
derived  from  the  decomposition  of  the  sulphide,  they  contain 

1  H.  W.  Turner,  Am.  Jour.  Sci.,  3d  ser.,  vol.  47,  1894,  p.  467. 

H.  D.  McCaskey,  Min.  Res.,  U.  S.  Geol.  Survey,  pt.  1,  1910,  p.  905. 

2  W.  Lindgren,  The  gold  belt  of  the  Blue  Mountains  of  Oregon,  Twenty- 
second  Ann.  Rept,,  U.  S.  Geol.  Survey,  pt.  2,  1901,  pp.  604,  708. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      489 

almost  invariably  pyrite  or  marcasite  and  frequently  stibnite,  but 
rarely  any  of  the  sulphides  of  the  base  metals  so  common  in  ore 
deposits.  Among  gangue  minerals  we  have  predominatingly  opal, 
chalcedony,  and  quartz,  also  calcite  and  dolomite,  more  rarely  ba- 
rite,  and  very  seldom  fluorite ;  zeolites  are  of  exceptional  occurrence. 

As  to  form  and  structure  the  ores  occur  in  irregular  and  "cham- 
bered" veins  and  brecciated  zones,  often  also  as  "stockworks" 
of  minute  seams,  or  as  disseminations  in  more  or  less  porous 
rocks.  The  irregularity  and  brecciated  character  of  the  deposits 
suggest  their  development  near  the  surface,  a  conclusion  that  is 
often  justified  by  other  geological  evidence. 

As  to  association  the  deposits  occur  in  rocks  of  any  kind  and 
any  age,  but  almost  always  in  close  connection  with  effusive 
rocks  or  in  regions  of  volcanic  activity.  Hot  springs  are  frequent 
in  many  quicksilver  districts,  and  this  conspicuous  association  has 
led  to  the  view  that  the  metal  is  a  last  product  of  differentiation 
in  many  magmas  nearing  the  surface  and  that  the  hot  springs  take 
it  into  solution  to  deposit  it  near  their  points  of  issue.  This 
theory  is  strongly  fortified  by  the  discovery  that  many  springs 
in  volcanic  regions  are  depositing  cinnabar  at  the  present  time. 

Although  most  of  the  quicksilver  deposits  have  been  formed 
at  a  relatively  late  time  and  in  connection  with  the  eruption  of 
Tertiary  and  recent  lavas,  it  does  not  necessarily  follow  that 
their  development  has  been  confined  to  late  geological  time. 
Older  surface  eruptions  were  undoubtedly  accompanied  in  places 
by  the  formation  of  quicksilver  deposits,  but  as  these  were  near 
the  surface  they  were  easily  eroded.  Attention  has  already  been 
called  to  this  elsewhere,1  and  it  has  been  suggested  that  the  de- 
posits at  Almaden,  in  Spain,  and  those  in  the  German  Palatinate 
may  belong  to  such  more  remote  volcanic  epochs. 

Distribution,  Production  and  Use. — Quicksilver  deposits  are 
widely  distributed,  although  the  main  production  of  the  metal 
comes  from  a  few  occurrences.  G.  F.  Becker,  who  first  studied 
the  occurrence  of  these  ores  concluded  that  their  distribution 
follows  the  main  structural  lines  of  the  continents,  especially  in 
the  Pacific  region  and  in  the  Alpine-Himalayan  chain.  While 
this  is  apparently  true,  it  is  more  correct  to  say  that  the  ores  fol- 
low the  belts  of  Tertiary  and  Quaternary  eruptions,  especially 
along  the  important  fracture  lines  of  the  globe. 

The  Coast  Range  of  California,  in  which  the  erogenic  move- 

1  Beyschlag,  Krusch,  and  Vogt,  Die  Lagerstatten,  etc.,  vol.  1,  1910,  p.  454. 


490  MINERAL  DEPOSITS 

ments  are  largely  post-Miocene,  contain  a  belt  of  quicksilver 
deposits  several  hundred  miles  in  length,  from  which  at  one  time 
a  large  production  was  obtained.1  A  second  belt,  perhaps  less 
well  defined  and  certainly  less  productive,  extends  from  north 
to  south  over  a  similar  length  in  western  Nevada. 

The  Mexican  area,  which  in  spite  of  comprising  many  deposits, 
yields  only  a  slight  production,  begins  in  western  Texas  in  the 
Terlingua  district2  and  may  be  considered  to  end  in  the  State  of 
San  Luis  Potosi,  Mexico.3 

Farther  south,  in  Peru,4  quicksilver  deposits  appear  again. 
The  Yauli  and  Huancavelica  districts  are  best  known;  the  latter 
at  one  time  was  an  important  producer. 

In  Europe  an  extensive  region  in  Italy,  Austria,  and  adjacent 
countries  contains  quicksilver  deposits;  this  area  includes  the 
deposits  of  Tuscany,5  Vallalta-Sagron,6  Idria  and  vicinity,7 

1  G.  F.  Becker,  The  quicksilver  deposits  of  the  Pacific  slope,  U.  S.  Geol. 
Survey,  Mon.  13,  1888. 

W.  Forstner,  The  quicksilver  resources  of  California,  Bull.  27,  California 
State  Min.  Bur.,  1903. 

H.  D.  McCaskey,  Mineral  Resources,  U.  S.  Geol.  Survey,  pt.  1,  1910- 
1916,  particularly  in  issue  of  1911,  with  literature. 

2W.  B.  Phillips,  The  quicksilver  deposits  of  Brewster  County,  Texas, 
Econ.  Geol.,  vol.  1,  1906,  pp.  155-162. 

H.  W.  Turner,  The  Terlingua  quicksilver  deposits,  Econ.  Geol.,  vol.  1, 
1906,  pp.  265-281. 

3  J.  .D.  Villarello,  Genesis  de  los  yacimientos  mercuriales  de   Palomas  y 
Huitzuco,  Mem.  Soc.  Ant.  Alzate,  vol.  20,  1903,  pp.  95-136. 

J.  D.  Villarello,  Descripci6n  de  los  criaderos  de  mercurio  de  Chiquilistan, 
Jalisco,  Idem,  vol.  20,  1904,  pp.  389-397. 

P.  A.  Babb,  Dulces  Nombres  quicksilver  deposit,  Mexico,  Eng.  and 
Min.  Jour.,  Oct.  2,  1909. 

F.  J.  H.  Merrill,  The  mercury  deposits  of  Mexico,  Mining  World,  vol.  24, 
1906,  p.  244. 

4  A.  F.  Umlauff,  El  cinabrio  de  Huancavelica,  Boi.  17,  Cuerpo  de  In- 
genieros  de  Minas,  Lima,  1904,  p.  61. 

6  V.  Spirek,  Die  Zinnobererzvorkommen  am  Monte  Amiata,  Zeitschr. 
prakt.  GeoL,  1897,  pp.  369-374;  idem,  1902,  pp.  297-299. 

B.  Lotti,  II  campo  cinabrifero  dell'  Abbadia,  etc.,  Rass.  Min.,  vol.  7, 
1898,  No.  11;  Zeitschr.  prakt.  Geol,  1898,  p.  258.  See  also  Rass.  Min., 
vol.  17,  1902,  No.  10. 

6  A.  Rzehak,  Die  Zinnoberlagerstatte  von  Vallalta-Sagron,  Zeitschr. 
prakt.  GeoL,  1905,  pp.  325-330. 

7F.  Kossmat,  Ueber  die  geologischen  Verhaltnisse  des  Bergbaugebietes 
von  Idria,  Jahrb.  K.  k.  geol.  Reichsanstalt,  vol.  49,  1899,  pp.  259-286. 

Geologisch-bergmannische  Karten,  etc.,  von  Idria.  Text  by  Plaminek. 
Published  by  the  Agricultural  Department,  Vienna,  1893.  (Literature.) 


DEPOSITS  FORMED  NEAR  THE  SURFACE      491 

Avala,1  in  Servia,  and  less  important  occurrences  in 
Bosnia. 

Isolated  yet  highly  productive  deposits  occur  in  Almaden,  in 
Spain.2 

Some  deposits  have  been  found  on  the  western  side  of  the 
Pacific,  mainly  in  Japan,  China,  Borneo,  Australia,  and  New 
Zealand. 

Before  the  war,  in  1913,  the  world's  production  of  quicksilver 
was  124,654  flasks  at  75  pounds.  Of  this  Spain  produced  43,799, 
Italy  29,513,  Austria  26,720  and  United  States  20,213.  In 
1917,  the  domestic  production  was  36,315  flasks.  California 
yielded  24,251  flasks.  The  average  price  in  1913  was  $40  per 
flask.  In  1917  the  average  for  the  year  was  $106.  The  principal 
use  of  quicksilver  is  for  gold  amalgamation,  drugs,  paints  and 
mercurial  fulminate  (Hg  (ONC)2)  an  explosive  used  for  priming 
shells. 

Geological  Features. — The  comparative  youth  of  the  deposits 
is  attested  by  the  fact  that  many  of  them  are  found  in  sedimen- 
tary or  volcanic  rocks  of  Tertiary  or  Quaternary  age.  They  are 
not  confined  to  these  rocks,  however,  and  may,  in  fact,  occur  in 
rocks  of  any  composition  or  age.  Sandstones,  shales,  limestone, 
serpentine,  granite,  andesite,  rhyolite,  or  basalt  may  harbor  the 
ores,  and  the  character  of  the  surrounding  rocks  seems  to  have 
little  influence  on  the  value  of  the  deposits. 

The  California  belt  contains  ores  in  Jurassic,  Cretaceous,  and 
Tertiary  sandstones  and  shale,  in  serpentine,  and  in  late  Tertiary 
or  Quaternary  basalt  and  andesite.  In  the  Nevada  belt  the  ores 
occur  in  Triassic  strata  in  Paleozoic  limestone  and  dolomite  or 
more  commonly  in  rhyolite,  probably  of  middle  Tertiary  age. 

In  Texas  and  Mexico  the  ore-bodies  are  in  Cretaceous  strata  or 
in  the  Tertiary  andesite,  basalt,  and  rhyolite  which  break  through 
them.  The  Peruvian  deposits  are  in  Jurassic  beds  or  in  Tertiary 
volcanic  rocks. 

In  the  Adriatic  region  of  Europe  the  ores  occur  in  rocks  of 
many  kinds :  In  Tuscany,  Mesozoic  and  Tertiary  limestones  and 
sandstones  with  trachyte  are  the  enclosing  rocks;  at  Idria,  the 

1  H.   Fischer,   Die  Quecksilberlagerstatten  am   Avala-Berge  in  Serbien, 
Zeitschr.  prakt.  Geol,  1906,  p.  245. 

2  There  is  no  modern  and  detailed  description  of  this  important  deposit. 
The  best  account  is  found  in    Beck's  "Lehre  von  den  Erzlagerstatten," 
vol.  1,  1909,  pp.  519-522. 


492  MINERAL  DEPOSITS 

disturbed  beds  of  the  Alpine  Triassic;  at  Avala,  the  serpentine 
and  probably  Cretaceous  limestone  cut  by  trachytic  dikes. 
De  Launay1  has  shown  that  these  Adriatic  ores  coincide  in  their 
extension  with  Tertiary  eruptives,  and  that  in  all  probability, 
even  where  these  eruptives  are  locally  absent,  as  at  New  Idria, 
the  deposits  owe  their  origin  to  the  after-effects  of  this  igneous 
activity  in  the  form  of  ascending  springs. 

In  the  Donetz  basin  in  southern  Russia2  the  cinnabar  ores  lie 
in  Carboniferous  strata  and  have  no  apparent  connection  with 
igneous  rocks. 

Mineralogy  of  Quicksilver  Ores. — Cinnabar  while  usually 
massive  sometimes  forms  well-defined  but  small  crystals;  these 
are  especially  common  in  porous  rocks  like  sandstone  and  tuff. 

Aside  from  pyrite  and  the  more  common  marcasite  the  ore 
mineral  most  generally  accompanying  cinnabar  is  stibnite;many 
stibnite  veins,  it  should  be  added,  also  contain  some  cinnabar. 
Quartz  is  usually  present,  but  far  more  commonly  the  silica 
appears  as  chalcedony  or  opal.  In  the  California  occurrences 
opal  is  particularly  abundant  and  here,  as  at  Avala,  much  of  it  is 
a  product  of  the  replacement  of  serpentine.  In  California  the 
cinnabar  is  not  often  found  in  the  opal  itself,  but  rather  in  the 
veinlets  of  quartz  or  chalcedony  traversing  it.  Calcite  is  not  an 
uncommon  gangue  material,  and  in  the  Coast  Ranges  of  Cali- 
fornia many  of  the  deposits  are  accompanied  by  calcium-mag- 
nesium carbonates  derived  by  replacement  from  serpentine  or 
allied  rocks. 

Among  the  sulphates  barite  is  fairly  abundant,  and  at  most 
places  there  is  also  more  or  less  gypsum,  which  may  often  be  a 
product  of  primary  deposition,  although  it  would  naturally  also 
be  generated  by  the  effect  of  decomposition  of  pyrite  in  a  cal- 
careous gangue.  Fluorite  is  rare,  but  is  recorded  from  Guad- 
alcazar  and  Idria.  In  many  deposits,  particularly  those  of  Idria 
and  the  California  belt,  hydrocarbons  are  characteristic;  they  are 
probably  derived  from  the  adjacent  sedimentary  beds,  but  are 
believed  to  have  exerted  some  influence  in  the  precipitation  of 
cinnabar  from  its  solutions.  Inflammable  gases,  mainly  hydro- 

1 L.  De  Launay,  La  metallogenie  de  1'Italie,  International  Geological 
Congress,  Mexico,  1906. 

2  Zeitschr.  prakt,  Geol,  1894,  p.  427,  after  Kulibin.  Tschernyschew  and 
Loutouguin,  Guide  des  excursions  du  VII  Congress  geol.  internat.,  No.  16. 
1897,  pp.  36-45. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      493 

carbons,  are  reported  from  several  localities  in  California,  notably 
New  Idria. 

Zeolites  are  not  unknown  in  quicksilver  deposits;  chabazite 
colored  by  cinnabar  is  mentioned  from  Almaden,  in  Spain,  and 
apophyllite  from  New  Almaden,  in  California. 

Alunite  has  been  found  associated  with  opal  and  cinnabar  in 
rhyolites  of  Nye  County,  Nevada,  near  Bullfrog.1 

Millerite,  or  sulphide  of  nickel,  is  not  uncommon  in  the  Cali- 
fornia occurrences;  this  metal  is  probably  derived  from  adjacent 
bodies  of  serpentine  or  peridotite. 

The  rare  tiemannite  and  onofrite  were  at  one  time  obtained 
near  Marysvale,  in  southern  Utah,  and  some  quicksilver  was 
recovered  from  such  ore.  The  minerals  occurred  as  fissure 
filling  in  limestone,  but  in  a  district  of  volcanism  with  rhyolitic 
and  andesitic  flows.  Selenides  are  also  reported  from  Guadalca- 
zar,  in  San  Luis  Potosi,  Mexico.  It  is  probable  that  careful 
examination  would  disclose  the  presence  of  selenides  at  many 
other  places.  The  mineral  livingstonite  (HgSb4S7)  occurred  in 
considerable  quantities  at  Huitzucq,  Guerrero,  Mexico.  Regard- 
ing oxidation  of  quicksilver  ores  see  p.  892. 

Quicksilver  ores  should  contain  at  least  0.5  per  cent,  of  the 
metal.  The  richest  ores  are  those  of  Almaden,  Spain,  which 
are  said  to  average  8  per  cent.,  while  at  New  Idria  ores  are  mined 
which  contain  less  than  0.5  per  cent. 

Structure. — In  their  structural  relations  the  majority  of  quick- 
silver deposits  clearly  indicate  their  origin  near  the  surface. 
Sharply  defined  continuous  fissure  veins  occur  only  exceptionally; 
far  more  common  are  irregular  and  "chambered"  veins — that  is, 
fissures  accompanied  by  brecciated  masses  in  which  the  ore 
minerals  have  lodged  (Fig.  150).  Another  common  mode  of 
occurrence  is  as  disseminations  in  porous  rocks,  like  sandstone 
and  tuffs,  or  again  as  "stockworks,"  the  ore  minerals  filling 
little  crevices  and  fissures  in  limestone,  serpentine,  or  other  rocks. 

Cinnabar  is  often  deposited  in  open  cavities  or  in  pores  or  in 
the  soft  mud  of  altered  rocks.  It  does  not  replace  limestone  on 
the  scale  of  the  lead  deposits,  but  that  replacement  has  occasion- 
ally occurred  seems  to  be  beyond  doubt. 

A  large  number  of  deposits  have  been  found  to  cease  or  become 
impoverished  at  a  depth  of  a  few  hundred  feet.  In  contrast  to 

1  Adolph  Knopf,  Some  cinnabar  deposits  of  western  Nevada,  Butt.  620, 
U.  S.  Geol.  Survey,  1915,  pp.  59-68. 


494 


MINERAL  DEPOSITS 


this,  the  celebrated  mines  of  Almaden,  Spain,  are  said  to  have 
found  richer  ore  in  depth,  and  the  workings  have  now  attained  a 
depth  of  1,300  feet. 

The  deposits  at  Almaden  occur  in  three  beds  of  steeply  dipping 
Silurian  quartzite  separated  by  bituminous  slates.  In  part  the 
cinnabar  may  occur  as  filling  of  the  pores  of  the  rock,  as  G.  F. 
Becker1  suggests,  but  Beck2  has  shown  convincingly  that  there 
has  also  occurred  an  actual  replacement  of  the  sandstone  grains 


FIG.  150. — Diagrammatic  vertical  cross  section  of  the  Redington  cinnabar 
mine,  California,  showing  brecciated  ore  chambers  near  surface,  changing 
in  depth  to  more  regular  fissures  filled  with  cinnabar.  Total  depth  about 
600  feet,  m,  Metamorphic  rock;  n,  Cretaceous  sandstone.  After  G.  F. 
Becker,  U.  S.  Geol.  Survey. 

by  the  ore  mineral  (Fig.  151).  The  ore-bodies  are  as  much  as 
45  feet  in  thickness,  and  the  average  tenor  of  the  ores  is  unusually 
high;  they  are  said  to  contain  8  per  cent,  quicksilver.  Granite 
and  diabase  break  through  the  sedimentary  series,  but  the 
geological  history  of  the  deposit  is  too  imperfectly  known  to 
draw  safe  conclusions  as  to  its  age  or  mode  of  origin.  Almaden 
is  the  richest  and  most  productive  quicksilver  region  in  the 
world.  The  value  of  its  metallic  product,  according  to  the 
handbook  of  Beyschlag,  Krusch,  and  Vogt,  during  the  period 
1564  to  1907,  is  estimated  at  212  million  dollars. 

1  G.  F.  Becker,  Mon.  13,  U.  S.  Geol.  Survey,  1888,  p.  399. 

2  R.  Beck,  Lehre  von  den  Erzlagerstatten,  vol.  1,  1909,  p.  521. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      495 

At  Idria,  in  southern  Austria,  is  located  another  of  the  great 
quicksilver  mines  of  the  world.  The  ores  are  contained  in  Triassic 
beds  of  shale,  marl,  and  dolomite;  they  are  apparently  connected 
with  and  in  part  occur  in  great  overthrusts  and  faults.  The 
ore-bodies,  which  apparently  do  not  extend  below  a  depth  of 
1,000  feet,  in  places  follow  the  stratification  and  were  formerly 
believed  to  be  of  syngenetic  origin.  The  ores  are  usually  desig- 
nated as  "impregnations"  in  shale  or  marls,  but  small  veins 
and  stockworks  are  also  found,  especially  in  the  dolomite.  The 
age  of  deposition  is  certainly  post-Cretaceous,  probably  Tertiary. 


FIG.  151. — Rich  ore,  Almaden,  Spain,  showing  cinnabar  between  grains 
of  quartzite  and  formed  by  replacement  hi  quartz,  c,  Cinnabar;  s,  sericite; 
p,  pyrite;  z,  zircon.  Magnified  70  diameters.  After  R.  Beck. 

Schrauf,  who  has  given  long  study  to  Idria,  believes  that  the 
ore  occurring  in  the  dolomite  is  a  later  migration  from  the  some- 
what older  deposit  in  the  shales. 

The  California  region  offers  types  of  almost  all  the  various 
structural  developments.  In  the  region  north  of  San  Francisco, 
near  Clear  Lake,  serpentine,  radiolarian  cherts,  altered  Franciscan 
sandstone  (Jurassic?),  and  Cretaceous  sandstones  prevail;  the 
rocks  are  greatly  shattered  and  late  Tertiary  to  Quaternary 
andesites  and  basalts  break  through  them.  The  occurrences  of 


496 


MINERAL  DEPOSITS 


cinnabar  are  numerous,  and  some  of  them  have  yielded  a  large 
production,  but  the  ore-bodies  generally  become  impoverished 
at  a  depth  of  a  few  hundred  feet.  The  deposits  form  fissure 
veins  largely  filled  with  attrition  material,  and  this  is  impregnated 
with  cinnabar,  pyrite,  opaline  silica,  and  calcite.  Masses  of  ore 
often  extend  into  the  country  rock  from  these  fissures  (Fig.  152). 
Or  again,  as  in  the  Great  Eastern  mine,  the  ore  forms  tabular 
masses  between  serpentine  and  sandstone,  or  pipes  in  opaline  or 
chalcedonic  rocks  between  the  same  formations  (Fig.  153),  or 
finally  it  may  be  developed  on  the  contact  of  basalt  and  sand- 
stone. The  Redington  mine  (Fig.  150)  was  operated  on  a  large 


FIG.  152. — Vertical  cross  section  through  workings  of  Napa  Consolidated 
mine,  California.  Irregular  veins  in  horizontal  Cretaceous  sandstone, 
widening  to  chambers  along  bedding  planes.  After  G.  F.  Becker,  U.  S. 
Geol.  Survey. 

chambered  deposit  at  the  surface  which  was  found  to  be  continued 
below  by  more  regular  and  narrow  veins.  Throughout  this 
region  hot  springs  are  found  in  and  around  the  ore  deposits. 

The  great  mine  of  New  Almaden,  in  Santa  Clara  County,  south 
of  San  Francisco,  is  opened  in  shattered  greenstone,  serpentine, 
radiolarian  chert,  and  sandstone  of  the  Franciscan  series.  Con- 
sidered in  detail  the  ore-bodies  are  stockworks,  but  they  are 
arranged  along  definite  fissures  and  have  on  the  whole  a  vein- 
like  character.  There  are  two  main  fissures  of  varying  dip  along 
and  from  which  the  ore-bodies  extend.  The  hanging  wall  is 


DEPOSITS  FORMED  NEAR  THE  SURFACE      497 

usually  an  impermeable,  slickensided  clay.  There  are  no  hot 
springs  and  no  eruptives  in  the  vicinity.  The  mine  has  been 
opened  to  a  depth  of  2,100  feet  and  a  continuous  ore-body 
extended  down  to  the  1,600-foot  level.  During  the  last  few  years 
little  work  has  been  done  in  the  lower  levels. 

At  New  Idria,  at  the  south  end  of  the  Mt.  Diablo  Range,  impor- 
tant deposits  have  been  worked  and  the  larger  part  of  the  produc- 
tion*of  California  is  now  derived  from  this  mine.  The  rocks  are 
disturbed  greenstones  and  sandstones  of  the  Franciscan  series, 
imconformably  covered  by  tilted  Chico  (Cretaceous)  and  Tejon 


FIG.  153. — Vertical  cross  section  of  the  Great  Eastern  mine,  California, 
showing  pipe  of  cinnabar  contained  in  opaline  replacement  gangue.  After 
G.  F.  Becker,  U.  S.  Geol.  Survey. 

(Eocene)  sandstone.  The  ores  appear  in  three  forms — as  normal 
veins,  as  irregular  stockworks,  and  as  impregnations  in  sand- 
stone. The  mine  is  opened  by  tunnels,  the  lowest  level  being  at  a 
vertical  depth  of  1,060  feet.  There  are  no  volcanic  rocks  in  the 
immediate  vicinity. 

In  the  Terlingua  district,  Texas,  near  the  Mexican  boundary, 
the  ores  are  found  in  the  Upper  Cretaceous  shales  and  the  Lower 
Cretaceous  limestone.  Volcanic  rocks  are  represented  by  sheets, 
dikes,  and  flows  of  andesite,  rhyolite,  and  basalt.  In  the  lower 


498 


MINERAL  DEPOSITS 


limestones  the  ores  are  mainly  in  nearly  vertical  calcite  veins, 
or  in  lodes  of  friction  breccia  (Fig.  154).  The  other  associated 
minerals  are  chalcedony,  gypsum,  aragonite  and  pyrite. 

Genesis. — The  uniform  character  of  the  quicksilver  deposits 
points  to  a  common  genesis  for  all  of  them.  The  earlier  belief 
that  the  ores  were  products  of  sublimation  is  generally  abandoned, 
for  the  usual  mode  of  occurrence,  with  minerals  of  aqueous  origin, 
such  as  calcite,  opal,  chalcedony,  and  often  barite,  is  decidedly 
opposed  to  such  a  view.  Becker  has  pointed  out  that,  as  the 
character  of  the  enclosing  rocks  has  little  influence  on  the  deposits 
they  are  most  probably  derived  from  a  common,  deep-seated 
source.  Their  structure  indicates  deposition  near  the  surface, 


Scale 
100  200 


300  Feet 


FIG.  154. — Vertical  cross  section  of  California  Hill,  Terlingua,  Texas, 
showing  cinnabar  veins  with  large  ore-bodies  below  impervious  shale. 
After  H.  W.  Turner. 

as  does  also  the  physiographic  evidence  at  many  places — for 
instance,  where  the  ore  appears  in  the  crevices  of  Quaternary 
and  little-eroded  lava  flows. 

When  it  is  noted  that  hot  springs  and  volcanic  surface  flows 
are  present  in  almost  all  regions  of  importance  (except  Almaden 
in  Spain,  Idria  in  Austria,  and  Nikitowka  in  Russia),  and  that 
cinnabar  in  considerable  quantities  is  associated  with  hot  spring 
deposits,  or  is  actually  found  deposited  by  hot  springs,  the  argu- 
ment becomes  very  strong  indeed  that  such  solutions  have  formed 
the  majority  of  the  deposits.  For  the  few  deposits  that  have 
no  such  clear  connection  with  volcanic  rocks  the  characteristic 


DEPOSITS  FORMED  NEAR  THE  SURFACE      499 

mineral  association  still  holds  good,  and  \ve  are  forced  to  the 
hypothesis  that  volcanism  and  hot-spring  action  are  the  causes  of 
these  also,  thoughthe  products  of  the  igneous  activity  may  have 
failed  to  reach  the  surface  and  the  hot  springs  may  have  subsided. 

The  evidence  relating  to  cinnabar  deposited  by  hot  springs  is 
summarized  in  the  following  paragraphs. 

At  Steamboat  Springs,  in  Nevada,  near  the  California  bound- 
ary, cinnabar  is  contained  in  the  hot  ascending  sodium  chloride 
waters,  together  with  antimony,  arsenic,  and  sulphur,  and  is 
actually  being  deposited  in  the  sinter.1  Close  by,  but  at  a 
higher  level,  is  a  low-grade  quicksilver  deposit  in  decomposed 
granite,  and  this  in  all  probability  was  also  formed  by  the  same 
springs  when  issuing  at  a  higher  level.  Underneath  the  sinters 
of  the  present  springs  the  gravels  contain  crystallized  stibnite 
and  pyrite. 

At  Sulphur  Bank,2  in  the  California  quicksilver  belt,  Le  Conte, 
Christy,  Rising,  Becker,  and  Posepny  have  studied  the  deposition 
of  cinnabar  and  sulphur  by  ascending  hot  sodium  carbonate  and 
borate  waters  and  have  all  arrived  at  the  conclusion  that  such 
deposition,  together  with  that  of  pyrite  and  opal,  is  actually 
taking  place.  The  Cretaceous  sandstones  and  associated 
Franciscan  metamorphic  rocks  are  here  overlain  by  flows  of 
both  normal  and  glassy  basalt  and  by  cinder  cones,  pointing  to 
very  recent  eruption.  The  hot  springs  have  altered  and  bleached 
the  basalt.  Sulphur  is  deposited  at  the  surface  by  the  oxidation 
of  H2S,  or  by  reaction  between  S02  and  H2S.  Below  the  super- 
ficial deposit  of  sulphur  cinnabar  is  found  in  the  basalt,  as  well 
as  in  the  underlying  shales  and  sandstones;  it  occurs  mostly  in 
veinlets  and  joints  together  with  the  pyrite  and  opal  above 
mentioned.  (Cfr.  p.  113.) 

The  Rabbit  Hole  sulphur  deposit,  in  Humboldt  County, 
Nevada,  described  by  G.  I.  Adams,3  is  evidently  a  product  of  hot 
springs,  and  near  it  are  considerable  areas  of  rhyolite.  The 
rocks  are  silicified,  and  opal,  alunite,  gypsum,  and  some  cinnabar 
are  present  as  associated  minerals. 

1  G.  F.  Becker,  Man.  13,  U.  S.  Geol.  Survey,  1888,  Chapter  XL 

s  J.  LeConte  and  W.  B.  Rising,  The  phenomena  of  metalliferous  vein 
formation  now  in  progress  at  Sulphur  Bank,  Cal.,  Am,  Jour.  Sci.,  3d  ser., 
vol.  24,  1882,  pp.  23-33. 

G.  F.  Becker,  op.  tit..  Chapter  VII. 

F.  Posepny,  The  genesis  of  ore  deposits,  2d  ed.,  1902,  pp.  32-36. 

3  Butt.  225,  U.  S.  Geol.  Survey,  1904,  pp.  497-502. 


500  MINERAL  DEPOSITS 

In  the  Hauraki  peninsula  of  New  Zealand,  near  OmapereLake,1 
where  basalts  overlie  Mesozoic  shales  and  sandstones,  mercury 
and  cinnabar  have  been  found  in  the  deposits  of  the  hot  springs 
at  several  places. 

E.  Cortese2  reports  the  occurrence  of  cinnabar  in  connection 
with  sulphur  deposits  which  result  from  still  active  hot  springs 
in  Chaguarama  Valley,  Venezuela.  The  cinnabar  occurs  in  Ter- 
tiary bleached  sandstone,  together  with  pyrite.  Borax  deposits 
are  also  said  to  occur  in  the  same  locality. 

A  careful  investigation  would  doubtless  disclose  the  presence 
of  cinnabar  in  many  other  spring  deposits.  If  it  is  found  in  more 
than  traces  the  best  way  to  test  such  material,  as  well  as  ores,  is 
by  the  miner's  pan,  in  which  the  bright-red  grains  of  cinnabar 
show  conspicuously. 

Quicksilver  is  apparently  contained  in  hot-spring  waters  carry- 
ing sodium  carbonate,  sodium  chloride,  or  sodium  borate;  some- 
times all  three  salts  as  well  as  carbon  dioxide  and  some  hydrogen 
sulphide  are  present.  Near  the  surface  these  springs  may  become 
acid  owing  to  the  oxidation  of  hydrogen  sulphide. 

Regarding  the  mode  in  which  mercury  is  carried  in  solution, 
Becker's  views,3  based  on  the  laboratory  experiments  of  W.  H. 
Melville,  still  appear  to  furnish  the  best  explanation.  While 
the  solubility  of  mercuric  sulphide  in  alkaline  compounds  con- 
taining sulphur  had  long  been  recognized,  the  evidence  was  to 
some  degree  conflicting. 

Becker  showed  that  mercuric  sulphide  is  freely  soluble  in 
solutions  of  sodium  sulphide,  as  well  as  in  a  mixture  of  Na2§  and 
NaOH,  and  also  in  warm  sodium  sulphydrate  (NaHS).  When 
neutral  sodium  carbonate  is  treated  with  hydrogen  sulphide, 
sodium  sulphydrate  and  probably  also  sodium  sulphide  will 
form;  these  dissolve  mercurial  sulphide,  and  double  salts  of  the 
general  formula  HgS.wNazS  doubtless  form.  Incidentally  it 
was  found  that  the  same  reagents  would  dissolve  metallic  gold, 
pyrite,  sphalerite,  and  cupric  sulphide.  The  solubility  of  the 

1 1.  M.  Bell  and  E.  de  C.  Clarke,  Bull.  8,  New  Zealand  Geol.  Survey, 
1909,  p.  87. 

A.  Liversidge,  Jour.  Roy.  Soc.  N.  S.  W.,  vol.  2,  p.  262. 

J.  Park,  Trans.,  N.  Z.  Inst.  Min.  Eng.,  vol.  38,  1904,  p.  27. 

Andre  P.  Griffiths,  The  Ohaeawai  quicksilver  deposits,  Trans.,  N.  Z 
Inst.  Min.  Eng.,  vol.  2,  1898,  p.  48. 

2  Eng.  and  Min.  Jour.,  Nov.  10,  1904. 

3  G.  F.  Becker,  Mon.  13,  U.  S.  Geol.  Survey,  1888,  Chapter  XV. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      501 

sulphides  of  arsenic  and  antimony  under  similar  conditions  is 
well  known. 

It  is  therefore  easy  to  perceive  that  a  spring  water  containing 
sodium  carbonate  and  hydrogen  sulphide  would  form  a  suitable 
solvent  for  the  compounds  mentioned.  The  precipitation  would 
be  easily  effected  by  oxidation  of  the  water  and  the  development 
of  free  acids,  by  dilution,  by  cooling,  or  by  the  presence  of  organic 
or  ammoniacal  compounds.  That  the  latter  two  agents  are 
active  in  many  cases  there  is  little  doubt. 

Relation  to  Other  Ore  Deposits. — Although  the  cinnabar  de- 
posits form  a  well-defined  group,  they  are  not  to  be  separated 
entirely  from  other  classes  of  ore  deposits.  Some  of  them  con- 
tain other  metallic  minerals,  and  there  are  many  that  show  a 
transition  to  the  stibnite  and  arsenical  veins.  The  Nevada  belt 
especially  furnishes  many  instances  of  a  close  relationship  to 
gold  and  silver  bearing  veins  on  one  hand  and  to  stibnite  veins 
on  the  other  hand.  It  is  true,  however,  that  no  cinnabar  deposit 
has  yet  been  found  to  change  gradually  into  ores  of  different 
character  as  depth  is  attained.  No  deposits  have  been  worked 
below  a  depth  of  2,000  feet  vertically  beneath  the  croppings. 
There  is  little  doubt  that  these  ores  were  formed  from  the  most 
volatile  parts  of  the  magmas,  carried  in  solution  by  ascending 
waters  until  they  came  close  to  the  surface.  But  the  irregular 
distribution  of  the  deposits  and  their  entire  absence  from  large 
areas  of  volcanism  form  problems  that  are  yet  unsolved. 

STIBNITE  DEPOSITS 

Mineralogy,  Production  and  Uses.— Stibnite  (Sb2S3  with  71.4 
per  cent.  Sb)  is  the  principal  ore  mineral  of  antimony.  Its  oxida- 
tion near  the  surface  results  in  various  oxides  (p.  899)  of  yellowish 
or  white  color  such  as  senarmontite  (Sb2Og),  cervantite  (Sb02), 
and  stibiconite  (H2Sb2O5).  While  stibnite  occurs  in  many  depos- 
its in  small  quantities,  especially  in  quicksilver  ores,  it  is  the 
characteristic  and  dominant  mineral  in  the  stibnite  veins  where 
it  is  accompanied  by  quartz  gangue  and  a  scant  amount  of  other 
sulphides,  such  as  arsenopyrite,  realgar,  pyrite  and  zincblende, 
more  rarely  jamesonite  and  similar  sulphantimonides.  Such 
ores  often  also  carry  gold  and  the  association  of  the  stibnite 
with  some  gold  quartz  veins  has  often  been  noted.  For  the 
purpose  of  making  pure  antimony  the  presence  of  arsenic  and 
copper  is  undesirable. 


502  MINERAL  DEPOSITS 

In  past  years  the  production  of  antimony  has  not  been  great 
owing  to  its  low  price  and  limited  usefulness.  Since  the  great 
war  began  the  price  has  increased  to  17  cents  per  pound.  The 
world's  production  of  antimony  metal  may  be  estimated  to  50,- 
000  metric  tons.  The  supply  is  mainly  obtained  from  China, 
from  Central  France,  and  from  the  State  of  Queretaro,  Mexico. 
In  the  United  States  but  little  pure  antimony  is  produced  though 
under  stress  of  war  the  production  of  such  ores  has  risen  to  several 
thousand  tons.  Antimony  is  used  for  bearing  metals,  type 
alloys,  shrapnel  bullets  and  its  salts  find  a  varied  use  in  the 
industries.  The  sulphide  is  used  in  pyrotechnics. 

Another  source  of  antimony  is  in  the  replacement  deposits  and 
veins  containing  mainly  galena  but  associated  with  tetrahedrite 
and  more  rarely  with  jamesonite,  bournonite,  boulangerite  and 
other  lead  sulphantimonides.  As  a  rule  these  are  related  to 
intrusive  action  and  the  small  amount  of  antimony  contained 
is  recovered  as  "hard  lead"  or  antimonial  lead  in  the  smelting 
operations.  From  2,000  to  3,000  tons  of  this  alloy  is  produced 
annually  in  the  United  States. 

No  antimonial  mineral  is  known  to  occur  in  magmatic  deposits; 
they  are  certainly  rare  in  the  contact-metamorphic  and  other 
high  temperature  deposits  though  in  these  jamesonite,  tetrahe- 
drite and  stibnite  have  been  occasionally  reported  (p.  738). 

Occurrence. — The  stibnite  veins  have  wide  distribution  but 
are  rarely  rich.  They  are  in  part  formed  near  the  surface,  but 
many  deposits  are  of  more  deepseated  origin  and  occur  in  or  near 
intrusive  rocks.  To  the  former  type  belong  the  stibnite  veins 
with  a  gangue  of  fine-grained  and  drusy  quartz  which  intersect 
flows  of  rhyolite  and  basalt  in  western  Nevada.  The  antimony 
sulphide  is  as  a  rule  beautifully  crystallized  in  acicular  and  pris- 
matic forms;  it  is  often  accompanied  by  a  little  pyrite,  zinc  blende, 
and  arsenopyrite,  sometimes  also  by  tetrahedrite  and  cinnabar. 
Such  veins  carry  a  little  silver  and  less  gold.  The  intimate  re- 
lationship of  these  veins  with  the  gold  and  silver  veins  proper  is, 
however,  shown  by  the  occurrence  in  one  of  them,  at  National, 
Nevada,1  of  a  shoot  of  remarkably  coarse  gold  of  the  electrum 
variety. 

Stibnite  veins  of  uncertain  affiliations  are  found  in  central 
Arkansas  but  are  of  no  great  importance.  The  veins  follow  the 
steep  stratification  of  Carboniferous  shale  and  sandstone,  and 

1  W.  Lindgren,  Bull.  601;  U.  S.  Geol.  Survey,  1915. 


DEPOSITS  FORMED  NEAR  THE  SURFACE       503 

the  stibnite  fills  the  spaces  between  the  quartz  combs.1  Stibnite 
shows  a  marked  tendency  to  form  replacements  in  limestone  and 
shale.  Such  deposits  in  Eocene  shale  below  a  thick  series  of  ande- 
sites  have  been  described  from  southern  Utah.2  They  are  un- 
doubtedly hot  spring  deposits.  Of  such  nature  are  also  the  deposits 
at  Pereta,  in  Tuscany,  where  the  mineral  is  associated  with  realgar 
and  cinnabar  and  occupies  veins,  seams,  and  irregular  pockets  in 
Tertiary  calcareous  and  detrital  rocks.  The  country  rock  is  in  part 
silicified,  in  part  altered  to  gypsum  or  alunite,  and  exhalations 
of  hydrogen  sulphide  testify  to  the  recent  age  of  the  deposit. 

Beck3  describes  important  deposits  of  stibnite  at  Kostainik, 
in  Serbia,  where  the  mineral  occurs  in  nests  and  veins  in  trachyte 
or  in  Triassic  clay  shales,  but  also  as  metasomatic  bodies  replacing 
the  beds  along  the  contact  of  limestone  and  shale.  The  gangue 
is  a  drusy  fine-grained  quartz. 

The  stibnite  veins  of  Japan,  renowned  for  their  beautiful  crys- 
tals, are  found  in  Mesozoic  and  Paleozoic  rocks  but  little  is  known 
about  their  affiliations. 

The  deposit  of  Djebel  Kami  mat,4  in  Algeria,  containing  sen- 
armontite,  and  that  of  Altar,5  Sonora,  from  which  stibnite  is 
reported  as  the  principal  ore  mineral,  appear  both  to  be  replace- 
ment deposits  in  limestone.  At  the  Algerian  locality  the 
replacement  veins  spread  out  in  Cretaceous  sediments,  while  at 
Altar  the  ore  is  said  to  occur  in  Carboniferous  limestone.  Both 
deposits  are  probably  oxidized  replacements  of  stibnite. 

Stibnite  veins  affiliated  with  intrusive  rocks  differ  but  little 
from  the  deposits  described  above.  They  are  known  from  central 
France,  where  narrow  veins  intersect  granite  and  surrounding 
schist.  Similar  deposits  are  not  uncommon  elsewhere  for  in- 
stance, in  Kern  County,  California,  where  the  quartz  veins  also 
contain  gold,  in  the  Coeur  d'  Alene  district  and  elsewhere. 

Stibnite  is  very  common  in  Alaska  and  generally  is  found  in 
gold-quartz  veins.  A.  H.  Brooks6  who  enumerates  67  occurrences 
suggests  that  the  stibnite  may  have  been  introduced  in  older 
gold-quartz  veins  during  a  later  and  Tertiary  mineralization. 

1  F.  L.  Hess,  Bull.  340,  U.  S.  Geol.  Survey,  1908,  pp.  241-256. 

2  G.  B.  Richardson,  idem,  pp.  253-256. 

3  R.  Beck  (after  W.  von  Fircks),  Zeitschr.  prakt.  Geol,  1900,  pp.  33-36. 

4  L.  De  Launay,  Gttes  mine'raux,  1,  1913,  p.  772. 

6  E.  T.  Cox,  Am.  Jour.  Sci.,  3d  ser.,  vol.  20,  1880,  pp.  421-423. 
6  Antimony  deposits  of  Alaska,  Bull.  649,  U.  S.  Geol.  Survey,  1916. 


504  MINERAL  DEPOSITS 


GOLD -QUARTZ  VEINS  IN  ANDESITE 

Transylvania.1 — In  northwestern  Hungary  and  in  adjoining 
parts  of  Transylvania  gold-bearing  veins  of  Tertiary  age  have 
been  developed  after  eruptions  of  andesites  and  dacites.  A 
mining  industry,  begun  centuries  ago,  still  flourishes  in  this 
region.  The  literature  is  very  extensive,  and  only  a  few  deposits 
can  be  mentioned  here  as  examples. 

The  geological  formations  in  the  western  part  of  the  gold- 
mining  region  of  Transylvania  consist  of  Mesozoic  melaphyres, 
Cretaceous  shales  and  sandstones,  and  Miocene  sediments,  all 
penetrated  by  late  Tertiary  eruptions  of  andesites  and  dacites. 
The  igneous  rocks  appear  as  lava  flows,  tuffs,  and  volcanic  necks. 
The  veins  near  Brdd,  at  present  the  most  productive  district,  fill 
well-defined  steeply  dipping,  in  places  branching  fissures  which 
generally  intersect  volcanic  rocks  or  Cretaceous  sediments.  The 
simple  veins  are  as  much  as  1  meter  in  thickness;  the  lodes 
attain  a  thickness  of  10  to  20  meters.  The  deposits  have  been 
worked  to  a  depth  of  about  270  meters.  The  fissures  are  tec- 
tonic, not  contraction  joints.  They  are  of  Miocene  age. 

The  surrounding  rocks,  particularly  the  andesite,  have  suffered 
extensive  propylitization,  the  femic  minerals  being  extensively 
decomposed,  while  feldspars  remain  fresh.  Pyrite  is  not  common 
except  near  the  veins.  Calcite  is  abundant.  Schumacher  does 
not  believe  that  propylitization  is  caused  by  "intensive  pene- 
tration by  gases  from  the  not  yet  wholly  solidified  intrusions," 
an  opinion  expressed  by  Stelzner  and  Bergeat.  He  nevertheless 
considers  the  process  distinctly  earlier  than  the  veins  and  inde- 
pendent of  them.  The  alteration  continues  to  the  greatest 
depths  attained.  "  Kaolinization "  near  the  veins  is  a  wholly 

1  Bela  von  Inkey,  Nagyag  und  seine  Erzlagerstatten,  Buda-Pest,  1885. 

Bela  von  Inkey,  De  la  relation  entre  l'6tat  propylitique  des  roches 
andesitiques  et  leur  filons  mineYaux,  Internat.  Geol.  Congress,  Mexico, 
1906. 

M.  v.  Palfy,  Das  Goldvorkommen  im  siebenburgischen  Erzgebirge,  etc. 
Zeitschr.  prakt.  Geol.,  1907,  pp.  144-148. 

C.  Semper,  Beitrage  zur  Kenntniss  der  Golderzlagerstatten  des  sieben- 
burgischen Erzgebirges,  Abh.  K.  preuss.  geol.  Landesanstalt,  Neue  Folge, 
Fasc.  33,  1900. 

F.  Schumacher,  Die  Golderzlagerstatten  der  Rudaer  Zwolfapostel- 
Gewerbschaft  zu  Brdd  im  Siebenburgen,  Zeitschr.  prakt.  Geol,  1912,  pp. 
1-85. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      505 

different  process  according  to  Schumacher  and  is  superimposed 
upon  propylitization.  Kaolin  and  sericite  are  both  present  in 
the  altered  rock,  but  the  possible  influence  of  descending  waters 
on  kaolinization  is  inadequately  treated.  The  alteration  of  the 
wall  rock  in  a  vein  0.5  meter  thick  extends  only  about  10  centi- 
meters from  the  vein,  but  many  smaller  veins  have  proportion- 
ately wider  zones  of  alteration. 

An  older  set  of  veins  is  composed  of  clastic  material  of  shale 
and  igneous  rocks  ("Glauch  veins")-  They  are  interpreted  as 
having  been  filled  by  ascending  liquid  muds.  Similar  veins  in 
the  Silverton  district,  Colorado,  have  been  described  by  Ransome. 


a/ 

FIG.  155. — Rich  gold-bearing  quartz,  Brad,  Transylvania,  a.  Granular 
quartz;  b,  gold  between  grains;  c,  plates  of  gold  accompanied  by  crushed 
quartz.  Magnified.  After  F.  Schumacher. 

The  gangue  of  the  ore-bearing  veins  is  composed  of  quartz 
(rarely  chalcedony),  calcite,  rhodochrosite,  and  barite,  a  fre- 
quently recurring  association  in  veins  of  this  class.  The  quartz 
is  usually  fine-grained,  sometimes  drusy,  cellular,  or  honey- 
combed, but  not  amethysthine.  Pyrite  in  small  crystalsl  is 
abundant;  the  concentrates  contain  10  grams  of  gold  and  69 
grams  of  silver  per  ton,  while  the  pyrite  in  the  country  rock 
contains  7  to  15  grams  of  gold  and  10  to  15  grams  of  silver  per 
ton;  both  kinds  are  therefore  poor  in  gold.  Marcasite^has^been 
notedjin  only  one  mine,  where  it  occurs  on  quartz,  associated 


506  MINERAL  DEPOSITS 

with  free  gold.  Zinc  blende  is  associated  with  pyrite  and  is 
poor  in  gold  but  contains  more  silver  than  the  pyrite.  Chalcopy- 
rite  and  galena  where  present  are  poor  in  gold  but  contain  several 
hundred  grams  of  silver  per  ton. 

The  principal  ore  mineral  is  native  gold,  which  occurs  com- 
monly in  crushed  quartz  or  in  little  fissures  (Fig.  155),  or  as 
sheets  or  wires  between  the  quartz  combs  of  veinlets.  Some  of 
it  is  found  in  coarse  quartz  and  is  apparently  older  than  the 
quartz  or  of  contemporaneous  origin.  It  occurs  also  in  sheets 
along  the  cleavage  planes  of  calcite  and  in  lumps  or  nodules  in 
clay.  In  part  it  is  therefore  of  relatively  late  origin.  The  gold 
contains  28  per  cent,  silver  and  the  ores  average  10  grams  of 
gold  per  ton.  Tellurides  and  rich  silver  minerals  are  rare. 


boale  Viktor  Adlt        ' 

0    0    10        20        30         40         50        00         70        SO         90  MetetS 

FIG.  156. — Section  of  stoped  area  in  vein  at  Brad,  Transylvania,  showing 
pockets  of  rich  ore;  also  rich  shoots  following  intersections  with  barren 
veins.  After  F.  Schumacher. 

The  structure  of  the  veins  is  irregularly  massive,  though  in 
places  crusted,  banded,  and  drusy.  Brecciated  structures  are 
common.  Small  prismatic  and  rectangular  pseudomorphs  of 
quartz  are  considered  as  replacements  of  gypsum  but  strongly 
resemble  the  similar  casts  of  celestite  at  Cripple  Creek,  Colorado. 

The  ore-shoots  are  irregular;  sometimes  they  are  narrow  but 
extend  with  steep  dip  for  100  or  200  feet  vertically.  Shoots 
often  occur  at  junctions  and  intersections  (Fig.  156).  At  a  junc- 
tion of  two  veins  with  a  narrow  pyritic  seam  was  found  a  pocket 
from  which  in  one  day  gold  weighing  55  kilograms  was  extracted. 
Near  the  surface  the  veins  were  poor.  The  richest  zone  extended 
from  about  100  meters  below  the  surface  down  to  a  depth  of 
320  meters.  The  remarkable  dependence  of  the  rich  ore  on 
narrow  seams  of  pyrite  is  evident  and  recalls  analogous  conditions 


DEPOSITS  FORMED  NEAR  THE  SURFACE      507 

in  the  Thames  district,  New  Zealand,  and  the  "indicators"  of 
Victorian  quartz  mines  and  many  other  gold  deposits  (Fig.  157). 

B.  von  Inkey  held  that  the  gold  was  concentrated  by  leaching 
from  the  country  rock.  Schumacher  believed  that  it  was  depos- 
ited by  ascending  hot  waters  in  the  vicinity  of  necks  of  intrusive 
rocks.  Beyond  the  intrusive  necks  the  veins  persist  but  contain 
only  gangue  minerals.  M.  Dittrich  examined  fresh  and  propy- 
litic  andesite,  using  the  cyanide  process,  but  found  gold  in  neither. 

While  much  of  the  gold  is  distinctly  later  than  the  gangue  it 
is  difficult  to  say  whether  we  have  to  deal  here  with  the  effect  of 
descending  waters  or  with  the  last  phases  of  vein  formation. 
A  similar  problem  is  offered  in  the  rich  pockets  of  veins  at  Thames, 
in  New  Zealand;  in  neither  place  is  enrichment  by  descending 
waters  satisfactorily  proved. 


FIQ.  157. — Pockets  of  native  gold  (a)  in  quartz  vein  (g)  along  intersec- 
tions with  pyritic  seams  (&).     After  F.  Schumacher. 

Hauraki  Peninsula,  New  Zealand.1 — The  Hauraki  region  in 
the  northern  island  of  New  Zealand  is  richly  mineralized  in  sev- 
eral districts.  Its  rocks  consist  mainly  of  andesite  and  dacite 
flows  of  Eocene  or  Miocene  age  covered  by  Pliocene  rhyolites. 

A  production  of  about  $30,000,000  is  recorded  from  the  Thames 
district,  though  but  little  gold  is  now  obtained  there.  The 

1  James  Park,  Geology  of  Hauraki  gold  field,  Trans.,  N.  Z.  Inst.  Min. 
Eng.,  vol.  1,  1897,  p.  3. 

P.  G.  Morgan,  Geology,  etc.,  of  Waihi,  Trans.,  Austr.  Inst.  Min.  Eng., 
vol.  8,  1902,  p.  166. 

J.  M.  Bell  and  C.  Fraser,  The  great  Waihi  gold  mine,  Bull  15,  New 
Zealand  Geol.  Survey. 

A.  M.  Finlayson,  Econ.  Geol,  vol.  4,  1909,  pp.  632-645  (with  literature). 

Arthur  Jarman,  The  geology  of  the  Waihi-Grand  Junction  mine,  Trans., 
Inst.  Min.  and  Met.  (London),  vol.  25,  1916,  pp.  3—40,  with  discussion. 


508  MINERAL  DEPOSITS 

veins  are  contained  in  broad  belts  of  soft,  propylitic  andesite 
(see  p.  480)  and  dip  40°  or  more.  Great  masses  of  low-grade 
quartz  occur,  but  the  gold  is  derived  mainly  from  rich  pockets 
occurring  down  to  a  depth  of  400  to  600  feet  below  the  surface. 
One  of  these  pockets  in  the  Caledonia  mine,  about  1871,  yielded 
9  tons  of  gold  in  15  months.  The  veins  have  been  followed  from 
a  height  of  1,500  feet  above  the  sea  to  640  feet  below  it,  but  owing 
to  intervening  faults  the  real  vertical  extent  is  only  1,200  feet. 
Park  states  that  the  veins  do  not  continue  into  the  underlying 
Jurassic  shale  and  that  they  are  thus  limited  to  the  thickness  of 
the  lava  flows  in  which  they  occur.  The  rich  shoots  occur  mainly 
where  the  veins  are  intersected  by  small  stringers  or  "leaders." 
Opinions  differ  widely  as  to  whether  this  concentration  in  pockets 
is  due  to  descending  waters  or  not.  In  all  probability,  however, 
it  was  one  of  the  latest  phases  of  the  primary  mineralization. 
The  principal  ore  mineral  is  gold  alloyed  with  30  to  40  per  cent, 
silver,  but  some  pyrite,  chalcopyrite,  zinc  blende,  galena,  stibnite, 
and  pyrargyrite  also  occur.  Arsenopyrite  and  native  arsenic, 
the  latter  secondary,  occur  at  Coromandel.  The  gangue  miner- 
als, besides  quartz,  are  dolomite  and,  occasionally,  rhodonite. 

The  Karangahake  deposits,  40  miles  south  of  the  Thames 
district,  are  also  in  propylitized  andesite  and  dacite  but  differ 
somewhat  from  the  type  described  and  consist  in  brief  of  calcite 
and  quartz  with  more  or  less  sulphides.  The  best  known  de- 
posits are  at  Waihi.  The  Waihi  lodes  are  conspicuous  and  were 
discovered  in  1878;  in  part  the  croppings  are  covered  by  rhyolite 
and  the  development  of  the  deposit  therefore  falls  between  the 
two  eruptions.  The  ore  proved  difficult  to  amalgamate  and  the 
mines  achieved  importance  only  after  the  introduction  of  the 
cyanide  process.  To  the  end  of  1917  the  total  production 
amounted  to  about  $57,000,000.  In  1917  the  ore  averaged  $8 
in  gold  and  1  ounce  of  silver  per  ton.  The  country  rock  is  a 
green  propylitic  dacite  with  some  pyrite,  calcite,  and  seams  of 
quartz  and  adularia.  This  rock  often  adjoins  the  veins  without 
further  alteration,  but  transitions  to  the  quartz  filling  by  silicifi- 
cation  are  said  to  exist. 

The  vein  system  is  complex,  and  sixteen  steeply  dipping  and 
interconnecting  veins  are  known.  Of  most  importance  is  the 
Martha  lode  (Fig.  158),  a  wide  fissure  vein  with  brecciated  walls; 
the  quartz  is  formed  largely  by  filling,  in  part  by  silicification. 
On  the  500-foot  level  the  lode  is  in  some  places  80  feet  wide;  for 


DEPOSITS  FORMED  NEAR  THE  SURFACE       509 

half  of  this  width  it  is  barren,  but  the  other  half  is  said  to  average 
$15  to  $20  per  ton.  The  proportion  of  gold  to  silver  by  weight 
is  1:3  or  1:4  and  this  average  was  maintained  from  the  surface 
down.  The  water  level  stood  within  200  or  300  feet  of  the  sur- 
face. The  lode  is  said  to  contain  ore  for  a  horizontal  distance 
of  1,700  feet.  The  developments  in  the  deepest  levels  are  said 
to  be  disappointing  as  to  the  quantity  of  ore,  but  the  lode  itself 
maintains  its  strength. 

A  lively  controversy  has  lately  developed  in  regard  to  near- 
surface  intrusions.  Bell  and  Frazer  consider  the  dacite  intrusive 
in  andesite  flows,  the  inference  being  that  ore  may  only  be  ex- 
pected in  that  rock.  Jarman  believes  there  are  no  intrusives 
but  only  a  series  of  flows. 


FIG.  158. — Cross-section  of  Waihi  mine,  New  Zealand,  showing  lode 
system  in  andesite  and  dacite  (G),  covered  by  post-mineral  rhyolite  (B). 
After  C.  Fraser. 

A  little  pyrite  was  found  in  the  first  level  in  the  Martha  lode ; 
on  the  second  level  the  sulphide  ore  on  the  foot-wall  was  a  few 
feet  wide;  on  the  500-foot  level  20  feet  of  sulphide  ore  was  exposed 
on  the  foot-wall,  while  the  remainder  of  the  vein,  at  this  place 
40  feet  wide,  was  thoroughly  oxidized,  with  much  black  man- 
ganese oxide.  This  sulphide  ore  is  of  nearly  the  same  value  as  the 
oxidized  ore,  containing  perhaps  a  little  more  gold  and  a  little 
less  silver. 

The  ore  consists  of  quartz  and  calcite,  with  3  per  cent,  of 
pyrite,  zinc  blende,  galena,  and  argentite.  The  sulphides  are 
often  banded  and  the  gold  values  are  mainly  in  the  pyrite;  the 


510 


MINERAL  DEPOSITS 


bullion  contains  some  selenium.  Throughout  the  oxidized  zone 
the  calcite  is  dissolved,  leaving  a  lamellar  quartz  ore  stained  by 
manganese,  but  this  change  is  produced  mainly  by  descending 
waters.  In  other  mines  of  the  district  there  are  indications  of  a 
pseudomorphic  deposition  of  silica,  similar  to  that  of  the  De 
Lamar  mine,  Idaho  (p.  513),  by  a  late  phase  of  ascending  solutions. 
In  at  least  some  mines  in  the  Karangahake  district  the  ore  be- 


Later    Atidesite 


FIG.  159. — Cross-section  of  the  San  Rafael  lode,  El  Oro,  Mexico,  show- 
ing branch  veins. 

comes  poor  when  the  zone  of  the  calcite,  unchanged  by  descend- 
ing waters,  is  reached. 

The  depth  of  the  oxidation  in  the  Waihi  mine  below  water  level 
is  noteworthy  and  probably  indicates  a  dry,  intervolcanic  epoch. 

El  Oro,  Mexico. — As  there  are  few  important  gold  deposits  in 
Mexico,  the  occurrence  at  El  Oro,  70  miles  northwest  of  the  fed- 


DEPOSITS  FORMED  NEAR  THE  SURFACE      511 

eral  capital,  is  of  special  interest.  The  district  is  situated  on  the 
volcanic  high  plateau  at  an  elevation  of  about  10,000  feet.  The 
barren  and  unaltered  andesites  of  this  plateau  overlie  the  ore- 
bearing  formation  which  consists  of  a  thick  flat  dipping  series  of 
well  stratified  black  bituminous  shale  with  some  sandstone; 
in  places  these  Jurassic  sediments  are  covered  by  earlier  andesites, 
which  near  the  vein  contain  pyrite  and  chlorite  and  in  other  places 
they  are  intruded  by  sills  of  similar  andesitic  rocks.  These 
earlier  andesites  are  held  to  be  of  Miocene  age. 

The  lodes,  about  ten  in  number,  only  outcrop  at  one  or  two 
places  and  have  been  opened  by  cross  cut  tunnels  and  shafts. 
Almost  all  of  the  important  work  has  been  done  since  1904. 
There  are  two  principal  master  lodes,  the  San  Rafael  and  Dos 
Estrellas  striking  N.N.W.  and  dipping  steeply  S.S.W.  The 
production  from  the  San  Rafael  alone  since  1904  is  approximately 
$40,000,000  from  not  less  than  5,000,000  metric  tons  of  ore. 
The  Dos  Estrellas  lode  for  many  years  yielded  about  $5,000,000 
per  annum. 

The  lodes  occupy  fault  fissures,  which  in  their  upper  parts  at 
least  were  open  and  much  of  the  ore  has  been  deposited  by  filling 
of  open  space.  In  depth  and  especially  in  andesite  rock  much  of 
the  quartz  is  formed  by  replacement.  In  1913,  the  greatest 
depth  attained  was  2,000  feet;  200  feet  of  which  was  in  barren 
cap  andesite.  The  filling  consists  of  fine-grained  quartz  inter- 
grown  with  much  coarse-grained  calcite.  There  is  only  a  small 
percentage  of  pyrite  and  zinc  blende.  The  gold  is  never  visible 
and  even  close  panning  often  fails  to  bring  a  color  from  rich 
ore. 

In  the  upper  levels  some  stopes  are  from  60  to  100  feet  wide 
but  in  depth  the  lode  contracts  to  smaller  dimensions  of  3  to  15 
feet. 

The  "branch  veins"  are  an  interesting  feature  of  the  large 
lodes  at  El  Oro.  They  are  steep  and  persistent  stringers  caused 
by  the  settling  of  the  hanging  wall  (Fig.  159)  and  are  usually 
rich  containing  from  $15  to  $35  in  gold  and  from  5  to  20  ounces 
of  silver  to  the  ton.  The  ore  from  the  main  lode  contains  only 
$5  to  $15  in  gold  with  2  or  3  ounces  of  silver  per  ton. 

The  ore  shoots  in  the  main  lodes  are  of  primary  origin  and  ex- 
tend horizontally,  to  a  depth  of  500  to  700  feet  below  the  capping 
andesite.  The  San  Rafael  has  been  stoped  continuously  for  one 
and  one-half  miles  through  three  properties.  These  horizontal 


512 


MINERAL  DEPOSITS 


shoots  are  probably  caused  by  an  upper,  impermeable  barrier, 
now  eroded,  of  andesite  or  clayey  rock. 

The  ore  of  the  branch  veins  is  usually  well  banded  by  crusti- 
fication  (Fig.  160)  and  is  much  richer  in  sulphides,  mainly  zinc 
blende  and  pyrite,  than  the  main  lode. 

Oxidation  of  the  main  lode  antedates  the  capping  of  younger 
andesite  and  is  practically  complete  to  a  depth  of  800  feet.  The 
calcite  is  dissolved  and  the  quartz  remains  as  a  porous,  cellular 


FIG.  160. — Photograph  of  branch  vein  on  1 ,000-foot  level,  El  Oro  Mining 
and  Railway  Company,  showing  pronounced  banding  by  sulphides;  vein 
three  feet  wide  contains  9%  ounces  gold  and  165  ounces  silver  per  ton. 
Open  cavity  in  middle. 

mass.  The  gold  being  in  the  quartz  there  is  a  considerable  ap- 
parent enrichment  of  gold  in  the  oxidized  zone.  Some  silver  has 
probably  been  leached,  but  no  evidence  was  found  of  transpor- 
tation of  gold.  There  is  little  manganese  in  the  ore. 

GOLD-QUARTZ  VEINS  IN  RHYOLITE 

The  Tertiary  rhyolites  in  the  Cordilleran  region  often  contain 
gold-bearing  veins  These  veins  are  poor  in  ore  minerals  other 
than  gold  but  usually  contain  some  argentite,  pyrargyrite,  and 


DEPOSITS  FORMED  NEAR  THE  SURFACE      513 

pyrite.  The  gold  is  frequently  coarse  and  accompanied  by  more 
or  less  silver.  Among  the  gangue  minerals  quartz  prevails, 
but  in  most  cases  it  is  associated  with  much  adularia,  probably 
derived  from  the  surrounding  potassic  rock.  Calcite  and  fiuorite 
are  also  common,  but  barite  is  rare.  The  veins  are  almost  char- 
acteristically pseudomorphic,  with  bladed  and  cellular  quartz 
and  adularia,  which  replace  calcite  and  fluorite.  Both  veins 
and  sheeted  zones  occur;  in  the  latter  there  is  little  gangue  and 
the  gold,  as  in  the  Jumbo  mine  at  Hart,  California,  may  be 
embedded  in  apparently  fresh  rhyolite. 

There  is  no  real  propylitic  alteration  of  the  country  rock  but 
often  extensive  silicification  and  much  finely  disseminated  pyrite. 
The  silicification  is  attended  by  concentration  of  potassium  as 
adularia  or  sericite.  The  decomposed  upper  zone  of  the  veins 


FIG.  161. — Vertical  section  of  the  vein  system  at  De  Lamar,  Idaho. 

contains  clay  seams  that  may  be  extremely  rich  in  gold  and 
secondary  silver  minerals,  as  at  De  Lamar,  Idaho,  and  Rawhide, 
Nevada. 

At  the  De  Lamar  mine1  a  series  of  parallel,  gently  dipping  veins 
of  the  kind  described  abut  against  a  fissure  filled  with  tough 
clay  (the  "iron  dike")  near  which  the  best  ore  is  found  (Fig.  161). 
Below  a  vertical  depth  of  about  800  feet  the  values  are  low, 
although  the  veins  persist.  Free  gold  is  rarely  seen.  The  pro- 
portion of  gold  to  silver  by  weight  is  1  : 20.  The  veins  are  ordi- 
narily 1  to  6  feet  thick  and  distinctly  filled,  though  transitions 
by  silicification  were  also  noted.  The  filling  is  wholly  quartz, 
pseudomorphic  after  calcite,  and  forms  a  cellular  mass  of  thin 
plates  coveerd  by  minute  crystals  (Figs.  147  and  148).  The 

1  W.  Lindgren,  Twentieth  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  3,  1900, 
p.  122. 


514 


MINERAL  DEPOSITS 


value  of  the  ore  ordinarily  ranges  from  $10  to  $20  per  ton.  The 
De  Lamar  mine  has  yielded  gold  and  silver  to  the  value  of  about 
$7,000,000,  but  is  now  closed. 

F.  C.  Schrader  has  described  similar  deposits  in  the  Black 
Mountains1  in  Mohave  County,  Arizona,  and  the  Jarbidge  dis- 
trict,2 Nevada.  At  both  places  adularia  is  extremely  abundant 
and  often  forms  over  50  per  cent,  of  the  gangue  (Fig.  162).  At 
the  Gold  Road  mine,  in  the  Black  Mountain  district,  the  vein 
is  wide  and  long;  the  replacement  of  calcite  and  fluorite  by 
quartz  and  adularia  is  very  clearly  shown  here.  The  ore  averages 
$10  per  ton  and  the  mine  has  yielded  gold  to  the  value  of  several 


FIG.  162. — Thin  section  showing  association  of  fine-grained  quartz  (Q), 
with  admixed  adularia,  argentite  (A),  and  native  gold  (G),  Jarbidge  district, 
Nevada.  Magnified  105  diameters.  After  F.  C.  Schrad&r. 

million  dollars.  At  Jarbidge  a  great  number  of  veins  have  been 
found  intersecting  an  older  rhyolite.  An  interesting  feature  is 
the  injection  of  rhyolite  into  some  of  the  veins,  which  are  dis- 
tinctly earlier  than  the  late  Tertiary  rhyolite  flows  and  were 
thus  formed  during  a  short  epoch  between  two  eruptions.  En- 
tirely similar  veins  are  found  at  Rawhide,  Gold  Circle,3  Round 
Mountain,4  and  many  other  places  in  Nevada.5 

1  Bull.  397,  U.  S.  Geol.  Survey,  1909. 

2  Idem,  497,  1912. 

3  A.  F.  Rogers,  Econ.  Geol.,  vol.  6,  1911,  p.  790. 

W.  H.  Emmons,  Bull.  408,  U.  S.  Geol.  Survey,  1910. 

4  F.  L.  Ransome,  Bull.  380,  U.  S.  Geol.  Survey,  1909,  pp.  44-47. 
6  S.  H.  Ball,  Bull.  308,  U.  S.  Geol.  Survey,  1907,  p.  46. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      515 

In  the  Bullfrog  district,1  Nevada,  a  thick  complex  of  tilted 
and  faulted  Miocene  rhyolite  flows  is  cut  by  gold-bearing  veins. 
These  veins  show  various  gradations  from  sheeted  zones,  in 
which  parallel  banded  veinlets  of  alternating  crusts  of  calcite 
and  quartz  are  separated  by  thin  slabs  of  rhyolite,  through 
irregular  stringer  lodes,  to  lodes  made  up  largely  of  angular 
fragments  of  rhyolite  cemented  by  quartz  and  calcite.  The 
calcite  is  in  part  replaced  by  cellular  quartz,  but  the  process  has 
not  been  carried  to  completion. 

The  extremely  rich  ore  of  the  National2  vein,  in  northwestern 
Nevada  (Fig.  163),  has  yielded  about  $3,000,000  from  a  narrow 


Fia.  163. — Dendritic  gold  (black)  in  extremely  fine-grained  quartz  of  prob- 
able colloid  deposition.  White  area  is  coarsely  crystalline  comb-quartz. 
National,  Nev.  Magnified  15  diameters. 


shoot  followed  to  a  depth  of  800  feet  on  the  vein.  The  veins  of 
that  district,  except  for  this  occurrence,  are  of  the  stibnite  type. 
The  native  gold  contains  50  per  cent,  silver  and  is  more  properly 
called  electrum. 

1  F.  L.  Ransoine,  W.  H.  Emmons,  and  G.  H.  Garrey,  Bull.  407,  U.  S. 
Geol.  Survey,  1910. 

2  W.  Lindgren,  Bull.  601,  U.  S.  Geol.  Survey,  1915. 


516 


MINERAL  DEPOSITS 


ARGENTITE-GOLD-QUARTZ  VEINS 

Tonopah,  Nevada.1 — The  Tonopah  district,  discovered  in  1900, 
is  situated  in  a  group  of  desert  hills  in  western  Nevada  about  30 
miles  north  of  Goldfield.  It  is  now  the  most  important  of  the 
western  silver-  and  gold-producing  localities.  In  1916  the  pro- 
duction amounted  to  nearly  $2,000,000  in  gold  and  8,700,000 


FIG.  164. — Vertical  cross-section  showing  Mizpah  vein  of  first  period  in 
Mizpah  Trachyte  (M.  T.),  cut  off  by  intrusive  sheet  of  West  End  rhyolite 
(W.  R.).  Later  normal  faulting  has  dislocated  vein  and  along  the  fault 
fractures  mineralization  of  the  second  period  has  taken  place  prolonging  the 
vein  for  short  distance  in  the  rhyolite.  After  J.  E.  Spurr. 

ounces  of  silver.  The  ore,  which  is  treated  by  concentration 
and  cyaniding,  yielded  $17  a  ton.  The  total  output  amounts  to 
$92,400,000.  In  1917  the  production  was  somewhat  smaller. 

1J.  E.  Spurr,  Geology  of  the  Tonopah  mining  district,  Prof.  Paper, 
42,  U.  S.  Geol.  Survey,  1905,  also,  Min.  and  Sci.  Press,  Apr.  22,  1911. 

J.  A.  Burgess,  Geology  of  the  producing  part  of  the  Tonopah  district, 
Econ.  Geol,  vol.  4,  1909,  pp.  681-712. 

A.  Locke,  The  geology  of  the  Tonopah  mining  district,  Trans.  Am.  Inst. 
Min.  Eng.,  vol.  43,  1913,  pp.  157-166. 

V.  C.  Heikes,  in  Min.  Res.,  U.  S.  Geol.  Survey,  annual  publication. 

J.  E.  Spurr,  Geology  and  ore  deposition  at  Tonopah,  Nevada,  Econ. 
Geol.,  vol.  10,  1915,  pp.  713-769. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      517 

According  to  Spun  the  veins  intersect  a  complex  volcanic 
series  of  flows  and  near-surface  intrusions.  The  oldest  rock  is  a 
highly  altered  trachyte  flow  (Mizpah  trachyte)  glassy  in  its 
lower  part.  This  is  the  "earlier  andesite"  of  former  reports  and 
contains  the  valuable  veins.  Andesite  and  "West  end"  rhyolite 
are  intruded  into  this  flow,  which  was  later  covered  by  the  "  later  " 
or  "Midway"  andesite  flow;  at  a  still  later  epoch  there  was  a 
series  of  flows  and  intrusions  of  rhyolite.  The  most  important 
of  the  intrusives  is  the  "Tonopah"  rhyolite  which  now  outcrops 
north  of  the  district  and  is  also  found  in  the  mine  workings. 
The  differentiation  of  flows  and  intrusions  near  the  surface  is 
most  difficult  and  have  led  to  conflicting  views.  Burgess  con- 
sidered the  complex  to  consist  of  a  series  of  flows.  A  Miocene 
age  is  attributed  to  the  volcanic  rocks. 

The  principal  quartz  veins  are  later  than  the  enclosing  trachyte 
and  earlier  than  the  following  intrusives.  They  are  also  older 
than  the  "Midway"  andesite.  A  second  set  of  veins  was  formed 
after  the  rhyolite  intrusion  and  before  the  "Midway"  andesite. 
A  few  of  these  are  productive.  A  third  set  of  quartz  veins  was 
formed  after  the  intrusion  of  the  Tonopah  rhyolite  and  contain 
a  small  amount  of  sulphides  of  lead,  zinc  and  copper.  All  these 
veins  formed  at  shallow  depths  and  the  different  types  are  held 
to  represent  various  stages  of  temperature;  part  of  the  second  and 
all  of  the  third  set  of  veins  are  believed  to  have  been  deposited 
at  higher  temperatures  corresponding  to  the  rhyolite  intrusions. 

The  faulting  is  complex  (Fig.  33).  Important  movements 
took  place  at  every  stage  and  were  caused  by  the  volcanic  dis- 
turbances. 

The  importance  of  the  volcanic  succession  is  clearly  seen. 
If  an  apparently  underlying  rock  is  really  intrusive  and  later 
than  the  vein  formation  no  continuation  of  the  bonanza  veins 
can  be  expected  in  it  (Fig.  164).  The  conditions  which  resulted 
in  the  consolidation"  of  large  masses  of  intrusive  magma  to  more 
or  less  glassy  rocks  seem  peculiar  and  perhaps  deserve  further 
investigation. 

The  productive  veins  of  the  earliest  period  show  few  outcrops 
at  the  surface;  they  have  an  easterly  strike  and  various  northerly 
dips.  The  veins  are  of  moderate  thickness  though  some  stopes 
are  30  feet  or  more  in  width.  Propylitic  alteration  (p.  479) 
affects  the  trachyte  and  the  andesite  but  next  to  the  veins  there  is 
much  silicification  with  sericite  and  adularia.  The  principal 


518  MINERAL  DEPOSITS 

gangue  mineral  is  white  quartz  of  fine  but  variable  grain,  with 
banded  structure  and  "chalcedonic  appearance,"  containing 
parallel  bands  of  finely  divided  sulphides.  The  veins  are  in 
part  filled,  but  in  part  appear  to  have  been  formed  by  replacement 
of  the  country  rock.  The  primary  ore  contains  some  black  par- 
ticles of  finely  divided  gold  alloyed  with  much  silver.  Argentite 
and  polybasite  are  the  principal  ore  minerals,  with  small  amounts 
of  pyrite,  chalcopyrite,  galena,  and  zinc  blende.  Selenium  is 
present,  probably  as  a  silver  selenide.  Among  gangue  minerals 
there  are,  besides  quartz,  rhodonite,  adularia,  and  various  car- 
bonates. The  secondary  ores  developed  by  oxidation  and  sul- 
phide enrichment  are  described  in  Chapter  31.  Hiibnerite  (tung- 
state  of  manganese),  and  scheelite  (tungstate  of  calcium),  are 
believed  to  belong  to  the  second  period  of  vein  formation.  The 
relations  of  the  metals  in  exceptionally  rich  concentrates  are  as 
follows: 

Ag 25.92  percent.             Sb 0.92  per  cent. 

Au 0.82  per  cent.            Fe 9. 81  per  cent. 

Pb 6. 21  per  cent.            MgO 1.49  per  cent. 

Zn 5 . 84  per  cent.            CaO 3 . 70  per  cent. 

Cu 1 . 32  per  cent.            S not   determined. 

Se 2. 56  per  cent.            CO2 6. 34  per  cent. 

As 0.19  per  cent.            SiO2 15. 18  per  cent. 

The  Comstock  Lode.1 — Among  other  deposits  of  this  type  the 
Comstock  lode  deserves  special  mention.  Discovered  in  1859,  it 
has  yielded,  to  the  end  of  1911,  a  total  production  of  $381,400,000 
in  silver  and  gold,  of  which  the  gold  amounted  to  $153,000,000. 
The  bonanza  period  fell  in  the  seventies  of  the  last  century  and 
although  the  production  since  then  has  declined  greatly,  yet 
during  the  last  few  years  a  systematic  unwatering  of  the  deep 
levels  has  resulted  in  a  noteworthy  rise  in  output.  In  1916  the 
lode  yielded  $483,000  in  gold  and  286,000  ounces  of  silver,  the 
ore  having  an  average  value  of  $10.64  per  t6n.  The  Comstock 
lode,  situated  near  the  summit  of  the  Virginia  Range,  east  of  the 
Sierra  Nevada,  is  a  fault  fissure  of  great  throw  (Fig.  165),  trace- 
able two  and  one-half  miles  and  in  places  several  hundred  feet 
wide,  the  vein  matter  of  the  lode  spreading  in  the  hanging  wall. 
Great  bonanzas  of  crushed,  quartz,  in  part  exceedingly  rich  in 
silver  minerals,  were  found  at  intervals  along  the^lode,  especially 

1  G.  F.  Becker,  Man.  3,  U.  S.  Geol.  Survey,  1882. 
J.  A.  Reid,  Bull.  4,  California  Univ.  Dept.  Geology,  1905,  pp.  177-199. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      519 

in  chambers  or  vertical  fissures  probably  produced  by  normal 
faulting  of  the  hanging  wall.  The  greatest  vertical  depth  at- 
tained below  the  outcrop  is  about  3,000  feet.  Mining  has  been 
greatly  hampered  by  enormous  quantities  of  hot  water  containing 
mainly  calcium  sulphate.  None  of  the  great  bonanzas  were 
found  below  a  depth  of  2,000  feet. 

The  lode  intersects  igneous  rocks  of  deep-seated  type,  showing 
transitions  and  variously  classified  as  diorite,  diabase,  and  augite 
andesite.1  These  are  covered  by  andesite  flows  of  distinctly 


FIG.  165. — Vertical  cross-section  through  the  Comstock  lode,  showing 
chambered  outcrop  and  bonanza  in  vertical  vein  in  hanging  wall.  After 
G.  F.  Becker,  U.  S.  Geol.  Survey. 

Tertiary  age,  which  are  also  mineralized.  Both  classes  of  rocks 
have  suffered  propylitic  alteration,  and  analyses  of  the  clay 
gouge  near  the  veins  show  that  sericitic  alteration  has  set  in  along 
the  principal  channels  which  the  solutions  followed.  The  ores 
consist  of  quartz  and  some  calcite,  in  places  banded  with  pyrite, 
galena,  chalcopyrite,  zinc  blende,  and  finely  distributed  rich 
silver  minerals.  The  valuable  minerals  are  mainly  native  gold, 
argentite,  stephanite,  and  polybasite. 

1  A.  Hague  and  J.  P.  Iddings,  Bull  17,  U.  S.  Geol.  Survey,  1885. 


520 


MINERAL  DEPOSITS 


There  are  in  places  two  generations  of  quartz,  as  shown  in  Fig. 
166,  the  older  quartz  containing  principally  pyrite.  Zeolites 
are  reported  in  the  altered  country  rock  but  are  apparently  not 
common.  According  to  Reid  the  descending  waters,  rich  in 
sulphates,  contained  notable  amounts  of  gold  and  silver,  and 
small  quantities  of  these  metals  were  also  present  in  the  ascending 
hot  sulphate  and  carbonate  wraters.  Opinions  still  differ  as  to 
the  relative  importance  of  sulphide  enrichment  in  this  place. 


FIG.  166. — Rich  ore,  Ophir  mine,  Comstock  lode,  showing  earlier  fractured 
quartz  with  fine-grained  pyrite  and  some  argentite  (a)  and  later  vein  with 
three  generations  of  galena  and  argentite  (6)  with  some  pyrite,  chalcopyrite, 
and  quartz  (q).  Drawn  from  specimen  in  collection  of  Massachusetts 
Institute  of  Technology,  natural  scale. 

ARGENTITE  VEINS 

The  argentite  veins  have  numerous  representatives  in  Mexico, 
as  at  Pachuca,  Real,  del  Monte,  and  Guanajuato.  In  general 
they  intersect  andesitic  rocks  of  supposedly  Miocene  age  but 
also  cut  adjacent  or  underlying  Mesozoic  sediments. 

The  rich  and  long  worked  veins  of  Pachuca1  have  come 
into  renewed  prominence  by  the  successful  application  of  the 

1  J.  Aguilera  and  E.  Ord6fiez,  El  mineral  de  Pachuca,  Boletin,  Inst.  geol. 
de  M&dco,  Nos.  7,  8,  9,  1879. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      521 

cyanide  process  to  their  ores.  A  complicated  vein  system  inter- 
sects andesite  flows  covering  Cretaceous  sediments.  The 
andesite  is  extensively  propylitized  and  this  altered  rock  also 
adjoins  the  veins,  near  which,  however,  a  silicification  is  often 
superimposed  upon  the  chloritization.  In  places  the  andesite  is 
entirely  silicified.  The  veins  are  filled  fissures,  crustified  or 
brecciated,  with  quartz,  sometimes  amethysthine,  as  the  prin- 
cipal constituent  of  the  gangue;  there  are  also  rhodochrosite, 
rhodonite,  adularia,  and  calcite,  the  last  named  being  the  most 
recent. 

The  ore  minerals  consist  of  argentite,  stephanite,  polybasite, 
pyrite,  galena,  and  zinc  blende.  The  veins  have  been  followed  to 
a  depth  of  2,000  feet  and  here  contain  argentite,  pyrite,  and  zinc 
blende.  The  ores  average  about  18  ounces  of  silver  to  the  ton. 
The  oxidation  is  marked  by  the  zones  of  the  "colorados"  and  the 
"negros,"  the  first  of  which  contains  limonite  with  silver  haloid 
salts  and  the  second  much  manganese.  The  "negros"  are  said  to 
contain  more  gold  than  the  deeper  ores,  which  are  very  low  in 
this  metal. 

There  are  many  other  old  and  famous  silver-mining  districts 
in  Mexico,  the  veins  of  which  are  similar  to  those  of  Pachuca. 
Among  them  are  Guanajuato,  Zacatecas,  Sombre rete,  Fresnillo, 
Batopilas,  and  Parral.  In  many  of  these  districts,  however,  the 
veins  are  contained  in  Cretaceous  or  Jurassic  slates  and  sand- 
stones, though,  without  much  doubt,  the  mineralization  is  genet- 
ically connected  with  the  surrounding  igneous  rocks.  The  latter 
are  in  some  cases  of  intrusive  origin. 

GOLD  TELLURIDE  VEINS 

Cripple  Creek.1 — The  veins  of  Cripple  Creek,  situated  in  an 
otherwise  barren  part  of  Colorado,  have  since  1891  annually 
yielded  a  large  amount  of  gold,  which  in  1900  reached  a  maxi- 
mum of  $18,000,000.  In  1916  the  production  of  gold  was  valued 
at  $12,120,000,  but  the  silver  recovered  amounted  only  to  $53,- 
000.  The  total  output  of  the  district  to  the  end  of  1916  is  $285,- 

1  W.  Cross  and  R.  A.  F.  Penrose,  Sixteenth  Ann.  Report,  U.  S.  Geol. 
Survey,  pt.  2,  1896,  pp.  1-209. 

W.  Lindgren  and  F.  L.  Ransome,  Prof.  Paper  54,  U.  S.  Geol.  Survey, 
1906. 

Horace  B.  Patton,  The  Cresson  Bonanza  at  Cripple  Creek,  Min.  and 
Sci.  Press,  Sept.  15,  1917. 


522 


MINERAL  DEPOSITS 


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500,000.  Individual  mines  have  pro- 
duced from  $10,000,000  to  $30,000,000 
each.  The  district  lies  on  a  granitic 
plateau  a  few  miles  southwest  of  Pikes 
Peak,  at  elevations  of  9,000  to  11,000  feet. 
Within  a  few  square  miles  are  a  large 
number  of  producing  mines;  64  reported 
in  1916  the  production  of  $46,000  short 
tons  of  ore,  averaging  $12  per  ton.  In 
earlier  years  the  average  value  was  $30 
to  $40  per  ton,  but  during  recent  years 
an  increasing  quantity  of  low-grade  ore 
from  dumps,  etc.,  has  been  treated,  some 
of  it  containing  only  $3  or  $4  per  ton. 
Six  or  eight  large  mines,  among  which 
are  the  Golden  Cycle,  the  Portland,  the 
Vindicator,  the  El  Paso,  and  the  Elkton, 
contribute  one-half  the  output,  the  ore 
having  a  value  of  $18  to  $30  per  ton. 
The  cyanide  process  preceded  by  roast- 
ing is  now  almost  universally  used  for 
the  ores  of  higher  grade;  most  of  the  ore 
is  reduced  in  large  mills  at  Colorado 
Springs,  although  there  are  many  smaller 
cyanide  plants  in  the  district.1 

The  mining  operations  have  always 
suffered  from  a  large  quantity  of  mine 
waters  and  the  greatest  depth  attained 
is  about  2,000  feet.  This  is  in  the  region 
of  the  Portland,  Golden  Cycle  and  Vin- 
dicator mines.  The  Roosevelt  tunnel, 
recently  completed,  is  nearly  3  miles 
long  and  now  drains  the  mines  to  an 
elevation  of  8,020  feet— that  is,  770  feet 
below  the  El  Paso  drainage  tunnel.  This 
tunnel,  in  1916,  discharged  about  10,000 
gallons  per  minute  and  has  now  nearly 
reached  the  Portland  mine  at  an  eleva- 
tion of  8,112  feet  and  164  feet  below  the 
1,900  foot  level. 

1  Data  from  reports  by  C.  W.  Henderson  in 
Min.  Res.,  U.  S.  Geol.  Survey,  annual  publication. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      523 

The  rocks  constituting  the  plateau  are  pre-Cambrian  red 
granite,  with  some  gneiss  and  fibrolite  schist.  Breaking  through 
this  basement  is  a  mass  of  Tertiary  volcanic  rocks,  the  area 
having  a  diameter  of  2  or  3  miles.  As  shown  by  the  mining  opera- 
tions the  contact  surface  between  the  granite  and  the  volcanic 
mass  is  steep,  or  even  vertical  or  overhanging  and  there  is  little 
doubt  that  this  "plug"  of  volcanic  material  represents  the  core 
of  a  Tertiary  volcano  which  formerly  rose  above  the  plateau, 
as  tentatively  indicated  in  Fig.  167.  The  bulk  of  the  remaining 
core  is  composed  of  tuffs  and  breccias  of  latite-phonolite  and  these 
are  cut  by  dikes  and  intrusive  masses  of  phonolite  and  syenite. 


FIG.  168.  FIG.  169. 

FIG.  168. — Vein  filling,  Portland  mine,  Cripple  Creek  (purple  quartz). 
/,'Fluorite;  q,  quartz  of  coarser  and  finer  grain;  p,  pyrite.  Magnified  50 
diameters. 

FIG.  169. — Filled  space  of  dissolution  in  granite,  Independence  mine, 
Cripple  Creek  (granite  ore),  o,  Orthoclase  of  granite;  m,  biotite  replaced  by 
adularia  and  pyrite;  v,  adularia  (secondary  orthoclase,  showing  crustifica- 
tion;  q,  quartz.  Magnified  14  diameters. 

The  latest  manifestations  of  volcanism  were  basic  dikes  of  mon- 
chiquite  and  vogesite,  and  the  veins  were  apparently  formed 
soon  after  these  dikes  had  been  intruded.  Many  of  the  rocks 
contain  a  notable  amount  of  combined  water.  The  deposits 
are  veins  which  followed  a  system  of  roughly  radiating,  steep 
fissures  (Fig.  37),  believed  to  have  resulted  from  compressive 
stress  developed  in  a  settling  volcanic  mass.  The  physiographic 
history  of  the  district  indicates  that  the  surface  at  the  time  of 
vein  formation  was  only  from  a  few  hundred  feet  to  1,000  feet 
above  the  present  surface. 


524 


MINERAL  DEPOSITS 


In  the  granite  adjoining  the  contact  are  also  found  irregular 
bodies  of  ore,  formed  by  replacement.  Most  of  the  veins  are 
closely  spaced  sheeted  zones  (Fig.  42)  a  few  feet  wide,  though 
some  attain  a  width  of  20  to  40  feet.  The  ore  deposition  has 
generally  taken  place  by  filling  along  the  narrow  fissures  and  only 
to  a  smaller  extent  by  replacement  of  the  intervening  or  adjoin- 
ing rock.  Low-grade  ores  are 
formed  by  mineralization  of 
narrow  seams  in  the  country 
rock. 

There  is  little  or  no  native 
gold,  except  in  the  oxidized 
zone  (p.  830).  The  principal 
ore  mineral  is  calaverite 
(AuTe2,  with  but  little  silver), 
of  yellowish  white  color  and 
often  well  crystallized.  Asso- 
ciated with  this  are  small 
quantities  of  pyrite,  zinc 
blende,  tetrahedrite,  stibnite, 
and  molybdenite,  rarely  hiib- 
nerite.  The  gangue  consists 
of  quartz  and  fluorite,  with 
some  dolomitic  carbonate. 
The  fluorite,  quartz,  and 
calaverite  are  often  inter- 
grown,  forming  a  fine-grained 
rock  which  has  the  purple 
color  of  the  fluorite  (Fig.  168). 


The  vein  structure  is  drusv 


FIG.  170.— Section  through  Stratton's 
Independence  mine,  Cripple  Creek, 
showing  relation  of  veins  to  granite-  and  the  calaverite  was  among 
breccia  contact.  After  Lindgren  and  the  latest  minerals  formed. 
Ransome,  U.  S.  Geol.  Survey.  The  vein  filling  consists  of 

about  60  per  cent,  quartz,  20 

per  cent,  dolomite,  20  per  cent,  fluorite,  0.1  per  cent,  gold,  and 
0.2  per  cent,  tellurium,  with  iron,  copper,  zinc,  and  molybdenum 
present  in  fractions  of  1  per  cent.  Extremely  rich  pockets  of  cala- 
verite are  sometimes  found.  The  most  notable  instance  is  the 
Cresson  bonanza  referred  to  in  the  list  of  literature  (p.  521). 

The  replacement  ore  consists  of  the  ordinary  red  granite,  often 
drusy  and  partly  replaced  by  adularia,  fluorite,  and  calaverite 


DEPOSITS  FORMED  NEAR  THE  SURFACE      525 


(Fig.  169).  In  the  upper  levels  there  are  a  great  number  of 
short  veins  all  of  which  carry  more  or  less  ore.  In  the  aggregate 
these  veins  contain  an  enormous  amount  of  ore,  some  of  which 
is  of  exceedingly  high  grade.  The  veins  are  less  abundant  in  the 
lower  levels  and  some  of  them  are  of  lower  grade.  Many  rich 
veins  continue,  however,  to  the  lowest  levels. 

As  shown  in  the  report  cited  the  ore-shoots  are  to  a  marked 
degree  influenced  by  intersections  with  other  veins  o?  dikes,  but 
many  of  the  largest  and  richest  shoots  have  no  such  relation. 


FIG.  171. — General  north-south  section  through  Stratton's  Independence 
mine,  Cripple  Creek,  showing  stopes  on  Independence  vein.  After  Lindgren 
and  Ransome,  U.  S.  Geol.  Survey. 

The  greatest  horizontal  extension  of  a  shoot  is  1,300  feet.  Many 
shoots  terminate  in  depth,  while  others  have  continued  to  the 
greatest  depths  attained.  The  relations  at  Stratton's  Inde- 
pendence mine  are  illustrated  in  Figs.  170  and  171. 

The  tuffs  and  breccias  are  generally  altered  and  contain  some 
fine-grained  pyrite,  which  has  little  value;  the  dark  silicates  alter 
to  carbonates,  fluorite,  and  pyrite  and  the  feldspars  to  sericite 


526  MINERAL  DEPOSITS 

and  adularia.  Cross  and  Penrose  thought  this  propylitic  altera- 
tion earlier  than  the  veins,  while  Lindgren  and  Ransome  con- 
sider it  to  be  caused  by  the  same  kinds  of  solutions  that  filled 
the  fissures.  Similar  differences  of  opinion  have  been  expressed 
in  relation  to  the  propylitization  at  other  places  (p.  483).  The 
alteration  close  to  the  veins  is  remarkably  slight  at  Cripple  Creek. 
There  is  no  evidence  that  there  has  ever  been  an  active  circula- 
tion of  surface  water  in  the  district.  The  porous  breccias  and  in 
general  the  whole  volcanic  plug  are  filled  with  stagnant  water, 
while  there  is  little  water  in  the  surrounding  granite.  The 
general  conclusion  of  Lindgren  and  Ransome  is  that  the  vein- 
forming  epoch  was  brief  and  that  the  remarkable  and  abundant 
telluride  ores  were  formed  by  alkaline  solutions  emanating  from 
deeper  igneous  masses,  the  last  effects  of  these  emanations  being 
the  exhalations  of  carbon  dioxide  and  nitrogen,  which  have  not 
yet  subsided.  The  waters  ascended  rapidly  in  the  deeper  parts 
of  the  volcanic  plug,  but  near  the  surface  they  spread  out  in  more 
numerous  fissures  and  precipitation  followed  by  cooling  or  mix- 
ture with  descending  solutions. 

GOLD  SELENIDE  VEINS 

Occurrence  of  Selenides. — In  minute  quantities  selenium  is 
present  in  many  deposits,  particularly  in  the  pyritic  copper 
deposits,  and  it  is  recovered  on  a  rather  large  scale  during  the 
electrolytic  refining  of  copper.  As  distinct  minerals  the  selenides 
are  apparently  confined  to  the  metallic  veins  formed  at  moderate 
or  shallow  depths.  Their  presence  in  some  rare  quicksilver 
deposits  has  already  been  mentioned.  In  the  silver  veins  of 
Mexico  selenides  of  silver  and  lead  have  been  found;  and  in 
some  silver-gold  veins  like  those  at  Tonopah  or  gold-silver  veins 
like  those  at  Waihi  they  are  important  constituents.  At  both 
Tonopah  and  Waihi  other  minerals  are  present  in  quantities. 

The  type  of  veins  described  in  these  paragraphs  is  remarkably 
free  from  ore  minerals  other  than  native  gold  and  selenides, 
and  it  is  rare,  only  two  examples  being  known,  that  of  Republic, 
in  Washington,  and  that  of  Radjang  Lebong,  in  Sumatra.  In 
some  respects,  however,  the  Tonopah  veins  are  allied  to  this  type. 

In  both  the  places  mentioned  there  is  a  predominating  gangue 
of  very  fine-grained  quartz,  beautifully  banded  by  crustification, 
but  not  markedly  drusy;  it  has  a  "  chalcedonic "  appearance, 
although  there  is  really  but  little  chalcedony  present,  and  it 


DEPOSITS  FORMED  NEAR  THE  SURFACE      527 


resembles  strongly,  both  in  hand  specimen  and  in  thin  section, 
some  kinds  of  sinter  deposited  at  the  orifices  of  hot  springs.  The 
gold  is  present  in  very  fine  distribution  and  the  gold  selenide 
has  not  yet  been  positively  identified.  These  veins  have  prob- 
ably been  deposited  close  to  the  surface. 

Republic,  Washington. — At  Republic1  a  series  of  Miocene 
andesite  and  latite  flows  filling  an  old  valley  have  been  intruded 
by  a  mass  of  latite  porphyry  belonging  to  the  same  general 


FIG.  172. — Typical  ore,  Republic  mine,  Republic,  Washington,  a,  Fine- 
grained quartz,  banded;  b,  streak  of  black,  finely  divided  sulphides  and 
selenides  (?);  c,  altered  latite  porphyry.  Natural  size. 

period  of  eruption.  Over  a  considerable  area  the  andesite  and 
porphyry  have  suffered  normal  propylitization,  chlorite,  earthy 
carbonates,  and  pyrite  being  the  principal  minerals  formed.  A 
series  of  parallel  fractures  dipping  from  38°  to  80°  have  been 
opened  in  the  volcanic  rocks  and  are  occupied  by  sharply  defined 
veins  averaging  3  or  4  feet  in  width.  Against  these  the  propy- 

1  Howland  Bancroft  and  W.  Lindgren,  Butt.  550,  U.  S.  Geol.  Survey, 
1914 


528  MINERAL  DEPOSITS 

litic  rock  borders,  usually  with  little  further  alteration.  The 
banded  filling  (Fig.  172)  consists  of  quartz  and  calcite  and  also 
includes  dark  masses  of  jasperoid  of  uncertain  derivation.  There 
is  some  adularia  in  the  filling  and  in  a  few  places  can  be  seen  the 
beginning  of  a  replacement  of  calcite  by  fine-grained  quartz  and 
adularia.  In  one  mine  the  quartz  filling  has  been  replaced  by 
laumontite  containing  much  silver. 

Free  gold  is  rarely  visible,  but  the  valuable  portions  of  the 
veins  lie  along  narrow  dark  bands  that  are  parallel  to  the  crusti- 
fication  and  are  believed  to  represent  finely  divided  gold  selenide. 
Local  crusts  are  rich  in  free  gold,  tetrahedrite,  and  chalcopyrite 
and  this  material  contains  about  2  per  cent,  of  selenium,  which, 
according  to  experiments  by  Dr.  Chase  Palmer,  of  the  United 
States  Geological  Survey,  is  probably  combined  with  gold. 

The  ores  of  Republic  have  proved  difficult  to  treat  by  the 
cyanide  process.  Their  grade  varies  greatly,  averaging  perhaps 
$11  per  ton.  The  proportion  of  gold  to  silver  by  weight  is  about 
1:3.  Oxidation  has  resulted  in  small  changes  but  has  set  some 
silver  free.  Since  1897  the  district  has  yielded  about  $7,000,000. 
The  greatest  depth  attained  is  800  feet. 

Sumatra. — The  Radjang-Lebong  field,1  in  southern  Sumatra, 
has  yielded  much  gold  in  recent  years.  The  annual  production 
is  about  50,000  ounces  of  gold  and  300,000  ounces  of  silver. 
Andesite  is  the  country  rock  and  the  principal  vein,  which  has  a 
width  of  17  feet,  is  divided  into  five  well-defined  seams  separated 
by  silicified  andesite.  According  to  R.  Beck  the  bluish-gray 
quartz  is  beautifully  banded  in  thin  concentric  crusts  of  "fibrous 
quartz."2  The  rich  ore,  like  that  of  Republic,  is  indicated  by 
thin  dark  dendritic  crusts  similar  in  appearance  to  the  quick- 
silver selenide  from  Lerbach,  the  silver  selenide  at  Tillingerode 
(both  localities  in  the  Harz  Mountains),  and  the  copper  selenide 
of  Skrikerum,  in  Sweden. 

The  ore,  of  which  only  a  small  part  is  amenable  to  amalgama- 
tion, contains  on  an  average  41  grams  of  gold  and  318  grams  of 
silver  per  metric  ton.  There  is  a  little  pyrite  and  chalcopyrite. 
The  bullion,  according  to  Truscott,3  contains 

1  R.  Beck,  Lehre  von  den  Erzlagerstatten,  vol.  1,  1909,  p.  488. 

2  The  expression  "fibrous  quartz"  used  by  Beck  suggests  a  possible  re- 
placement of  primary  calcite  by  quartz  and  adularia. 

3  M.  Maclaren,  Gold,  London,  1909,  p.  298. 

S.  J.  Truscott,  Trans.  Inst.  Min.  and  Met.,  London,  vol.  10,  1902,  p.  53. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      529 

Gold  and  silver 91 .52 

Selenium 4.35 

Copper 1 . 82 

Lead 1.65 

Zinc 0.48 

Iron..                                                             .  0.14 


Total 99.96 

THE  BASE-METAL  VEINS 

Among  the  deposits  formed  relatively  near  the  surface  by 
ascending  thermal  waters  in  genetic  connection  with  igneous 
rocks,  ores  rich  in  the  baser  metals  and  worked  principally  for 
these  metals  are  rather  exceptional.  Heavy  deposits  of  pyrite 
and  chalcopyrite  ore  are  very  seldom  found;  one  instance  is 
furnished  by  the  Nacozari  district,  in  Mexico,  where  at  the  Los 
Pilares  mine1  a  large  body  of  low-grade  pyrite  and  chalcopyrite 
is  worked.  The  ores  occur  mainly  in  the  interstices  of  a  brec- 
ciated  rhyolite  in  a  series  of  fractures  arranged  in  almost  circular 
form.  The  pyrite  is  often  crystallized;  the  gangue  is  quartz; 
the  ore  is  poor  in  gold  and  silver. 

The  veins  and  stocks  of  the  San  Juan  region,  Colorado,  de- 
scribed in  more  detail  below,  are  sometimes  rich  in  lead,  zinc,  and 
copper,  but  yield  principally  gold  and  silver.  Both  galena  and 
zinc  blende  occur,  but  the  copper  is  derived  mainly  from  tetra- 
hedrite  or  enargite. 

The  Schemnitz  deposits,  in  Hungary,  consist  of  a  strong  vein 
system  intersecting  rhyolite  and  andesite  above  Triassic  slates 
and  Eocene  strata.  The  gangue  minerals  are  fine-grained  quartz 
and  amethyst,  together  with  later  calcite,  ankerite,  rhodochrosite, 
rhodonite,  siderite,  and  barite,  with  much  pyrite,  galena,  chalco- 
pyrite, and  zinc  blende.  Among  the  rarer  minerals  are  adularia, 
fluorite,  and  diaspore.  The  proportion  of  gold  to  silver  by  weight 
is  1:23. 

The  Bull-Domingo  and  Bassick  deposits,2  at  Silver  Cliff, 
Colorado,  yielded  principally  gold  and  silver.  The  ore  consisted, 
however,  largely  of  sulphides  and  tellurides,  which  in  the  Bass- 
ick mine  occurred  in  what  is  considered  a  volcanic  neck.  The 
Bassick  deposit  was  mined  to  a  depth  of  800  feet  and  yielded 

1  S.  F.  Emmons,  Econ.  Geol.,  vol.  1,  1906,  pp.  629-643. 

2  S.  F.  Emmons,  The  mines  of  Ouster  County,  Colorado,  Seventeenth  Ann. 
Rept..  U..S.  Geol.  Survey,  pt.  2,  1896,  pp.  430-447, 


530  MINERAL  DEPOSITS 

rich  ore.  The  cross-section  covered  about  25  by  100  feet  and 
the  ores  encrusted  the  fragments  of  volcanic  rocks  filling  the  shoot. 
When  the  solutions  depositing  veins  in  volcanic  rocks  leave 
the  flows  and  enter  into  the  surrounding  limestones  and  other 
sedimentary  rocks,  deposition  by  selective  precipitation  comes 
into  play  and  ores  rich  in  sulphides,  particularly  galena,  may  be 
formed.  Examples  of  this  are  found  in  several  deposits  of  the 
Ouray  district,  Colorado,  described  by  J.  D.  Irving. 

THE  SAN  JUAN  REGION,  COLORADO1 

General  Features. — One  of  the  most  interesting  metallogenetic 
provinces  is  that  of  the  rugged  San  Juan  region,  in  southwest 

1  W.  Cross  and  C.  W.  Purington,  U.  S.  Folio  57,  Telluride;  Cross,  Spencer, 
and  Purington,  U.  S.  Folio  60,  La  Plata;  Cross,  Howe,  and  Ransome,  U. 
S.  Folio  120,  Silverton;  Cross,  Spencer,  and  Ransome,  U.  S.  Folio  130, 
Rico;  Cross,  Howe,  Irving,  and  W.  H.  Emmons,  U.  S.  Folio  131,  Needle 
Mountains;  Cross  and  Hole,  U.  S.  Folio  171,  Engineer  Mountain;  Cross 
and  Howe,  U.  S.  Folio  153,  Ouray. 

C.  W.  Purington,  Report  on  mining  industries,  Telluride  quadrangle, 
Eighteenth  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  3,  1897,  pp.  745-850. 

Cross  and  Spencer,  Geology  of  the  Rico  Mountains,  Twenty-first  Ann. 
Rept.,  U.  S.  Geol.  Survey,  pt.  2,  1900,  pp.  7-165. 

F.  L.  Ransome,  Economic  geology  of  the  Silverton  quadrangle,  Bull. 
182,  U.  S.  Geol.  Survey,  1901. 

J.  D.  Irving,  Ore  deposits  of  the  Ouray  district,  Bull.  260,  U.  S.  Geol. 
Survey,  1905. 

W.  H.  Emmons,  Neglected  mine,  etc.,  Durango  quadrangle,  Bull.  260, 
U.  S.  Geol.  Survey,  1905. 

J.  D.  Irving  and  H.  Bancroft,  Geology  and  ore  deposits  near  Lake  City, 
Bull.  478,  U.  S.  Geol.  Survey,  1911. 

Arthur  Lakes,  Geology  of  western  ore  deposits,  Denver,   1905. 

T.  A.   Rickard,  Across  the  San  Juan  Mountains,  New  York,  1903. 

T.  A.  Rickard,  The  Enterprise  mine,  Rico,  Colo.,  Trans.  Am.  Inst.  Min. 
Eng.,  vol.  20,  1897,  pp.  906-980. 

C.  W.  Purington,  The  Camp  Bird  mine,  Trans.  Am.  Inst.  Min.  Eng., 
vol.  33,  1904,  pp.  499-528. 

C.  W.  Purington,  Ore  horizons  in  the  San  Juan  Mountains,  Econ.  Geol., 
vol.  1,  1905,  pp.  129-133. 

A.  Winslow,  The  Liberty  Bell  mine,  Trans.  Am.  Inst.  Min.  Eng.,  vol. 
29,  1900,  pp.  285-307. 

J.  E.  Spurr,  summary  in  Prof.  Paper  63,  U.  S.  Geol.  Survey,  1908, 
pp.  111-168. 

W.  H.  Emmons,  A  preliminary  report  on  the  geology  and  ore  deposits 
of  Creede,  Colo.,  Bull.  530,  U.  S.  Geol.  Survey,  1912,  p.  42-65. 

C.  W.  Henderson  in  Min.  Res.,  U.  S.  Geol.  Survey,  annual  publication. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      531 


Colorado,  including  the  mining  districts  of  Telluride,  Ouray, 
Silverton,  Lake  City,  Rico,  Needle  Mountains,  La  Plata,  and 
Creede  (Fig.  173).  Space  is  not  available  to  describe  these 
fully,  but  some  of  the  more  important  relations  should  be  pointed 


luu 


Scale  of  Miles 

FIG.  173. — Map     showing    approximate    distribution    of     the     principal 
muiing  regions  in  Colorado.     After  J.  E.  Spurf,  U.  S.  Geol.  Survey. 

out.     The  districts  are  known  mainly  through  the  labor  of  the 
geologists  of  the  Federal  Survey. 

The  San  Juan  Mountains  consist  chiefly  of  volcanic  rocks, 
poured  out  over  a  basement  of  Paleozoic  and  Mesozoic  sediments; 
pre-Cambrian  rocks  are  also  exposed  in  places  (Fig.  174).  The 
volcanic  flows,  occupying  over  3,000  square  miles,  have  a  total 


532 


MINERAL  DEPOSITS 


thickness  of  many  thousand  feet  and  were  erupted  at  intervals 
during  the  whole  Tertiary  period.  The  lowest  formation,  called 
the  San  Juan  tuff,  consists  of  about  3,000  feet  of  andesitic  mate- 
rial. Covering  this  are  andesites,  latites,  and  rhy elites,  called 
the  Silverton  series,  4,000  feet  thick;  this  in  turn  is  overlain  by 
the  Potosi  series,  consisting  of  rhyolite  and  latite.  The  eruptions 
were  separated  by  epochs  of  erosion  and  were  closed  by  the 
effusion  of  the  Hinsdale  series  of  rhyolites,  andesites,  and  basalts. 
Deep  erosion  has  laid  bare  these  flows  to  a  depth  of  several 
thousand  feet  and  exposed  a  number  of  smaller  intrusive  stocks 
and  sheets  of  dioritic  or  monzonitic  character  and  granular  or 
porphyritic  texture.  The  very  latest  intrusions  are  represented 
by  dark  dikes  of  lamprophyric  character. 


Scale  of  Miles 


FIG.  174. — Section  at  Telluride,  Colorado.  Ag,  Algonkian  quartzite;  Jd, 
Dolores  formation  (Jurassic);  Kmc,  Mancos  shale  (Cretaceous);  Esm,  San 
Miguel  conglomerate  (Eocene);  Sj,  San  Juan  tuff;  Pr,  Potosi  rhyolite;  gd, 
gabbro-diorite;  Sj,  Pr,  gd,  Eocene  and  Miocene.  After  W.  Cross,  U.  S.  Geol. 
Survey. 

This  vast  uplifted  and  eroded  dome  of  volcanic  rocks  is  trav- 
ersed by  numerous  systems  of  strong  fissure  veins,  many  of 
them  continuous  for  several  miles.  Their  development  followed 
closely  after  the  latest  epoch  of  volcanic  activity  and  they  inter- 
sect several  of  the  intrusive  masses.  While  the  vein  systems 
bear  the  marks  of  deposition  within  a  moderate  distance  of  the 
original  surface,  there  are  some  features  that  tend  to  connect 
them  with  deposits  formed  at  greater  depth  and  in  more  direct 
genetic  connection  with  igneous  intrusions.  Many  types  are 
represented,  including  normal  veins,  stocks,  replacement  deposits 
in  sedimentary  formations,  and  some  small  contact-metamorphic 
deposits  near  the  contacts  of  the  intrusive  masses.  Some  of  the 
veins  are  exposed  over  vertical  distances  of  several  thousand  feet. 
The  Virginius  vein,  above  Ouray,  for  instance,  has  been  mined 


DEPOSITS  FORMED  NEAR  THE  SURFACE      533 


for  a  vertical  distance  of  3,300  feet;  the  Revenue  tunnel  is  2,400 
feet  below  the  outcrops. 

The  predominating  val- 
ues are  in  gold  and  silver, 
but  in  some  districts  much 
lead,  copper  and  zinc  are 
also  present,  mainly  as 
galena,  tetrahedrite,  and 
zinc  blende.  The  gangue 
is  mainly  quartz,  but  rho- 
dochrosite,  rhodonite,  ba- 
rite,  and  fluorite  are  often 
present. 

Telluride  District. — In 
the  Telluride  district  the 
present  annual  production 
is  about  $2,000,000  in  gold 
and  3,000,000  ounces  of 
silver,  with  some  copper 
and  lead,  mainly  from  the 
strong  veins  worked  by. 
the  Liberty  Bell,  Smuggler- 
Union,  and  Tomboy  mines. 
The  veins  are  filled  fissures, 
averaging  3  or  4  feet  in 
width,  with  crustification 
banding,  drusy  structure, 
sericitization,  and  silicifi- 
cation  of  the  walls.  The 
larger  lodes  appear  often 
as  a  number  of  parallel 
plates  of  filled  veins  sep- 
arated by  sheets  of  altered 
rock.  The  Smuggler  vein, 
for  instance,  cuts  through 
the  San  Juan  tuff,  the 
andesite,  and  the  rhyolite, 
a  vertical  distance  of  2, 000 
feet  (Fig.  175).  What 
aspect  the  veins  assume  in  the  underlying  sedimentary  forma- 
tion is  as  yet  unknown. 


534 


MINERAL  DEPOSITS 


The  gangue  minerals  are  quartz,  calcite,  siderite,  rhodochrosite, 
udularia,  barite,  and  fluorite,  the  last  being  abundant  in  the  Tom- 
boy mine  (Fig.  176).  Native  gold,  pyrite,  galena,  zinc  blende, 
and  chalcopyrite  are  the  principal  metallic  minerals.  The  ores 
contain  2  or  3  per  cent,  sulphides  and  yield  about  $6  in  gold 
and  a  few  ounces  of  silver  per  ton.  The  treatment  consists  of 
a  combination  of  amalgamation,  concentration,  and  cyaniding. 
There  are  several  minor  silver  veins  and  replacement  deposits 
in  the  district. 


1 


1'  2   3  4 


1  Ft. 


FIG.  176. — Section  showing  succession  of  ore  minerals  in  Mendota 
workings,  Smuggler  vein  Telluride,  Colorado.  1,  Country  rock;  1',  seri- 
citized  and  impregnated  country  rock;  2,  zinc  blende  with  calcite;  2', 
zinc  blende  with  galena;  3,  white  quartz;  4,  rhodochrosite;  5,  blue  quartz 
with  finely  disseminated  sulphides.  After  C.  W.  Purington,  U.  S.  Geol. 
Survey. 

Silverton  District. — The  Silverton  district  is  rich  in  veins, 
which  contain  more  base  metals  than  those  of  the  Telluride  dis- 
trict. The  present  annual  production  is  about  $500,000  in 
gold,  500,000  ounces  of  silver,  3,500,000  tons  of  lead,  and  some 
copper  and  zinc.  The  deposits  are  simple  veins  or  lodes,  averag- 
ing about  3  feet  in  thickness.  The  structure  is  commonly  mas- 
sive but  sometimes  banded  (Fig.  177)  and  drusy;  on  the  whole  it 


DEPOSITS  FORMED  NEAR  THE  SURFACE      535 


FIG.  177. — Cross  section  of  banded  vein  near  London  shaft,  Silverton, 
Colorado,  a,  Country  rock;  b,  quartz  and  chalcopyrite;  c,  tetrahedrite; 
d,  d',  quartz;  e,  galena.  Vein  6  inches  wide.  After  F.  L.  Ransome,  U.  S. 
Geol.  Survey. 


FIG.  178. — Thin  section  of  ore  from  ividgeway  mine,  Silverton,  Colorado. 
Large  black  areas,  pyrite;  small  black  areas,  argentite,  with  a  little  galena 
and  zinc  blende;  shaded  grains,  quartz.  Magnified  17  diameters.  After 
F.  L.  Ransome,  U.  S.  Geol  Survey. 


536  MINERAL  DEPOSITS 

resembles  that  of  deeper-seated  veins.  Many  of  the  veins  are 
rich  in  sulphides. 

The  prevailing  gangue  mineral  is  quartz  and  this  is  of  coarser- 
grained  texture  than  is  common  in  the  veins  deposited  near 
the  surface  (Fig.  178).  The  gangue  also  includes  much  calcite, 
dolomite,  rhodochrosite,  rhodonite,  barite,  and  fluorite.  The  ore 
minerals  are  pyrite,  galena,  chalcopyrite,  zinc  blende,  tetrahedrite, 
enargite,  argentite,  and  native  gold,  more  rarely  hiibnerite, 
molybdenite,  and  various  sulphantimonides  and  bismuthides. 
Tetrahedrite  and  galena  are  very  abundant.  Tellurides  are  rare. 
Rock  alteration  in  this  district  is  discussed  in  some  detail  on 
pages  486-487. 

Ouray  District. — The  Camp  Bird  lode,  in  Ouray  County, 
probably  represents  the  continuation  of  one  of  the  Telluride  lodes. 
For  many  years  the  Camp  Bird  mine  has  yielded  rich  returns, 
amounting  from  1903  to  1910  to  $16,500,000,  principally  in  gold, 
though  some  silver,  copper  and  lead  are  recovered.  The  ore 
contains  about  $22  in  gold  per  ton.  The  lode  intersects  San 
Juan  tuff  and  andesite  and  is  described  as  a  sheeted  zone  4  or  5 
feet  thick  made  up  of  alternating  fissure  filling  and  altered  rock; 
filling  was,  however,  the  predominating  process.  The  gangue 
is  quartz,  often  crusted  and  banded,  with  rhodochrosite,  calcite, 
and  fluorite.  The  metallic  minerals  are  very  fine  native  gold, 
with  a  few  per  cent,  of  galena,  pyrite,  and  zinc  blende,  also 
some  finely  distributed  tellurides.  Mining  was  discontinued  in 
1916. 

Irving  describes  a  number  of  replacement  deposits  in  the  Ouray 
district  just  outside  of  the  volcanic  area.  They  are  contained 
in  quartzite  or  limestone  below  impervious  beds  and  yield 
sulphides,  ores  with  galena,- tetrahedrite,  chalcopyrite,  jasperoid, 
and  barite. 

Rico  District. — The  Rico  Mountains  are  a  domelike  uplift  of 
sedimentary  rocks  ranging  from  Algonkian  to  Jurassic  in  age, 
intruded  by  stocks,  sheets,  and  sills  of  monzonite,  or  monzonite 
porphyry  (Fig.  179).  The  ores  are  therefore  rather  of  the  deep- 
seated  type,  but  nevertheless  their  genetic  connection  with  the 
other  deposits  of  the  San  Juan  region  is  clear.  The  deposits 
form  lodes,  bed-veins  (blankets),  and  replacements.  The 
blankets  often  lie  parallel  to  the  sheets  of  intruded  rocks  or  below 
impervious  shales.  The  abundant  ore  minerals  consist  of  pyrite, 
galena,  zinc  blende,  and  tetrahedrite,  in  a  gangue  of  quartz, 


DEPOSITS  FORMED  NEAR  THE  SURFACE       537 


LaPlata          Dolores  Hico  Hermosa        Devonian      Algonkian      Monzonite. 

porphyry 
Scale  of  Miles 
0  %  1  2 


FIG.  179. — Geological  section  through  part  of  the  Rico  dome,  Colorado. 
After  Cross  and  Spencer,  U.  S.  Geol.  Survey. 


1TY 


BLENDE  u.-j  GAL 


*   m. 


FIG.  180.— Banded  ore,  Rico,  Colorado.     A/ier  T.  A.  Rickard. 


538  MINERAL  DEPOSITS 

rhodochrosite,  calcite,  and  fluorite.  The  filling  is  often  beauti- 
fully banded  (Fig.  180).  A  very  limited  vertical  range  is  char- 
acteristic of  the  deposits.  The  greater  part  of  the  production 
has  come  from  the  blankets,  a  short  distance  below  which  the 
veins  have  become  impoverished  in  the  Hermosa  (Pennsylvanian) 
formation  of  sandstone,  shale,  and  limetone.  Besides  the 
silver-lead  ores  the  district  contains  larger  bodies  of  low-grade 
pyritic  ores.  The  rich  silver  minerals  were  mainly  the  result  of 
oxidation  and  sulphide  enrichment  of  the  galena  and  tetrahedrite. 

La  Plata,  Durango,  and  Needle  Mountains  Quadrangles. — 
South  of  Rico  the  sandstones  of  the  Jurassic  and  Triassic  (Dolores 
formation)  are  abundantly  intruded  by  sheets  of  monzonitic 
rocks.  Late  dikes  of  basic  character  intersect  the  monzonite 
porphyries.  Veins,  cutting  both  porphyries  and  sediments  con- 
tain native  gold,  tellurides  (petzite,  sylvanite,  and  calaverite), 
argentiferous  tetrahedrite,  tennantite,  stephanite,  amalgam, 
pyrite,  marcasite,  chalcopyrite,  galena,  and  zinc  blende.  In  the 
gangue  quartz  prevails,  with  some  rhodochrosite  and  barite 
The  similarity  to  the  telluride  veins  of  Lake  City  is  striking. 
Entirely  similar  telluride  veins  occur  in  the  northeastern  part 
of  the  Needle  Mountains  quadrangle  south  of  Silverton. 

Lake  City  District. — The  production  of  this  district  is  now 
small,  but  it  has  yielded  a  total  output  of  $1,250,000  in  gold, 
4,000,000  ounces  .of  silver,  and  40,000  tons  of  lead,  the  value  of 
the  lead  exceeding  that  of  the  silver. 

The  deposits  range  from  simple  veins  to  more  complicated 
lodes,  but  filling  with  banding  was  the  predominating  process  in 
their  development.  The  width  ranges  from  a  few  inches  to  20  feet . 
Many  of  the  fissures  are  short  and  were  found  to  pinch  out  at 
relatively  slight  depth.  The  alteration  of  the  wall  rock  is  slight 
but  is  marked  by  some  silicification  and  sericitization.  Pyritiza- 
tion  of  the  surrounding  rhyolite  or  andesite  extends  to  consider- 
able distances.  The  veins  intersect  all  rocks  below  the  Potosi 
series  and  also  cut  the  intrusions  of  monzonite  porphyry.  The 
following  types  of  veins  are  recognized : 

1.  Tetrahedrite-rhodochrosite  veins.     The  ores  contain  mainly 
galena,  pyrite,  and  tetrahedrite,  the  last  rich  in  silver,  with  some 
pyrite  in  a  gangue  of  quartz,  barite,  and  rhodochrosite. 

2.  Quartz-galena-zinc  blende  veins.     The  ores  of  these  veins 
contain  dominant  galena,  pyrite,  and  zinc  blende,  with  subor- 
dinate chalcopyrite,  in  a  quartz  gangue. 


DEPOSITS  FORMED  NEAR  THE  SURFACE       539 

3.  Telluride  veins.  The  few  veins  of  this  type  contain  gold 
and  silver  tellurides  disseminated  through  a  quartz  gangue  with 
subordinate  sulphides,  tetrahedrite,  and  barite. 

Creede  District. — In  the  Creede  district,  in  the  eastern  part  of 
the  great  volcanic  area,  strong  and  beautifully  banded  veins 
intersect  rhyolite.  They  carry,  in  a  gangue  of  amethystine 
quartz,  barite,  and  some  fluorite,  a  considerable  amount  of  galena 
and  zinc  blende.  The  average  ore  in  1910  contained  0.09  ounce 
of  gold  and  12.29  ounces  of  silver  per  short  ton,  0.02  per  cent, 
copper,  6.5  per  cent,  lead,  and  1.9  per  cent,  zinc.1  Some  of  the 
ores  are  considerably  richer  in  galena.  Most  of  the  ore  is  con- 
centrated, and  the  total  production  in  1910  had  a  value  of  about 
$1,000,000.  In  1916  the  total  value  was  $471,000. 

Summary. — The  deposits  of  the  San  Juan  region  consist  of 
gold-bearing  quartz  veins,  gold-telluride  veins,  and  base-metal 
veins  generally  carrying  galena  and  tetrahedrite.  The  descrip- 
tions show  a  merging  of  the  types  and  certain  minerals  common 
to  most  of  them,  such  as  rhodochrosite,  rhodonite,  and  barite. 
The  ores  are  usually  well  banded  and  crustified. 

The  ores  are  clearly  independent  of  the  character  of  the  country 
rock,  as  has  been  already  noted  by  J.  D.  Irving.  They  also 
intersect  the  intrusive  monzonites,  where  these  are  exposed  by 
erosion,  and  they  are  apparently  derived  from  a  deeper  source. 
The  ores  were  probably  deposited  in  one  main  epoch  following 
the  intrusive  activity. 

Spurr  believes  that  fissuring  and  mineralization  were  caused  by 
and  followed  the  dome-like  uplifts  and  that  these  were  due  to 
deep-seated  intrusions,  not  yet  exposed  by  erosion.  The  metals 
in  the  deposits  would  then  have  been  derived  from  the  ascending 
emanations  of  these  deep-seated  intrusives.  The  region  shows, 
as  few  others  do,  the  relation  of  the  upper  and  deeper  vein  zones. 


GOLD-ALUNITE  DEPOSITS 

General  Features. — In  volcanic  regions  it  is  not  uncommon  to 
find  considerable  areas  of  bleached  and  altered  lavas  which  con- 
tain more  or  less  alunite  (K20.3A12O3.4S03.6H2O),  an  earthy  or 
compact,  rarely  coarsely  crystalline  mineral  of  inconspicuous  ap- 
pearance (p.  479).  Occasionally  diaspore  or  gibbsite  is  associated 

1  C.  W.  Henderson  in  Min.  Res.,  U.  S.  Geol.  Survey,  pt.  1,  1910,  p.  424. 


540  MINERAL  DEPOSITS 

with  ahmite.1  In  most  cases  this  basic  sulphate,  which  is 
.  insoluble  in  water,  is  probably  formed  by  the  action  of  waters 
containing  free  sulphuric  acid  on  aluminous  rocks.  It  is  also 
found  in  places  in  the  oxidized  zones  of  veins  containing  pyrite.2 
In  such  altered  zones  in  volcanic  rocks  alunogen,  jarosite,  halo- 
trichite,  and  other  sulphates  of  iron  and  aluminum  are  often 
encountered  as  products  of  solution  and  oxidation.3  Pyrite 
sometimes  appears  as  a  primary  constituent,  its  iron  being 
probably  derived  from  the  ferromagnesian  silicates  of  the  rock. 

Although  the  alunite  itself  is  used,  in  large  deposits,  for  the 
production  of  alum  and  similar  salts,  it  is  unusual  to  find  rare 
metals  associated  with  areas  of  alunitization.  Only  one  such 
deposit  has  been  discovered,  and  that  is  the  remarkable  bonanza 
of  Goldfield,  described  by  F.  L.  Ransome.4 

Goldfield,  Nevada.— The  Goldfield  district,  discovered  in  1902, 
lies  in  a  low  range  of  desert  hills  in  western  Nevada.  The  total 
production  to  the  end  of  1917  was  $80,700,000  in  gold  and 
1,250,000  ounces  of  silver.  In  1911  the  district  yielded  gold  and 
silver  valued  at  $10,300,000;  in  1917  only  $2,000,000. 

The  geological  features  consist  of  a  succession  of  volcanic  flows,  of 
which  15  members  are  recognized,  resting  on  a  basement  of  granite 
and  Cambrian  shale.  The  age  of  these  lava  flows  and  intercalated 
lacustrine  beds  probably  ranges  fropi  Eocene  to  the  latest  Plio- 
cene. On  the  basement  rest  several  flows  of  rhyolite;  then  come 

1  Whitman  Cross,  Geology  of  Silver  Cliff  and  the  Rosita  Hills,  Seven- 
teenth Ann.  Report,  U.  S.  Geol.  Survey,  pt.  2,  1896,  pp.  263-403. 

L.  de  Launay,  La  m6talloge'nie  de  1'Italie,  10th  Int.  Geol.  Congress, 
Mexico,  1907,  pp.  125-132. 

E.  S.  Larsen,  Alunite  in  the  San  Cristobal  quadrangle,  Colorado,  Bull. 
530,  U.  S.  Geol.  Survey,  1913,  pp.  179-183. 

R.  T.  Hill,  Camp  Alunite,  Nevada,  Eng.  and  Min.  Jour.,  vol.  86,  1908, 
pp.  1203-1206. 

2  W.  Lindgren  and  F.  L.  Ransome,  Prof.  Paper  54,  U.  S.  Geol.  Survey, 
1906,  p.  125. 

For  more  complete  list  of  literature  see  F.  L.  Ransome,  Prof.  Paper  66, 
U.  S.  Geol.  Survey,  1909.  p.  132,  and  B.  S.  Butler  and  H.  S.  Gale,  Alunite,  a 
newly  discovered  deposit  near  Marysvale,  Utah,  Bull.  511,  U.  S.  Geol.  Survey, 
1912. 

»  C.  W.  Hayes,  Bull.  315,  U.  S.  Geol.  Survey,  1907,  p.  215. 

4  Prof.  Paper  66,  U.  S.  Geol.  Survey,  1909. 
Econ.  Geol.,  vol.  5,  1910,  pp.  301-311,  438-470. 

1  Alunitized  areas  have  been  described  from  Vancouver  island,  in  Jurassic 
and  Triassic  lavas.  These  altered  rocks  contain  a  little  gold.  See  C.  H. 
Clapp,  Econ.  Geol.,  vol.  10,  1915,  pp.  70-88. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      541 

heavy  masses  of  andesite  or  latite.  In  the  andesite  is  intruded 
a  thick  sheet  of  dacite  in  which  most  of  the  ore-bodies  are  found. 
The  andesite  is  overlain  by  1,000  feet  of  lacustrine  beds,  and  still 
later  than  these  are  thin  flows  of  basalt  and  rhyolite.  The  dacite 
sheet,  almost  700  feet  thick,  consists  of  a  rock  of  intermediate 
composition  with  glassy  groundmass.  Silica  amounts  to  60 
per  cent.,  and  the  potash  about  equals  the  soda.  The  pre-lacus- 
trine  rocks  are  considered  Eocene.  The  lacustrine  conditions 
ended  by  deformation  and  a  low  dome-like  uplift  resulted. 

The  principal  producing  area  is  small;  most  of  the  deposits 
are  in  the  dacite,  though  a  few  are  contained  in  rhyolite,  andesite, 
or  latite.  The  deposits  are  probably  younger  than  the  lake  beds 
but  older  than  the  latest  rhyolite  and  basalt;  the  age  is  considered 
late  Miocene  or  early  Pliocene.  They  were  thus  formed  in  inter- 
volcanic  time,  like  so  many  others  of  the  ore  deposits  connected 
with  effusive  rocks.  The  deposits  are  contained  in  an  area  of 
highly  altered  rock  and  are  irregular  silicified  and  fractured 
rock  masses  to  which  the  term  "ledges"  is  applied;  though 
irregular  (Fig.  181),  the  ledges  show  a  more  or  less  marked  elon- 
gation in  one  direction  and  are  often  vertical.  The  ledges  occur 
in  dacite,  in  andesite,  and  in  some  of  the  rhyolites;  they  are  then 
parts  of  the  country  rock  which  have  suffered  more  silicification 
than  the  adjoining  rock,  which  is  also  altered.  The  altered  rock, 
below  the  zone  of  oxidation,  is  a  porous  dark-gray  material 
with  softer  white  spots  to  indicate  the  feldspar  phenocrysts. 

The  four  minerals  characteristic  of  this  type  of  alteration  are 
quartz,  kaolinite,  alunite,  and  pyrite.  The  altered  dacite  con- 
tains approximately  49  per  cent,  quartz,  24  per  cent,  kaolinite, 
16  per  cent,  alunite,  and  7  per  cent,  pyrite,  and  the  replacement 
has  been  attended  by  losses  of  much  CaO,  MgO,  and  NagO  and  of 
some  SiO2,  A12O3,  and  K2O.  Water,  sulphuric  acid,  and  sulphur 
have  been  added.  There  is  no  change  in  titanium  and  phos- 
phorus. Under  the  assumption  that  no  change  in  volume  has 
taken  place  the  altered  rock  has  lost  5.3  per  cent,  in  weight  and 
now  has  a  porosity  of  10  per  cent. 

The  ore-shoots  form  irregular  bodies  in  the  ledges  and  have 
no  visible  limits  other  than  those  indicated  by  the  assay,  but 
the  ore  is  usually  marked  by  more  distinct  brecciation  and  seam- 
ing than  the  surrounding  ledge  rock.  During  the  earlier  years 
little  of  the  ore  contained  less  than  $30  per  ton,  but  in  1916  ores 
yielding  only  $8  per  ton  were  treated.  Some  shoots  are  roughly 


542 


MINERAL  DEPOSITS 


Ore  Ledge  matter  Dacite  Latite 


FIG.  181. — Plan  of  the  109-foot  level  and  vertical  section  of  the  January 
mine,  Goldfield,  Nevada.     After  F.  L.  Rcmsome,  U.  S.  Geol.  Survey. 


DEPOSITS  FORMED  NEAR  THE  SURFACE       543 

equidimensional  masses;  others  are  tabular,  though  often  twisted 
and  warped.  In  the  lower  levels  (a  depth  of  1,750  feet  has  now 
been  attained  in  the  principal  mine)  the  ore-bodies  are  said  to  be 
more  veinlike  and  regular,1  and  the  ore  contains  more  copper. 
Much  low-grade  ore  containing  gold,  copper  and  silver  is  said 
to  have  been  found  on  the  contact  of  latite  and  shale.  The 
rich  shoot  in  the  Mohawk  ground  at  a  depth  of  200  feet 
formed,  an  irregular  series  of  chambers  about  100  feet  high  with 
a  stope  length  of  75  feet;  below  the  245-foot  level  this  shoot  merges 
with  other  shoots  to  form  an  irregular  chain  of  workings.  A 

Surface 


Latite 
280-foot  leve 


380-foot  level 


FIG.  182.  —  Vertical  section  of  the  Combination  ledge,  Goldfield,  Nevada. 
After  F.  L.  Ransome,  U.  S.  Geol.  Survey. 

shoot  in  the  Combination  mine  (Fig.  182)  on  the  130-foot 
level  is  500  feet  long,  up  to  40  feet  wide,  and  100  feet  high. 
Ransome  states  that  there  is  a  gradual  though  irregular  decrease 
in  the  tenor  of  the  ore  in  depth  and  that  at  a  depth  of  about 
1  ,000  feet  the  workings  are  likely  to  pass  into  underlying  rocks 
—  andesite,  or  pre-Tertiary  basement  rocks  —  and  this  would 
probably  be  attended  by  changes  in  the  mineralization.  This 
prediction  has  been  verified.  While  the  shoots  are  not  large  com- 
pared to  those  of  many  other  mines,  the  ore  has  been  extremely 
rich.  Much  of  it  has  averaged  $419  per  ton,  containing  20 
1  A.  Locke,  Eng.  and  Min.  Jour.,  Oct.  26,  Nov.  2,  1912. 


544 


MINERAL  DEPOSITS 


ounces  of  gold  and  3  ounces  of  silver  to  the  ton.     One  ship- 
ment of  14^  tons  is  said  to  have  brought  $45,783. 1 

The  unoxidized  ore  contains  fine-grained  pyrite  and  marcasite, 
bismuthinite,  goldfieldite  (5CuS.(Sb,  Bi,  As)2(Se  Te)3,  an  arsen- 
ical famatinite  (Cu3SbS.i),  and  native  gold,  also  tellurides,2  all  in  a 


FIG.  183. — Photomicrograph  of  silicified  dacite,  Goldfield,  Nevada, 
showing  quartz  of  varying  grain,  p,  Pyrite;  t,  famatinite;  q,  quartz; 
h,  vug.  Magnified  40  diameters.  After  F.  L.  Ransome,  U.  S.  Geol.  Survey. 

dark-gray  flinty  quartz  gangue  (Fig.  183).  Concentric  shells  of 
ore  minerals  with  much  finely  divided  yellowish-brown  native 
gold  about  greatly  altered  fragments  of  rock  are  rather  characteris- 

1  F.  L.  Ransome,  op.  cit.,  p.  171. 

2  F.  L.  Ransome,  op.  cit.,  pp.  115-116. 

W.  J.  Sharwood,  Gold  tellurides,  Min.  and  Sci.  Press,  vol.  94,  1907, 
p.  731. 


DEPOSITS  FORMED  NEAR  THE  SURFACE      545 


tic  of  the  rich  ore.  Other  sulphides  like  galena  and  zinc  blende, 
which  elsewhere  are  common,  are  rare  at  Goldfield.  The  fol- 
lowing analysis  or  rich  ore  shows  well  its  unusual  character. 

ANALYSIS  OF  ORE  FROM  THE  MOHAWK  MINE,  GOLDFIELD,  NEVADA 


Per  cent. 

Per  cent. 

SiO2       .      . 

66  30 

Bi 

0  35 

A12O3       

9  09 

Te 

2  42 

CaO  

none 

Sb 

trace 

MgO 

0  24 

Au 

2  OO1 

H2O              .    .    . 

7  00 

Ae 

0  25 

Cu  

2  08 

s 

6  30 

Fe     . 

3  83 

Zn  

trace 

99  86 

1  Equal  to  541  fine  ounces  per  ton. 

While  the  deposition  of  pyrite,  alunite,  and  kaolinite  proceeded 
during  the  whole  epoch  of  ledge  formation,  the  richest  ores  were 
deposited  somewhat  later  in  brecciated  and  shattered  parts. 
The  water  level  stands  100  to  150  feet  below  the  surface;  above 
this  the  ores  are  oxidized  and  contain  some  gypsum  but  do  not 
differ  greatly  in  tenor  from  the  fresh  ores  below  water  level. 

These  remarkable  deposits  are  believed  to  have  been  formed 
by  ascending  alkaline  waters,  containing  hydrogen  sulphide, 
which  derived  their  load  of  rare  metals  from  deep-seated  magmas. 
Through  oxidation  near  the  surface  sulphuric  acid  was  generated 
from  the  hydrogen  sluphide  and  this  acid  attacked  the  rocks, 
causing  the  alunitic  alteration.1  The  sulphuric  acid,  descending 
and  mingled  with  surface  waters,  acted  as  a  precipitant  for  the 
gold  solution,  and  this  combination  of  ascending  alkaline  and 
descending  acid  waters  has,  according  to  Ransome,  probably 
resulted  in  the  development  of  this  unusual  type. 

The  surface  at  the  time  of  ore  deposition  was  probably  only  a 
few  hundred  feet  above  the  present  surface. 

1  Alunite  is  said  to  continue  as  a  gangue  mineral  down  to  the  low  grade 
ore-bodies  on  the  shale  contact.  These  also  contain  much  pyrite  and  fama- 
tinite  but  little  free  gold  and  tellurides. 


CHAPTER  XXV 

METALLIFEROUS  DEPOSITS  FORMED  AT  INTERMEDI- 
ATE DEPTHS  BY  ASCENDING  THERMAL  WATERS  AND  IN 
GENETIC  CONNECTION  WITH  INTRUSIVE  ROCKS 

GENERAL  FEATURES 

It  is  exceedingly  common  to  find  metalliferous  deposits  in  or 
near  bodies  of  intrusive  rocks.  These  deep-seated  rocks  have 
been  exposed  by  long-continued  erosion  and  in  places  it  is  possi- 
ble to  arrive  at  a  good  estimate  of  the  thickness  of  rocks  removed, 
especially  where  the  total  thickness  of  the  sedimentary  series  in 
which  the  intrusion  occurred  is  known.  That  the  deposits  are 
not  of  recent  development,  but  were  formed  a  short  time  after 
the  intrusion,  can  in  most  cases  be  proved  conclusively,  and 
from  this  it  follows  that  they  were  developed  under  a  great 
thickness  of  covering  rocks.  Their  depth  below  the  actual  sur- 
face at  the  time  of  mineralization  may,  roughly  speaking,  be 
considered  as  ranging  from  4,000  to  12,000  feet.  In  most  of 
these  deposits  the  absence  of  high-temperature  minerals,  such 
as  magnetite,  garnet,  pyroxene,  or  tourmaline,  shows  that  a 
high  degree  of  heat  did  not  prevail  at  the  time  of  genesis.  The 
depth  below  the  surface  indicates  that  the  normal  rock  tem- 
peratures would  be  from  50°  to  125°  C.,  but  in  all  cases  the 
vicinity  of  recently  intruded  rocks  had  forced  the  temperature 
curves  nearer  to  the  surface;  the  heated  waters  which  deposited 
.the  ores  either  emanated  from  the  intrusive  magma  or  at  least 
derived  their  high  temperatures  from  it.  It  is  manifestly 
impossible  to  give  accurate  figures,  but  reasoning  from  what  is 
known  of  the  stability  of  minerals  characteristic  of  this  class  of 
deposits  we  may  say  with  some  confidence  that  the  actual 
temperatures  may  have  ranged  from  175°  to  300°  C.  When  the 
high-temperature  curves  were  near  the  surface,  these  deposits 
may  have  originated  at  a  depth  of  only  a  few  thousand  feet; 
when  the  intrusions  were  deeper  seated  the  depth  at  which  the 
deposits  were  formed  may  have  exceeded  12,000  feet. 

546 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  547 

The  pressure  was  necessarily  strong;  as  calculated  on  a  hydro- 
static basis  it  ranged  from  140  to  400  atmospheres.  Communi- 
cation with  the  surface  was  probably  established  in  many  places; 
where  it  was  lacking  the  water  and  gases,  propelled  from  the 
magma,  may  have  been  under  still  higher  pressure. 

When  the  temperature  exceeded  the  upper  limit  stated  above, 
silicate  minerals  characteristic  of  greater  heat  undoubtedly 
developed  and  the  resulting  deposit  is  of  a  different  type.  When 
the  temperature  fell  below  the  lower  limit  stated,  the  general 
type  of  mineralization  must  have  approached  that  of  the  deposits 
found  near  the  surface. 

The  structure  of  the  deposits  is  what  might  be  expected  from 
the  opening  of  fissures  under  pressure  at  considerable  depth. 
The  fissures  are  fairly  regular  in  strike  and  dip,  having  neither 
the  extreme  brecciated  structure  common  to  deposits  formed  close 
to  the  surface  nor  the  lenticular  form  and  irregular  openings  of 
the  deep-seated  deposits.  Smooth  walls  and  slickensides  are 
abundant.  As  the  fissures  were  opened  in  the  zone  of  fracture, 
open  spaces  are  present  in  many  deposits,  though  the  walls 
usually  come  together  within  short  intervals.  In  calcareous 
rocks,  more  rarely  in  igneous  rocks  or  quartzite,  replacement 
deposits  were  often  developed;  they  are  more  common  here  than 
in  the  deposits  formed  close  to  the  surface,  though  the  solutions, 
on  the  whole,  spread  much  less  widely  through  the  igneous  rocks 
in  this  group  than  in  the  shallow  deposits. 

The  metals  contained  are  principally  gold  and  silver,  often 
with  large  amounts  of  copper,  lead,  and  zinc.  In  the  deep-seated 
deposits  molybdenum,  bismuth,  tungsten,  and  arsenic  are  not 
uncommon  associates;  we  find  the  same  metals  here,  though 
they  are  much  less  prominent;  in  addition  there  is  also  much 
antimony,  and  in  places  tellurium.  The  ore  minerals  are  sul- 
phides, arsenides,  sulphantimonides,  and  sulpharsenides.  Pyrite, 
chalcopyrite,  arsenopyrite,  galena,  zinc  blende,  tetrahedrite, 
tennantite,  and  native  gold  are  the  most  common  and  on  the 
whole  there  is  not  much  variety  and  complexity.  Scarcely  ever 
do  we  find  the  oxides  such  as  magnetite,  specularite,  and  ilrnenite. 
The  metallic  minerals  develop  both  in  the  filling  and  in  the  altered 
country  rock,  but  in  the  fissure  veins  proper  it  is  common  to 
find  the  valuable  ores  mainly  in  the  filled  spaces.  The  pre- 
dominating gangue  mineral  is  quartz,  but  carbonates  are  also 
common,  such  as  calcite,  dolomite,  and  ankerite,  more  rarely 


548  MINERAL  DEPOSITS 

siderite;  fluorite  and  barite  are  occasionally  of  importance. 
Chalcedony  and  opal  are  rarely  found. 

Among  the  minerals  of  this  type  of  metallization  are  found  no 
biotite;  no  pyroxenes  or  amphiboles;  no  garnet,  tourmaline,  or 
topaz;  no  zeolites  or  kaolin. 

Very  frequently  these  veins  follow  lamprophyric  dikes,  which 
are  usually  the  last  manifestation  of  igneous  activity. 

This  class  yields  a  large  proportion  of  the  gold  production  of 
the  world,  as  well  as  much  of  its  silver,  copper,  and  zinc.  Its 
deposits  are  by  far  the  most  abundant  in  the  Cordilleran  region, 
as  well  as  in  other  parts  of  the  world  where  intrusive  activity  has 
been  followed  by  deep  erosion. 

The  gold-quartz  veins  of  California  and  of  Victoria  (Australia) 
and  many  of  those  of  the  Cordilleran  region,  the  zinc-lead-silver 
replacement  deposits  atLeadville,  Park  City,  and  Aspen,  theCoeur 
d'Alene  lead  veins,  and  many  other  types  belong  to  this  class. 

The  intrusive  bodies  may  be  laccoliths,  stocks  or  batholiths; 
the  latter  term  being  reserved  for  intrusive  cross-cutting  masses 
of  large  size  like  those  of  the  Helena-Butte  region  in  Montana, 
the  great  Idaho  granite  mass,  or  the  enormous  intrusive  masses 
following  the  Pacific  Coast  from  lower  California  to  Alaska. 
It  has  long  been  known  that  the  interior  parts  of  great  batholiths 
are  relatively  barren  and  that  mineral  deposits  rarely  occur  in 
laccoliths.  This  appears  to  be  dependent  upon  the  principles  of 
magmatic  differentiation  for  the  volatile  constituents  of  the 
magma  from  which  the  ore  deposits  have  been  formed  have  a 
tendency  to  rise  to  the  roof  of  the  intrusive  or  to  accumulate  in 
the  cupolas  of  its  upper  parts.1 

Attention  has  recently  been  directed  to  these  problems  by  But- 
ler,2 who  points  out  that  in  Utah  those  stocks  which  are  truncated 
by  erosion  near  their  apices  or  points  nearest  to  the  surface  con- 
tain ore  deposits  of  great  importance  while  those  which  have 
been  truncated  by  erosion  to  deeper  levels  show  relatively  few 
deposits.  When  the  mobile  constituents  reached  a  point  when 
the  magma  was  sufficiently  consolidated  to  fracture  they  were 
guided  by  the  fissures  and  on  reaching  favorable  physical  and 
chemical  environments  began  to  deposit  the  metals  in  solution. 

1  R.  A.  Daly,  Igneous  rocks  and  their  origin,  New  York,  1914,  p.  244  and 
253. 

2  B.  S.  Butler,  Relation  of  ore  deposits  to  different  types  of  intrusive  bodies, 
Econ.  Geol.,  vol.  10,  1915,  pp.  101-122. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  549 

Billingsley  and  Grimes1  have  lately  applied  the  same  theory 
to  the  Boulder  Batholith,  Montana,  and  appear  to  have  shown 
that  the  maximum  deposition  took  place  near  the  roof  of  that 
batholith. 

Many  veins  of  this  class  show  interesting  changes  in  depth  and 
laterally  from  the  focus  of  the  ascending  waters.  In  general 
the  ore  is  likely  to  grow  more  siliceous  and  pyritic  in  depth  though 
many  cases  are  known  where  there  is  practically  no  change  in 
the  composition  for  a  vertical  distance  of  several  thousand  feet. 
Gold,  copper  and  zinc  appears  to  have  been  deposited  by  rela- 
tively hot  waters  while  lead  and  rich  silver  deposits  as  a  rule 
appear  farther  away  from  the  intrusive  suggesting  deposition  at 
lower  temperatures  (p.  190). 


METASOMATIC  PROCESSES 

General  Character. — The  alteration  of  the  country  rock  is 
usually  very  intense  next  to  the  ore,  but  seldom  yields  coarsely 
crystalline  products  as  in  some  high  temperature  deposits 
(p.  659).  In  feldspathic  and  ferromagnesian  rocks  the  principal 
product  is  sericite,  the  fine-grained  foliated  form  of  muscovitel 
in  many  deposits  carbonates,  such  as  calcite,  dolomite,  and  ank- 
erite,  develop  in  large  amounts.  The  dark  minerals  are  first 
altered,  their  iron  being  usually  recombined  as  pyrite.  The 
feldspars  are  also  rather  easily  altered;  quartz  grains  are  attacked, 
though  less  easily  than  the  feldspars,  and  are  partly,  at  least, 
converted  to  a  similar  aggregate  of  sericite  and  carbonates. 

The  granodiorite  adjoining  a  gold-quartz  vein  may  be  altered, 
for  instance,  to  a  rock  composed  of  16  per  cent,  quartz,  42  per 
cent,  sericite,  33  per  cent,  calcium  (magnesium,  iron)  carbonate, 
and  9  per  cent,  pyrite.  While  the  orthoclase  and  the  soda-lime 
feldspars  are  conspicuously  absent  as  vein-forming  minerals, 
albite  is  not  uncommon,  especially  in  some  gold-quartz  veins. 
This  mode  of  alteration  is  frequently  observed  in  amphibolitic 
rocks,  which  contain  much  sodium  and  presumably  much  albite 
developed  during  previous  dynamo-metamorphism.  In  many 
cases  the  new  albite  and  dolomitic  or  ankeritic  carbonates  form 
together.  Pyrite  is  a  common  metasomatic  mineral  and  is  often 

1  P.  Billingsley  and  J.  A.  Grimes.  The  ore  deposits  of  the  Boulder  Batho- 
lith, Trans.,  Am.  Inst.  Min.  Eng.,  vol.  58,  1918,  pp.  284-368. 


550  MINERAL  DEPOSITS 

associated  with  the  ferromagnesian  minerals,  but  may  also  form 
in  quartz  and  feldspars.  Other  metallic  minerals  are  not  com- 
mon; the  apatite  and  zircon  of  the  igneous  rocks  resist  alteration; 
while  titanite  and  ilmenite  yield  rutile.  In  many  vein  types  of 
the  interior  Cordilleran  province  the  metasomatic  carbonates  are 
scarce  or  absent,  as  in  the  copper  veins  of  Butte,  Montana,  and 
Clifton,  Arizona.  Serpentine  is  sometimes  altered  to  magnesite 
and  dolomite. 

Among  sedimentary  rocks  quartzite  and  sandstone  are  little 
affected,  except  in  veins  of  the  Coeur  d'Alene  type,  where  the 
quartz  grains  are  replaced  by  siderite.  Clay  slates  always  con- 
tain metasomatically  developed  pyrite  in  cubes;  whether  they 
are  otherwise  altered  or  not  depends  upon  their  composition:  if 
they  contain  feldspathic  sediment,  sericitic  and  carbonatic  altera- 
tion will  ensue;  if  only  kaolin,  sericite,  and  quartz  are  present 
there  will  be  little  noticeable  alteration,  except  in  some  instances 
where  almost  complete  silicification  takes  place. 

Limestone  and  other  calcareous  rocks  are  almost  always  sub- 
ject to  silicification  by  the  replacement  of  the  carbonates  with 
fine-grained  quartz  aggregates;  the  resulting  rocks  are  usually 
called  "jasperoids"  and  look  more  or  less  like  chert  (Figs.  57 
and  58).  Ore  minerals  develop  abundantly  by  metasomatic 
action  in  such  rocks. 

Along  with  or  preceding  this  silicification,  dolomitization  often 
takes  place;  the  solutions  apparently  abstract  a  part  of  the 
calcite  in  the  limestone  and  replace  it  by  the  less  easily  soluble 
magnesium  carbonate.  Limestone  is  sometimes  converted  to 
magnesite,  siderite,  manganosiderite  or  fluorite. 

The  alteration  is  accompanied  by  strong  leaching  of  sodium 
and  by  concentration  of  potassium.  Where  there  is  little  car- 
bonatization  much  calcium  and  magnesium  are  also  leached. 
Aluminum  in  most  cases  remains  about  constant. 

Alteration  of  Wall  Rocks  Adjoining  Gold-Quartz  Veins. — In 
veins  characterized  by  quartz  filling  with  free  gold  and  simple 
sulphides  or  arsenopyrite,  the  country  rock  next  to  the  walls  is 
usually  rich  in  carbonates,  sericite,  and  pyrite,  but  rarely  con- 
tains much  gold.  Extensive  alteration  zones  are  not  common, 
and  sometimes  fresh  rock  adjoins  the  vein.  The  relative  quan- 
tity of  alteration  products  may  differ  considerably,  even  in  the 
same  mine. 

Clay  slate,  with  more  or  less  carbonaceous  matter,  is  thought 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  551 

by  some  to  have  a  precipitating  and  enriching  influence  on  the 
vein,  but  to  what  extent  this  is  true  is  doubtful.  While  this  influ- 
ence can  apparently  be  recognized  in  some  districts  like  Gympie, 
in  Queensland,  it  is  not  clearly  shown  along  the  Mother  Lode  of 
California.  The  black  clay  slates  near  the  veins  are  often  rich 
in  crystallized  cubes  of  pyrite. 

Metasomatic  rocks  containing  albite  result  from  the  alteration 
of  amphibolites  at  Angels  Camp,  Calaveras  County,  California, 
where  they  constitute  low-grade  ores.  The  Utica,  Lightner, 
and  Melones  mines  are  the  best  known  of  those  working  on  these 
deposits.  The  metasomatic  processes  were  similar  to  those  that 
affected  the  amphibolites  of  Kalgoorlie.  Ransome1  has  described 
the  Angels  Camp  deposits. 

Specimens  from  Angels  Camp  and  Melones  show  that  some 
of  the  altered  rocks  consist  of  sericite  with  embedded  grains 
of  calcium-magnesium-iron  carbonates  and  pyrite.  In  other 
places  the  carbonates  prevail  over  the  sericite,  while  in  a  third 
and  very  common  type  much  of  the  sodium,  abundant  in  the 
amphibolite,  has  been  retained  as  albite  in  the  altered  rock. 
Large  grains  of  carbonates  are  separated  by  a  granular  mass  of 
quartz,  albite,  pyrite,  and  sericite. 

In  the  ordinary  course  of  alteration  the  ferromagnesian  min- 
erals are  first  converted  into  larger  foils  of  sericite.  A  chlorite 
rich  in  iron  is  also  formed,  which  during  a  later  stage  is  converted 
into  sericite.  The  feldspars  are  then  attacked  along  cracks  and 
cleavage  planes,  and  a  finely  felted  aggregate  of  sericite  and  cal- 
cite  invades  the  grains  until  the  replacement  is  complete.  A  tex- 
ture often  observed  consists  of  interlacing  sericite  foils,  the  tri- 
angular or  polygonal  interstices  of  which  are  filled  with  calcite 
(Fig.  184).  Orthoclase  is  almost  always  more  resistant  than 
the  soda-lime  feldspars. 

1  F.  L.  Ransome,  Folio  63,  U.  S.  Geol.  Survey.  See  also  W.  Lindgren, 
Econ.  Geol,  vol.  1,  1906,  p.  543.  Of  these  veins,  Ransome  says: 

"  They  are  in  the  main  complex  stringer  leads.  But  the  country  rock 
is  usually  much  more  altered  and  may  be  heavily  impregnated  with  pyrite 
near  the  vein.  It  is  often  changed  to  a  soft  grayish  rock  consisting  chiefly 
of  carbonates  of  lime  and  magnesia,  with  sericite  and  sometimes  a  little 
chlorite.  Such  altered  and  pyritized  country  rock  is  too  poor  in  gold  to 
pay  for  working  alone,  but  is  often  run  through  the  mills  for  the  sake  of  the 
rich  stringers  which  intersect  it.  These  veins  are  usually  richer  in  carbonates 
than  those  in  the  black  slate  areas  and  in  certain  parts  of  the  district  are 
rich  in  tellurides." 


552 


MINERAL  DEPOSITS 


The  quartz  is  also  attacked,  but  with  more  difficulty,  and  is  in 
no  case  completely  replaced.  Magnetite  appears  to  be  converted 
to  siderite  and  titanite  to  rutile.  A  part  of  the  ferromagnesian 
minerals  are  transformed  into  pyrite  Sharp  cubes  of  pyrite 
develop,  however,  not  only  in  the  sericitic  aggregate,  but  also  in 


FIG.  184. — Altered  granodiorite,  Bellefountain  mine,  Nevada  City, 
California,  m,  Fine  aggregate  of  sericite  with  some  calcite  and  quartz, 
replacing  orthoclase  and  andesine;  b,  original  biotite  altered  to  sericite;  q, 
original  quartz ;  black,  pyrite  with  included  sericite.  Magnified  15  diameters. 

the  fresh  feldspars  or  even  in  the  quartz.  Arsenopyrite  is  almost 
the  only  other  sulphide  which  is  enabled  to  form  in  the  altered 
rock,  and  it  develops  in  sharp  rhombic  crystals. 

From  many  analyses  the  eight  given  on  page  553  are  selected.1 

1  W.  Lindgren,  The  gold-silver  veins  of  Ophir,  Cal.,  Fourteenth  Ann.  Rept., 
U.  6.  Geol.  Survey,  pt.  2,  1894,  pp.  243-284. 

W.  Lindgren,  Metasomatic  processes  in  fissure  veins,  Trans.  Am.  Inst. 
Min.  Eng.,  vol.  30,  1900,  p.  666. 


DEPOSITS^FORMED  AT  INTERMEDIATE  DEPTHS  553 

TABLE  I.— ANALYSES   OF   METASOMATIC   ROCKS   FROM  CALIFORNIA  GOLD 
QUARTZ  VEINS 


A 

Aj 

B 

B! 

*"1 

c, 

D 

D! 

SiO2  
TiO2  
A1203  
Fe203  
FeO  

65.54 
0.39 
16.52 
1.40 
2.49 

46.13 
0.67 
15.82 
0.89 
2.27 
1.61 

45.56 
1.11 
14.15 
1.20 
9.83 
7.86 
0.10 
0.25 
trace 
2.30 
trace 
trace 
6.76 
1.18 
1.57 
trace 
0.23 
4.84 
0.14 
0.03 
3.04 

37.01 
0.85 
12.99 
0.43 
3.57 
7.99 
trace 
0.24 
trace 
9.78 
trace 
trace 
5.49 
4.02 
0.13 
trace 
0.13 
1.92 
0.06 
0.04 
15.04 

66.65 
0.38 
16.15 
1.52 
2.36 
0.02 

56.25 
0.25 
17.65 
0.76 
2.64 
2.87 

51.01 

0.98 
11.89 
1.57 
6.08 
'1.73 
trace 
trace 

10.36 
none 
none 
8.87 
0.15 

417 

45.74 
0.36 

5.29 
0.13 
2.06 
0.49 

0.2G 

23.85 
none 
trace 
0.94 
1.29 
0.11 
trace 
0.22 
1.07 
0.07 

18.91 

FeS2 

Cu2S  
MnO  
NiO,  ZnO  
CaO  
SrO  
BaO  
MgO  
K2O  
Na2O  
Li2O 

0.06 

4.88 
not  det. 
not  det. 
2.52 
1.95 
4.09 

0.09 
trace 
10.68 
trace 
trace 
2.13 
5.30 
0.17 
trace 
0.12 
2.42 
0.10 

0.10 

4.53 
trace 
0.07 
1.74 
2.65 
3.40 
trace 
0.18 
0.72 
0.10 

none 
4.46 

0.03 
1.69 
6.01 
0.30 

H2O- 

0.30 
2.36 
0.21 

0.24 
2.09 
0.17 

H20+  
P20S  
S03  
CO2 

0.59 
0.18 

11.24 

4.82 

Total. 


100.6199.64100.1599.69100.57100.60    99.35100.79 


1  Probably  present  as  Fe7S8. 

A.  Fresh  granodiorite,  Lincoln,  Placer  County,  California.     Though  not 
adjoining  the  vein,  it  indicates  closely  the  actual  composition  of  the  fresh 
wall  rock.     W.  F.  Hillebrand,  analyst. 

AI.  Altered  granodiorite,   Plantz  vein,   Ophir,   Placer  County.     W.   F. 
Hillebrand,  analyst. 

B.  Amphibolite  schist,  Conrad  vein,  Ophir,  Placer  County.     Fairly  fresh, 
but  contains  pyrite  and  calcite.     W.  F.  Hillebrand,  analyst. 

Bi.  Completely  altered  amphibolite  schist,  Mina  Rica  vein,  Ophir,  Placer 
County.     W.  F.  Hillebrand,  analyst. 

C.  Fresh  granodiorite,   Nevada  City,   Nevada  County.     W.   F.   Hille- 
brand, analyst. 

Ci.  Altered   granodiorite,    Bellefountain   mine,    Nevada   City.     George 
Steiger,  analyst. 

D.  Fresh  diabase,  Grass  Valley.     H.  N.  Stokes,  analyst. 

DL  Altered  diabase,  North  Star  mine,  Grass  Valley.     W.  F.  Hillebrand, 
analyst. 


554 


MINERAL  DEPOSITS 


From   the   chemical   and   microscopical    data   the   following 
compositions  may  be  calculated: 

TABLE  II.— CALCULATED  MINERALOGICAL  COMPOSITION  OF  THE 
ALTERED  ROCKS  OF  TABLE  I 


A» 

P, 

Ct 

D, 

Quartz  

16.00 

24.00 

25.00 

35.00 

Sericite  (with  a  little  chlorite) 

41.76 

46.97 

61.46 

21.20 

CaCO3  

17.53 

18.87 

7.23 

42.15 

MgCO3  

9.67 

2.93 

2.70 

0.71 

FeC03  

5.76 

3.67 

0.58 

MnCO3  

0.42 

0.14 

Rutile  

0.85 

0.67 

0.25 

0.36 

Pyrite  

7.99 

1.61 

2.87 

0.50 

Apatite  

0.13 

0.22 

0.46 

0.15 

Total  

100.11 

99.08 

100.55 

100.07 

As  it  seems  probable  that  the  alumina  has  remained  fairly 
constant  in  the  first  six  analyses  in  Table  I,  they  may  be  directly 
compared  for  an  approximate  review  of  the  chemical  changes. 

Analysis  Di  differs  from  the  rest  in  showing  an  exceptionally 
high  percentage  of  introduced  lime  and  carbon  dioxide  and  a 
corresponding  loss  of  magnesia.  Moreover,  the  alumina  is  so 
low  that  it  must  be  supposed  to  have  been  removed. 

The  characteristic  features  of  the  process  seem  to  consist  in 
the  decrease  of  silica,  magnesia,  and  soda  and  increase  of  lime, 
potash,  and  carbon  dioxide — the  calcitic  altered  rock  strongly 
contrasting  with  the  quartz-filled  veins.  There  is  some  evidence 
of  partial  leaching  of  titanium  and  phosphorus.  Sufficient  data 
are  not  available  for  the  accurate  determination  of  change  of 
volume  during  the  process,  and  of  the  actual  losses  and  gains. 
It  seems  probable  that,  in  most  cases,  the  added  material  has 
more  than  balanced  the  losses.  Unquestionably  there  has  been 
a  strong  addition  of  calcium  and  potassium,  and  the  vein-filling 
process  probably  began  with  deposition  by  solutions  extremely 
rich  in  these  constituents,  as  well  as  in  carbon  dioxide.  The 
quartz  filling  sometimes  shows  imprints,  along  its  walls,  of 
calcite  crystals,  from  which  it  may  be  concluded  that  during  the 
process  of  filling  the  nature  of  the  solutions  changed  to  the  later 
phase,  in  which  almost  nothing  but  quartz  was  deposited. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  555 

Interior  Types. — In  the  Blue  Mountains  of  Oregon  the  wall 
rocks  of  the  veins  are  altered  to  products  in  all  respects  similar 
to  those  of  California.  In  the  veins  of  Idaho,  Utah  and  Colorado 
genetically  connected  with  Cretaceous  or  early  Tertiary  intru- 
sions of  quartz  monzonite  or  similar  porphyries,  the  carbonati- 
zation  is  far  less  marked  and  both  calcium  and  mangnesium  are 
leached.  The  accompanying  analyses,  Table  III,  illustrate  the 
chemical  changes  in  two  prominent  types. 

TABLE  III.— ANALYSES  OF  FRESH  AND  ALTERED  ROCKS  FROM  IDAHO 

GOLD-QUARTZ  VEINS 
[Analyst,  W.  F.  Hillebrand] 


E 

Jljj 

F 

F! 

SiO2 

65.23 

66.66 

57.78 

58.01 

TiO2 

0.66 

0.49 

1.01 

1.08 

A1203  

16.94 

14.26 

16.28 

15.72 

Fe2O3  

1.60 

0.67 

1.02 

0.64 

FeO  

1.91 

1.33 

4.92 

3.87 

CoO,  NiO  



0.02 

0.12 

MnO  

trace 

trace 

0.15 

0.17 

CaO  

3.85 

3.37 

6.65 

2.15 

SrO  

0.07 

none 

BaO  

0.19 

none 

0.12 

trace? 

MgO  

1.31 

0.95 

4.60 

2.07 

K20  

3.02 

4.19 

2.22 

4.79 

Na2O  

3.57 

none 

3.25 

0.10 

H2O  -  

0.18 

0.36 

0.34 

0.31 

H20+  

0.88 

2.16 

0.92 

2.71 

PA  

0.19 

0.17 

0.30 

0.31 

CO2  

0.25 

3.67 

0.15 

2.86 

s  

none 

0.95 

0.02 

1.25 

Fe  

0.84 

1.52 

Pb  

0.86 

Cu  

0.05 

As  

1.65 

Total 

99.78 

100.07 

99.82 

100.24 

E.  Fresh  granitic  rock  immediately  adjoining  the  Silver  Wreath  quartz 
vein,  Willow  Creek,  Idaho. 

EL  Altered  rock  adjoining  the  same  vein. 

F.  Fresh  quartz-pyroxene  diorite  adjoining  the   Croesus  vein,  Hailey, 
Idaho. 

Fi.  Altered  rock  adjoining  the  same  vein. 


556 


MINERAL  DEPOSITS 


E  and  Ei  represent  the  fresh  and  altered  rock  from  the  Willow 
Creek  district,  Boise  County,  where  the  narrow  quartz  veins  carry 
scarcely  any  free  gold,  but  much  auriferous  galena,  pyrite,  arseno- 
pyrite,  and  zinc  blende.  F  and  Fi  represent  the  fresh  and  al- 
tered rock  from  the  Croesus  mine,  Wood  River  district,  Elaine 
County,  where  the  narrow  streaks  of  filling  consist  of  quartz 
siderite,  pyrrhotite,  and  chalcopyrite,  with  a  little  galena,  arseno- 
pyrite,  and  zinc  blende.  Here  again  only  a  fraction  of  the  gold 
is  in  the  free  state.  The  ore  contains  very  little  silver.1 

The  specific  gravity  of  E  is  2.714.  From  the  mineralogical 
composition  given  in  the  report  quoted  the  specific  gravity  is 
calculated  to  2.720,  which  is  a  close  agreement,  the  difference 
possibly  indicating  a  very  slight  porosity.2 

TABLE  IV.— MINERALOGICAL  COMPOSITION  OF  Et  AND  Fi,  IN  TABLE  III 


E, 

F, 

Quartz 

42  00 

36  18 

Sericite  
Chlorite     .    . 

46.84 

38.18 
11  76 

CaCO3  
MgCO3  

4.80 
1  96 

3.11 
1  26 

FeC03  
RutUe  

1.45 
0.49 

2.19 
1.08 

Apatite 

0  72 

Pyrite 

1  78 

0  58 

Pyrrhotite 

0  15 

Zinc  blende  
Galena  
Chalcopyrite  
Arsenopyrite  

trace 
0.99 
0.15 

3.58 

Total . 


99.32 


99.93 


The  measured  specific  gravity  of  EI  is  2.774,  indicating  that 
the  rock  alters  to  denser  minerals.     The  calculation  of  the  same 

1  For  full  calculations  and  description  of  E  and   EI  see  W.  Lindgren, 
Eighteenth  Ann.  Kept.,  U.  S.  Geol.  Survey,  pt.  3,  1898,  p.  640;  for  F  and  F1 
see  W.  Lindgren,  Twentieth  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  3,  1900, 
pp.  211-232. 

2  In  this  calculation  the  following  figures  for  specific  gravity  are  used : 
quartz,  2.65;  sericite,  2,83;  biotite,  3.00;  oligoclase,  2.65;  orthoclase,   2.56. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  557 

specific  gravity  Table  from  IV  gives  2.796,  which  shows  a  decided 
porosity  of  the  altered  rock.  Under  these  circumstances,  no 
evidence  of  pressure  being  noted,  it  may  be  assumed  that  not 
much  change  in  volume  has  taken  place.  By  multiplying  the 
percentages  of  E  and  Ei  by  2.714  and  2.774,  respectively,  and 
comparing  the  results,  the  absolute  gains  and  losses  per  cubic 
meter  may  be  obtained  (see  Table  V). 

In  the  same  manner  the  measured  specific  gravities  of  F  and 
Fi  are  compared  with  the  calculated  specific  gravities.1 

By  multiplying  the  percentages  of  F  and  Fi  by  the  measured 
specific  gravities,  and  comparing  these  data,  the  absolute  gains 
and  losses  are  again  obtained. 

The  calculation  shows  that  during  the  alteration  of  E  to  Ei  291 
kilograms  were  added  and  229  lost  per  cubic  meter,  the  net  gain 
being  62  kilograms.  During  the  alteration  of  F  to  Fi  416  kilo- 
grams were  added  and  333  lost  per  cubic  meter,  the  net  gain  being 
83  kilograms. 

Similar  changes  resulted  in  the  two  rocks :  a  moderate  addition 
of  silica  and  a  strong  gain  of  potash;  nearly  complete  loss  of  soda, 
baryta,  and  strontia;  partial  loss  of  alumina,  magnesia,  and  lime, 
F,  however,  losing  much  more  lime  than  E.  In  Ei  the  amounts 
lost  of  Fe203  and  FeO  are  almost  completely  converted  into  Fe 
(in  FeSa).  In  F  these  losses  are  less  and  not  sufficient  to  account 
for  the  gain  of  Fe;  consequently  iron  must  have  been  added. 
Phosphoric  acid  is  constant,  consistently  with  the  fresh  state 
of  the  apatite. 

These  figures  give  some  idea  of  the  intensity  of  the  transfer  of  ma- 
terial, though  the  balance  of  material  added  is  comparatively  small. 

Similar  alteration  in  the  veins  of  Bingham,  Utah,  has  been 
described  by  Boutwell.2  The  Last  Chance  lode  is  from  1  to 
14  feet  wide  and  contains  galena,  zinc  blende,  pyrite,  and  some 
calcite.  The  alteration  and  bleaching  extend  about  1  or  2  feet 
into  the  country  rock,  which  is  a  monzonite,  consisting  of  ortho- 
clase,  plagioclase,  augite,  biotite,  and  hornblende.  The  altera- 
tion begins  by  chloritization  and  dissemination  of  pyrite,  but  the 
end  product  consists  largely  of  sericite  and  pyrite.  The  analy- 
sis indicates  an  unusual  and  almost  complete  removal  of  mag- 
nesia and  extensive  leaching  of  sodium  and  calcium.  There  has 

1  Twentieth  Ann.  Rept,  U.  S.  Geol.  Survey,  pt.  3,  1900,  pp.  221-232. 
-  J.  M.  Boutwell,  Bingham  mining  district,  Prof.  Paper  38,  U.  S.  Geol. 
Survey,  1905,  p.  178. 


558 


MINERAL  DEPOSITS 


evidently  been  an  addition  of  potassium,  as  there  is  considerably 
more  than  is  called  for  in  the  ordinary  composition  of  sericite. 
As  usual  TiO2  remains  constant,  and  the  altered  rock  contains 
practically  no  carbonates. 

ANALYSES  SHOWING  ALTERATION  OF  MONZONITE  AT    BINGHAM,  UTAH 

[Analyst,  E.  T.  Allen] 


SiO2 58.64 

TiO2 0.83 

A12O3 ;  15.35 

Fe2O3 :  3.25 

FeO |  2.54 

CaO 5.37 

BaO 0.18 

MgO 3.84 

K20 4.23 

Na20 3.60 

H2O- 0.86 

H2O  + !  .  . '  1.50 

CO2 none 

P2O5 !  0.02 

S I  0.05 

100.26 

O  equivalent  to  S 0 . 02 

'    Total...  100.24 


56.78 
0.81 

16.90 
6.87 
2.34 
1.18 
0.14 
0.03 
7.02 
0.37 
1.32 
2.23 
0.26 
0.04 
5.93 


102.22 


100.00 


Traces  MnO,  Cr2O3. 

I.  British  Tunnel,  Last  Chance  mine. 

II.  British  Tunnel,  Last  Chance  mine,  at  wall  of  lode. 

One  of  the  great  mineral  belts  of  Colorado  extends  in  a  north- 
easterly direction  from  Leadville  to  Boulder  by  way  of  Park. 
Clear  Creek,  and  Gilpin  counties  (p.  617).  It  is  characterized  as 
a  whole  by  an  abundance  of  heavy  sulphide  ores,  principally 
pyrite,  zinc  blende,  and  galena,  with  subordinate  chalcopyrite 
and  a  notable  content  of  gold  and  silver.  The  gangue  is  sub- 
ordinate and  consists  of  a  little  quartz  and  more  or  less  of  a 
sideritic  carbonate.  The  ores  appear  in  replacement  deposits 
and  veins.  At  Leadville,  where  the  ores  replace  limestone  at 
the  contacts  with  intrusive  porphyry,  the  alteration  of  the  car- 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  559 

bonate  rock  is  remarkably  slight.  There  may  be  locally  a  little 
pyrite  in  the  limestone  near  the  ore;  at  other  places  the  lime- 
stone is  recrystallized  and  contains  much  manganosiderite  or  is 
silicified.  The  contact  between  ore  and  limestone  is  usually  very 
sharp — indeed,  in  spite  of  the  completeness  of  the  replacement, 
practically  unaltered  limestone  may  lie  next  to  the  ore. 

At  Breckenridge,  Georgetown,  and  Central  City  the  deposits 
are  fissure  veins,  generally  filled  with  massive  sulphides,  and, 
in  feldspathic  rocks,  they  are  adjoined  by  an  altered  zone  from 
a  few  inches  to  20  feet  or  more  in  width.  The  alteration  prod- 
ucts are  quartz,  sericite,  and  a  sideritic  carbonate,  with  more  or 
less  pyrite.  In  a  few  of  the  Georgetown  veins  Spurr1  detected 
adularia,  but,  on  the  whole,  this  mineral  is  absent. 

The  siderite  is  thought  to  have  been  derived  from  biotite  and 
magnetite,  and  the  gangue  minerals  are  believed  to  have  been 
derived  from  the  adjoining  rocks.  Kaolin  is  considered  to  have 
resulted  from  alteration  by  descending  waters  during  the  proc- 
esses of  weathering  and  sulphide  enrichment. 

The  course  of  alteration  near  the  veins  has  been  studied  in 
detail  by  Ransome2  at  the  Wellington  lode,  350  feet  below  the 
surface  and  below  the  zone  of  oxidation.  The  vein  is  here  5 
feet  wide  with  good  walls  and  contains  zinc  blende  and  galena  in 
a  little  gangue  of  siderite  and  barite  with  more  or  less  included 
country  rock.  The  alteration  spreads  20  feet  -from  the  vein. 
The  fresh  rock  is  a  dark-gray  monzonite  porphyry,  the  ground- 
mass  of  which  consists  of  labradorite,  orthoclase,  biotite,  and 
diopside.  The  altered  rock  is  light  gray,  with  disseminated  par- 
ticles of  sulphides.  The  rock,  while  retaining  a  faint  trace  of  its 
structure,  is  changed  to  a  mass  of  carbonate,  sericite,  and  quartz. 

By  multiplying  the  figures  of  the  percentage  composition  by 
the  specific  gravities  of  the  rock  mass  the  constituents  per  100 
cubic  centimeters  of  fresh  and  altered  rock  are  obtained.  These 
figures  compared  give  the  gains  and  losses  for  each  constituent 
during  the  alteration  of  100  cubic  centimeters  of  fresh  rock,  and 
from  these  again  may  be  calculated  the  gains  and  losses  in  per- 
centage of  the  original  mass  of  276.3  grams  of  fresh  rock.  These 
gains  or  losses  in  percentages  may  be  applied  directly  by  addition 

1  J.  E.  Spurr,  Economic  geology  of  the  Georgetown  quadrangle,   Prof. 
Paper  63,  U.  S.  Geol.  Survey,  1908. 

2  F.  L.  Ransome,  The  Breckenridge  district,  Prof.  Paper  75,  U.  S.  Geol. 
Survey,  1911,  pp.  95-101, 


560 


MINERAL  DEPOSITS 


or  subtraction  to  the  figures  of  the  chemical  analysis  of  fresh 
rock,  and  this,  as  shown  in  column  III,  will  express  the  nature 
of  the  change  more  clearly.  There  has  been  a  notable  loss  of 
silica,  calcium,  potassium,  and  sodium.  The  additions  comprise 
carbon  dioxide,  sulphides,  ferrous  iron,  and  magnesium,  which 
would  hardly  bear  out  Spurr's  assertion  that  the  siderite  in  the 
vein  is  derived  from  the  adjoining  country  rock.  As  usual 
apatite  remains  unaltered,  and  the  ilmenite  is  converted  to 
rutile.  Some  paragonite  has  probably  developed  besides  the 
sericite,  if  indeed  the  rock  does  not  contain  albite. 

TABLE  SHOWING  ALTERATION  OF  DIORITE  PORPHYRY  AT  BRECKENRIDGE, 
COLORADO 


I 

II 

III 

SiQ,.  
TiO2  ,. 
Al  O 

57.35 
1.07 
16  29 

46.62 
1.01 
12  66 

49.48 
1.07 
13  44 

Fe2O3  

3.15 

trace 

0.02 

FeO 

4  36 

11   15 

11  78 

MnO  
Cap  
BaO  
SrO 

0.12 
5.66 
0.10 
0  05 

0.92 
1.55 
none 

0.97 
1.66 

MgO 

2  41 

4  02 

4  25 

K20  
Na2O  
H2O-  
H2O+  
CO,  

3.39 
4.50 
0.15 
0.70 
0.46 

1.68 
1.35 
0.31 
3.41 
11.48 

1.79 
1.45 
0.33 
3.60 
12.11 

P,CK 

0  70 

0  50 

0  53 

FeS2 

0  09 

1  99 

2  10 

ZnS  
PbS  

none 

0.97 
0.52 

1.02 
0.55 

Total 


100.55 


100.14 


106.15 


Specific  gravity: 

J 

Mass  

2  .  763 

2.930 

Powder  j           2.799 

2.940 

I.  Diorite  porphyry,  25  feet  from  vein,  Wellington  mine. 

II.  Altered  porphyry,  close  to  vein,  Wellington  mine. 

III.  Composition  of  same  volume  of  altered  rock  in  percentage  of  original 
rock  mass. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  561 


An  approximate  calculation  shows  the  altered  rock  to  be 
composed  as  follows : 

APPROXIMATE  MINERALOGICAL  COMPOSITION  OF  ALTERED  DIORITE 
PORPHYRY  AT  BRECKENRIDGE,  COLORADO 


Sericite 
Quartz...  . 
Kaolinite . . 
Carbonate . 
Rutile... 


30.5 
31.6 

2.8 
29.3 

1.0 


Apatite 
Pyrite 
Sphalerite . . 
Galena 

Total.. 


1.3 
2.0 
1.0 
0.5 


....100.0 


Abscissas  Represent  Distance  from  V 


FIG.  185. — Diagram  showing  alteration  of  diorite  porphyry  by  vein- 
forming  solutions  at  Breckenridge,  Colorado.  After  F.  L.  Ransome,  U.  S. 
Geol.  Survey. 

The  carbonate  consists  of  63.9  per  cent.  FeCO3,  29.6  per  cent. 
MgC03,  5.2  per  cent.  MnCO3,  and  1.3  per  cent.  CaCO3,  all  in 
isomorphous  mixture. 

The  majority  of  the  large  copper  deposits  of  the  West  are 


562  MINERAL  DEPOSITS 

genetically  connected  with  intrusives  and  have  been  formed  at 
intermediate  depths,  though  some  of  them  like  Ely  and  Clifton 
and  Cactus  show  unmistakable  affiliations  with  the  high  tem- 
perature deposits.  The  thermal  alteration  of  the  feldspathic 
rocks  results  universally  in  sericite,  quartz  and  pyrite  with  prac- 
tically no  carbonates.  Omitting  minor  constituents  the  compo- 
sition of  the  altered  rocks  would  average,  in  per  cent.,  about 
65.0Si02,  16.0A12O3,  2.0FeO,  0.5Fe2O3,  l.OMgO,  0.25CaO,  0.5 
Na20,  5.0K20  and  S.OFeS*.1 

From  all  this  it  follows  that  the  mineralization  in  mineral  de- 
posits of  this  class  has  been  effected  by  solutions  of  relatively 
uniform  character  capable  of  substituting  K20  for  Na20.  In 
some  deposits,  particularly  those  carrying  gold,  silver  and  lead 
the  alkaline  earths  have  been  fixed  as  carbonates,  while  in  copper 
deposits  there  is  usually  no  carbonates  in  the  metasomatic 
products. 

The  total  result  indicates  action  by  hot  ascending  solutions  con- 
taining more  potash  than  soda  and  having  a  variable  amount 
of  alkaline  carbonates  and  free  carbon  dioxide. 

PARAGENESIS 

It  has  long  been  observed  that  the  minerals  are  formed,  in 
the  main,  in  an  orderly  succession,  which  sometimes  is  repeated. 
This  was  first  emphasized  by  Breithaupt,  who  recorded  the 
series  for  different  mineral  deposits  in  his  book  on  the  paragenesis 
of  minerals.  The  study  of  the  succession  has  an  evident  bearing 
on  scientific  and  economic  problems,  and  much  work  has  been 
done  lately  on  this  subject  in  connection  with  the  examination 
of  ores  in  polished  sections  by  metallographic  methods.  These 
have  disclosed  the  wonderful  extent  to  which  metallic  minerals 
are  replaced  by  others  during  the  process  of  metallization. 

When  both  filling  and  replacement  have  been  in  action  it  is 
natural  that  the  country  rock  would  first  be  attacked  by  the 
waters.  There  was  first  a  process  of  dialysis  by  means  of  which 
the  solutions  were  filtered  through  a  porous  rock  which  many 
elements  found  it  difficult  to  penetrate.  Thus  we  find  that  the 
composition  of  the  metasomatic  rocks  near  the  fissure  differs 

i  Clifton-Morenci,  W.  Lindgren,  Prof.  Paper  43,  U.  S.  Geol.  Survey; 
Butte,  W.  H.  Weed,  idem,  74;  San  Francisco  district,  Utah,  B.  S.  Butler, 
idem,  80;  Ely,  A.  C.  Spencer,  idem,  96;  Ray  and  Miami,  F.  L.  Ransome, 
idem,  in  press. 


DEPOSITS  FORMED  AT  INTERMEDIATE'DEPTHS  563 

considerably  from  that  of  the  filling.  In  gold  quartz  veins,  for 
instance,  there  is  as  a  rule  little  gold  in  the  metasomatic  rocks 
while  the  filling  may  be  rich.  The  carbonates,  pyrite,  and  seri- 
cite  in  the  rock  seem  to  have  developed  about  contemporaneously. 
I  am  unable  to  accept  Rogers'1  suggestion  that  sericite  is  a  mineral 
of  late  origin. 

In  general  the  arsenopyrite  and  pyrite  appear  early  in  the  vein 
filling  and  are  preceded  and  followed  by  quartz.  The  latter 
mineral  nearly  always  appears  in  several  generations.  Calcite, 
dolomite  and  siderite  are  usually  the  latest  gangue  minerals.  Chal- 
copyrite  and  bornite  are  always  later  than  the  pyrite  and  galena 


FIG.  186.— Drawing  of  polished  surface  of  ore  from  Gilpin  County, 
Colorado,  showing  earlier  pyrite  traversed  by  later  veins  of  chalcopyrite, 
sphalerite  (s),  and  quartz.  After  E.  S.  Bastin. 

is  one  of  the  latest  of  the  simple  sulphides  (Fig.  186).  At  times  it 
crystallizes  with  the  zinc  blende.  Argentite  included  in  galena  is 
evidently  of  simultaneous  formation.  Gold  is  frequently  later 
than  the  earlier  quartz  generations,  and  replaces  pyrite  and 
arsenopyrite. 

The  sulphantimonides  and  sulpharsenides  are  almost  invariably 

1  A.  F.  Rogers,  Sericite,  a  low  temperature  hydrothermal  mineral,  Econ. 
Geol,  vol.  11,  1916,  pp.  118-150. 


564  MINERAL  DEPOSITS 

the  last  metallic  minerals  to  form  during  the  primary  metalliza- 
tion. The  occurrence  of  jamesonite,  tetrahedrite,  pearcite 
and  ruby  silver  in  vugs  and  replacing  veinlets  is  a  common 
observation.  The  periods  of  deposition  of  the  various  minerals 
frequently  overlap  and  recur. 

GOLD -QUARTZ  VEINS  OF  THE  CALIFORNIA  AND  VICTORIA  TYPE 

Principal  Characteristics. — As  quartz  and  gold  may  be  deposited 
together  within  a  considerable  range  of  temperature,  there  are 
several  types  of  gold-quartz  deposits.  The  deposits  formed  at 
higher  temperatures,  distinguished  by  such  gangue  minerals  as 
tourmaline,  apatite,  garnet,  biotite,  and  amphibole,  will  be  de- 
scribed in  subsequent  pages.  Those  formed  near  the  surface  at 
temperatures  not  much  above  100°  C.  have  been  discussed  in  the 
preceding  chapter.  Between  the  two  kinds  stands  the  large 
group  of  important  deposits  whose  geological  relations  point  to 
development  at  considerable  depth  and  whose  mineral  association 
points  to  moderate  temperatures — perhaps  200°  to  300°  C. 

The  first  type  is  represented  by  certain  Appalachian  and  Bra- 
zilian gold-quartz  veins;  the  second  by  veins  like  those  of  the 
Comstock,  Bodie,  and  Tonopah,  usually  appearing  in  Tertiary 
lavas.  Between  the  two  stand  the  gold-quartz  veins  of  Cali- 
fornia, eastern  Australia,  and  many  localities  in  the  interior 
Cordilleran  region  of  North  America. 

The  general  characteristic  of  these  intermediate  deposits  is 
the  association  of  a  preponderant  gangue  of  milky,  coarsely 
crystalline  quartz,  sometimes  drusy,  though  rarely  showing 
comb  structure,  with  free  gold  and  auriferous  simple  sulphide 
minerals.  Where  the  country  rock  is  suitable  for  replacement 
carbonates  and  sericite  appear  with  pyrite  in  the  altered  rocks. 

The  veins  occur  in  deeply  eroded  regions  and  in  or  surrounding 
intrusives  of  quartz  monzonitic  or  dioritic  or  gabbroitic  kind. 
The  absence  of  biotite,  magnetite,  epidote,  garnet,  and  tourma- 
line is  also  notable.  The  only  silicates  present  are  albite  and 
chlorite,  and  these  only  locally.  The  destruction  of  the  outcrops 
by  erosion  usually  results  in  rich  placers,  in  which  large  nuggets 
of  gold  are  often  found. 

The  free  gold  always  contains  a  little  silver,  the  average  fineness 
being  0.800;  the  sulphides  are  likely  to  carry  more  silver  in 
proportion  than  the  native  gold.  Some  types  of  these  veins 
carry  a  notable  amount  of  silver,  but  scarcely  ever  such  amounts 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  565 

as  are  common  in  the  Tertiary  veins  deposited  near  the  surface  in 
Tertiary  lavas. 

Gold-Quartz  Veins  of  the  Sierra  Nevada.1 — The  greatest 
development  of  the  gold-quartz  veins  is  found  in  California; 
they  begin  in  the  southern  end  of  the  State  in  San  Diego  County 
and  continue  with  interruptions  to  the  northern  end,  where,  in 
Trinity  and  Siskiyou  counties,  there  is  a  productive  area  of  no 
small  value.  The  gold  belt  also  continues  into  southwestern 
Oregon,  but  farther  north  disappears  under  the  Tertiary  lavas 
and  Cretaceous  and  Tertiary  sediments. 

Most  typically  the  veins  are  developed  in  the  Sierra  Nevada, 
which,  with  its  gentle  western  slope  and  abrupt  eastern  escarp- 
ments, separates  the  deserts  of  the  Great  Basin  from  the  central 
valleys  of  California  (Fig.  187). 

The  crest  and  main  mass  of  this  range  form  parts  of  an  enor- 
mous batholith  of  massive  granodiorite  and  allied  rocks,  intruded 
into  Mesozoic  and  Paleozoic  metamorphosed  sediments.  These 
sedimentary  rocks  are  closely  folded  and  compressed  and  occupy 
a  belt  on  the  western  slope,  which  gradually  widens  and,  in 
Plumas  County,  spreads  over  a  width  of  60  miles.  The  great 
batholith  itself  contains  extremely  few  quartz  veins;  mineraliza- 
tion is  confined  to  the  belt  of  metamorphic  rocks  on  the  western 
lope  and  often  begins  abruptly  at  the  contact;  this  is  shown,  for 
instance,  by  the  river  gravel,  which  becomes  auriferous  where 

1  H.  W.  Fairbanks,  Geology  of  the  Mother  Lode  region,  Tenth  Rept., 
California  State  Min.  Bur.,  1890,  pp.  23-90. 

W.  Lindgren,  Characteristic  features  of  the  California  gold-quartz  veins, 
Bull,  Geol.  Soc.  Am.,  vol.  6,  1896,  pp.  221-240. 

W.  Lindgren,  Gold-silver  veins  of  Ophir,  Fourteenth  Ann.  Rept.,  U.  S. 
Geol.  Survey,  pt.  2,  1893,  pp.  243,  284. 

W.  Lindgren,  The  gold-quartz  veins  of  Nevada  City  and  Grass  Valley, 
Seventeenth  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  2,  1896,  pp.  1-262. 

F.  L.  Ransome,  The  Mother  Lode  district,  Folio  63,  U.  S.  Geol.  Survey. 

W.  H.  Storms,  The  Mother  Lode  region  of  California,  Bull.  18,  California 
State  Min.  Bur.,  1900. 

W.  H.  Storms,  Pocket  mines,  Min.  and  Sci.  Press,  June  6,  1908;  idem, 
Possibilities  of  the  Mother  Lode  in  depth,  Nov.  18,  1911. 

W.  H.  Storms,  The  occurrence  of  gold  at  intersections,  Min.  and  Eng. 
World,  Nov.  25,  1911. 

H.  W.  Turner  and  F.  L.  Ransome,  Sonora,  Folio  41,  U.  S.  Geol.  Survey, 
and  other  folios  of  the  same  region. 

Charles  G.  Yale,  Mine  production  of  California,  Mineral  Resources, 
U.  S.  Geol.  Survey,  annual  publication. 


566 


MINERAL  DEPOSITS 


the  streams  enter  the  metamorphic  areas.  The  highly  productive 
part  of  the  belt  does  not,  usually,  adjoin  the  granitic  rocks,  but 
appears  lower  down  in  the  foothill  region  near  smaller  intrusive 
areas. 

The  metamorphic  rocks  are  a  complex  body,  for  besides  the 
prevailing  Paleozoic  slates  with  occasional  lenses  of  limestone 
and  in  the  lower  foothills  a  narrow  belt  of  late  Jurassic  Mariposa 


FIG.  187. — Map  of  Nevada  and  northern  part  of  California,  showing  promi- 
nent mining  districts. 

slate,  they  contain  Paleozoic  lava  flows  and  a  vast  quantity 
of  tuffs,  diabases,  and  old  andesites  erupted  by  volcanoes  of 
Jurassic  age. 

Later  than  these  rocks,  and  probably  dating  from  earliest 
Cretaceous  time,  are  numerous  smaller  intrusions  of  gabbro, 
diorite,  and  granodiorite,  which  are  massive  and,  in  a  general 
way,  contemporaneous  with  the  main  batholith  of  the  range. 
The  basic  intrusions  appear  to  be  somewhat  older  than  those 
containing  more  silica. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  567 


FIG.  188. — Map  ef  principal  vein  systems  near  Ophir  and  Auburn,  California. 
A,  Small  area  of  amphibolite.     Scale  1  inch  =2.7  miles. 


FIG.  189. — Vertical  cross-section  of  the  Mother  Lode  near  the  Argonaut 
shaft,  showing  reverse  fault  along  vein  fissure.  "Schist"  is  amphibolite, 
white  area,  Mariposa  clay  slate  (Jurassic). 


568  MINERAL  DEPOSITS 

In  and  around  these  smaller  intrusions,  as,  for  instance,  at 
Grass  Valley,  Nevada  City  (Fig.  190),  Ophir  (Fig.  188),  and  West 
Point,  the  gold-bearing  veins  often  cluster  and  may  occur  in  any 
kind  of  rocks;  there  are  also  several  long  lines  along  which 
fracturing  and  subsequent  mineralization  have  taken  place. 
One  of  these  follows  the  so-called  "serpentine  belt,"  a  dike-like 
intrusive  mass  70  miles  long;  another  extends  from  the  Forest 
Hill  divide,  in  Placer  County,  up  into  Sierra  County,  passing  the 
town  of  Washington.  The  most  important  line  is  that  followed 
by  the  Mother  Lode,  in  the  foothills  of  Mariposa,  Tuolumne 
Calaveras,  Amador,  and  Eldorado  counties,  for  a  distance  of 
130  miles.  The  Mother  Lode  is  by  no  means  a  single  vein,  but 
rather  a  system  of  linked  veins,  placed,  however,  within  a  narrow 
belt  about  a  mile  wide,  and  maintaining  a  remarkably  straight 


20CMJ 
2UUU 


FIG.  190. — Geological  section  at  Nevada  City,  California.  Cc,  Carbonif- 
erous slate;  Jm,  Jurassic  slate;  pt,  porphyrite;  gb,  gabbro;  pts,  amphibolite; 
s,  serpentine;  grd,  granodiorite.  Scale  1  inch  =  2,400  feet. 

course;  it  cuts  Paleozoic  slates  and  greenstones,  but  on  the  whole 
follows  fairly  closely  a  narrow  belt  of  the  Jurassic  Mariposa 
slate  and  in  places  lies  between  this  slate  and  the  greenstone. 
There  is  a  notable  displacement  along  the  Mother  Lode,  in  the 
nature  of  a  reverse  fault  (Fig.  189). 

The  strike  of  the  veins  is  predominantly  north-northwest, 
parallel  to  the  range  and  to  the  strike  of  the  steeply  inclined 
strata,  but  the  dip  usually  intersects  that  of  the  beds  and,  in 
the  Mother  Lode,  is  about  60°  east.  In  many  districts  other 
directions  of  strike  and  dip  prevail.  The  veins  are  easily  trac- 
able  by  prominent  quartz  outcrops,  and  many  of  them  are  re- 
markably straight  and  continuous  in  strike  and  dip.  It  is  not 
uncommon  to  find  veins  continuous  along  the  strike  for  1  or  2 
miles. 

Many  of  the  veins  have  been  successfully  worked  to  a  vertical 
depth  of  2,000  feet.  In  the  Kennedy  mine,  on  the  Mother  Lode 
belt,  a  vertical  depth  of  4,000  feet  has  been  attained,  good  ore  ap- 
pearing in  the  lower  levels.  The  Argonaut  is  now  developed  by  a 
4,500-foot  incline  reaching  4,000  feet  in  vertical  depth.  At  the 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  569 

Central  Eureka  mine,  near  the  Kennedy,  rich  ore-bodies  were 
found  below  the  1,000-foot  level,  though  there  was  very  little 
ore  above  that  horizon.  The  North  Star  vein  at  Grass  Valley 
has  been  followed  along  its  flat  dip  for  over  6,350  feet  and,  at 
that  depth,  yields  a  high  production. 

There  are  many  structural  types;  the  most  common  is  the 
simple  filled  vein  (Fig.  191),  which  may  carry  from  a  mere  film 
of  quartz  to  a  thickness  of  10  or  20  feet.  Many  of  the  outcrops 


FIG.  191. — Argonaut  vein  in  slate  country  rock,  Amador  County,  California, 
at  650-foot  level.     Photograph  by  0.  H.  Packer.  j§ 

appear  to  be  much  thicker  than  the  figures  just  given,  but  these 
large  masses  are  poor  in  gold.  Again,  there  are  composite  veins 
or  lodes  in  which  certain  belts  of  country  rock  are  filled  by  branch- 
ing veinlets  of  quartz  or  which  may  contain  altered  slabs  of 
country  rock  (Fig.  193).  In  crushed  clay  slates  the  veins  are 
sometimes  broken  and  folded  (Fig.  192).  Large  bodies  of  rock 
changed  by  replacement  to  gold  ores  are  comparatively  rare; 
such  ores  are  mined  in  several  places  at  Angels  Camp,  Calaveras 
County,  but  even  here  the  gold  is  mainly  contained  in  thin 
quartz  seams  in  the  altered  rock.  Again,  gold-bearing  quartz 
seams  may  follow  joints  of  certain  direction  in  large  masses  of 
rock;  many  such  masses  have  been  worked  by  the  simple  process 


570 


MINERAL  DEPOSITS 


of  hydraulic  washing  of  the  upper,  weathered  part.     Such  de- 
posits are  called  seam-diggings. 

More  rarely  the  veins  follow  narrow  dikes  of  albite  aplite;  or 
they  are  developed  on  joint  planes  across  the  strike  of  thicker 
dikes  in  the  manner  of  ladder  veins. 


FIG.  192. — Bunker  Hill  vein,  Amador  County,  California,  showing  folded 
vein  in  crushed  clay  slate. 

The  association  of  gold  with  dikes  consisting  mainly  of  albite 
rock  has  been  described  by  Turner1  and  Reid.2  Turner  de- 
scribes such  dikes  on  Moccasin  Creek,  in  Tuolumne  County,3 

1  H.  W.  Turner,  Notes  on  the  gold  ores  of  California,  Am.  Jour.  Sci., 
3d  sen,  vol.   47,  1894;  idem,  3d  ser.,  vol.  49,  1905;  replacement  deposits 
in  the  Sierra  Nevada,  Jour.  Geol,  vol.  7,  1899,  pp.  389-400. 

2  John  A.  Reid,  The  east  country  of  the  Mother  Lode,  Min.  and    Sci. 
Press,  March  2,  1907. 

3  H.  W.  Turner,  Seventeenth  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  1,"  1896, 
p.  664. 


DEPOSITS  FORM  ED  [AT  INTERMEDIATE  DEPTHS  571 

at  the  Shaw  mine,  in'  Eldorado  County,  and  at  other  places,  but 
the  associated  ores  are  generally  of  low  grade  and  the  mineraliza- 
tion is  everywhere  later  than  the  dike. 

Reid  observed  numerous  thin  dikes  in  Calaveras  slate  near 
Blue  Canyon,  Placer  County,  consisting  largely  of  albite,  which 
are  cut  or  followed  by  seams  or  veins  containing  pyrite  or  arseno- 
pyrite  and  native  gold.  The  gangue  in  these  veins  consists  of 
quartz  and  albite,  with  some  manganiferous  ankerite. 

Along  the  walls  there  is  always — in  feldspathic  and  femic 
rocks  adapted  to  such  processes — more  or  less  replacement 


FIG.  193.— Section  of  Pittsburgh  vein,  Nevada  City,  California. 

extending  a  few  inches  or  a  few  feet  from  the  vein;  bleaching, 
softening,  and  dissemination  of  pyrite  indicate  such  replacement, 
in  which  the  principal  feature  is  the  development  of  calcium- 
magnesium-iron  carbonates  with  much  sericite  (p.  552).  Occa- 
sionally, in  sodic  amphibolites,  much  albite  develops,  and  in 
the  vicinity  of  Angels  Camp,  on  the  Mother  Lode,  such  replace- 
ments may  contain  enough  gold  to  be  called  ore.  In  serpentine 
the  alteration  to  a  coarse  aggregate  of  ankerite  and  bright-green 
chromium  mica  (mariposite)  is  characteristic;  this  product  of 
replacement  constitutes  ore  in  only  a  few  places,  such  as  the 
Rawhide"  mine,  southeast  of  Angels  Camp,  where  it  was  pene- 
trated by  gold-bearing  quartz  stringers. 

The  ore-shoots  are  irregularly  distributed;  many  veins  are  of 


572  MINERAL  DEPOSITS 

pockety  character,  containing  rich  bonanzas  at  certain  points, 
which  may  be  determined  by  intersections  or  by  the  crossing 
of  certain  beds  of  the  schist  series.  Other  veins  have  large  and 
regular  shoots  generally  with  a  steep  pitch,  and  sometimes  with 
a  pitch  length  of  2,000  or  3,000  feet.  In  isolated  cases,  such  as 
the  Idaho-Maryland  vein  at  Grass  Valley,  the  pitch  of  the  rich 
pay-shoot  was  flat  on  the  plane  of  the  vein  (p.  185).  In  many 
districts,  especially  at  Grass  Valley,  the  rule  is  that  the  shoot 
pitches  to  the  left  of  an  observer  looking  down  the  dip. 

Including  the  placer  gold  yielded  by  the  outcrops  disintegrated 
during  Tertiary  and  Quaternary  time,  the  production  of  the 
California  gold-quartz  deposits  is  exceedingly  large,  being  more 
than  $1,200,000,000  in  value.  The  actual  mining  of  the  quartz 
veins  has  yielded  much  less,  perhaps  only  $400,000,000.  A  long 
list  of  celebrated  mines  could  be  cited,  each  one  having  yielded 
from  $5,000,000  to  $20,000,000.  Among  them  are  the  North 
Star,  Empire,  and  Idaho-Maryland,  of  Grass  Valley,  and  the 
Plymouth  Consolidated,  Kennedy,  Keystone,  Eureka  Consoli- 
dated, Gover,  and  Zeile  on  the  Mother  Lode.1  The  present 
annual  production  from  deposits  of  this  class  in  California  is 
increasing  and  now  amounts  to  about  $13,000,000. 

The  principal  and  almost  exclusive  gangue  mineral  is  milk- 
white  quartz  with  coarse  massive  texture,  occasionally  drusy. 
In  thin  section  the  quartz  shows  partly  idiomorphic  forms  (Fig. 
48),  and  some  individuals  include  earlier  slender  prisms.  Comb 
structure  is  sometimes  seen,  but  never  the  delicate  banding  of 
the  veins  formed  near  the  surface.  In  places  sulphides  en- 
crust rock  fragments  enclosed  in  quartz.  A  rough  banding 
may  result  from  irregular  distribution  of  the  sulphides,  from 
the  inclusion  of  narrow  strips  of  black  slate,  or  from  subse- 
quent shearing  of  the  vein  (Fig.  49) ;  the  last  is  not  uncommon 
and  is  indicated  in  thin  section  by  the  crushing  of  the  primary 
individual  crystals  (Fig.  54).  Fluid  inclusions  are  plentiful, 
and  seem  to  consist  of  an  aqueous  solution.  Carbon  dioxide  has 
been  reported  in  one  or  two  cases.  Calcite,  dolomite,  and  anker- 
ite  are  formed  in  subordinate  quantities,  though  they  may  be 
present  abundantly  in  the  replaced  country  rock  adjoining  the 
vein.  Barite,  fluorite,  and  tourmaline  are  practically  absent, 

1  The  Kennedy  mine  to  the  close  of  1915  has  yielded  $6,378,000  from  792,- 
000  tons  of  ore.  The  North  Star  mine  from  1884  to  1915  inclusive  has  yielded 
$17,450,526  from  1,358,394  tons  of  ore. 


DEPOSITS  FORM  ED  AT  INTERMEDIATE  DEPTHS  573 

as  are  biotite,  garnet,  amphibole,  epidote,  zeolites,  rhodonite, 
and  rhodochrosite.  No  bituminous  material  has  been  reported. 
Mariposite,  a  chromium  mica,  is  common  near  serpentine  in  the 
altered  rock ;  roscoelite,  a  vanadium  mica,  is  sometimes  associated 
with  native  gold.  Rutile  is  generally  confined  to  the  altered 
rock.  Specularite  and  magnetite  are  absent,  except  in  isolated 
cases.  Scheelite  is  known  to  occur  at  several  places. 

The  native  gold  is  the  principal  ore  mineral  and  occurs  in  all 
ores  and  at  all  depths.  Sometimes  large  masses  are  found.  A 
mass  of  solid  gold  valued  at  $40,000  was  taken  out  from  the 
Bonanza  mine,  near  Sonora,  in  a  pocket  which  yielded  $360,000. 
This  mine  produced  more  than  $2,000,000  in  gold,  the  greater 
part  of  which  was  pounded  out  of  the  quartz  in  hand  mortars. 
Still  heavier  masses  of  gold  were  found  in  the  Monumental 
mine,  Sierra  County,  and  below  the  croppings  of  the  Carson  Hill 
veins  on  the  Mother  Lode.  In  some  veins  the  gold  is  distributed 
in  microscopic  particles;  in  others  it  is  visible  (Fig.  194)  and  occurs 
in  threads  and  plates.  Coarse  gold  replacing  quartz  and  arseno- 
pyrite  is  described  by  Ferguson1  from  the  pocket  mines  of  Al- 
legany,  in  Sierra  County  (Fig.  194,  C).  Very  rarely,  in  some 
pocket  mines,  gold  of  a  fineness  exceeding  0.9002  is  encountered, 
but  the  average  fineness  is  0.800,  and  it  is  rarely  as  low  as  0.700, 
the  remainder  per  mille  being  principally  silver. 

Variable  but  always  comparatively  small  quantities  of  metallic 
minerals  accompany  the  gold,  ordinarily  making  up  2  to  3  per 
cent,  of  the  mass.  Pyrite  is  universally  present;  pyrrhotite 
rarely,  and  then  only  in  veins  in  granite  rocks.  Chalcopyrite, 
zinc  blende,  and  galena  are  most  abundant  next  to  pyrite; 
arsenopyrite  is  not  quite  so  common.  Tetrahedrite  is  frequently 
found,  while  stibnite  and  molybdenite  are  rare.  Compounds  of 
tin,  uranium,  boron,  phosphorus,  and  fluorine  are  lacking. 
Tellurides  like  altaite,  hessite,  calaverite,  petzite,  and  melonite 
are  sometimes  associated  with  native  gold. 

The  sulphides  obtained  by  concentration  from  the  ore  are 
usually  rich,  often  having  a  value  of  $100  to  $300  per  ton,  but 
their  value  is  only  a  small  part  of  the  value  of  the  ore.  C.  G. 
Yale3  gives  the  following  figures  for  the  Mother  Lode  mines. 
In  the  five  Mother  Lode  counties  1,393,788  tons  of  ore  were 

1  Bull.  580,  U.  S.  Geol.  Survey,  1915,  pp.  153-182. 

2  Gold  from  the  San  Giuseppe  mine,  near  Sonora,  was  0.990  fine. 

3  Mineral  Resources,  U.  S.  Geol.  Survey,  pt.  1,  1916,  p.  217. 


574 


MINERAL  DEPOSITS 


A  B 

Fia.  194. — A,  Thin  section  of  gold-bearing  quartz,  Keltz  mine,  Tuolumne 

County,  California.     Q,  quartz;  P,  pyrite;  black,  gold,  deposited  later  than 

pyrite.  Magnified  70  diameters.  After  W.  J.  Sharwood. 

B,  Thin  section  of  gold-bearing  quartz  (Q),  Omaha  mine,  Grass  Valley, 

California,  showing  gold  (black)  deposited  contemporaneously  with  pyrite 

(P).     Magnified  17  diameters. 


C,  Gold  replacing  arsenopyrite  and  quartz,  Alleghany  district,  California. 
After  H.  G.  Ferguson. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  575 

mined  in  1916,  with  a  total  recovery  of  $5,853,618.  The  gold 
recovery  on  the  amalgamating  plates  or  from  the  cyanide  bullion 
averaged  $2.69  per  ton,  while  36,974  tons  of  concentrates  (mainly 
pyrite)  obtained  from  the  ore  averaged  in  value  $57  per  ton; 
the  total  value  recovered  in  gold  (with  a  very  small  quantity  of 
silver)  averaged  $4.20  per  ton. 

The  ore  in  the  large  mines  of  the  Mother  Lode  thus  averages 
almost  $4  per  ton,  which  would  be  considered  a  very  low  grade  of 
ore  in  districts  like  Tonopah,  Cripple  Creek  or  Pachuca.  In 
some,  districts,  like  Grass  Valley,  where  narrower  veins  are  mined, 
the  ore  assays  $10  to  $15  per  ton. 

The  gold-quartz  veins  of  the  Sierra  Nevada  were  formed 
shortly  after  the  intrusion  of  the  granodiorite  batholith  in  latest 
Jurassic  or  earliest  Cretaceous  time.  They  have, with  the  sur- 
rounding rocks,  been  subjected  to  an  intense  erosion,  the  vertical 
measure  of  which  amounts  to  several  thousand  feet.  The 
exposures  by  unequal  erosion  or  by  mining  operations  show,  in 
many  districts,  that  the  vertical  range  of  gold  deposition  without 
notable  change  in  richness  of  shoots  was  over  4,000  feet;  the 
relations  in  some  districts  lead  to  the  conclusion  that  the  deepest 
parts  now  mined  were  formed  7,000  feet  or  more  below  the 
surface. 

The  permeation  of  the  metamorphic  series  by  gold-bearing 
quartz  is  remarkable,  although  the  greatest  richness  is  concen- 
trated, as  stated  above,  in  certain  districts  or  along  certain  lines. 

No  hypothesis  of  lateral  secretion  can  account  for  the  great 
masses  of  quartz,  nor  for  the  occurrence  of  the  veins  in  the  most 
diverse  rocks.  For  an  explanation  of  their  origin  we  are  com- 
pelled to  look  to  the  great  batholithic  intrusion,  or  rather  to  the 
many  minor  intrusions  on  the  flank  of  the  range.  At  Grass 
Valley  this  conclusion  is  inevitable;  on  the  Mother  Lode  it  is  less 
positive.  The  Mother  Lode  is,  however,  a  profound  dislocation, 
and  we  may  well  assume  that  it  extends  to  a  great  depth  and 
probably  derived  its  metallic  contents  from  underlying  intrusive 
bodies.  It  must  also  be  conceded  that  in  many  places  the 
evidence  points  to  the  gabbros  and  peridotites  (from  which  the 
serpentine  was  derived)  and  to  the  numerous  albite  aplite  dikes 
which  accompanied  the  basic  intrusions  as  a  source  of  at  least 
part  of  the  gold.  A  remarkable  feature,  nevertheless,  is  the 
absence,  in  the  veins,  of  the  usual  mineralizers  like  chlorine, 
fluorine,  and  boron. 


576 


MINERAL  DEPOSITS 


Regarding  the  nature  of  the  depositing  solutions  fora  ; 
gold-quartz  veins  we  have,  of  course,  no  direct  information. 
They  must  have  been  aqueous,  to  produce  crustification  of  quartz 
and  calcite.  They  must  further  have  been  competent  to  cause 
replacement  by  pyrite,  sericite,  and  earthy  carbonates.  Hot 
waters  containing  carbon  dioxide,  alkaline  carbonates,  and 
hydrogen  sulphide  would  fulfil  these  requirements.  They 
probably  carried  gold  dissolved  in  alkaline  sulphides,  a  form  in 
which  the  gold  solution  is  stable  to  ordinary  reducing  agents  such 
as  carbon  and  pyrite.  Such  solutions  will  deposit  the  gold  by 
contact  with  acids  or  by  exposure  to  oxidation,  probably  also 
by  decrease  of  temperature.  W.  Skey,  T.  Egleston.  G.  F.  Becker, 
and  lately  V.  Lenher1  have  drawn  attention  to  this  solvent. 

The  Gold-Quartz  Veins  of  the  Interior  Cordilleran  Region.2 — 
A  great  number  of  intrusive  masses  of  quartz  monzonitic  or 


FIG.  195. — Section  of  Hidden  Treasure  vein,  Neal  district,  Idaho. 

granodioritic  type  are  found  in  the  interior  Cordilleran  region  of 
the  United  States.  They  are,  as  a  rule,  of  more  recent  age  than 
the  great  coast  batholith,  their  epoch  of  intrusion  falling  at  the 
end  of  the  Cretaceous  or  the  beginning  of  the  Tertiary.  In  or 
around  these  intrusives  gold-quartz  veins  are  often  found,  clearly 
related  to  the  California  type,  but  differing  from  it  in  some 
respects.  Frequently  they  follow  lamprophyric  dikes  (Fig.  195). 
They  contain  more  sulphides,  though  of  the  same  kinds,  and  they 

1  Econ.  Geol,  vol.  7,  1912,  pp.  744-750;  vol.  13,  1918,  pp.  161-184. 

2  W.  Lindgren,  The  mining  districts  of  Idaho  Basin,  etc.,  Eighteenth  Ann. 
Rept.,  U.  S.  Geol.  Survey,  pt.  2,  1897,  pp.  617-744. 

W.  Lindgren,  The  gold  belt  of  the  Blue  Mountains  of  Oregon,  Twenty- 
second  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  2,  1901,  pp.  551-776. 

C.  E.  Weaver,  The  Blewett  mining  district.  Bull.  6,  Washington  Geol. 
Survey,  1911. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  577 

carry,  as  a  rule,  more  silver  in  the  sulphides  than  the  veins  of 
the  California  type;  there  is  less  free  gold,  and,  in  some  instances, 
only  a  small  proportion  of  the  total  gold  is  amenable  to  direct 
amalgamation.  The  gold  contains  silver  and  rarely  has  a  fine- 
ness above  0.700.  Rich  silver  minerals  often  form  in  the  oxi- 
dized zones.  The  precious  metals  are  contained  chiefly  in  the 
quartz  filling,  but  the  altered  rock  adjoining  the  veins  some- 
times carries  gold  and  silver,  which  means  that  it  is  in  part 
replaced  by  gold-  and  silver-bearing  sulphides.  In  feldspathic 
and  ferromagnesian  rocks  sericitic  and  pyritic  alteration  (Fig. 
196)  affects  the  wall  rocks;  carbonatization  is  often  observed, 


FIG.  196. 


FIG.  197. 


FIG.  196. — Pyrite  (p),  forming  by  replacement  along  calcite  veinlets 
(black);  calcite  forms  lining  around  pyrite  crystals.  In  chloritic  diabase, 
Great  Northern  mine,  Canyon,  Oregon.  Magnified  10  diameters. 

FIG.  197. — Quartz  (q)  with  native  gold  (black),  replacing  vein  of  calcite  (c) ; 
Great  Northern  mine,  Canyon,  Oregon.  Magnified  10  diameters. 


but  is  rarely  as  intense  as  in  the  veins  of  the  Sierra  Nevada. 
Pyrite,  arsenopyrite,  chalcopyrite,  galena,  and  blende  are  the 
common  ore  minerals,  but  tetrahedrite  is  also  plentiful  and 
cinnabar  is  known  to  occur.  Tellurides  are  sometimes  present 
and  are  almost  intergrown  with  native  gold.  Quartz  is  the 
prevailing  and  usually  the  only  gangue  mineral.  Quartz  with 
coarse  native  gold  has  been  observed  to  replace  an  earlier  calcite 
gangue  (Fig.  197).  Tourmaline,  magnetite,  and  pyrrhotite 
are  not  known.  The  grade  of  the  ore  is  from  $5  to  $15  per  ton. 


578  MINERAL  DEPOSITS 

Victoria,  Australia.1 — The  principal  gold-bearing  region  of 
Victoria,  though  of  much  smaller  extent  than  the  California 
gold  belt,  is  believed  to  have  produced  about  the  same  amount, 
namely,*  $1,300,000.000  in  gold.  Here,  too,  the  placers  have 
yielded  by  far  the  greater  production.  Both  gravel  deposits 
and  quartz  veins  still  yield  a  gradually  diminishing  output. 
In  1915  the  production  of  gold  from  quartz  mines  was  only  about 
218,660  ounces.  The  ores  averaged  $7.50  per  ton. 

This  most  productive  region  includes  the  celebrated  districts 
of  Ballarat  and  Bendigo  and  is  situated  in  the  low  ranges  of  the 
mountains  rising  between  the  basaltic  and  Tertiary  terranes  on 
the  south  and  the  Murray  Plains  on  the  north  (Fig.  198). 

Little-altered  Ordovician  slates  and  sandstones  prevail  and 
form  sharply  compressed  folds.  Intruded  in  them  are  two  bath- 
oliths  of  granitic  rock,  probably  quartz  monzonite,  the  largest 
being  that  between  Bendigo  and  Castlemaine;  there  are  also 
many  smaller  bodies  of  the  same  kind.  The  intrusions  are 
probably  of  late  Silurian  age,  and  erosion  of  at  least  3,000  feet 
has  planed  the  region  to  an  undulating  surface. 

Within  the  folded  Ordovician  rocks  quartz  veins  are  abundant 
and  generally  follow  the  strike  of  the  strata,  being  massed  along 
certain  productive  "reef  lines."  Frequently  they  are  conforma- 
ble between  shale  and  sandstone,  but  some  of  them  cut  across 
the  strike.  A  common  type  has  one  well-defined  wall  from  which 
flat  and  irregular  bodies  of  quartz  project  into  the  hanging  or 
foot  wall.  These  flat  "makes"  are  particularly  characteristic 
and  usually  contain  the  best  pay  at  Ballarat  East  and  other 
places.  The  saddle  reefs  constitute  an  interesting  division,  in 
which  masses  of  quartz  fill  cavities  produced  at  anticlines  (Fig. 

1  E.  J.  Dunn,  Report  on  the  Bendigo  gold  field,  Dept.  of  Mines,  Mel- 
bourne, 1896. 

T.  A.  Rickard,  The  Bendigo  gold  field.  Trans.  Am.  Inst.  Min.  Eng.,  vol. 
20,  1891,  pp.  463-545. 

J.  W.  Gregory,  The  Ballarat  East  gold  field,  Mem.  4,  Victoria  Geol. 
Survey,  1907. 

W.  Baragwanath,  The  Castlemaine  gold  field,  Mem.  2,  Victoria  Geol. 
Survey,  1903. 

O.  A.  L.  Whitelaw,  The  Wedderburn  gold  field,  Mem.  10,  Victoria 
Geol.  Survey,  1911. 

W.  Lindgren,  Characteristics  of  gold-quartz  veins  in  Victoria,  Eng. 
and  Min.  Jour.,  March  9,  1905. 

F.  L,  Stillwell,  Replacement  in  the  Bendigo  quartz  veins,  etc.,  Econ. 
Geol.,  vol.  13,  1918,  pp.  100-111. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  579 

199)  or  less  commonly  at  synclines  (Fig.  200)  by  stresses  subse- 
quent to  the  principal  folding;  they  are  often  connected  with 
irregular  "makes"  and  spurs  (Fig.  201)  of  quartz.  These  open 
cavities,  subsequently  filled  by  quartz,  are  the  necessary  result 


AUSTRALIA: 

Broken  Hill 
!'  NEW  SOUTHNVALES 


FIG.   198. — Sketch  map  of  eastern  Australia,  showing  location  of  important 
mining  districts. 

of  stresses  applied  to  folded  masses  of  little-altered  sediments, 
the  strata  of  which  vary  considerably  in  hardness. 

The  best  instances  of  saddle  reefs,  many  of  them  superimposed 
upon  and  following  three  or  four  distinct  lines  of  anticlines,  are 
found  at  Bendigo  (Fig.  36)  and  Castlemaine.  The  Bendigo  veins 
have  been  worked  to  a  depth  of  4,600  feet  in  the  Victoria  reef, 


580 


MINERAL  DEPOSITS 


situated  on  the  New  Chum  reef  line,  but  sinking  has  been  sus- 
pended. A  body  of  quartz,  containing  at  best  $17  per  ton,  was 
mined  at  a  depth  of  about  4,200  feet,  but  it  is  said  that  on  the 
whole  little  profitable  mining  has  been  done  at  Bendigo  below  a 
depth  of  2,500  feet.  The  granite  rocks  rarely  contain  quartz 


FIG.  199. — Saddle  reef  in  slate  and  sandstone,  Bendigo,  Victoria. 
L  After  E.  J.  Dunn. 

veins.  The  vein-filling  is  a  massive  milk-white,  sometimes  glassy 
quartz  of  coarse  crystalline  texture.  It  contains  native  gold, 
often  coarse,  and  also  a  little  pyrite  and  arsenopyrite ;  sometimes 
also  a  little  galena,  zinc  blende,  molybdenite,  and  stibnite.  No 
tellurides  are  reported.  There  is  neither  barite  nor  fluorite. 
Ankerite  or  calcite  with  some  magnesium  and  iron  is  common 
but  subordinate,  usually  appearing  near  the  walls.  Albite  and  a 
vermicular  chlorite  are  present  in  places,  the  former  in  vugs,  the 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  581 


FIG.  200. — Trough  reef,  in  slate  and  sandstone,  Bendigo,  Victoria. 
After  E.  J.  Dunn. 


Fro.  201. — Spur  reef  in  slate  and  sandstone,  Bendigo,  Victori; 
After  E.  J.  Dunn. 


582  MINERAL  DEPOSITS 

latter  enclosed  in  massive  quartz.  There  is  little  evidence  of 
banded  structure,  except  that  near  the  walls  of  the  veins  thin 
lamellae  of  slate  may  be  interlaminated  with  quartz. 

At  Ballarat  rich  ore-bodies  occur  at  the  intersection  of  flat 
bodies  of  quartz  with  certain  thin  pyritic  and  carbonaceous 
seams  of  slate,  the  so-called  "indicators."  It  has  been  held 
that  the  gold  has  been  precipitated  by  the  carbon  in  the  indicator. 
A  more  plausible  view  is  that  the  indicators  are  narrow  fissures, 
later  than  the  flat  "makes"  and  enriching  them  at  the  inter- 
section. Similar  features  have  been  noted  at  other  points  in 
Victoria  and  seem  to  point  to  a  process  of  enrichment, 
although  probably  not  caused  by  surface  waters.  At  Ballarat 
the  developments  at  depths  below  1,500  feet  have  not  been 
encouraging. 

Australian  geologists  have  presented  strong  evidence  that  the 
deposition  of  the  quartz  was  completed  before  the  Devonian 
rocks  were  laid  down,  and  this  determines  the  age  of  the  veins 
within  narrow  limits.  The  granitic  intrusion  and  the  formation 
of  the  quartz  veins  were  closely  associated  events.  The  fact 
that  so  few  lodes  occur  in  the  granitic  rocks  is  probably  ex- 
plained by  the  great  resistance  of  the  hard  intrusive  bosses  to 
compressive  stresses,  compared  with  the  yielding  nature  of  the 
soft  sedimentary  rocks. 

J.  R.  Don  (p.  13)  has  shown  that  the  sediments  away  from  the 
veins  contain  no  gold,  and  that  the  increasing  traces  of  gold  found 
as  the  veins  are  approached  are  dependent  upon  the  amount  of 
pyrite  introduced  from  the  veins. 

Metasomatic  processes  play  but  a  small  part  in  these  veins. 
The  slates  are  little  altered,  except  by  the  introduction  of  pyrite 
and  occasionally  of  some  carbonates  of  calcium  and  magnesium. 
Dunn  held  that  the  veins  had  been  opened  by  the  crystallizing 
force  of  the  quartz.  Recently  Still  well  has  advanced  the  view  that 
the  laminated  "ribbon  quartz"  has  been  formed  by  replacement 
of  shale. 

New  South  Wales  and  Queensland. — A  large  number  of  well- 
known  and  productive  districts  are  found  in  New  South  Wales 
and  Queensland,  in  which  the  gold  occurs  in  quartz  veins  asso- 
ciated with  intrusive  rocks.  Some  of  these  veins  carry  quan- 
tities of  sulphides  besides  free  gold;  occasionally  fluorite  and 
barite  are  reported.  The  almost  universal  conditions  are  a 
deeply  eroded  region  with  diorite  or  granodiorite  or  their  basic 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS   583 

dikes  intruded  into  Paleozoic  sediments  which  are  usually 
more  highly  altered  than  in  Victoria;  the  veins  occur  either  in 
intrusive  or  in  sedimentary  rocks  or  in  both.  Placers  are  usually 
present. 

At  Hill  End,1  north  of  Bathurst  in  New  South  Wales,  folded 
Silurian  slates  and  tuffs  are  intruded  by  dikes  and  sills  of  quartz 
porphyry.  The  lenticular  quartz  veins  lie  in  slate  or  at  the 
contact  with  the  intrusive  rocks.  Coarse  gold  prevails;  one  mass 
extracted  in  1872  consisting  of  solid  gold  mixed  with  some 
quartz  weighed  630  pounds  and  was  valued  at  $60,000.  Five 
and  one-half  tons  of  solid  gold  were  recovered  at  this  place  from 
10  tons  of  quartz,  the  value  of  the  gold  being  $3,300,000.  Similar 
geological  conditions  exist  at  Hargraves,  but  the  quartz  here 
occurs  as  saddle  reefs.  Here,  as  at  Ballarat,  flat  "makes"  are 
present  and  are  enriched  where  they  are  crossed  by  "indicators" 
or  narrow  bands  of  dark-greenish  slate. 

At  Hill  Grove,2  in  the  New  England  district,  in  the  north- 
eastern part  of  New  South  Wales,  near  the  wolframite  deposits 
mentioned  elsewhere  (p.  671),  slates  and  quartzites  are  intruded 
by  quartz-mica  diorite  and  the  veins  occur  in  the  sedimentary 
rocks,  often  near  lamprophyric  dikes.  The  veins,  which  average 
6  feet  in  width,  contain,  besides  quartz  and  free  gold,  scheelite, 
arsenopyrite,  and  much  stibnite.  Andrews  regards  them  as 
due  to  the  last  emanations  from  the  same  granitic  magma,  the 
earlier  high-temperature  emanations  having  produced  the  cassit- 
erite-molybdenite-wolframite  deposits. 

At  Charters  Towers,3  in  Queensland,  the  veins  intersect  granitic 
rocks  ranging  from  granites  to  tonalites  or  quartz-mica  diorites. 
The  veins  have  been  highly  productive  and  have  been  worked  to 
a  depth  of  3,000  feet  along  the  dip;  they  contain  about  7  per  cent, 
of  sulphides  (pyrite,  galena,  zinc  blende,  pyrrhotite,  and  arseno- 
pyrite). As  usual  in  granitic  rocks,  a  considerable  part  of  the 
gold  of  the  ore  is  contained  in  the  sulphides.  The  veins  are 
regular  but  narrow,  averaging  about  3  feet  in  thickness.  The 
average  value  of  the  ore  is  probably  less  than  $15  per  ton. 

1  E.  F.  Pittman,  Mineral  resources  of  New  South  Wales,  Geol.  Survey, 
N.  S.  W.,  1901,  p.  31.     Comprehensive  summary  in  Maclaren's  "Gold," 
1908,  pp.  341-358. 

2  E.  C.  Andrews,  Records,  Geol.  Survey,  N.  S.  W.,  vol.  8,  1909,  p.  143. " 

3  Jack,  Rands,  and  Maitland,  Ann.  Rept.,  Geol.  Surv.,  Queensland,  1892. 
W.  E.  Cameron,  Publ,  No.  224,  Geol.  Survey,  Queensland,  1909. 


584  MINERAL  DEPOSITS 

Nova  Scotia. — The  gold-quartz  veins  of  Nova  Scotia,1  from 
which  during  the  last  fifty  years  a  moderate  production  has  been 
derived,  are,  in  many  respects,  of  special  interest.  The  veins  are 
contained  in  folded  sedimentary  rocks — slate  and  quartzite— 
probably  of  Cambrian  age,  and  these  are  intruded  by  granitic 
rocks  of  Silurian  age.  The  gold  belt  extends  for  a  distance  of 
280  miles  along  the  south  coast,  and  its  average  width  is  about 
30  miles.  The  numerous  quartz  veins,  many  of  which  can  be 
traced  for  long  distances,  often  occur  in  the  manner  of  saddle 


FIG.  202. — Folded  quartz  vein  in  slate,  Mexican  mine,  Goldenville,  Nova 
Scotia.    After  T.  A.  Rickard. 

reefs  along  the  anticlines.  The  anticlinal  axes  are  in  places 
marked  by  structural  elliptical  domes  in  which  the  strata  pitch 
both  ways  on  the  strike,  and  gold-bearing  quartz  veins  are 
usually  found  in  such  domes.  The  veins  are  ordinarily  parallel 

1  E.  R.  Faribault,  The  gold  measures  of  Nova  Scotia  and  deep  mining, 
Jour.,  Canadian  Min.  Inst.,  vol.  2,  1899,  pp.  119-161. 

J.  E.  Woodman,  Geology  of  Moose  River  gold  district,  Nova  Scotia, 
Inst.  Nat.  Sci.,  vol.  11,  1903,  pp.  18-88. 

W.  Malcolm,  Goldfields  of  Nova  Scotia,  Mem.  20-E,  Canada  Dept.  Mines, 
Geol.  Survey  Branch,  1912. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  585 

to  the  stratification,  but  some  of  them,  while  parallel  in  strike, 
cut  across  the  dip.  Corrugated  and  crenulated  veins  are  common 
(Fig.  202)  and  the  term  "barrel  quartz"  is  used  to  describe  the 
material  in  them;  the  corrugation  is  believed  to  have  been 
caused  by  deformation  subsequent  to  the  deposition.  The 
gangue  is  always  quartz;  arsenopyrite,  pyrite,  chalcopyrite, galena 
and  zinc  blende  are  fairly  common,  but  the  principal  valuable 
mineral  is  native  gold. 

Veins  with  stibnite  occur  in  the  auriferous  belt,  and  scheelite 
has  been  recently  discovered. l  Faribault  holds  that  the  intrusive 
granite,  in  which  no  gold-quartz  veins  have  been  found,  is  later 
than  the  veins.  If  this  is  the  correct  interpretation,  the  relation- 
ship is  the  reverse  of  that  in  all  other  districts  containing  simi- 
lar veins.  T.  A.  Rickard,  however,  has  expressed  a  contrary 
opinion  and  believes  that  the  formation  of  the  gold-bearing  veins 
succeeded  the  granitic  intrusion. 

Under  the  microscope  the  glassy  quartz  shows  intense  defor- 
mation and  the  corrugated  veins  are  probably  simply  the  result 
of  the  crumpling  of  harder  beds  in  a  plastic  medium  (cfr.  Fig.  14) . 
There  were  probably  two  epochs  of  folding,  one  preceding  and  the 
other  following  the  deposition  of  quartz.  T.  A.  Rickard,2 
on  the  other  hand,  believes  that  the  crenulations  are  the  product 
of  complex  fractures  in  rocks  of  uneven  texture. 

GOLD -ARSENOPYRITE    DEPOSITS 

The  frequent  association  of  arsenopyrite  with  gold  has  been 
noted  above. 

In  some  veins  arsenopyrite  predominates  and  the  ore  may  be 
utilized  for  the  recovery  of  arsenic  as  a  by-product.  Some  of 
these  deposits  contain  also  pyrite,  chalcopyrite,  pyrrhotite, 
zinc  blende,  galena,  realgar  and  stibnite  in  the  order  of  succession 
named.  Quartz  is  the  prevailing  gangue  mineral.  Arsenopyrite 
is  always  the  oldest  ore  mineral.3 

Similar  ores  are  also  found  as  lenses  and  replacement  veins  in 
schist  and  then  usually  show  affiliations  with  the  high  tempera- 

1  V.  G.  Hills,  Tungsten  mining  in  Nova  Scotia,  Proc.  Colorado  Sci.  Soc., 
vol.  10,  1912,  pp.  203-210. 

2  The  domes  of  Nova  Scotia,  Trans.  Inst.  Min.  and  Met.,  London,  vol.  21, 
1912,  pp.  506-560. 

3  J.  E.  Spurr,  The  ore  deposits  of  Monte  Cristo,  Twenty-second  Ann.  Rept.. 
U.  S.  Geol.  Survey,  pt.  2,  1901,  pp.  777-865. 


586  MINERAL  DEPOSITS 

ture  deposits.  Many  such  deposits  of  arsenopyrite  occur  in 
Ontario,1  for  instance,  at  the  Deloro  Mine,  which  for  long  time 
was  worked  for  gold  and  arsenic. 

GOLD-BEARING  REPLACEMENT  DEPOSITS  IN  LIMESTONE 

Deposits  in  which  limestone  is  replaced  by  jasperoid  or  fine- 
grained silica  and  which  carry  gold  or  silver  or  both  are  some- 
times found  in  the  Cordilleran  States  where  intrusive  porphyries 
invade  calcareous  sediments.  Few  examples  are  known  else- 
where. These  ores,  which  are  usually  very  poor  in  sulphides, 
are  at  several  places  of  great  economic  importance. 

In  the  Mercur  district,2  situated  in  the  Oquirrh  Range  in  Utah, 
such  siliceous  silver  ores  are  found  at  the  lower  contact  of  a 
thin  sheet  of  granite  porphyry  with  Carboniferous  limestone. 
The  jasperoid  rock,  in  places  55  feet  thick,  contains  more  or  less 
silver  throughout  but  has  not  been  extensively  worked.  It  car- 
ries barite  and  calcite  in  places  and  large  cavities  are  sometimes 
covered  by  crystals  of  these  minerals.  The  ore  contains  some 
stibnite,  also  a  little  copper  and  very  small  amounts  of  arsenic, 
molybdenum,  and  tellurium.  No  pyrite  was  observed. 

In  the  same  district,  below  an  upper  sheet  of  porphyry  which 
like  the  lower  is  greatly  decomposed  by  processes  of  weathering, 
is  found  a  sheet  of  jasperoid  rock  locally  25  feet  thick  which 
contains  minutely  divided,  generally  invisible  gold  with  some 
fine-grained  pyrite,  a  little  barite,  and  some  realgar  and  cinnabar. 
In  part  the  porphyry  itself  constitutes  ore  and  the  ore  may  ex- 
tend into  the  limestone  above  the  porphyry.  Spurr  suggests  that 
the  ores  gained  access  to  the  sheet  through  vertical  fissures,  now 
filled  with  calcite.  The  deposits  are  evidently  later  than  the  por- 
phyry intrusion,  and  the  ores  are  similar  to  those  of  the  Black 
Hills. 

From  1890  to  the  end  of  1913  about  4,900,000  tons  of  this  gold 
ore  averaging  about  $3.58  per  ton  in  gold  have  been  mined  in 
the  Mercur  district.3  The  total  yield  having  had  a  value  of 
$19,000,000.  The  mines  are  now  closed  and  dismantled. 

1  W.  G.  Miller  and  C.  W.  Knight,  Ontario  Bur.  Mines,  vol.  22,  pt.  2, 
1914,  p.  110;  also  idem,  vol.  11,  1902,  p.  105. 

2  R.  C.  Hills,  Proc.,  Colorado  Sci.  Soc.,  Aug.  6,  1894. 

J.  E.  Spurr,  Sixteenth  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  2,  1895,  pp. 
349-455. 

*  V.  C".  Heikes,  Mineral  Resources,  U.  S.  Geol.  Survey,  1913  and  previous 
years,  Production  of  gold  and  silver,  chapter  on  Utah. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  587 

W.  H.  Weed1  describes  similar  deposits  in  the  Moccasin  district, 
in  Montana,  where  rhyolite  porphyry  and  phonolitic  dikes  in- 
trude Carboniferous  limestone.  Near  these  intrusives  the  lime- 
stone is  replaced  by  fluorite  (Fig.  64)  and  by  jasperoid;  the 
replaced  rock  contains  gold  and  has  been  mined  successfully  for 
a  number  of  years.  Some  of  the  ore  deposits  here,  too,  lie  at  the 
lower  contacts  of  intrusive  sheets. 

The  so-called  refractory  siliceous  ores  of  the  Black  Hills  of 
South  Dakota,  described  by  J.  D.  Irving,2  constitute  the  best 
examples  of  this  type  of  replacement  ores.  For  many  years 
these  ores  have  yielded  annually  about  $2,000,000  in  gold  and 
100,000  ounces  of  silver,  from  about  600,000  short  tons.  The 
ores  are  treated  by  the  cyanide  process.  The  deposits  form 


Clay  Shale 


Dolomite  and  Ore 


Quartzite 


Schist 


FIG.  203. — Cross  section  of  shoot  of  siliceous  ore  replacing  Cambrian  dolo- 
mite. Black  Hills,  South  Dakota.  Spread  of  ascending  solutions  on  under 
side  of  impervious  shale  makes  shoot  wider  at  top.  After  J.  D.  Irving.'^ 


replacements  of  dolomite  at  two  horizons  in  the  Cambrian  section 
of  the  Black  Hills,  in  a  region  which  is  intruded  on  a  large  scale 
by  dikes,  sheets,  and  laccoliths  of  rhyolite  porphyry,  syenite 
porphyry,  and  phonolite  of  probable  Eocene  age.3  The  more 
important  lower  horizon  is  15  to  25  feet  above  the  basement  of 
pre-Cambrian  schists. 

1  W.  H.  Weed  and  L.  V.  Pirsson,  Geology  and  mineral  resources  of  the 
Judith  Mountains  of  Montana,  Eighteenth  Ann.  Report,  TJ.  S.  Geol.  Survey, 
pt.  3,  1898,  pp.  437-616. 

J  Prof.  Paper  26,  U.  S.  Geol.  Survey,  1904. 

3  T.  A.  Jaggar,  Idem,  pp.  24-26. 


588  MINERAL  DEPOSITS 

The  ores  occur  immediately  below  more  or  less  impervious 
beds  of  shale  or  below  sills  of  intrusive  rocks.  While  the  richest 
ore  replaces  the  dolomite,  ores  of  lower  grade  may  also  replace 
the  underlying  basal  Cambrian  quart zite  and  the  overlying  shale; 
the  replaced  bodies  are  at  most  18  feet  thick,  averaging  6  feet. 
These  channel-like  ore-bodies  have  a  width  attaining  300  feet 
but  averaging  much  less.  Their  length  is  considerable,  one  shoot 
being  followed  for  three-fourths  of  a  mile;  many  parallel  shoots 
may  be  found  in  one  locality,  each  shoot  corresponding  to  a  fissure 
or  series  of  fissures  (vertical)  which  intersect  the  basal  beds  but 
which  do  not  carry  the  ore  below  the  quartzite  and  rarely  above 
the  shale  (Fig.  203). 

The  ore  is  a  hard,  brittle  fine-grained  siliceous  rock,  often 
reproducing  the  dolomite  texture  with  great  fidelity  (Fig.  56). 
The  fresh  ore  is  locally  bluish  and  contains  finely  divided  pyrite; 
much  of  it  contains  solution-cavities  lined  with  quartz  crystals. 
Fluorite  is  always  present,  frequently  also  barite.  Other  asso- 
ciated minerals  are  stibnite,  occasionally  wolframite,  and  prob- 
ably arsenopyrite  and  tellurides  in  fine  distribution.  Much 
of  the  ore  is  mined  at  shallow  depths  and  it  is  largely  oxidized. 
Interesting  data  as  to  the  form  and  distribution  of  the  ore-shoots 
are  also  given  by  J.  D.  Irving  in  a  later  paper.1 


GOLD-BEARING  REPLACEMENT  DEPOSITS  IN  QUARTZITE 

It  is  not  uncommon  in  mineralized  districts  to  find  gold  ores  in 
quartzite.  Usually  they  take  the  form  of  gold-quartz  veins, 
but  replacement  ores  may  also  occur.  Such  a  deposit  is  that  of 
the  Delamar  mine,2  in  central  Nevada,  which  for  many  years 
had  a  large  production,  aggregating  several  million  dollars  in 
gold,  but  which  is  now  closed.  At  this  place  Paleozoic  quartzite 
is  intruded  by  two  dikes  of  granite  porphyry  and  one  narrow 
lamprophyric  dike.  Along  the  latter  a  strong  fracture  has 
developed.  The  ore  occurred  in  an  irregular  chimney  on  both 
sides  of  this  fracture  and  is  later  than  the  dikes  (Fig.  204).  The 
ore  has  been  formed  by  recrystallization  of  the  quartzite  and 
does  not  appear  in  the  dikes.  The  rock  of  granular  texture  is 
replaced  by  a  fine-grained  drusy  aggregate,  with  a  little  pyrite 

Replacement  ore-bodies,  Econ.  Geol,  vol.  6,  1911,  pp.  527-561. 

2S.  F.  Emmons,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  31,  1901,  pp.  658-675. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  589 

and  a  telluride  of  gold.  Only  the  oxidized  ore  was  worked;  the 
tenor  of  this  gradually  decreased  until  on  the  tenth  level  it  was 
only  $4  to  $5  per  ton  and  below  working  cost. 


FIG.  204. — Plan  of  seventh  level  of  Delamar  mine,  Nevada. 
After  S.  F.  Emmons. 


GOLD-BEARING  REPLACEMENT  DEPOSITS  IN  PORPHYRY 

Replacement  deposits  in  aluminous  rocks  like  granite  porphyry, 
gneiss,  and  amphibolite  are  not  uncommon  in  connection  with 
gold-bearing  veins,  as  shown,  for  instance,  at  Cripple  Creek, 
Colorado,  and  along  the  Mother  Lode  in  California  (pp.  523  and 
571).  Larger  bodies  of  rock  are  more  rarely  replaced.  W.  H 
Emmons1  has  described  an  example  of  this  in  the  Little  Rocky 
Mountains  of  northeast  Montana,  a  small  outlier  on  the  Great 
Plains.  Stocks  and  sheets  of  syenite  porphyry  are  intruded  in  a 
Paleozoic  sedimentary  complex.  Broad  zones  in  this  porphyry 
are  replaced  and  cemented  by  quartz,  pyrite,  secondary  ortho- 
clase,  and  fluorite.  The  deposits  are  really  wide  replacement 
lodes,  some  of  them  traceable  for  1,200  feet  and  varying  from  a 
few  feet  to  100  feet  in  width.  The  gold  is  finely  distributed  and 
probably  occurs  as  a  telluride,  the  ores  averaging  about  $3  per 
ton  in  gold  and  one  ounce  of  silver.  The  operations  have  thus  far 

lBull.  340,  U.  S.  Geol.  Survey,  1908,  pp.  96-116.  See  also  W.  H. 
Weed  and  L.  V.  Pirsson,  Jour,  Geol,  vol.  4,  1896,  pp.  399-428. 


590  MINERAL  DEPOSITS 

been  confined  to  the  oxidized  zone,  which  descends  to  a  depth  of 
200  feet.     The  relationship  to  the  Cripple  Creek  ores  is  evident. 

THE  SILVER-LEAD  VEINS 

General  Features. — The  silver  deposits  of  intermediate  depths 
include  many  types  between  which  so  many  transitions  exist 
that  a  classification  is  difficult.  Certain  forms  occurring  as 
fissure  veins  parallel  closely  the  gold-bearing  quartz  veins;  but 
many  of  the  silver  deposits  contrast  with  those  of  gold  in  be- 
ing associated  with  carbonate  gangue,  more  frequently  ankerite 
or  other  magnesium-calcium-iron  carbonates  than  calcite  or 
siderite. 

The  replacement  deposits  in  limestone  very  often  contain  rich 
silver  ores,  though  rarely  much  gold.  The  two  ore  minerals 
most  common  in  silver  deposits  are  galena  and  tetrahedrite ;  with 
these  zinc  blende  is  usually  associated.  Galena  and  zinc  blende 
may  so  predominate  that  the  base  metals  yield  the  principal 
value  of  the  deposits.  Chalcopyrite,  pyrite,  and  arsenopyrite 
play  subordinate  parts.  Native  silver  is  probably  never  a  pri- 
mary mineral,  although  abundantly  formed  by  secondary  reac- 
tions effected  by  descending  waters,  and  rich  sulphantimonides 
like  proustite,  pyrargyrite,  and  polybasite  are  also  largely, 
though  not  wholly,  of  similar  secondary  origin. 

The  following  types  merely  serve  as  centers  around  which  the 
descriptions  may  be  grouped. 

Quartz-Tetrahedrite -Galena  Veins. — Prominent  veins  carrying 
milky  quartz  and  sparsely  disseminated  tetrahedrite,  galena,  and 
zinc  blende,  with  subordinate  pyrite,  are  common  in  the  Cordil- 
leran  region  in  or  near  intrusive  bodies  of  granitic  texture.  Many 
such  veins  are  found  in  the  great  batholiths  of  Idaho  and  Mon- 
tana; also  in  New  Mexico — for  instance,  at  Organ,1  where  quartz 
monzonite  breaks  through  Paleozoic  limestones.  The  deposits 
are  on  the  whole  poor  and  rarely  worked,  although  from  1870 
to  1890  the  enriched  surface  zones  in  many  places  yielded  much 
silver  chloride,  native  silver,  and  ruby  silver.  The  Granite- 
Bimetallic  vein  in  Montana  is  a  famous  representative  (p.  888). 
With  rising  silver  prices  many  such  veins  will  be  re-opened. 

TetrahedriterGalena-Siderite  Veins  (Wood  River  Type).— 
The  association  of  siderite  gangue  with  galena  and  zinc  blende  and 

1  Prof.  Paper  68,  U.  S.  <?eol.  Survey,  1910,  p.  209. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  591 

with  a  smaller  quantity  of  tetrahedrite  rich  in  silver  (freibergite) 
is  not  uncommon  in  veins  associated  with  intrusions  of  quartz 
monzonite,  granodiorite,  diorite,  or  lamprophyric  dikes  The 
latter  dikes  are  the  latest  igneous  rocks,  and  the  ores  appear  to 
have  been  introduced  shortly  after  their  intrusion. 

The  deposits  are  usually  veins  in  which  the  ores  appear  in  part 
as  filling,  but  largely  as  replacements  of  the  country  rock.  Sider- 
ite  is  the  characteristic  gangue  mineral,  but  calcite  and  inter- 
mediate carbonates  of  calcium,  magnesium,  and  iron  are  often 
present ;  quartz  enters  into  the  gangue  when  the  veins  intersect 
the  granitic  rocks.  Among  the  ore  minerals  tetrahedrite  is  the 
principal  carrier  of  silver  and  is  often  intimately  intergrown  with 
galena.  The  galena  is  mostly  coarse  grained  and  also  carries 
silver,  while  the  zinc  blende,  with  4  or  5  per  cent,  of  iron,  is  rela- 
tively poor  in  silver,  but  is  sometimes  recovered  as  a  by-product 
in  concentration. 

Chalcopyrite  is  less  abundant  than  tetrahedrite;  pyrite  is  not 
conspicuous;  while  arsenopyrite  and  pyrrhotite  occasionally  ap- 
pear, particularly  in  granitic  country  rock. 

The  ore-bodies  often  replace  calcareous  shales  along  the  vein, 
but  these  shales  appear  to  have  been  little  altered,  except  for 
the  introduction  of  metallic  minerals  and  some  siderite.  In 
granular,  feldspathic  rocks,  close  to  the  vein,  sericite,  carbon- 
ates, and  a  chlorite  rich  in  iron  develop  in  large  amounts  and 
sulphides  are  introduced.  A  complete  replacement  by  sulphides 
is  unusual.  Sodium  is  almost  wholly  removed,  but  potassium 
fixed  as  sericite  and  calcium  fixed  as  carbonate  remain. 

The  structure  of  the  ore  is  generally  massive,  and  large  bodies 
of  galena  are  common ;  in  one  stope  of  the  Minnie  Moore  mine,  in 
the  Wood  River  district,  Idaho,  16  feet  of  solid  galena  was  shown. 
Smaller  veins  may  show  banded  structure,  but  rarely  comb 
structure.  Sometimes  a  thin  layer  of  quartz  may  be  found  along 
the  wall,  then  a  narrow  comb  of  calcite,  while  the  mass  of  the 
vein  consists  of  massive  galena,  alternating  with  bands  of  zinc 
blende  and  containing  intergrown  tetrahedrite.  The  tetrahe- 
drite appears  to  replace  galena.  The  galena  frequently  shows 
phenomena  of  pressing,  gliding  and  recrystallization. 

The  width  of  the  veins  rarely  exceeds  a  few  feet,  and  part 
of  this  is  usually  crushed  country  rock.  Their  outcrops  are 
inconspicuous. 

The  ore-shoots  are  markedly  irregular  and  the  cost  of  mining 


592 


MINERAL  DEPOSITS 


is  therefore  high.  A  marked  deterioration  may  often  be  ob- 
served in  depth;  the  large  bodies  of  rich  silver  ore — aside  from 
those  affected  by  the  surface  enrichment — are  found  compara- 
tively near  the  surface  and  the  lower  levels  commonly  show 
pinched  veins  or  a  predominance  of  zinc  blende,  pyrite  and  quartz. 
The  upper,  oxidized  parts  of  the  veins  are  usually  enriched  by 
secondary  silver  chloride,  native  silver,  and  pyrargyrite. 


Hailey 


FIG.  205. — Vein  systems  near  contact  of  intrusive  mass  in  Carboniferous 
sediments,  Wood  River  district,  Idaho. 

Wood  River,  Idaho.1 — The  silver-lead  veins  near  Hailey,  Idaho, 
on  the  Wood  River,  north  of  the  Snake  River  lava  plains,  were 
discovered  about  1878  and  for  many  years  yielded  a  high  produc- 
tion which  now  has  decreased  greatly. 

The  district  lies  a  few  miles  east  of  the  eastern  contact  of  the 
great  granitic  batholith  of  central  Idaho  and  the  prevailing  rocks 
are  calcareous  shale,  quartzite,  and  limestone  of  Carboniferous 

1  W.  Lindgren,  Wood  River  mining  district,  Twentieth  Ann.  Rept.,  U.  S. 
Geol.  Survey,  pt.  3,  1900,  pp.  218-231. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  593 

age,  compressed  in  northwestward-striking  folds.  These  sedi- 
mentary rocks  are  intruded  by  a  minor  batholith  of  diorite  and 
quartz  monzonite,  following  the  general  direction  of  the  strata 
and  from  2  to  3  miles  wide.  The  deposits  are  fissure  veins  ar- 
ranged in  two  parallel  linked  systems  (Fig.  205)  along  the  con- 
tacts of  the  batholith  and  in  places  cutting  across  the  contacts 
into  the  granitic  rock.  Some  of  the  veins  follow  lamprophyric 
dikes.  Their  strike  generally  cuts  the  stratification  at  an  acute 
angle  and  their  dip  is  prevailing  50°  southwest.  The  croppings 
are  inconspicuous.  Most  of  the  veins  are  in  calcareous  shale. 


FIG.  206. — Section  of  vein  in  Wood  River  district,  Idaho. 


The  ore  consists  of  galena,  zinc  blende,  and  tetrahedrite,  with 
but  little  pyrite  and  chalcopyrite ;  the  gangue  is  siderite,  or 
intermediate  calcium,  iron,  and  magnesium  carbonates,  with  a 
little  quartz.  The  ore  minerals  have  massive  structure,  some- 
times roughly  banded.  Second-class  ore  consists  of  seams  of 
carbonate  gangue  with  small  grains  of  galena.  As  there  are  no 
smelting  works  in  the  district  the  ores  and  concentrates  must  be 
shipped ;  the  shipments  consist  of  high-grade  ore  containing  40  to 
50  per  cent,  of  lead  and  50  ounces  of  silver  ton;  a  little  gold  is 
usually  present. 

Most  of  the  veins  are  narrow,  although  they  may  in  places 


594  MINERAL  DEPOSITS 

widen  out  into  bodies  of  galena  many  feet  wide.     The  typical 
structure  of  a  filled  vein  is  shown  in  Fig.  206. 

The  ore-bodies  are  irregularly  scattered  along  the  veins  and 
are  for  the  most  part  replacements  of  calcareous  shale  by  galena. 
Some  of  these  replacement  bodies  lie  obliquely  across  the  strike 
of  the  vein  and  may  be  several  hundred  feet  long  and  10  to  30 
feet  wide.  In  a  few  places  the  developments  have  been  carried 
far  below  the  adit  levels,  and,  on  the  whole,  the  lowest  levels  have 
shown  fewer  and  poorer  ore-bodies  than  the  upper  parts  of  the 
veins.  There  is,  however,  little  indication  of  sulphide  enrich- 
ment, and  the  oxidized  zone  is  shallow. 

While  some  of  the  veins  in  the  granitic  rocks  have  the  same 
character  as  those  in  the  shales,  others  carry  gold  as  the  principal 
metal.  These  gold-bearing  veins  occur  both  in  the  diorite  of  the 
small  batholith  and  in  the  main  batholith  of  quartz  monzonite; 
they  contain  quartz,  calcite,  siderite,  pyrrhotite,  arsenopyrite, 
and  chalcopyrite  and  are  in  part  free  milling.  Without  doubt, 
these  gold  deposits  belong  to  the  same  epoch  of  metallization  as 
the  silver-lead  veins,  and  they  show  the  same  type  of  metasomatic 
alteration,  namely,  sericitization  and  carbonization. 

Slocan,  British  Columbia.1 — The  veins  of  the  Slocan  district 
are  mainly  contained  in  the  clay  slates  of  the  Slocan  series,  the 
age  of  which  is  possibly  Carboniferous.  The  sedimentary 
rocks  are  intruded  by  granite,  quartz  porphyry,  and  lampro- 
phyric  dikes.  The  fissure  veins  have  a  general  northeast  direc- 
tion and  high  southeast  or  northwest  dips.  Where  the  veins 
inter  sect  the  igneous  rocks  quartz  is  the  prevailing  gangue  mineral. 
In  the  sedimentary  rocks  the  gangue  is  mainly  siderite  or  manga- 
nosiderite.  A  specimen  gave,  for  instance,  59  per  cent.  FeCOs, 
27  per  cent.  MnCO3,  12  per  cent.  MgCO3,  and  2  per  cent.  CaCO3. 
The  ore  minerals  are  zinc  blende,  galena,  and  tetrahedrite  rich 
in  silver.  Pyrite  and  chalcopyrite  are  fairly  common ;  pyrrhotite 
is  less  abundant  and  is  confined  mainly  to  the  vicinity  of  intrusive 
rocks.  Native  silver,  of  secondary  origin,  is  present  and  some- 
times coats  the  cleavage  planes  of  zinc  blende. 

In  the  Slocan  district  also  gold-bearing  veins  occur  together 
with  the  silver-lead  veins  and  are  apparently  of  the  same  age. 

1  O.  E.  Leroy,  Summary  report  for  1909,  Geol.  Survey  Canada,  1910, 
pp.  131-133;  also  Geol.  Survey  Canada,  Map  62A,  1912. 

W.  R.  Ingalls,  Report  of  the  Zinc  Commission,  Ottawa,  1906,  p.  238,  etc. 
W.  L.  Uglow,  Econ.  Geol.,  vol.  12,  1917,  pp.  643-662. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  595 

Many  of  the  Slocan  veins  have  proved  less  rich  in  depth  than 
near  the  surface  and  contain  more  siderite,  quartz,  and  pyrite; 
this  is  probably  a  change  in  the  primary  mineralization.  The 
succession  of  minerals  is:  Siderite,  sphalerite,  galena,  tetrahedrite. 

Galena-Siderite  Veins. — The  galena-siderite  veins  form  a  small 
but  important  type,  represented  in  the  United  States  in  the 
Coeur  d'Alene  district1  in  Idaho.  In  contrast  to  the  Wood 
River  type  these  veins  contain  little  tetrahedrite  and  are  poor  in 
silver;  on  the  other  hand,  they  yield  about  one-third  of  the  lead 
production  of  the  United  States  and  in  the  aggregate  also  much 
silver.  In  1916  the  ore  treated  amounted  to  2,500,000  short  tons 
which  yielded  about  178,000  tons  of  lead  and  11,600,000  ounces 
of  silver;  the  estimated  yield  of  zinc  was  43,000  tons.  Among 
the  principal  mines  are  the  Bunker  Hill  &  Sullivan,  the  Inter- 
state-Callahan,  the  Hecla,  the  Morning,  and  the  Hercules.  Some 
of  the  mines  have  been  worked  to  a  depth  of  3,300  feet  below  the 
outcrops. 

The  prevailing  country  rock  is  a  fine-grained  sericitic  quartzite, 
referred  to  the  Burke  and  Revett  formations  of  the  thick  and 
closely  folded  pre-Cambrian  Belt  series  of  northern  Idaho. 
These  rocks  are  traversed  by  many  faults,  which,  however,  are  not 
mineralized;  the  veins  follow  subordinate  fissures  of  small  throw. 
Two  masses  of  monzonite  of  probable  Cretaceous  age,  the  larger 
not  more  than  3  miles  in  length,  intrude  the  Belt  series  and  cause 
some  contact  metamorphism  by  the  development  of  biotite, 
garnet,  and  pyroxene  in  the  quartzites.  The  last  phases  of  the 
intrusion  are  represented  by  lamprophyric  dikes. .  These  are 
later  than  the  mineralization  and  intersect  the  ore. 

It  is  held  probable  that  the  intrusions  of  monzonite  connect  and 
widen  below  the  surface.  The  ore  deposits  are  composite  veins 
or  lodes,  often  of  considerable  thickness,  formed  partly  by 
filling,  but  largely  by  replacement  of  the  country  rock  along 
nearly  vertical  shear  zones  with  northwesterly  trend.  The 

1  F.  L.  Ransome  and  F.  C.  Calkins,  The  geology  and  ore  deposits  of  the 
Coeur  d'Alene  district,  Idaho,  Prof.  Paper  62,  U.  S.  Geol.  Survey,  1908. 

J.  R.  Finlay,  The  mining  industry  of  the  Coeur  d'Alenes,  Idaho,  Trans. 
Am.  Inst.  Mm.  Eng.,  vol.  33,  1903,  pp.  235-271. 

O.  H.  Hershey,  Genesis  of  the  silver-lead  ores  in  the  Wardner  district, 
Idaho,  Min.  and  Sci.  Press,  June  1,  8  and  15,  1912;  also  Sept.  17,  Oct.  4, 
1913;  May  20,  1916. 

V.  C.  Heikes,  Mines  report  on  Idaho  in  Mineral  Resources,  U.  S.  Geol. 
Survey,  pt.  1,  Annual  publication. 


596 


MINERAL  DEPOSITS 


longest  of  the  veins  is  the  Bunker  Hill,  which  is  traceable  for 
7,000  feet. 

The  ore-shoots  are  large,  and  many  of  them  are  roughly  verti- 
cal; some  have  been  followed  for  3,000  feet  in  pitch  length.  At 
the  Bunker  Hill  &  Sullivan  mine  (Fig.  207)  the  ore-bodies  do  not 
always  follow  the  main  wall,  which  dips  38°  SSW.,  but  may  lie  in 
the  shattered  country  rock  within  250  feet  above  it.  The  width 
of  the  ore  is  in  places  as  much  as  40  feet,  9  feet  being  the  average 
in  some  of  the  larger  mines. 

Galena,  with  some  pyrite  and  zinc  blende,  and  in  places  a  little 
tetrahedrite  rich  in  silver  are  the  principal  ore  minerals.  Chalco- 


FIG.  207. — Vertical  cross-section  of  part  of  Bunker  Hill  and  Sullivan 
vein  showing  relation  of  ore-bodies  (black)  to  structure.  L.  R.,  Lower 
Revett  formation;  U.  B.,  Upper  Burke  formation.  Lines  between  faults 
indicate  intersections  of  stratification  planes  with  vertical  plane.  After 
Oscar  H.  Hershey. 


pyrite  is  present  in  small  amounts;  in  some  mines  pyrrhotite 
takes  the  place  of  pyrite.  Siderite  and  quartz  are  the  predomi- 
nant gangue  minerals;  barite,  calcite,  and  dolomite  are  rare. 
Some  of  the  siderite  contains  several  per  cent,  of  manganese. 
The  ores  are  in  large  part  formed  by  replacement  of  sericitic 
quartzite  along  the  tight  shear  planes  of  the  lodes.  The  siderite 
develops  first,  replacing  both  sericitic  cement  and  quartz  grains 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  597 

in  the  quartzite.  Rhornbohedrons  of  siderite  may  often  be  seen 
cutting  across  the  clastic  grains.  According  to  Ransome  the 
galena  is  in  part  later  than  the  siderite  and  replaces  that  mineral. 
Replacement  of  quartzite  by  galena  is  shown  in  Fig.  60.  Some 
ore-bodies  consist  of  almost  massive  galena,  but  the  ordinary 
ore  is  an  aggregate  of  siderite  and  galena,  which  must  be 
concentrated.  The  bulk  of  the  ores  range  from  3  to  14  per  cent, 
lead  and  from  2.5  to  6  ounces  of  silver  to  the  ton.  These  are 
concentrated  to  a  product  containing  about  50  per  cent,  of  lead. 
The  lowest  grade  which  can  be  worked  at  present  carries  5  per 
cent,  lead  and  3  ounces  of  silver  to  the  ton.  A  few  of  the  mines 
like  the  Interstate-Callahan  and  the  Morning  yield  much  zinc 
blende. 

The  metasomatic  action  indicated  by  the  presence  of  siderite 
in  the  quartzite  often  spreads  for  100  feet  or  more  beyond  the  ore. 
There  are  some  indications  of  change  of  ores  in  depth;  Ransome 
finds  that  in  the  lower  levels  of  many  mines  pyrite,  pyrrhotite, 
and  zinc  blende  become  more  abundant. 

Ransome  traces  a  genetic  connection  between  the  ore  deposits 
and  the  intrusive  monzonite.  Contact-metamorphic  ores  in 
irregular  bodies  are  found  in  two  mines  close  to  the  monzonite, 
in  the  contact  zone.  These  ores  contain  galena,  zinc  blende, 
pyrite,  pyrrhotite,  chalcopyrite,  and  magnetite,  with  a  gangue 
of  garnet,  biotite,  and  diopside.  In  veins  near  the1  intrusive 
mass  pyrrhotite  and  magnetite,  as  well  as  garnet  and  biotite, 
are  found.  Siderite  occurs  only  outside  of  the  contact  zone.  In 
the  Wardner  mines,  which  are  several  miles  from  the  contact, 
siderite  is  most  plentiful.  The  deposits  were  formed  within  the 
epoch  of  granitic  intrusions,  as  shown  by  the  occasional  inter- 
section of  ore-bodies  by  lamprophyric  dikes;  and  since  the  time 
of  ore  formation  erosion  has  probably  removed  several  thousand 
feet  of  rock. 

The  facts  briefly  set  forth  are  of  highest  importance  and  serve 
to  connect  the  high-temperature  deposits  with  those  of  inter- 
mediate, conditions. 

0.  H.  Hershey  believes  that  the  metals  of  the  Wardner  dis- 
trict, in  the  western  part  of  the  Coeur  d'  Alene  region,  were 
originally  disseminated  in  the  Belt  sediments  and  that  these  dis- 
seminations were  eventually  concentrated  into  ore-bodies  by  hot 
waters  ascending  on  thrust  faults.  He  also  holds  that  the  con- 
tact metamorphic  deposit  of  the  Success  Mine  has  been  formed 


598  MINERAL  DEPOSITS 

prior  to  the  monzonite  intrusion  and  was  invaded  and  meta- 
morphosed by  it.1 

Lead-Silver  Veins  with  Calcite,  Siderite,  and  Barite. — 'Veins 
containing  galena  and  zinc  blende  with  a  gangue  of  calcite, 
siderite,  or  barite  are  abundant  in  many  mining  regions  and  are 
frequently  connected  with  replacement  deposits  in  limestone. 
In  many  places  they  have  a  distinct  connection  with  intrusive 
rocks  and  were  formed  shortly  after  the  irruption,  but  some  of 
them  are  similar  to  the  Mississippi  Valley  lead-zinc  deposits  and 
may  well  have  been  deposited  by  the  ascending  waters  of  the 
ordinary  circulation.  Among  numerous  examples  the  deposits 
of  Clausthal  and  Przibram  may  be  briefly  mentioned. 

Lead-Silver  Veins  of  Clausthal.2 — The  mines  of  Clausthal, 
in  the  Harz  Mountains  of  Germany,  which  have  been  in  opera- 
tion since  the  thirteenth  century  and  still  maintain  a  moderate 
production,  are  working  on  a  vein  system  which  intersects  a 
folded  complex  of  Devonian  and  Carboniferous  sedimentary 
beds,  the  prevailing  rocks  being  clay,  slate  and  graywacke. 
The  general  strike  of  the  veins  is  east-west,  and  the  dip  is  steep. 
The  numerous  veins  extend  over  an  area  15  miles  in  length 
and  5  miles  in  width;  they  are  in  general  composite  veins  or 
lodes  and  the  important  fissures  are  also  faults  of  considerable 
throw.  Mining  operations  in  this  district  have  attained  a  depth 
of  3,000  feel. 

The  ores  contain  chiefly  galena  and  zinc  blende,  with  some 
marcasite,  pyrite,  chalcopyrite,  and  tetrahedrite.  Arsenical 
minerals  are  generally  absent.  In  one  group  of  veins  calcite  and 
quartz  predominate;  in  another  barite  and  siderite.  The  galena 
contains  only  0.05  per  cent,  silver,  though  richer  ores  with  as 
much  as  0.3  per  cent,  silver  are  found  in  some  mines.  Symmet- 
rical banding  is  exceptional,  the  normal  ore  having  an  irregularly 
massive  structure.  The  tendency  appears  to  be  toward  an 

1  J.  B.  Umpleby,  Genesis  of  the  Success  zinc-lead  deposits,  Econ.  Geol. 
vol.  12,  1917,  pp.  138-153. 

O.  Hershey,  Idem,  pp.  348-558. 

2  A.  von  Groddeck,  Ueber  die  Erzgange  des  Oberharzes,  Zeitschr.  Deutsch 
geol.  Gesell.,  1866,  pp.  693-776. 

F.  Klockmann,  Beitrage  zur  Erzlagerstattenkunde  des  Oberharzes, 
Zeitschr.  prakt.  Geol.,  1893,  pp.  466-471. 

B.  Baumgurtel,  Oberharzer  Gangbilder,  Leipzig,  1907,  pp.  23. 

See  also  Stelzner  and  Bergeat,  Die  Erzlagerstatten,  1,  1905,  pp.  763- 
771,  and  R.  Beck,  Lehre  von  den  Erzlagerstatten,  1,  1909,  p.  363-367. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS   599 

increase  in  the  percentage  of  zinc  in  depth.  Within  the  lodes  the 
clay  slates  are  slightly  altered  by  mechanical  and  chemical  proc- 
esses. The  change  in  composition  is  slight1  and  appears  to  con- 
sist largely  of  an  increase  in  sericite  at  the  expense  of  an  original 
chloritic  mineral  in  the  clay  slate. 

Opinions  differ  as  to  the  genetic  interpretation  of  the  veins 
of  Clausthal.  There  are  no  intrusive  rocks  in  the  immediate 
vicinity  aside  from  a  dike  of  kersantite,  which  is  faulted  by  the 
vein  fissures,  and  the  mass  of  "Brocken"  granite  in  the  eastern 
part  of  the  district.  A  genetic  connection  of  these  intrusives 
with  the  veins  seems  probable,  but  cannot  be  regarded  as  proved. 
According  to  von  Koenen2  the  fissures  are  of  Miocene  age  and 
some  movement  on  these  fissures  seems  to  be  taking  place  at 
present.  The  mineral  association  would  indicate  deposition  at 
less  depth  or  at  lower  temperature  than  in  the  veins  of  the  Coeur 
d'Alene  district,  for  instance. 

Lead-Silver  Veins  of  Przibram,  Bohemia.3 — The  mines  of  Przi- 
bram,  which  have  been  worked  for  several  hundred  years  and  still 
maintain  a  small  output,  are  situated  40  miles  south-southwest 
of  Prague,  in  the  "Silurian  syncline,"  well  known  in  the  early 
history  of  geology.  The  predominating  rocks  are  Cambrian  (?) 
graywacke  and  clay  slate,  a  folded  and  faulted  complex  intruded 
by  a  stock  of  diorite.  Dikes  of  diabase  are  exceedingly  numerous 
and  are  followed  by  the  veins;  dikes  of  diorite  and  kersantite  are 
also  present.  The  intrusive  diorite  produces  a  decided  contact 
metamorphism  in  Paleozoic  sediments. 

The  veins  have  a  steep  dip  and  have  been  followed  down  to  a 
depth  of  3,773  feet;  about  forty  of  these  veins  have  been  worked, 
and  they  are  contained  within  a  narrow  area  4  or  5  miles  in  length. 
The  width  of  the  veins  attains  25  feet,  but  averages  much  less. 
Fig.  208  gives  an  idea  of  their  structure.  The  ore  minerals  con- 
sist of  galena  and  zinc  blende  with  some  pyrite  and  chalco- 
pyrite  and  occasionally  many  other  minerals  like  arsenopyrite, 

1  A.  v.  Groddeck,  Jahrb.  Preuss.  geol.  Landes-Anstalt,  1885,  pp.  1-52. 
W.  Lindgren,  Trans.  Am.  lust.  Min.  Eng.,  vol.  30,  1901,  p.  683. 

2  A.  von  Koenen,  Die  Dislokationen  W.  und  S.  W.  vom  Harz.,  etc.,  Jahrb. 
Preuss.  geol.  Landes-Anstalt,  1903,  pp.  68-82. 

3  J.  Schmidt,  Bilder  von  den  Erzlagerstatten  von  Przibram.    Published  by 
Austrian  Agricult.  Dept!,  Vienna,  1887. 

F.  Posepny,  Archiv  fur  prakt.  Geol.,  2,  Freiberg,  1895,  pp.  609-745. 
A.  Hofman  and  F.  Slavik,  Ueber  Diirrerze  von  Przibram,  Bull,  internal. 
15,  Acad.  Sci.  d'e  Boheme,  1910. 


600 


MINERAL  DEPOSITS 


stibnite^  urariinite,  cobalt  arid  nickel  minerals,  wurtzite,  and 
millerite.  Rich  silver  minerals  like  argentite  and  pyrargyrite, 
-as  well  as  native  silver,  were  plentiful  in  the  oxidized  zone.  The 
galena  is' the  carrier  of  silver  and  contains  about  0.5  per  cent,  of 
this  metal.  Among  gangue  minerals  calcite,  siderite,  and  quartz 
predominate,  but  barite  arid  ankerite  are  also  known.  The 
structure  is  in  part  banded  and  drusy. 

The  quartz  and  zinc  blende  appear  to  increase  in  depth  and  the 
.ores  become  "dry."     These  dry  ores  contain  about  50  per  cent. 


b     b 


FIG.  208. — Section  of  the  Adalbert  vein  at  Przibram,  Bohemia.  G, 
Graywacke;  D,  diorite;  q,  quartz;  c,  calcite;  g,  galena;  b,  zinc  blende.  After 
J.  Zadrazil  and  J.  Schmidt. 

quartz,  17  per  cent,  siderite,  17  per  cent,  galena,  0.26  per  cent, 
silver,  and  also  primary  boulangerite,  tetrahedrite,  pyrargyrite, 
diaphorite,  specularite,  chlorite,  and  cassiterite. 

The  deep  workings  are  practically  dry,  but  there  existed  for- 
merly a  rich  zone  of  oxidation  descending,  in  spite  of  a  high 
present  water  level,  to  depths  of  200  to  900  feet. 

The  genetic  connection  of  the  veins  with  the  intrusive  diorite 
and  its  satellites  of  diabasic  and  lamprophyric  dikes  appears  to  be 
clearly  indicated;  the  presence  of  cassiterite  points  in  the  same 
direction.  That  the  region  is  a  metallogenetic  province  con- 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  601 

nected  with  intrusions  is  also  suggested  by  the  occurrence  of 
gold-bearing  quartz  veins  in  the  diorite  or  granite. 

Pyritic  Galena-Quartz  Veins. — In  the  surroundings,  of  granitic 
and  dioritic  intrusions  a  certain  type  of  lead-bearing  veins  is 
especially  common,  distinguished  by  pyrite,  galena,  dark  zinc 
blende,  arsenopyrite,  and  some  chalcopyrite,  with  subordinate 
arsenopyrite,  in  a  gangue  of  quartz,  with  a  small  amount  of 
calcite  or  dolomite. 

Freiberg,  Saxony. — The  type  just  mentioned  corresponds 
closely  to  the  "Kiesige  Bleiformation "  of  Freiberg,1  there  repre- 
sented by  numerous  veins  of  considerable  persistency  contained 
in  a  flat  dome  of  biotite  gneiss.  K.  Dalmer  and  other  geologists 
have  pointed  out  their  probable  genetic  connection  with  the 
intrusive  (Carboniferous)  granites  of  the  Erzgebirge  .and  their 
well-established  relationship  to  the  tin  veins  which  are  situated 
closer  to  or  within  the  intrusives.  The  Freiberg  veins  of  this 
type  are  narrow,  being  seldom  3  feet  wide,  and  have  been  mined 
to  a  depth  of  2,100  feet.  The  pyrite,  arsenopyrite,  and  zinc 
blende  are  poor  in  silver,  but  the  galena  contains  0.1  to  0.2  per 
cent,  of  this  metal. 

Stelzner  and  Schertel  ascertained  that  the  zinc  blende  contains 
microliths  of  cassiterite.  The  vein  structure  is  irregularly  mass- 
ive, without  marked  banding  or  crustification. 

The  ores  are  of  low  grade,  and,  after  a  period  of  activity  extend- 
ing over  nearly  750  years,  the  mines  are  now  practically  closed. 

The  silver-lead  deposits  of  Freiberg  comprise  a  complicated 
system  of  fissure  veins  of  different  types  and  ages,  which  have 
been  carefully  studied  by  such  men  as  A.  G.  Werner  (1791), 
A.  von  Weissenbach  (1836),  J.  C.  Freiesleben  (1843),  F.  C.  von 
Beust  (1840),  B.  von  Cotta  (1861),  and  H.  Miiller  (1849-1901). 

The  veins  are  classified  as  follows: 

(1)  Older  Veins. — Noble2  quartz  formation:  Fine-grained 
quartz  with  argentite,  pyrargyrite,  native  silver,  pyrite,  and 
arsenopyrite. 

Pyritic  lead  formation:  Quartz,  pyrite,  galena,  zinc  blende, 
arsenopyrite,  and  chalcopyrite. 

Tin  formation:  Quartz,  fluorite,  arsenopyrite,  cassiterite,  and 
chalcopyrite. 

1  Herman  Mtiller,  Die  Erzgange  des  Freiberger  Bergrevieres,  Erltuter- 
ungen  zur  geol.  Spedal-Karte  Sachsens,  Leipzig,  1901,  p.  350. 

2  The  word  "Edel,"  or  noble,  refers  to  the  high-grade  silver  ores. 


602  MINERAL  DEPOSITS 

Noble  lead  formation :  Quartz,  ankerite,  rhodochrosite,  galena, 
zinc  blende,  pyrite,  tetrahedrite,  pyrargyrite,  proustite,  and 
polybasite. 

(2)  Younger  Veins. — Barytic  lead  formation:  Barite,  fluorite, 
quartz,  calcite,  galena  (poor  in  silver),  chalcopyrite,  tetrahedrite, 
zinc  blende.  These  veins  are  often  of  considerable  width. 

The  barite  veins  are  distinctly  later  than  the  older  group,  and 
their  minerals  occur  in  beautifully  banded  and  drusy  form. 
Miiller  is  doubtless  right  in  ascribing  a  Tertiary  age  to  these 
veins  and  a  possible  connection  with  the  basaltic  eruptions  of 
that  age  along  the  Bohemian  frontier.  The  barytic  lead  veins 
sometimes  carry  nickel  and  cobalt  minerals  and  Miiller  is  inclined 
to  correlate  them  with  the  cobalt  and  nickel  veins  of  Annaberg. 

The  older  group  appears  to  be  genetically  connected  with  the 
granitic  intrusions  of  Carboniferous  age,  or  perhaps  also  with 
the  Permian  and  Carboniferous  porphyries  (intrusive  and  effu- 
sive) of  the  same  region.  The  "noble  quartz  formation"  alone 
is  intersected  by  dikes  of  quartz  porphyry,  while  the  other  veins 
appear  to  be  later  than  the  porphyry.  The  granite  stocks  of  the 
region  are  intersected  by  veins  similar  to  those  of  Freiberg;  but 
no  granite  occurs  in  the  Freiberg  district.  It  is  interesting  to 
note  that  in  parts  of  the  district  dikes  of  kersantite  and  minette 
are  plentiful  and  that  the  veins  are  later  than  these  dikes. 

Between  the  various  members  of  the  older  group  many  transi- 
tions exist,  and  it  seems  justifiable  to  regard  them  as  genetically 
connected  with  the  granitic  eruptions  of  Carboniferous  age  and 
as  formed  shortly  after  the  last  lamprophyric  dikes  of  that  parent 
magma  had  been  intruded.  The  mineral  association  of  the 
"noble  quartz  formation"  and  the  "noble  lead  formation,"  with 
apparently  primary  argentite  and  pyrargyrite  continuing  to  the 
greatest  depth  reached,  far  beyond  the  zone  of  oxidation,  seems 
to  suggest  that  these  veins  have  been  formed  at  relatively  low 
temperature.  They  do  not  correspond  to  the  types  usually  asso- 
ciated with  intrusive  masses. 

The  ore-shoots  of  the  Freiberg  veins  are  irregular;  the  richest 
parts  were  often  at  intersections  of  fissures  (Fig.  72).  The  oxi- 
dized ores  worked  in  the  early  history  of  the  mines  were  rich  in 
argentite  and  native  silver. 

^Pyritic  Galena-Quartz  Veins  in  the  United  States. — Near  intru- 
sive areas  in  the  central  and  eastern  Cordilleran  States  are  many 
veins  of  the  Freiberg  type,  just  described,  although  they  ordina- 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS   603 

rily  also  carry  gold  together  with  silver.  Few  of  them  are, 
however,  of  the  first  importance.  More  common,  perhaps,  are 
veins  which  carry  mainly  massive  galena  and  zinc  blende  asso- 
ciated with  but  little  pyrite,  or  veins  in  which  the  pyrite  entirely 
predominates.  Examples  of  this  kind  are  given  in  the  descrip- 
tion of  the  Leadville  region  (p.  613). 

Excellent  examples  of  the  type,  described  by  F.  C.  Schrader, 
occur  near  Kingman,1  in  northwestern  Arizona,  in  the  Wal- 
lapai  mining  district.  These  veins  were  discovered  in  1872. 
Their  upper  parts  yielded  rich  silver  ores,  but  of  late  years  the 
silver  production  has  declined  as  the  leaner  primary  sulphides 
were  encountered,  and  in  the  ores  now  extracted  zinc  blende 
is  the  most  valuable  constituent.  The  greatest  depth  attained 
is  about  1,400  feet.  The  rocks  are  pre-Cambrian  granite,  gneiss, 
and  schist  intruded  by  granite  porphyry,  probably  of  Mesozoic 
age,  and  by  a  great  number  of  lamprophyric  dikes  of  minette 
and  vogesite,  which  in  part  are  followed  by  the  veins. 

The  deposits  are  well-defined  fissure  veins  with  steep  dip,  form- 
ing conjugated  systems  with  northwesterly  strike;  they  are 
straight  and  have  well-defined  walls,  and  some  of  them  are 
traceable  for  considerable  distances.  The  gangue  is  quartz,  in 
places  shattered  and  cemented  by  a  later  generation  of  calcite, 
occasionally  also  siderite.  Among  the  primary  sulphides  are 
pyrite,  galena,  zinc  blende,  and  chalcopyrite,  rarely  molybdenite 
and  stibnite.  The  ore  may  contain  $10  in  gold  and  silver,  8  per 
cent,  lead,  and  5  to  16  per  cent.  zinc.  It  is  in  part  shipped  crude, 
in  part  concentrated. 

The  structure  is  irregularly  massive,  in  places  with  rough  band- 
ing by  arrangement  of  the  sulphides.  The  veins  are  narrow, 
though  in  some  places  ore-bodies  20  feet  wide  have  been  worked. 
The  pay-shoots  are  irregular,  but  often  coincide  with  inter- 
sections of  veins.  The  water  level  is  from  100  to  400  feet  below 
the  surface  and  above  it  were  rich  oxidized  lead  ores,  horn  silver, 
native  silver,  argentite,  and  ruby  silver.  The  decrease  of  galena 
and  increase  of  chalcopyrite  noted  in  the  lower  levels  suggest  a 
gradual  change  in  the  primary  filling. 

The  ore  is  mainly  deposited  by  filling  of  cavities;  the  wall  rocks 
contain  little  ore  but  are  sericitized  and  filled  with  pyrite.  close 
to  the  veins. 

1  F.  C.  Schrader,  Mineral  deposits  of  the  Cerbat  Range,  Black  Mountains, 
etc.,  Bull.  397,  U.  S.  Geol.  Survey,  1909. 


604  MINERAL  DEPOSITS 

THE  SILVER-LEAD  REPLACEMENT  DEPOSITS  IN  LIMESTONE 

General  Features. — Limestone,  dolomites,  and  calcareous  shales 
are  easily  soluble  by  waters  circulating  above  the  water  level 
along  stratification  planes,  joints,  veins,  or  zones  of  brecciation; 
caves  and  open  passages  will  result.  Below  the  water  level 
more  slowly  .circulating  solutions  often  replace  limestone  by 
dolomite  or  cherty  or  jasperoid  silica.  If  the  solutions  carry 
metallic  sulphides  these  are  easily  precipitated,  and  by  a  simul- 
taneous operation  the  carbonate  goes  into  solution  while  a  corre- 
sponding volume  of  sulphides  takes  its  place  (Fig.  29).  Some 
of  these  replacement  deposits  that  have  no  genetic  connection 
with  igneous  rocks  have  been  described  above  (p.  444). 

In  districts  where  metallization  is  caused  by  igneous  activity 
the  limestone  is  often  replaced  close  to  the  contact  by  sulphides, 
particularly  copper  sulphides,  associated  with  high-temperature 
minerals;  these  deposits  are  described  in  Chapter  XXVI.  .Fre- 
quently, however,  replacement  by  sulphides  is  also  found  at 
greater  distances  from  the  igneous  rock,  but  the  circulating  solu- 
tions which  caused  the  replacement,  while  probably  derived  from 
the  magma,  had  a  lower  temperature  and  therefore  no  high-tem- 
perature minerals  could  form.  Such  deposits,  which  contain 
mainly  lead,  zinc,  and  silver,  may  appear  in  connection  with  erup- 
tions of  lavas  and  may  form  relatively  close  to  the  surface,  but 
they  are  more  common  in  the  vicinity  of  intrusive  rocks  now  ex- 
posed by  erosion.  The  process  is  therefore  favored  by  higher 
temperature  and  pressure. 

For  the  development  of  replacement  deposits,  pathways  that 
can  be  followed  by  the  solutions  are  necessary.  Joints  and  seams 
may  provide  them,  but  more  commonly  the  fissures  which  were 
formed  during  or  after  the  intrusion  guide  the  solutions  to  the 
limestone.  When  the  waters  have  entered  a  fissure  the  processes 
of  replacement  begin  immediately  but  the  products  of  interchange 
are  hot  confined  to  this  fracture.  On  the  contrary,  they  spread 
in  all  directions,  guided  by  minor  structural  planes,  and  replace- 
ment deposits  in  limestone  are  therefore  characteristically  irregu- 
lar; it  often  happens  that  the  original  fissure  may  be  difficult 
to  discover,  though  genetically  it  is  the  key  to  the  extent  and  the 
continuation  of  the  deposit.  The  mining  of  such  deposits 
demands  thorough  knowledge  of  the  geological  structure. 

There  are  a  great  number  of  such  deposits  in  districts  of  the 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  605 

Cordilleran  region  of  the  Americas.  Many  of  them  are  small  and 
are  soon  exhausted,  while  others  are  among  the  great  ore  deposits 
of  the  world.  The  districts  of  Aspen  and  Leadville,  Colorado; 
Eureka,  Nevada;  Lake  Valley,  New  Mexico;  Elkhorn,  Montana; 
Park  City  and  Tintic,  Utah;  and, Sierra  Mojada,  Mexico,  may 
serve  as  examples. 

At  some  places  these  silver-lead  deposits  follow  dikes  or  in- 
trusive sheets,  but  such  deposits  were  usually  formed  after 
the  rock  had  congealed  and  cooled.  At  other  places  they  are 
dependent  upon  impervious  overlying  beds  like  shale.  The  latter 
mode  of  occurrence  is  exceedingly  common  (Fig!  70)  and  indicates 


FIG.  209. — Irregular  replacement  deposit  in  the  Garrison  mine,  Cortez,. 
Nevada.  Ore  consists  of  galena,  zinc  blende,  pyrite,  stromeyerite,  etc., 
and  their  oxidation  products:  After  W.  H.  Emmons,  U.  S.  Geol.  Survey. 

that  the  solutions  were  ascending  and  that  deposition  followed 
the  ponding  or  stagnation  of  the  solution  or  at  least  was  favored 
by  less  rapid  circulation.  Sections  of  two  smaller  replacement 
deposits  are  shown  in  Figs.  209  and  210. 

The  primary  minerals  of  these  replacement  deposits  are  com- 
paratively few  and  simple.  Deep  oxidation  is,  however,  common 
in  limestone  and  descending  waters  may  effect  many  changes  and 
develop  a  great  number  of  rare  oxidized  minerals  in  the  oxidized 
zone,  while  complex  secondary  sulphides  may  form  in  the  lower 
parts  of  the  deposit.  The  gangue  minerals  are  few:  Dolomite 
is  often  present  as  a  coarser  aggregate  and  at  many  places  the 
process  of  replacement  was  begun  by  a  dolomitization  of  the  lirriej 


606  MINERAL  DEPOSITS 

stone.  Dense,  cherty  quartz  is  exceedingly  common,  much  more 
so  than  coarser  crystalline  quartz.  In  accordance  with  the 
suggestion  of  Spurr,  this  siliceous  gangue  is  called  jasperoid, 
though  this  term  is  really  a  misnomer,  for  the  rocks  are  gray 
rather  than  red  or  brown.  Other  gangue  minerals  are  calcite, 
barite,  sometimes  fluorite,  various  carbonates  allied  to  ankerite, 
and  more  rarely  rhodochrosite.  The  replacement  deposits  carry- 
ing siderite  are  described  on  page  595.  The  common  primary 
ore  minerals  are  pyrite,  galena,  zinc  blende,  chalcopyrite,  and 
more  rarely  arsenopyrite.  Tetrahedrite,  tennantite,  enargite, 
bornite,  bismuthinite,  wolframite,  molybdenite,  and  stibnite 
are  of  local  importance.  But  when  argentite,  ruby  silver,  steph- 
anite,  polybasite,  and  native  silver  as  well  as  various  sulph- 

^Alphave-n 
W 


Limestone      IwvlJf 

Diabase   Rhy°llte 
^Blackis  ore 


*   Ryepatch  vein 


FIG.  210. — Section-  of  Ryepatch  mine,  Union ville,  Nevada.  Ore-body 
between  faults  250  feet  wide;  consists  of  calcite,  quartz,  pyrite,  galena, 
zinc  blende,  tetrahedrite,  etc.  After  F.  L.  Ransome,  U.  S.  Geol.  Survey. 

antimonides  of  lead  appear  the  probability  is  that  they  are 
secondary  minerals.  Chalcocite  is  in  these  deposits  probably 
always  secondary.  Gold  is  sometimes  present  as  a  primary 
mineral,  but  the  ores  carry  ordinarily  much  more  silver  than  gold. 
Galena  is  very  common  and  is  usually  rich  in  silver.  The  silver 
content  of  galena  is  usually  caused  by  a  primary  intergrowth 
with  small  grains  of  argentite.  In  many  so-called  lead  deposits 
the  lead  really  predominates  only  in  the  oxidized  zone,  while  the 
primary  ore  carries  far  more  pyrite  and  zinc  blende  than  galena. 
Such  are  the  relations  at  Leadville,  for  instance. 

The  ore  which  replaces  limestone  is  usually  coarse-grained, 
while,  as  mentioned  above,  the  replacement  of  limestone  by 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  607 

silica  yields  rocks  with  fine  grain.  Crustified  or  drusy  structures 
are  unusual,  though  common  in  the  oxidized  parts  of  these 
deposits.  Before  the  importance  of  replacement  as  a  geological 
process  was  recognized  many  of  these  ores  were  considered  as 
fillings  of  limestone  caves.  Some  of  the  deposits  consist  of  mas- 
sive sulphides,  while  in  others,  presumably  formed  at  lower 
temperature,  the  gangue  may  prevail. 

Replacement  deposits  are  not  confined  to  calcareous  rocks. 
They  occur  also  in  quartzite,  shale,  and  igneous  rocks,  but  they 
are  certainly  more  common  in  carbonate  rocks  than  elsewhere. 
Very  hot  solutions  may  replace  any  rtbck,  but  most  of  the  deposits 
described  in  this  chapter  were  probably  laid  down  by  solutions 
having  a  temperature  of  less  than  200°  C.,  and  under  such  cir- 
cumstances limestone  would  be  replaced  while  other  rocks  would 
be  little  affected.  Siliceous  rocks  are  more  easily  replaced  than 
aluminous  material;  it  is  evidently  difficult  to  carry  away  large 
amounts  of  alumina  even  at  high  temperatures,  and  while  in 
limestone  the  replacement  is  often  complete,  ores  in  aluminous 
rocks  contain  much  residual  material. 

Park  City,  Utah.1 — The  Park  City  district  lies  near  the  summit 
of  the  Wasatch  Range.  Since  1870  it  has  yielded  silver  to  the 
value  of  $97,000,000  and  lead  valued  at  $53,500,000,  lately  also 
much  copper  and  zinc  and  it  still  remains  one  of  the  most  impor- 
tant metal-producing  districts  of  the  United  States.  Its  ores  are 
in  part  shipped  as  mined,  but  much  is  also  concentrated,  the 
total  output  of  crude  ore  for  1917  being  240,600  tons.  The  con- 
centrating ore  contains  from  6  to  8  per  cent,  lead,  6  to  8  per  cent, 
zinc,  6  to  10  per  cent,  iron,  and  9  ounces  of  silver  per  ton;  also 
some  gold  and  copper.  The  deepest  shafts  attain  1,500  and  2,000 
feet  and  the  workings  of  the  district  probably  aggregate  100  miles 
in  length. 

A  huge  anticline  of  late  Carboniferous,  Permian,  and  Triassic 
sediments,  mainly  limestone,  quartzite,  and  shale,  the  total  thick- 
ness of  beds  exceeding  8,000  feet,  is  intruded  by  laccolithic  stocks 
of  diorite  porphyry,  probably  of  late  Cretaceous  age,  which  have 
caused  contact  metamorphism  in  the  adjoining  limestone  and 
shales. 

The  ores  occur  as  lode  deposits  and  closely  associated  bedded 

1  J.  M.  Boutwell,  Prof.  Paper  77,  U.  S.  Geol.  Survey,  1912. 

V.  C.  Heikes,   Mine  production  of  Utah,  in  Mineral  Resources,  U.  S. 
Geol.  Survey,  pt.  1,  Annual  publication. 


60S     . 


MINERAL  DEPOSITS 


deposits,  in  two  parallel  zones  extending  northeastward.  The 
bedded  deposits,  mainly  in  limestone,  have  been  mined  to  a  depth 
of  900  feet;  the  lode  deposits  continue  to  the  greatest  depths 
attained.  The  lode  deposits  intersect  the  sediments  and  the 
porphyry  as  well,  have  a  steep  dip,  ,and  often  lie  in  quartzite  or 
between  limestone  and  quartzite.  The  ores:  are  in  part  deposited 
by  filling  of  seams  in  shattered  ground,  in  part  by  replacement. 
The  stopes  are  as  much  as  30  feet  in  width. 


'  FIG.  211. — Vertical  section  of  rich  lead  ore  occurring  in  veins  and  in 
replacement  deposits,  Kearns-Keith  mine,  Park  City,  U-tah.  a,  Tunnel; 
b,  diorite  porphyry,  sheeted  and  pyritic ;  c,  hanging-wall  fissure  d,  lead  ore 
in  siliceous  gangue;  e,  breccia  zone  with  ore  fragments;/,  marmorized  lime- 
stone, Thaynes  formation;  g,  h,  banded  replacement  ore,  in  part  oxidized. 
After  J.  M.  Boutwett,  U.  S.  Geol.  Survey. 

•  The  bedded  deposits  are  massive  sulphides  replacing  limestone 
strata  in  two  of  the  calcareous  formations  and  are  from  a  few 
inches  to  10  feet  thick,  500  to  800  feet  in  the  direction  of  the  strike 
and  at  most  200  feet  along  the  dip.  The  relation  between  the 
two  types  is  shown  in  Fig.  211.  The  layers  of  the  bedded  ore 
are  made  up  of  ore  and  gangue  minerals  in  granular  texture 
exactly  like  that  of  the  original  limestone.  There  is  evidence  of 
two  epochs  of  deposition,,  for  some  of  the  bedded  ores  near  the 
porphyry  contacts  contain  garnet  with  calcite  as  gangue,  while 


DEPOSITS  FORMED' AT  INTERMEDIA  TE  DEPTHS      609 

the  lode  deposits  and  the  bedded  ores  associated  with  them  are 
free  from  garnet  and  were  formed  after  the  cooling  of  the  porphyry. 

The  ore  minerals  are  galena,  zinc  blende,  tetrahedrite,  and  a 
little  chalcopyrite.  Tetrahedrite  is  often  intergrown  with  coarse 
galena.  The  gangue  is  mainly  quartz  and  jasperoid;  fluorite, 
calcite,  and  rhodonite  occur  locally.  Sericitization  is  noted  where 
the  lodes  intersect  porphyry.  The  richest  ore  was  formed  in 
the  bedded  deposits;  the  ore  in  depth  is  of  leaner  grade,  but 
carries  more  copper  and  zinc. 

The  Park  City  mines  are  very  wet  and  the  water  level  is  high. 
In  view  of  this  it  is  remarkable  that  the  oxidation  is  deep  and 
partial  oxidation  has  been  noted  to  a  depth  of  1,200  feet.  The 
oxidized  zone  contained  apparently  but  little  native  silver  and 
cerargyrite. 

Tintic,  Utah.1 — The  replacement  deposits  qf  the  Tintic  dis- 
trict, situated  in  a  desert  range  70  miles  south  of  Salt  Lake  City, 
exemplify  another  type,  which  has  been  so  modified  by  oxida- 
tion that  the  original  character  of  the  ore  is  sometimes  difficult 
to  interpret.  Paleozoic  limestones  are  intruded  by  a  monzonite 
stock  that  formed  the  core  of  a  volcano  of  early  Tertiary  age, 
the  surface  flows  of  which  are  largely  eroded.  A  number  of  nar- 
row fissures  traverse  both  monzonite  and  limestone;  in  the 
former  the  deposits  are  pyritic  veins  with  sericitized  walls,  while 
in  the  limestone  the  inconspicuous  fractures  widen  locally  into 
large  or  small  ore  bodies  characterized  as  chambers,  chimneys, 
pipes,  pockets  or  pods,  which  in  large  part  replace  the  adjacent 
rock  or  follow,  for  a  distance,  stratification  planes,  fissures  or 
joints  (Fig.  212).  In  the  Iron  Blossom  mines  the  galena  ore 
forms  a  horizontal  pipe-like  ore  shoot  with  a  greatest  width  of 
150  feet.  This  has  been  mined  for  nearly  8,000  feet  and  follows 

1  G.  W.  Tower,  Jr.,  and  G,  O.  Smith,  Geology  and  mining  industry  of  the 
Tintic  district,  Utah,  Nineteenth  Ann.  Rept.  U.  S.  Geol.  Survey,  pt.  3,  1898, 
pp.  603-785. 

W.  P.  Jenney,  The  mineral  crest,  etc.,  Trans.  Am.  Inst.  Min.  Eng.,  vol. 
33,  1903,  pp.  46-50;  475-483. 

V.  C.  Heikes,  Op.  cit.' 

W.  Lindgren  and  G.  F.  Loughlin,  Prof.  Paper  107,  U.  S.  Geol.  Survey, 
1919. 

W.  Lindgren,  Processes  of  mineralization  and  enrichment  in  the  Tintic 
district,  Econ.  Geol,  vol.  10,  1915,  pp.  225-240. 

G.  W.  Crane,  Geology  of  the  ore  deposits  of  ;the  Tintic  district,  Trans., 
Am.  Inst.  Min.  Eng.,  vol.  54,  1917,  pp.  342-355. 


010 


MINERAL  DEPOSITS 


r\          ^    r\<-\   \V\ 

\VW\X 

•E  700'    ,  \     \  N    V      \ 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS   611 

the  intersection  of  an  obscure  fissure  with  a  certain  bed  of  pure 
limestone. 

The  primary  ore  minerals  are  galena  and  zinc  blende  with  very 
little  pyrite.  The  gangue  minerals  are  fine  grained  quartz  or 
jasperoid  and  barite.  The  galena  is  rich  in  silver  (20-50  ounces 
per  ton).  Other  large  replacement  bodies  consist  mainly  of 
quartz  and  jasperoid  and  carry  gold  and  silver  with  a  little  lead. 
Near  the  intrusive  mass  the  ore  contains  mainly  enargite 
(Cu3AsS4)  with  a  little  gold. 

The  jasperoids  along  the  deposits  result  from  the  replacement 
of  limestone  and  dolomite  by  colloidal  silica,  which  later  crystal- 
lized to  chalcedony  and  quartz.  The  crushing  of  this  silica 
resulted  in  brecciation  and  silica  of  a  second  phase  was  deposited 
in  open  spaces  as  colloid  material  which  later  crystallized  to 
quartz.'  The  development  of  crystalline  barite,  galena  and  other 
sulphides  accompanied  both  phases. 

Complete  or  partial  oxidation  continues  to  a  depth  of  about  2,000 
feet,  which  coincides  with  the  water  level.  In  the  mines  in 
monzonite  the  water  level  stands  much  higher. 

During  the  deep  oxidation  cerussite  and  anglesite  formed  in  the 
lead  deposits.  Secondary  zinc  minerals,  mainly  smithsonite, 
usually  develop  in  the  limestone  outside  of  the  primary  lead 
shoots.1  Complex  copper  and  copper-lime  arsenates  in  the 
copper  bearing  deposits  and  these  minerals  are  usually  accom- 
panied by  more  or  less  chalcocite  and  covellite. 

The  mines  of  Tintic  yield  annualy  $6,000,000  to  $9,000,000, 
their  complex  smelting  ores  containing  gold,  silver,  copper, 
lead,  and  zinc.  The  annual  ore  production  is  about  300,-> 
000  tons.  The  total  value  of  the  silver,  lead,  gold  and  cop- 
per produced  from  1869  to  1916,  inclusive,  is  approximately 
$170,000,000. 

Aspen,  Colorado.2 — The  ore  deposits  at  Aspen,  in  the  central 
part  of  Colorado  (Fig.  173),  for  many  years  yielded  a  large 
amount  of  lead  and  silver,  and  the  annual  output  is  still  of  con- 
siderable value.  During  recent  years  zinc  blende  has  been  added 
to  the  products  of  this  district.  The  ores  average  5  ounces  of 

1  G.  F.  Loughlin,  The  oxidized  zinc  ores  of  the  Tintic  district,  Econ.  Geol, 
vol.  9,  1914,  pp.  1-19. 

1  J.  E.  Spurr,  Mon.  31,  U.  S.  Geol.  Survey,  1898. 

J.  E.  Spurr,  Ore  deposition  at  Aspen,  Colo.,  Econ.  Geol,  vol.  4,  1909, 
pp.  301-320. 


612  MINERAL  DEPOSITS 

silver  per  tqii  and  6^  per  cent.  lead.  The  geological  column  at 
Aspen  includes  200  to  400  feet  of  Cambrian  quartzite;  250 
to  400  feet  of  Silurian  dolomite;  60  feet  of  Devonian  quartzite 
and  shale  (" Parting  quartzite") ;  250  feet  of  lower  Carboniferous 
dolomite  and  150  feet  of  limestone  of  the  same  age;  1,000  feet  or 
more  of  thin-bedded  Carboniferous  limestones  and  shales  called 
the  Weber  formation;  and  a  great  thickness  of  Carboniferous, 
Triassic,  and  Cretaceous  sandy  and  calcareous  sediments.  The 
entire  series  is  sharply  upturned.  A  sheet  of  diorite  porphyry 
intrudes  the  lower  Paleozoic-  formations  and  a  sheet  of  rhyolite 
porphyry  lies  at  the  base  of  the  Weber  formation.  Both  intru- 
sives  are  of  late  Cretaceous  or,  early  Tertiary  age.  Complicated 
faulting  and  local  doming  accompanied  the  intrusion.  -During 
the  short  epoch  of  ore  deposition  sulphides  were  deposited  along 
the  faults  and  fractures,  the  most  important  horizon  of  miner- 
alization being  at  the  base  of  the  Weber  shales,  where  the  deposit- 
ing waters  were  dammed  by  the  relatively  impervious  shale  and 
it  may  also  have  acted  as  a  precipitant.  In  his  later  paper 
Spurr  differentiates  the  complicated  deposits  as  (1)  barite 
veins;  (2)  silver  sulphides,  sulphantimonides,  and  sulphar- 
senides;  (3)  galena  and  zinc  blende  veins.  This  series  is  be- 
lieved to  have  been  deposited  under  conditions  of  gradually 
rising  temperature.  Faulting  continued  after  the  short  epoch 
of  ore  deposition. 

The  barite  veins  are  generally  barren.  After  their  develop- 
ment rich  sulphides  such  as  argentite,  polybasite,  and  tetrahedrite 
were  deposited.  A  remarkable  shoot  of  polybasite  ore  yielding 
many  million  dollars  was  mined  in  the  Molly  Gibson  mine,  at  a 
depth  of  a  few  hundred  feet  below  the  surface.  This  occurrence 
of  rich  silver  sulphides  certainly  suggests  enrichment  by  descend- 
ing surface  waters,  but  Spurr  insists  on  its  primary  origin.  The 
last  phase  of  mineralization  consisted  in  the  deposition  along 
zones  of  fracture  and  brecciation  of  lead  and  zinc  ores  of  milling 
grade  and  poor  in  silver.  The  ore  minerals  occur  in  disseminated 
form,  replacing  limestone,  either  without  gangue  or  with  a  little 
jasperoid  and  dolomite. 

The  mine  water  is  abundant  and  the  water  level  stood  originally 
about  300  feet  below  the  surface.  Down  to  1,000  feet  (the  great- 
est depth  thus  far  reached)  native  silver  with  some  barite  has 
been  deposited  by  descending  solutions,  probably  by  the  reducing 
influence  of  the  carbonaceous  Weber  shales. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  613 

Leadville,  Colorado.1 — Since  the  discovery  of  the  ore  deposits 
of  Leadville,  Colorado,  in  1875  this  district  has  yielded  an  enor- 
mous production  of  lead  and  silver,  also  much  gold,  copper,  and 
zinc.  Previous  to  that  date  placers  were  worked  in  the  district 
and  gold  to  the  value  of  several  million  dollars  was  washed  from 
the  gravel  of  the  gulches.  For  a  long  time  after  1875  the  oxidized 
lead  ores,  containing  much  iron  and  manganese,  were  worked. 


Vertical  and  Horizontal  Scale 

400  800  1200  Feet 

FIG.  213. — Vertical  section  showing  geological  structure  and  occurrence 
of  ore-bodies  at  Leadville,  Colorado.  1,  Wash;  2,  lake  beds;  3,  Leadville 
blue  limestone  (Carboniferous);  4,  parting  quartzite  (Devonian);  5,  white 
limestone  (Silurian) ;  6,  lower  quartzite  (Cambrian) ;  7,  gray  porphyry  (early 
Tertiary);  8,  white  porphyry  (early  Tertiary);  9,  granite  (pre-Cambrian); 
ore-bodies  in  black.  After  S.  F.  Emmons  and  J.  D.  Irving,  U.  S.  Geol. 
Survey. 

At  the  present  time  the  main  product  is  heavy  sulphide  ore 
containing  pyrite,  zinc  blende,  galena,  and  chalcopyrite.     Bodies 

1  S.  F.  Emmons,  Geology  and  mining  industry  of  Leadville,  Colorado, 
Mon.  12,  U.  S.  Geol.  Survey,  1886. 

S.  F.  Emmoos  and  J.  D.  Irving,  The  Downtown  district,  Bull.  320, 
U.  S.  Geol.  Survey,  1907. 

A.  A.  Blow,  Trans.  Am.  Inst.  Min.  Eng.,  vol.  18,  1890,  pp.  145-181. 

Philip  Argall,  Eng.  and  Min.  Jour.,  vol.  89,  1910,  p.  261. 

Philip  Argall,  Min.  and  Sci.  Press,  July  11  and  15,  1914. 

Max  Boehmer,  Trans.  Am.  Inst.  Min.  Eng.,  vol.  41,  1911,  pp.  162-165. 

C.  J.  Moore,  Econ.  Geol.,  vol.  7,  1912,  pp.  590-592. 

J.  D.  Irving,  Prof.  Paper,  in  course  of  publication,  U.  S.  Geol.  Survey. 


614  MINERAL  DEPOSITS 

of  oxidized  zinc  minerals  such  as  calamine  and  smithsonite  have 
recently  been  discovered.1 

Out  of  a  total  of  477,000  tons  mined  in  1916,  310,000  tons  were 
sulphide  ores,  and  18,000  tons  iron-manganese  ores  of  the  oxi- 
dized type  containing  a  little  silver  and  used  for  flux.  The  heavy 
sulphide  ores  are  in  part  smelted  directly  and  in  part  concentrated ; 
they  vary  considerably  but  consist  mainly  of  pyrite  and  zinc 
blende,  with  less  than  1  per  cent,  of  copper,  1  to  4  per  cent,  of 
lead,  and  2  to  9  ounces  of  silver  per  ton.  The  silver  is  mostly 
in  the  galena  and  zinc  blende.  The  crude  zinc  ore  shipped  aver- 
aged 30  per  cent,  of  that  metal.  In  1916  the  district  yielded 
$1,470,000  in  gold,  2,784,000  ounces  of  silver,  1,300  tons  of  cop- 
per, 11,000  tons  of  lead,  and  38,000  tons  of  zinc — a  total  value 
of  $15,600,000.2 

The  geological  section  consists,  according  to  S.  F.  Emmons,  of 
Paleozoic  rocks  resting  on  granite  and  gneiss.  The  following 
formations  are  important  in  the  study  of  the  ore  deposits. 

Weber  shales  and  grits,  lower  Carboniferous 2,500  feet. 

Blue  limestone,  lower  Carboniferous 200  feet. 

Parting  quartzite,  Devonian. . . . .' 40  feet. 

White  limestone,  Silurian 160  feet. 

Lower  quartzite,  Cambrian 150  to  -200  feet. 

These  formations  are  intruded  by  numerous  sheets  of  porphyry, 
which  in  the  main  lie  parallel  to  the  bedding,  but  in  places  cut 
diagonally  across  it.  Some  sheets  are  thin,  and  others  are  neady 
1,000  feet  in  thickness.  The  "white  porphyry"  is  a  siliceous 
granite  porphyry,  which  Spurr3  calls  alaskite  porphyry.  The 
"gray  porphyry"  is  similar  but  contains  remains  of  resorbed 
ferromagnesium  minerals  and  is  a  little  lower  in  silica.  The  white 
porphyry  is  normally  intruded  in  the  blue  or  Leadville  limestone; 
the  gray  porphyry  forms  thinner  sheets  at  various  horizons. 

The  intrusions  and  the  ore  deposition  were  followed  by  a 
marked  doming  of  the  strata  and  faulting  of  great  complexity,  so 
that  the  district  now  consists  of  numerous  blocks  successively 
dropping  off  toward  the  Arkansas  Valley  (Fig.  213).  The 

1  G.  M.  Butler,  Some  recent  developments  at  Leadville,  Earn.  Geol,  vol. 
7,  1912,  pp.  315-323;  vol.  8,  1913,  pp.  1-18. 

2  C.    W.   Henderson,   Mines   report   of   Colorado  in  Mineral  Resources, 
U.  S.  Geol.  Survey,  pt.  1,  Annual  publication. 

3  Prof.  Paper  63,  U.  S.  Geol.  Survey,  1908,  p.  70. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS   615 


ore  deposits  are  found 
the  contact  with  the 
overlying  white  por- 
phyry (Fig.  214). 
The  upper  surface  of 
the  ore  is  often  re- 
markably regular  and 
sharp,  being  formed  by 
the  porphyry  contact, 
while  the  lower  sur- 
face is  irregular.  Al- 
though this  is  the 
normal  development, 
replacement  ores  are 
also  found  in  other 
positions,  along  fis- 
sures in  limestone  (Fig. 
215),  along  fault  planes 
or  below  the  gray  por- 
phyry or  in  fissure 
veins  extending  below 
the  sedimentary  beds. 
The  fissure  veins  are 
confined  to  the  part  of 
the  district  near  the 
Ibex  mine;  many  of 
them  are  rich  in  na- 
tive gold. 

Some  of  the  ore- 
bodies  are  of  large 
size,  especially  in  a 
horizontal  direction. 
Owing  to  their  mode 
of  occurrence  and  to 
the  great  quantity  of 
mine  water  a  depth  of 
only  1 ,500  feet  has  been 
attained;  it  was  con- 
sidered useless  to  go  be- 
low the  basement  of  the 
Paleozoic  formations. 


mainly  in  the  blue  limestone  at  or 
o 


near 


616 


MINERAL  DEPOSITS 


Blow  has  shown  that  the  ore-shoots  on  Iron  Hill  follow  north- 
eastward-trending zones  parallel  with  cross  cutting  sheets  of 
gray  porphyry. 

The  usual  ore  is  a  massive  granular  mixture  of  sulphides,  among 
which  pyrite  and  zinc  blende  prevail.  There  is  a  scant  gangue 
of  quartz,  jasperoid,  and  little  barite.  The  limestone  near  the 
deposits  often  contain  much  manganosiderite  spreading  from  the 
ore-bodies.  The  contact  between  ore  and  limestone  is  usually 
surprisingly  sharp,  though  irregular.  Among  the  rarer  minerals 
is  native  gold,  molybdenite,  wolframite  and  scheelite;  the  ores 
contain  a  little  antimony,  arsenic,  and  bismuth,  and  the  presence 
of  traces  of  tellurium  in  the  pyrite  has  been  shown  by  Pearce. 


Monzonite 
+  Porphyry 


Blue  Limestone 


FIG.  215. — Cross-section  of  shoot  at  Oro  La  Plata  mine,  Leadville,  Colorado, 
showing  irregular  ore-bodies  along  fractures.     After  J.  D.  Irving. 

The  oxidized  ores  of  the  upper  levels  which  are  still  mined  to 
some  extent  contain  limonite,  manganese  oxide,  cerussite,  and 
zinc  carbonates.  They  were  frequently  rich  in  silver  chloride. 
The  genesis  of  the  Leadville  ores  has  been  discussed  extensively. 
Emmons  held  that  they  were  formed  by  aqueous  solutions  coming 
from  above  and  that  they  derived  their  mineral  content  mainly 
from  the  igneous  rocks,  but  he  did  not  deny  the  possibility  that 
the  solutions  may  originally  have  come  from  great  depths,  nor 
did  he  assert  that  they  were  necessarily  derived  from  the  erup- 
tive rocks  in  immediate  contact  with  the  deposits.  He  also 
fully  recognized  that  at  the  time  of  ore  formation  the  present 
deposits  were  covered  by  about  10,000  feet  of  overlying  rocks. 
Other  writers,  among  them  A.  A.  Blow,  have  sought  to  prove 
that  the  deposits  were  formed  by  ascending  solutions. 


DEPOSITS  FORMED  A  T  INTERMEDIA  TE  DEPTHS      617 

The  deposits  of  Leadville  are  unusual  in  that  the  sulphide 
replacement  is  so  complete  and  that  the  contacts  with  the  lime- 
stone and  porphyry  are  so  sharp.  They  strongly  resemble  the 
contact-metamorphic  deposits  except  in  the  association  of 
gangue  minerals,  which  points  clearly  to  moderate  temperatures 
at  which  calcium  silicates  could  not  form,  and  this  seems  to  prove 
that  the  ore  deposition  did  not  take  place  immediately  after  the 
intrusion.  The  recent  discoveries  of  vein  deposits  near  the  Ibex 
Mine  and  ores  containing  magnetite  and  epidote  also  point  to 
deep-seated  sources  for  the  metals  far  below  the  present  ore 
horizon ;  but  how  the  solution  could  penetrate  along  the  contacts  for 
so  long  a  distance  without  visible  passageways  is  as  yet  a  mystery. 

The  Leadville-Boulder  County  Belt.— The  Leadville  deposits 
form  only  a  single  unit  in  a  belt  of  deposits  which  extends  for 
80  miles  in  a  northeasterly  direction  and  comprises  a  great  many 
districts,  including  the  Kokomo,  Alma,  Fairplay,  Breckenridge, 
Montezuma,  and  Argentine  and  continuing  through  Clear  Creek, 
Gilpin,  and  Boulder  counties  (Fig.  173).  The  deposits  include 
replacement  bodies  and  veins  and  are  found  in  rocks  of  the  most 
diverse  kinds.  A  common  feature  of  the  whole  belt  is  a  series 
of  intrusives,  appearing  as  sheets  in  the  sedimentary  formations 
and  dikes  or  smaller  stocks  in  the  pre-Cambrian  granite  and 
schists.  The  inference  that  these  intrusives  are  genetically 
connected  with  the  deposits  seems  well  founded.  S.  F.  Emmons 
and  Whitman  Cross  first  called  attention  to  this  belt  of  intrusives ; 
S.  H.  Ball1  and  F.  L.  Ransome2  have  discussed  the  petrography 
of  the  porphyries  and  Ransome  has  presented  a  diagram  showing 
the  composition  of  all  analyzed  varieties.  The  most  abundant 
intrusives  are  alaskite  porphyry,  granite  porphyry,  bostonite 
porphyry,  monzonite  porphyry,  and  quartz  monzonite  porphyry. 
The  dikes  commonly  extend  in  a  northwesterly  direction,  but 
show  no  great  individual  continuity.  Ball  has  indicated  on  a 
map3  all  the  occurrences  of  porphyry  within  this  belt.  Emmons 
held  that  the  intrusions  were  probably  of  Jurassic  age,  but  later 
evidence  discovered  by  Cross  and  others  has  shown  that  they 
must  be  later,  falling  either  in  the  very  latest  Cretaceous  or  the 
earliest  Tertiary.  The  ore  deposits  are  later  than  the  porphyries 
but  were  probably  formed  shortly  after  their  consolidation. 

1  Prof.  Paper  63,  U.  S.  Geol.  Survey,  1908,  pp.  67-70. 

2  Prof.  Paper  75,  U.  S.  Geol.  Survey,  1911,  pp.  60-62. 

3  Op.  tit.,  PI.  XI. 


618      ':;'.''  MINERAL  DEPOSITS 

The  prevailing  type  is  a  sulphide  ore  with  abundant  pyrite  and 
zinc  blende  and  lesser  amounts  of  galena  and  tetrahedrite. 
Chalcopyrite  is  subordinate,  arsenopyrite  rare.  Telluride  ores 
occur  occasionally  in  the  eastern  end  of  the  belt.  Silver  prevails 
in  the  southeastern  part  and  gold  is  the  important  metal  in  Gilpin 
and  Boulder  counties.  The  gangue  is  made  up  of  quartz,  sider- 
ite,  manganosiderite  and  other  carbonates,  but  not  much  rhodo- 
chrosite  is  present.  In  places  there  is  considerable  barite. 

The  replacement  deposits  of  Leadville,  in  Carboniferous  lime- 
stone below  porphyry  sheets,  have  already  been  mentioned.  In 
the  Tenmile  district1  replacement  deposits  and  fissures  veins 
appear  in  the  upper  Carboniferous  formations,  with  pyrite,  zinc 
blende,  and  galena  in  a  gangue  of  quartz,  calcite,  rhodochrosite, 
and  barite.  The  Red  Cliff  district2  has  ores  similar  to  those  of 
Leadville,  also  replacement  deposits  carrying  gold  in  Cambrian 
quartzite.  The  Breckenridge  district3  contains  fissure  veins 
intersecting  Cretaceous  shale  and  monzonite  sheets,  with  pyrite, 
zinc  blende,  and  galena  as  the  principal  ore  minerals.  Remark- 
able pockets  of  crystallized  gold  are  thought  to  be  deposited  by 
descending  waters.  Northeast  of  Breckenridge  is  the  Monte- 
zuma  district,4  in  pre-Cambrian  rocks  with  northeasterly  trending 
veins  carrying  pyrite,  chalcopyrite,  galena,  and  zinc  blende,  in 
places  with  ruby  silver  or  similar  rich  silver  minerals,  which  are 
apparently  of  later  origin  than  the  rest  of  the  ore.  The  gangue 
consists  of  quartz,  siderite,  and  barite.  Farther  up,  at  the  Con- 
tinental Divide,  is  the  Argentine  district,  with  veins  in  gneiss, 
which  carry  similar  ore  minerals  in  a  gangue  of  quartz,  calcite, 
and  fluorite.  These  veins  contain  both  silver  and  gold. 

The  Clear  Creek  district8  lies  also  in  the  pre-Cambrian  area. 
The  rocks  comprise  an  older  division  of  sedimentary  origin,  the 
Idaho  Springs  formation,  intruded  by  granite,  diorite,  and 
pegmatite.  A  complex  system  of  dikes  of  the  kinds  mentioned 
above  is  followed  by  the  veins,  which  are  principally  silver 
deposits  containing  galena,  zinc  blende,  pyrite,  tetrahedrite, 
and  chalcopyrite,  in  a  more  or  less  scant  gangue  of  earlier 

1  S.  F.  Emmons,  Folio  48,  U.  S.  Geologic  Atlas,  1898. 

2  A.  H.  Means,  Earn.  Geol,  vol.  10,  1915,  pp.  1-27. 

3  F.  L.  Ransome,  Prof.  Paper  75,  U.  S.  Geol.  Survey,  1911. 

4  H.  B.  Patton,  First  Rept.,  Colo.  Geol.  Survey,  1909,  pp.  112-144. 

6  J.  E.  Spurr,  G.  H.  Carrey,  and  S.  H.  Ball,  Economic  geology  of  the 
Georgetown  quadrangle,  Prof.  Paper  63;  U.  S.  Geol.  Survey,  1908. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS    619 


FIG.  216. — Section  of  Pelican  vein,  Georgetown,  Colorado,  a,  Main  vein; 
b,  ore,  mainly  zinc  blende;  c,  ore  zone  of  brecciated  and  silicified  alaskite 
porphyry;  d,  fractured  and  silicified  porphyry;  e,  sheeted  porphyry;  /, 
altered  gneiss.  After  J.  E.  Spurr,  U.  S.  Geol.  Survey. 


S-jf'Ty^,- 

i~^/c^'-; 

*£%%#< 

x  -  r-  / 


z  feet 


Granite  Galena  ore    Brown  sphalerite  ore      Quartz  Clay  selvage 

oarrying  a  little  galena 

FIG.  217.-^-Cross-section  of  Frostburg  vein,  Georgetown,  Colorado,  show- 
ing deposition  of  galena  and  zinc  blende.  After  J.  E.  Spurr,  U.  S.  Geol. 
Survey. 


620  MINERAL  DEPOSITS 

quartz  and  later  siderite,  ankerite,  and  calcite  (Figs.  216  and  217). 
Silver,  gold,  copper,  lead,  and  zinc  are  produced. 

In  Gilpin  County1  gold-bearing  veins  prevail;  they  carry  abun- 
dant pyrite,  with  some  chalcopyrite,  tennantite  and  enargite, 
rarely  pitchblende,  in  a  scant  quartz  and  siderite  gangue.  The 
ores  average  about  $8  in  gold  to  the  ton  and  contain  a  small 
amount  of  silver.  A  later  vein  formation  carries  galena  and  zinc 
blende,  with  silver,  in  a  quartz,  siderite  and  calcite  gangue. 
Quartz-telluride  ores  are  also  known  (Fig.  41). 

In  all  the  districts  the  alteration  of  the  feldspathic  country 
rock  adjacent  to  the  vein  is  of  the  sericitic  type.  The  lowest 
workings  in  Clear  Creek  and  Gilpin  counties  are  2,000  feet 
below  the  outcrops. 

In  Boulder  County,  the  present  production  of  which  is  small, 
there  are  some  gold-bearing  veins  similar  to  those  of  Gilpin 
County,  and  also  some  veins  which  have  produced  rich  silver 
ores,  but  the  most  interesting  types  are  the  telluride  veins,  which 
are  rare  in  the  other  districts  mentioned,  and  the  tungsten  veins, 
which  are  absent  elsewhere. 

The  Enterprise  vein,2  which  is  typical  of  the  telluride  deposits, 
consists  of  several  narrow  seams,  forming  a  sheeted  zone  along 
which  filling  and  replacement  have  occurred.  The  width  of 
this  zone  is  from  1  to  3  feet  and  the  filling  is  often  beautifully 
banded  with  abundant  druses.  The  country  rock  is  pre-Cam- 
brian  granite.  The  minerals  are  crystallized  quartz  and  dense 
jasperoids  with  barite,  adularia,  and  roscoelite  (vanadium  mica) ; 
there  is  a  little  pyrite  and  much  molybdenite;  the  most  valuable 
minerals  are  the  gold  and  silver  tellurides. 

The  Tungsten  Deposits  of  Boulder  County. — Some  wolframite 
occurs  in  the  gold-bearing  veins  of  Boulder  County,  but  there  is  a 
fairly  well-defined  area  near  Nederland3  which  is  characterized 
by  tungsten  ores.  The  tungsten  mining  in  Boulder  County  is  of 
recent  origin  but  has  developed  rapidly,  and  has  especially  been 

1  E.  S.  Bastin  and  J.  M.  Hill,  Economic  geology  of  Gilpin  County,  etc. 
Prof.  Paper  94,  U.  S.  Geol.  Survey,  1917. 

2  W.  Lindgren,  Econ.  Geol,  vol.  2,  1907,  pp.  453-463. 

T.  A.  Rickard,  Trans.  Am.  Inst.  Min.  Eng.,  vol.  33,  1903,  pp.  567-577. 

3  W.  Lindgren,  Op.  tit. 

R.  D.  George,  The  main  tungsten  area  of  Boulder  County,  First  Ann. 
Kept.,  Colorado  Geol.  Survey,  1908,  pp.  9-103. 

F.  L.  Hess,  chapter  on  production  of  tungsten,  Mineral  Resources,  U.  S. 
Geol.  Survey,  Annual  publication. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  621 

stimulated  by  war  conditions.  In  1918,  5,020  tons  of  tungsten 
concentrate  were  produced  in  the  United  States  of  which  the 
larger  part  came  from  the  deposits  of  Boulder  County.  The  re- 
mainder of  the  domestic  production  comes  largely  from  Atolia, 
Kern  County,  California,  where  scheelite  (CaWO4)  occurs  in 
veins  associated  with  gold-bearing  quartz  veins  of  the  middle 
depths.  Queensland,  Argentina,  Portugal  and  Burma  furnish 
the  bulk  of  the  foreign  production.  The  ore  from  these  four 
countries  comes  from  high-temperature  veins  related  to  the  cassit- 
erite  veins.  For  concentrates  with  60  per  cent.  WO3  the  price 
in  1910  was  from  $400  to  $500  per  ton;  in  1918  about  $1,200 
per  ton.  Tungsten  is  used  mainly  for  high-speed  tool  steel,  the 
alloy  ferro-tungsten  or  metallic  tungsten  being  first  produced. 
It  is  also  used  in  incandescent  lamps,  etc.  (p.  673) . 

The  veins  of  Boulder  County  are  narrow,  often  brecciated 
fissures  in  granite,  some  of  them  following  porphyry  dikes.  The 
gangue  is  made  up  of  quartz  and  fine-grained  silica.  Kaolin  is 
abundant  as  a  secondary  mineral.  There  is  a  little  pyrite,  but 
the  principal  mineral  is  the  iron  tungstate,  ferberite,  which  occurs 
in  a  fine-grained  mixture  with  quartz  or  as  beautiful  crystals 
coating  druses  and  also  associated  with  quartz  crystals. 

Summary. — The  mineral  belt  described  in  the  preceding 
paragraphs  illustrates  well  the  intimate  connection  between  veins 
and  replacement  deposits.  With  all  their  differences  the  de- 
posits are  evidently  of  approximately  contemporaneous  origin. 
The  southwest  end  at  Leadville  represents  the  deposits  formed 
at  considerable  depth,  probably  not  less  than  10,000  feet  as  es- 
timated by  Emmons.  Toward  the  northeast  end  the  sedimen- 
tary series  is  absent  and  probably  never  covered  the  Front  Range 
of  Colorado.  There  is  evidence  in  this  range  of  a  tertiary  surface 
higher  than  the  present  but  of  less  relief;  and  Spurr  believes  that 
effusive  rocks  once  rested  on  this  surface.  S.  H.  Ball1  estimates 
that  in  Clear  Creek  County  the  total  erosion  since  the  intrusion 
of  the  porphyries  has  been  about  5,000  feet,  and  it  is  probable 
that  it  was  less  in  Boulder  County.  Corresponding  to  this 
difference  in  geologic  conditions  is  a  difference  in  structure  and 
composition  of  the  ore  deposits.  At  the  southwestern  end  of 
the  belt  heavy  sulphide  ores  prevail,  mostly  as  replacements.  In 
Clear  Creek  and  Gilpin  counties  the  banded  and  drusy  structure 
of  the  veins  begins  to  be  apparent,  and  in  Boulder  County  we  find 

1  Pro/.  Paper  63,  U.  S.  Geol.  Survey,  1908,  p.  145. 


622  MINERAL  DEPOSITS 

veins  like  the  telluride  deposits  and  the  tungsten  veins,  which  dis- 
tinctly resemble  the  deposits  formed  at  slight  depth  below  the 
surface,  having  drusy,  banded  structure  and  containing  fine- 
grained quartz  and  tellurides.  These  relations  are,  to  say  the 
least,  very  suggestive  of  progressive  change  in  original  depth  of 
deposition  from  the  western  to  the  eastern  end  of  the  belt. 

The  rich  silver  minerals,  which  are  found  in  some  of  the  dis- 
tricts, are  regarded  by  Spurr  and  also  by  E.  S.  Bastin1  as  products 
of  deposition  by  descending  surface  waters.  Spurr  believes  that 
the  metals  in  the  Clear  Creek  deposits  were  derived  from  deep- 
seated  magmatic  sources  and,  dissolved  in  magmatic  waters, 
ascended  the  fissures  after  each  intrusion  of  porphyry.  He  also 
believes  that  most  of  the  gangue  minerals — quartz,  carbonates, 
and  barite — were  formed  through  the  leaching  of  the  country 
rock  by  the  ascending  waters. 

Ransome  is  less  positive  regarding  the  genesis  of  the  deposit  at 
Breckenridge  and  points  out  that  many  of  the  fissures  are  short 
along  the  strike  and  die  out  before  reaching  great  depth,  so  that 
instead  of  directly  ascending  solutions  there  was  probably  con- 
siderable lateral  movement  during  which  the  solutions  had 
opportunity  to  search  the  surrounding  rocks  more  or  less  thor- 
oughly. The  gangue  minerals  are  also  regarded  by  him  as  de- 
rived from  the  country  rock,  while  he  considers  a  magmatic  origin 
possible  for  the  metals  arid  elements  like  sulphur  and  fluorine. 
In  view  of  the  faint  contact-metamorphic  effects  shown  at  the 
western  end  of  the  belt  in  the  limestones,  it  is  probable  that  little 
if  any  of  the  mineral-bearing  solution  was  derived  from  the  small 
.bodies  of  intruding  porphyries  and  that  the  metals  were  rather 
derived  from  deep-seated  magma  basins. 

DEPOSITS  WITH  NATIVE  SILVER 

Native  silver  is  not,  as  a  rule,  a  primary  mineral  in  the  deposits 
which  contain  it,  nor  is  it  restricted  to  any  particular  class  of 
deposits.  As  a  secondary  product  due  to  reactions  within  the 
oxidized  zone  it  is  common  in  many  kinds  of  deposits — for 
instance,  in  argentiferous  galena  ores,  in  tetrahedrite  ores,  and  in 
the  argentite  veins  in  the  Tertiary  lavas.  It  is  ordinarily  found 
some  distance  below  the  surface;  cerargyrite  (AgCl)  is  more 
abundant  in  the  outcrops.  The  native  silver  often  occurs  at 

1  Econ.  Geol,  vol.  8,  1913,  p.  51. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  623 

depths  far  below  the  zone  of  oxidation,  properly  speaking.  At 
Aspen,  Colorado,  it  is  abundant  in  fissures  and  vugs  of  lime- 
stone and  shale  900  feet  below  the  surface  and  is  distinctly  later 
than  the  primary  lean  galena-zinc  blende  ores;  along  the  delicate 
threads  of  the  metal  small  barite  crystals  are  often  suspended. 
In  certain  deposits  the  native  silver  is  the  predominating  ore 
mineral  down  to  considerable  depths.  Some  of  these  occurrences 
are  described  below  and  may  be  divided  into  two  groups: 

1.  The  silver-bearing  deposits  with  zeolitic  enrichment. 

2.  The  silver-bearing  cobalt-nickel  deposits. 

The  Zeolitic  Enrichments. — Zeolites  are  ordinarily  foreign  to 
ore  deposits  connected  with  igneous  rocks.  Their  occurrence  in 
some  contact-metamorphic  deposits  and  in  fissure  veins  is 
mentioned  on  page  427.  Zeolites  are  found  in  the  deposits  of 
Kongsberg,  Norway;  Andreasberg,  Germany;  the  Arqueros  and 
other  mines  in  Chile;  and  Guanajuato,  Mexico,  all  of  which  are 
worked  for  silver.  They  are  rarely  noted  in  the  western  part  of 
the  United  States.  Much  remains  to  be  learned  about  their 
relation  to  the  metallization.  In  general,  it  seems  certain  that 
the  zeolites  were  deposited  later  than  the  other  minerals,  prob- 
ably not,  however,  by  descending  waters,  but  rather  by  remaining 
stagnant  parts  of  the  original  vein-forming  solutions.  A  con- 
centration of  silver  ores  often  accompanied  their  development. 
Calcite,  quartz,  barite,  and  fluorite  are  the  principal  gangue 
minerals  in  the  typical  localities;  the  presence  of  antimony  and 
quicksilver  is  often  mentioned. 

The  renowned  silver  mines  of  Kongsberg,  in  southern  Norway, 
which  have  been  worked  for  several  hundred  years  and  still 
remain  productive,  have  been  described  by  Vogt.1  The  deposits 
are  narrow  veins  in  gneiss  and  mica  schist,  often  breaking  up 
when  entering  amphibolite.  Along  certain  lines  following  the 
schistosity  the  rocks  contain  disseminated  sulphides,  mainly 
pyrite  and  pyrrhotite,  and  the  veins  become  enriched  where 
crossing  these  "fahlbands, •'  probably  on  account  of  their  pre- 
cipitating influence.  The  mines  have  been  worked  to  a  depth 
of  3,000  feet.  Quartz,  chlorite,  and  axinite  crystallize  next  to 
the  walls,  but  the  prevailing  gangue  is  calcite  with  some  barite 
and  fluorite,  rarely  adularia  and  albite.  Zeolites  also  occur  and 
are  among  the  latest  gangue  minerals,  prehnite,  stilbite,  harmo- 
tome,  and  laumontite  being  among  those  identified.  The  prin- 
1  J.  H.  L.  Vogt,  Z&itschr.  prakt.  Geol.,  1899,  pp.  113-123;  177-181, 


624  MINERAL  DEPOSITS 

cipal  ore  mineral  is  native  silver,  mostly  in  wire  form;  this  is 
believed  to  be  derived  from  the  more  scarce  argentite  by  a  process 
of  enrichment.  Less  prominent  are  ruby  silver,  stephanite, 
pyrite,  pyrrhotite,  arsenopyrite,  chalcopyrite,  zinc  blende,  and 
galena,  the  latter  poor  in  silver.  A  certain  part  of  the  silver  is 
believed  by  Vogt  to  result  from  primary  deposition.  The  native 
silver  contains  quicksilver.  Anthracite  is  also  one  of  the  gangue 
minerals  deposited  during  the  early  stages. 

Vogt  supposes  the  native  silver  to  be  derived  from  argentite 
and  proustite  as  follows: 

Ag2S+02  =  2Ag+S02. 
Ag2S+H20  =  2Ag+H2S+0. 

=  3Ag+As+3H2S+30. 


The  list  of  minerals  given  shows  clearly  that  the  veins  have  had 
a  complicated  history,  beginning  with  the  deposition  of  high-tem- 
perature minerals  like  axinite  and  ending  with  that  of  min- 
erals like  zeolites,  probably  formed  at  about  100°  C.  This 
history  has  evidently  not  yet  been  followed  in  detail;  it  is  stated 
by  Vogt  that  axinite  crystallized  together  with  the  zeolites,  but 
this  seems  a  curious  association.  The  presence  of  free  oxygen  at 
great  depths  might  also  well  be  questioned. 

The  other  notable  occurrence  is  at  Andreasberg,  in  the  Harz 
Mountains,  best  described  by  A.  Bergeat.  l  The  veins  at  Andreas- 
berg  are  simple  filled  fissures,  at  most  0.5  meter  thick,  chiefly 
in  Silurian  clay  slates  and  quartzites.  They  appear  not  far 
from  the  intrusive  mass  of  the  "Brocken"  granite.  The  veins 
are  included  between  two  great  divergent  dislocations,  forming 
impermeable  walls,  against  which  the  silver  veins  split  and  cease. 
The  mines,  which  attained  a  depth  of  2,700  feet,  are  now  closed. 

In  general  the  veins  carry  argentiferous  galena  and  tetra- 
hedrite;  sometimes  they  yield  large  druses  full  of  rich  silver  ores, 
calcite,  and  zeolites.  Bergeat  distinguishes  five  phases: 

1.  In  crevices  near  the  veins,  in  part  also  in  the  fissures  them- 
selves, are  garnet,  epidote,  axinite,  and  albite. 

2.  Earliest  bituminous  calcite  with  simple  antimonides,  arsen- 
ides, and  sulphides:  Niccolite,  smaltite,  lollingite,  breithauptite 
(NiSb),   zinc  blende,   galena,   pyrite,   pyrrhotite,   chalcopyrite. 

3.  Tetrahedrite  with  quartz  (replacing  calcite)  and  fluorite, 

1  Stelzner  and  Bergeat,  Die  Erzlagerstatten,  vol.  2,  1906,  pp.  718-720 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  625 

chalcopyrite,  galena,  zinc  blende;  native  silver  and  millerite, 
encrusting  tetrahedrite. 

4.  Sulphantimonides  and  sulpharsenides:  Pyrargyrite,  prous- 
tite,   miargyrite,   polybasite,  stibnite,   argentite   (in  part  from 
pyrargyrite),  fluorite. 

5.  Native  silver,  realgar,  calcite,  apophyllite,  analcite,  chaba- 
zite,  heulandite,  brewsterite,  harmotome,  stilbite,  natrolite;  also 
fluorite  and  chalcopyrite. 

This  clear  exposition  of  paragenesis  indicates  a  long  epoch  of 
deposition  with  gradually  diminishing  temperature  and  em- 
phasizes the  connection  of  the  native  silver  with  zeolitization. 

Zeolites  occur  in  a  number  of  Chilean  silver  veins,  particularly 
at  Arqueros  and  Rodaito,  in  association  with  native  silver,  cal- 
cite, barite,  and  silver  amalgam.  The  only  occurrence  known 
in  the  United  States  is  at  the  South  Republic  mine,  Republic, 
Washington,  where  the  gold  selenide  veins  hi  andesite  are  filled 
by  closely  banded,  extremely  fine  grained  quartz  (p.  527).  At 
the  mine  mentioned  the  vein  has  in  part  been  dissolved  and  re- 
placed by  a  loose  aggregate  of  calcite  and  laumontite  much 
richer  in  silver  than  the  original  quartz. 

The  occurrence  of  native  silver  in  the  zeolitic  copper  and  silver 
deposits  of  the  Lake  Superior  region  is  described  on  page  437. 
In  short,  zeolitization,  probably  effected  by  warm  or  tepid  waters, 
seems  particularly  'adapted  to  the  concentration  of  silver  and 
the  deposition  of  both  native  silver  and  rich  silver  minerals. 

The  Silver-Bearing  Cobalt-Nickel  Veins  of  Saxony. — In 
different  parts  of  the  world  occur  narrow  veins  with  calcite  or 
barite  gangue  and  arsenides  or  sulphides  of  cobalt  and  nickel;  the 
cobalt  and  nickel  minerals  contain  silver,  and  this  metal  is  often 
separated  as  seams  of  native  silver  enclosed  in  the  older  metallic 
minerals.  At  the  present  time  it  seems  probable  that  the  silver 
is  primary  and  that  it  was  deposited  by  the  same  solutions  from 
which  the  earlier  arsenides  were  formed. 

The  cobalt  veins  of  Annaberg,1  in  Saxony,  appear  in  gneiss 
intruded  by  dikes  of  granitic  and  lamprophyric  character;  they 
are  younger  than  the  veins  in  the  same  region  carrying  cassit- 
erite  and  those  yielding  pyritic  ores  with  galena.  The  gangue 
minerals  are  barite,  calcite,  fluorite,  quartz,  and  dolomitic  car- 

1  H.  Miiller,  Die  Erzgange  des  Annaberger  Bergrevieres,  Geol.  Landes- 
anstalt,  Leipzig,  1894. 


626  MINERAL  DEPOSITS 

bonates.  The  principal  ore  minerals  are  chloanthite,  smaltite, 
bismuthinite,  also  rich  silver  minerals  such  as  argentite,  pyrar- 
gyrite,  and  native  silver;  the  latter  are  distinctly  later  than  the 
primary  nickel-cobalt-bismuth  ores. 

Most  of  the  rich  silver  ores  appear  where  the  veins  intersect 
certain  flat  crushed  zones  in  the  gneiss,  which  contain  carbona- 
ceous material  and  finely  divided  sulphides  like  pyrite  and  chalco- 
pyrite.  The  greatest  depth  attained  was  1,400  feet. 

The  veins  of  Schneeberg,  in  Saxony,1  are  contained  in  contact- 
metamorphic  clay  slates  and  tend  to  impoverishment  in  the 
underlying  granite.  The  primary  gangue  consisted  of  calcite, 
ankerite,  barite,  and  fluorspar,  but  these  minerals  are  now  largely 
replaced  by  hackly  and  platy  fine-grained  quartz  by  a  process 
similar  to  that  to  which  many  later  Tertiary  gold-silver  veins 
have  been  subjected.  This  is  thought  to  be  the  only  locality  where 
such  a  replacement  has  been  carried  on  in  veins  of  more  deep- 
seated  deposition.  The  ore  minerals  are  smaltite,  chloanthite, 
niccolite,  bismuthite,  and  native  bismuth.  Native  silver  and 
rich  silver  minerals  are  subordinate  in  the  silicified  veins,  but 
appear  in  the  primary  barytic  veins.  From  this  it  is  perhaps 
permissible  to  draw  the  conclusion  that  the  silicification  has  been 
accompanied  by  solution  and  removal  of  silver.  The  process 
was  evidently  not  effected  by  surface  waters,  but  rather  by 
ascending  siliceous  solutions. 

Uranium  ores,  mainly  uraninite  or  pitch  blende,  are  found  at 
Schneeberg  and,  more  abundantly,  in  the  somewhat  similar  veins 
at  Joachimsthal,2  Bohemia.  The  geological  relations  at  the  two 
places  are  similar.  At  both  places  the  cobalt  and  nickel  minerals 
are  the  older  and  the  rich  silver  minerals  the  younger.  Between 
them  in  point  of  age  lie  the  uranium  ores. 

The  Silver-Bearing  Cobalt-Nickel  Veins  of  Ontario,  Canada.3 
— At  a  number  of  localities  in  Ontario  silver-bearing  veins  have 
been  found.  Many  of  them  appear  to  be  connected  genetically 
with  igneous  rocks  of  Keweenawan  age,  and  the  occurrence  of 

1  K.  Dalmer,  E.  Kohler,  and  H.  Miiller,  Section  Schneeberg,  Geol.  Spez. 
Karte  Sachsen,  1883. 

2  J.  Step  and  F.  Becke,  K.  Akad.  Wis.  Wien  Sitzungsber.,  vol.  113,  1904, 
pp.  585-618;  also  E.  S.  Bastin  and  J.  M.  Hill,  Prof.  Paper  94,  U.  S.  Geol. 
Survey,  1917,  p.  122. 

3  E.  D.  Ingall,  Report  on  mine  and  mining  on  Lake  Superior  (Silver 
Islet),  Ann.  Rept.  Canada  Geol.  Survey,  pt.  H,  1887. 

Willett  G.  Miller,  Cobalt-nickel  arsenides  and  silver  deposits  of  Tern- 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  627 

native  silver  in  the  copper  mines  working -the  amygdaloids  of 
Michigan  may  be  recalled. 

The  veins  of  Silver  Islet  and  those  at  points  southwest  of  Port 
Arthur,  on  the  north  shore  of  Lake  Superior,  have  been  known 
for  many  years.  The  Silver. Islet  vein,  at  one  time  a  heavy  pro- 
ducer, intersects  greenstone  and  contains,  in  a  gangue  of  calcite. 
ankerite,  and  quartz,  native  silver,  argentite,  tetrahedrite,  galena, 
zinc  blende,  pyrite,  and  some  cobalt  and  nickel  minerals.  Graph- 
ite, it  is  stated,  occurs  also  in  the  vein." 

In  1903  the  silver  veins  of  Cobalt,  Ontario,  were  discovered 
and  they  soon  became  extraordinarily  productive.  Up  to  1917, 
inclusive,  the  total  production  had  reached  about  275,000,000 
ounces,  and  the  output  in  that  year  was  19,250,000  ounces. 
The  ore  is  extremely  rich,  its  tenor  often  reaching  several  thou- 
sand ounces  per  ton.  Some  of  the  veins  contained  slabs  of  native 
silver  of  great  size.  One  specimen,  now  in  the  Parliament  build- 
ing at  Toronto,  is  5  feet  long  and  weighs  1,640  pounds;  it  contains 
9,715  troy  ounces  of  silver.  The  La  Rose  vein,  in  a  horizontal 
distance  of  100  feet  and  above  the  60-foot  level,  yielded  532  tons, 
which  contained  658,000  ounces  of  silver.  So-called  low-grade 
ores  average  about  200  ounces  per  ton,  while  that  which  is  con- 
centrated before  shipping  contains  about  30  ounces  per  ton. 
Owing  to  the  complex  character  of  the  ore  the  smelting  charges 
have  been  very  high.  Lately  the  cyanide,  amalgamation  and 
flotation  processes  have  been  adopted  with  much  success. 
Besides  silver,  cobalt,  nickel  and  arsenic  are  recovered. 

In  the  last  few  years  similar  cobalt-silver  veins,  productive 
in  part,  have  been  discovered  in  other  portions  of  Ontario,  par- 
ticularly at  Gowganda  and  South  Lorrain. 

iskaming,  Fourteenth  Ann.  Rept.,  Ontario  Bur.  Mines,  pt.  2,  1905.  Idem. 
Sixteenth  Ann.  Rept.,  pt.  2,  1907;  Nineteenth  Ann.  Rept.,  pt.  2,  1913. 

H.  V.  Ellsworth,  A  study  of  certain  minerals  from  Cobalt,  Twenty- 
fifth  Ann.  Rept.,  idem,  1916,  pp.  200-243. 

A.  A.  Cole,  The  mining  industry  of  part  of  Ontario  served  by  T.  and 
N.  O.  Ry.,  Annual  publication. 

R.  E.  Hore,  Origin  of  Cobalt  silver  ores,  Trans.  Canadian  Min.  Inst., 
vol.  2,  1908,  pp.  275-286. 

S.  F.  Emmons,  Cobalt  district,  Ontario,  Min.  and  Sci.  Press,  March 
18,  1911,  reprinted  in  "Types  of  Ore  Deposits,"  San  Francisco,  1911. 

W.  Campbell  and  C.  W.  Knight,  A  microscopic  examination  of  the 
cobalt-nickel  arsenides,  Econ.  Geol,  vol.  1,  1906,  pp.  767-776. 

E.  S.  Bastin,  Significant  mineralogical  relation  in  silver  ores  of  Cobalt, 
Econ.  Geol,  vol.  12,  1917,  pp.  219-236. 


628  MINERAL  DEPOSITS 

The  geology  of  the  Cobalt  region  is  summarized  by  W.  G. 
Miller  as  follows:  The  oldest  rocks,  known  as  the  Keewatin 
series,  are  basic  volcanic  rocks,  greenstones,  and  schists,  with 
more  or  less  cherty  or  jaspery  sediments.  On  the  eroded  Kee- 
watin were  deposited  the  conglomerate  and  graywacke  (arkose 
sandstone)  of  Temiskaming  (Huronian)  age.  A  thickness  of 
300  feet  of  these  gently  dipping  strata  is  exposed  at  Cobalt. 

After  the  deposition  of  the  Huronian  beds  an  irruption  of 
diabase  took  place,  assuming  the  form  of  sills  from  100  to  500 
feet  thick.  The  veins  were  formed  after  the  intrusion  of  this 
diabase,  which  is  regarded  by  some  authors  as  of  Keweenawan 


eewatin   Basement  Rocks.          Ep£3  Huronian,  Pragmental  Rocks 
|  Veins  j  Hypothetical  Veins 

FIG.  218. — Geological  section  of  Cobalt  district,  Ontario,  showing  also 
probable  geological  relations  before  erosion  had  produced  the  present  land 
surface.  After  W.  G.  Mitter. 

age;  the  fractures  occupied  by  the  veins  are  believed  by  some  to 
have  been  caused  by  cooling  or  shrinking  of  the  diabase,  as 
well  as  the  Keewatin  and  Huronian.  Fig.  218  illustrates  the 
geological  relations  and  the  subsequent  erosion  of  the  region  to 
the  present  surface.  The  veins  in  the  Huronian  conglomerate 
have  been  the  most  productive,  but  ores  occur  also  in  the  Kee- 
watin and  in  diabase. 

Almost  the  whole  production  has  been  derived  from  veins  in 
the  lower  wall  of  the  sill,  the  veins  once  cutting  through  the  upper 
or  hanging  wall  having  been  mostly  removed  by  erosion.  Few 
of  the  veins  have  been  followed  more  than  500  feet  horizontally 
and  very  little  ore  has  been  taken  out  below  the  500-foot  level. 
Some  ore  has,  however,  been  found  at  a  depth  of  1000  feet. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  629 

The  greatest  depth  reached  in  the  district  is  1600  feet  (1917). 
About  100  veins  are  known  in  the  district. 

In  the  narrow  veins,  which  are  seldom  more  than  a  few  inches 
in  width,  the  filling  is  often  "frozen  to  the  walls."  (Fig.  219.) 
In  some  places  the  veins  form  a  network  of  stringers.  The  silver, 
cobalt,  and  nickel  are  very  irregularly  distributed  in  the  veins; 
the  adjoining  country  rock  often  contains  carbonates  of  dolomitic 
character  and  small  specks  of  the  various  minerals,  including 


FIG.  219. — Photograph  of  vein  of  arsenides  with  inclusions  of  greywacke, 
Cobalt  district.    After  W.  L.  Whitehead. 

native  silver.  The  gangue  is  calcite  or  a  calcium-magnesium  car- 
bonate; graphite  is  reported;  quartz  occurs  sparingly.  The  ore 
minerals  are  abundant  and  consist  of  native  silver,  native  bis- 
muth, niccolite  (NiAs),  breithauptite  (NiSb),  smaltite  (CoAs2), 
chloanthite  (NiAs2),  argentite,  millerite,  arsenopyrite,  cobalt- 
ite  (CoAsS),  dyscrasite  -(Ag6Sb),  pyrargyrite,  stephanite,  poly- 
basite,  proustite,  and  tetrahedrite.1  Pyrite  is  rare.  Smaltite 
and  niccolite  are  most  common.  The  minerals  have  a  marked 

1  The  silver  and  the  dyscrasite  contains  in  places  a  little  mercury,  rising 
in  exceptional  cases  to  5  per  cent.  It  is  believed  to  be  present  as  silver 
amalgam.  G.  H.  Clevenger,  Econ.  Geol,  vol.  10,  1915,  p.  770. 


630  MINERAL  DEPOSITS 

tendency  to  concentric  arrangement  and  dendritic  replacements 
of  calcite  by  smaltite  and  silver  are  often  seen.  Coatings  of 
green  nickel  arsenate  and  pink  cobalt  arsenate  mark  the  outcrops. 

Carload  lots,  containing  from  100  to  6,000  ounces  per  ton,  aver- 
aged 6  per  cent.  Co,  3.66  per  cent.  Ni,  and  27  per  cent.  As. 

An  analysis  by  A.  R.  Ledoux  of  two  carloads  shipped  to  the 
smelter  is  as  follows: 

Si02 3.34             Bi trace 

Fe 1.78             Ag 5.31 

A1,O3 0.27             Sb 1.46 

CaO 5.85            As.... 42.46 

MgO 4.63            C02 9.26 

Cu 0.09            Cl 0.08 

Ni 13.87             S 1.89 

Co 8. 03 

The  majority  of  the  authors  have  expressed  the  belief  that  the 
ore  deposition  is  genetically  connected  with  the  intrusion  of  the 
diabase  sills  which  are  probably  of  Keewenawan  age1  and  it  is 
also  generally  recognized  that  the  native  silver  is  later  than  the 
cobalt  and  nickel  arsenides.  Some  veins  rich  in  arsenides  are  poor 
in  silver.  The  order  of  succession  was  first  roughly  established  by 
Campbell  and  Knight.  Ellsworth  concludes  that  the  mineraliz- 
ing solutions  were  at  first  rich  in  arsenic  and  deposited  smaltite 
and  chloanthite.  Later  the  monarsenides,  niccolite  and  brei- 
thauptite  were  formed.  Finally  sulphur  became  prominent  and 
the  sulpharsenides  such  as  arsenopyrite  and  cobaltite  were  pre- 
cipitated. Calcite  was  then  deposited  and,  after  a  period  of 
fracturing,  solutions  which  may  have  contained  sulphate  or 
carbonate  of  silver  began  to  deposit  native  silver  mainly  by  re- 
placement of  the  diarsenides  and  monarsenides  while  the  sul- 
pharsenides and  pyrite  were  inert.2 

Native  bismuth,  argentite,  proustite,  etc.,  were  deposited  with 
the  silver.  The  process  of  deposition  of  the  various  arsenides 
was  practically  continuous  but  the  silver  distinctly  marks  another 
epoch.  Whether  this  is  a  case  of  enrichment  by  descending 
solutions  which  have  leached  the  upper,  now  eroded,  parts  of  the 

1  C.  K.  Leith  has  pointed  out  the  remarkable  association  of  silver  de- 
position in  the  Lake  Superior  region  with  the  Keewenawan  intrusions. 

2  Chase  Palmer,  Diarsenides  as  silver  precipitants,  Econ.  Geol.,  vol.  12, 
1917,  pp.  207-218. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  631 

vein,  or  whether  it  is  a  separate  phase  of  the  primary  deposition 
is  not  yet  fully  established.  Emmons,  Van  Hise,  and  Bastin 
have  held  the  former,  whereas  some  observers  like  W.  G.  Miller, 
A.  R.  Whitman  and  W.  L.  Whitehead  (unpublished  manuscripts) 
are  inclined  to  the  other  view. 

While  the  question  is  a  difficult  one  it  does  not  seem  im- 
probable that  the  silver  may  have  been  deposited  by  the  last  as- 
cending solutions  of  magmatic  origin. 


FIG.   220. — Replacement   veinlet   of   silver  traversing  ferruginous,  calcite 
and  smaltite  (with  niccolite).    Arsenides  are  dotted.     After  E.  S.  Bastin. 


QUARTZ-ADULARIA-ZEOLITE  VEINS  (ALPINE  TYPE) 

Occurrence  and  Mineral  Association. — The  so-called  Alpine 
veins  have  little  or  no  importance  as  a  source  of  metalliferous 
ores,  but  many  of  them  contain  beautifully  developed  crystals, 
represented  in  all  large  mineral  collections,  and  are  prospected 
and  worked  to  some  extent  for  such  specimens.  They  occur  in 
the  crystalline  rocks  in  Tyrol,  Switzerland,  and  the  French  Alps. 
There  are  several  types.  One  class  found  in  the  Zillerthal,  in 
Tyrol,  contains  pyrite  and  galena  with  quartz,  adularia,  albite, 
epidote,  calcite,  prehnite,  desmine,  and  laumontite.  Another, 
in  the  French  Alps,  yields  axinite,  titanite,  ilmenite,  and  many 
other  minerals  which  indicate  deposition  at  high  temperature. 

The  veins  in  Switzerland  have  been  studied  in  detail  by  J. 


632  MINERAL  DEPOSITS 

Konigsberger,1  whose  investigations  have  shed  much  light  on  the 
conditions  under  which  these  beautiful  minerals  have  been  formed. 

The  "veins"  are  approximately  horizontal  filled  crevices  in  a 
biotite  gneiss;  they  are  really  local  openings  in  joints,  which  can 
be  followed  for  considerable  distances  and  which  lie  perpendicular 
to  the  schistosity  of  the  rock.  These  crevices  are  surrounded  by 
zones  of  altered  rock  not  more  than  double  the  width  of  the  open- 
ing; they  usually  contain  open  cavities  into  which  the  crystals 
project. 

The  biotite  of  the  gneiss  is  altered  to  chlorite;  sometimes 
also  to  specularite,  and  with  the  chlorite  are  epidote  and  quartz; 
the  plagioclase  alters  to  sillimanite,  kaolin,  and  epidote;  albite 
and  adularia  crystallize  in  vugs;  quartz  and  orthoclase  are  not 
attacked,  although  next  to  the  vein  the  orthoclase  is  often  covered 
by  secondary  adularia.  While  Konigsberger  does  not  say  so, 
it  is  probable  that  the  kaolin  is  secondary,  for  the  veins  have  for 
a  long  time  been  within  reach  of  oxidizing  waters. 

It  will  be  observed  that  the  type  of  alteration  is  not  that  of 
ordinary  veins;  it  seems  most  closely  related  to  some  veins  formed 
near  the  surface — for  instance,  the  Cripple  Creek  deposits. 
Sillimanite  has  not  been  shown  to  form  in  the  wall  rocks  of 
metalliferous  veins. 

The  general  succession  is:  (1)  Smoky  quartz  and  adularia 
(oldest);  (2)  calcite;  (3)  zeolites.  The  zeolites  are  not  always 
noted  in  descriptions  of  Alpine  veins,  because  they  have  often 
been  leached  out  or  softened  by  oxidizing  waters.  The  long 
list  of  other  minerals  includes  fluorite,  chlorite  (often  dusting  the 
faces  of  quartz  and  adularia),  apatite,  albite,  stilbite,  heulandite, 
apophyllite,  laumontite,  chabazite,  specularite,  and  rarely  galena, 
chalcopyrite,  and  molybdenite. 

Origin. — Konigsberger  concludes  that  the  minerals  were  de- 
rived chiefly  from  the  rock  itself.  The  minerals  were  deposited 
by  cooling  of  hot  solutions  containing  carbon  dioxide.  The 
crystallization  in  the  crevice  took  place  at  temperatures  of  290° 
to  130°C.  It  is  not  improbable  that  the  Alpine  veins  were 
produced  by  intrusive  after-effects  at  various  temperatures. 

Some  of  the  quartz  crystals  contain  large  fluid  inclusions;  an 
analysis  of  the  liquid  gives  the  following  result.  The  total  solids 
amount  to  10  per  cent.,  a  rather  concentrated  solution. 

1 J.  Konigsberger,  Die  Minerallagerstatten  im  Biotitprotogin  des  Aar 
massives,  Neues  Jahrb.,  Beil.  Bd.,  vol.  14,  1901,  pp.  43-49. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  633 

COMPOSITION  OF  FLUID  INCLUSION  IN  QUARTZ 


H20 
C02 
Na 


C03 

Cl 

SO. 


85.0 
5.0 
2.5 
1.5 
3.5 
1.5 
0.7 


The  dissolved  salts  then  consist  of  alkaline  carbonates,  chlorides, 
and  sulphates  in  the  order  named. 

As  the  smoky  quartz  is  bleached  at  300°  C.  it  is  assumed  that 
it  could  not  have  been  formed  above  this  temperature.  The 
bubbles  of  the  inclusions  disappear  at  225°  C.  Because  the 


Galena 

He 

ilandlte 

Desmine 

Chlorite 

Apatite 

Fluorite 

,  Epidote 

~*          Calcite 

Adularia 

Quartz  X 

Laumontite 

)( 

Scolecite 
— X — 

Chabazite 


300°  C 


250°  C 


200°C 


150CC 


FIG.  221. — Order  of  crystallization  in  the  zeolitic  veins  of  Aar,  Switzerland. 
After  J.  Konigsberger. 

liquid  must  have  rilled  the  cavity  at  the  time  of  formation  and 
because  the  pressure  did  not  differ  much  from  that  of  saturated 
water  gas,  the  temperature  of  formation  is  about  the  same  as 
that  at  which  the  bubbles  disappear. 

The  red  color  of  the  fluorite  disappears  at  175°  C.,  independent 
of  the  surrounding  medium  and  pressure.  The  zeolites  were 
probably  formed  at  about  130°  C.  Konigsberger's  view  as  to 
the  order  of  crystallization  is  expressed  in  Fig.  221. 

Analogous  minerals  are  found  in  the  miarolitic  cavities  in 
certain  granites;1  in  these  occurrences  the  succession,  beginning 

1  A.  Schwantke,  Drusen  Mineralien  des  Striegauer  Granits,  Inaug. 
Dissert.,  Leipzig,  1890. 


634  MINERAL  DEPOSITS 

with  the  earliest,  is  (1)  adularia,  albite,  quartz,  epidote;  (2) 
fluorite,  apatite,  calcite,  chlorite,  epidote;  and  (3)  zeolites. 

THE  COPPER  VEINS 

The  copper-bearing  veins  in  which  filling  has  been  the  prin- 
cipal process  do  not  have  the  importance  of  the  corresponding 
class  of  gold  quartz  veins.  Only  when  replacement  has  converted 
wider  bodies  of  rock  into  ore  and  when  enrichment  by  descending 
surface  waters  have  acted  upon  the  primary  ore  do  we  find  large 
and  productive  members  in  this  class. 

Many  of  the  great  copper  deposits  are  mainly  pyritic  replace- 
ments of  igneous  or  sedimentary  rocks  and  are  described  on  the 
following  pages  or  with  the  high-temperature  deposits  in  Chapter 
XXVI.  Still  others  derive  their  importance  from  the  accumu- 
lation of  secondary  chalcocite,  either  in  wide  replacement  veins 
or  in  broad  mineralized  zones.  Many  of  these  in  fact  also 
belong  to  the  high-temperature  deposits  as  far  as  the  poor, 
primary  mineralization  is  concerned. 

Chalcopyrite -Quartz  Veins. — Simple  veins  containing  mainly 
chalcopyrite,  pyrite  and  quartz,  with  some  bornite,  more  rarely 
tetrahedrite  and  siderite,  are  common  enough  in  many  districts, 
but  are  rarely  of  great  importance  unless  also  containing  gold 
and  silver.  The  extensive  group  of  tourmaline-bearing  copper 
veins  belongs  to  the  high-temperature  deposits. 

J.  B.  Umpleby1  describes  the  Lost  Packer  vein,  in  central 
Idaho,  which  cuts  through  mica  schist  and  which,  in  a  gangue 
of  quartz  and  siderite,  contains  chalcopyrite  and  some  pyrrho- 
tite  and  pyrite.  The  chalcopyrite  and  the  quartz  contain,  in 
the  ore  shoots,  about  3  ounces  of  gold  per  ton.  The  vein  is  inter- 
sected by  dikes  and  is  probably  a  high-temperature  deposit. 

Somite-Quartz  Veins. — Quartz  veins  containing  bornite  are 
not  uncommon  but  are  rarely  of  great  economic  importance. 
Veins  of  this  kind  occur  in  the  Virgilina  district,2  Virginia  and 
North  Carolina,  where  F.  B.  Laney  noted  interesting  inter- 
growths  of  bornite  and  chalcocite.  The  deposits  are,  however, 
lenticular  veins  and  probably  were  formed  at  high  temperatures. 

Pyrite-Enargite  Veins. — The  enargite-bearing  veins  constitute 
a  less  common  yet  in  places  a  most  important  class. 

1  Bull.  530,  U.  S.  Geol.  Survey,  1913,  pp.  70-73. 

2W.  H.  Weed,  Bull.  455,  U.  S.  Geol.  Survey,  1911,  pp.  67-93. 

F.  B.  Laney,  Butt.  21,  North  Carolina  Geol.  and  Econ.  Survey,  1910; 
Bull.  14,  Virginia  Geol.  Survey,  1917. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  635 

Enargito  is,  on  the  whole,  a  rare  mineral  and  rather  favors 
the  deposits  formed  relatively  near  the  surface.  At  Mancayan, 
in  Luzon,  in  the  Philippine  Islands,  enargite  is  found  in  a  large 
replacement  deposit  in  "quartz  porphyry"  and  "andesite," 
formerly  worked  on  a  large  scale  and  containing,  in  a  quartz 
gangue,  tetrahedrite,  bornite,  and  enargite  (luzonite).  Another 
deposit  is  that  of  Bor,1  in  Serbia,  which  is  also  an  irregular  quartz- 
oze  replacement  in  a  rock  that  is  called  andesite  but  is  really 
an  intrusive  porphyry  with  holocrystalline  groundmass.  The 
principal  minerals  are  pyrite,  enargite,  quartz,  barite,  and  second- 
ary covellite.  Finally  should  be  mentioned  the  occurrence  in 
the  Sierra  Famatina,2  Argentina,  where  veins  carrying  quartz, 
enargite,  and  famatinite  (Cu3SbS4)  break  through  clay  slate 
intruded  by  granitic  rocks  and  "dacite." 

The  most  prominent  examples  of  this  type  are  found  in  the 
Butte  district3  (Figs.  46  and  47),  Montana,  now  the  most  pro- 
ductive copper  camp  in  the  world.  The  veins  are  contained  in 
sericitized  quartz  monzonite  and  are  formed  in  part  by  replace- 
ment of  the  intrusive  rock.  The  primary  ores  are  pyrite, 
enargite  (Cu3AsS4),  and  perhaps  chalcocite,  with  smaller  amounts 
of  bornite  and  sphalerite.  On  account  of  the  importance  of 
secondary  processes  in  their  formation  the  description  of  the 
Butte  copper  veins  is  placed  in  Chapter  XXXI.  Similar  veins 
occur  at  Cerro  de  Pasco,  and  Morococha,  in  Peru,  and  at 
Chuquicamata  in  Chile. 

THE  PYRITIC  REPLACEMENT  DEPOSITS 

While  pyrite  is  a  persistent  mineral,  crystallizing  within  a 
wide  range  of  temperatures,  it  is  easily  apparent  that  the  deposits 
containing  large  masses  of  pyrite  have  not  been  formed  close  to 
the  surface,  but  rather  at  considerable  depths  and  at  temperatures 
well  above  100°C.  The  deposits  of  this  type  are  mainly  confined 
to  regions  that  have  been  deeply  eroded  since  the  deposits  were 
formed  and  many  of  them  show  a  mineral  association  indicating 
high  temperature. 

In  many  text-books  the  pyritic  deposits  are  treated  as  a  dis- 
tinct class,  and  are  assumed  to  have  a  similar  origin.  We  know 

1  M.  Lazarevic,  Zeitechr.  prakt.  GeoL,  1912,  pp.  337-370. 

2  A.  W.  Stelzner,  Beitrage   zur   Geologie   der  Argentinischen   Republik, 
pt.  1,  1885,  p.  228. 

3  W.  H.  Weed,  Geology  and  ore  deposits  of  the  Butte  district,  Montana, 
Prof.  Paper  74,  U.  S.  Geol.  Survey,  1912.     See  also  footnotes,  p.  862. 


636  MINERAL  DEPOSITS 

now  that  they  comprise  deposits  of  widely  differing  origin  and 
history. 

In  a  broad  way  we  may  distinguish  (1)  those  associated  with 
silicates  such  as  amphibole,  pyroxene,  epidote,  tourmaline,  and 
garnet,  the  iron  sulphide  being  in  part  present  as  pyrrhotite,  and 
(2)  those  associated  with  calcite,  barite,  and  quartz  as  gangue 
minerals.  The  deposits  of  the  first  class  were  undoubtedly 
formed  at  considerably  higher  temperatures  than  those  of  the 
second  and,  in  general,  probably  also  at  greater  depth. 

Class  1  comprises  (A)  some  deposits  of  purely  magmatic 
origin  like  those  of  Sudbury,  Ontario;  (B)  a  large  division,  of 
contact-metamorphic  type,  like  the  deposits  of  Ducktown, 
Tennessee;  Granby,  British  Columbia;  and  the  Highland  Boy 
mine  at  Bingham,  Utah;  (C)  a  third  division,  difficult  to  interpret 
but  thought  by  many  to  represent  a  phase  of  igneous  injec- 
tion; this  may  not  be  firmly  established,  but  their  close  connec- 
tion with  igneous  rocks  can  hardly  be  questioned;  among  these 
are  the  deposits  at  Vigsnas,  Sulitelma,  and  Roros  in  Norway,  Fah- 
lun  and  Bersbo  in  Sweden,  and  Bodenmais  in  Bavaria.  In  each 
of  these  three  divisions  the  deposit  may  have  been  subjected  to 
dynamic  metamorphism,  with  the  attendant  development  of 
amphibole  and  garnet  and  of  schistose  structure.  Many  of  these 
metamorphosed  deposits  have  had  a  complicated  history  and  are 
among  the  most  difficult  to  interpret  and  classify. 

Class  2  is  also  connected  with  the  eruption  of  igneous  rocks, 
but  the  high-temperature  minerals  are  absent;  the  deposits  of 
Rammelsberg  in  the  Harz,  Germany,  of  Mount  Lyell,  Tasmania, 
of  Rio  Tinto,  Spam,  and  of j  Shasta  County,  California,  may 
serve  as  examples.  The  deposit  at  Kyschtim,1  in  the  Ural  Moun- 
tains, and  that  at  Tyee,2  on  Vancouver  Island,  also  appear  to 
belong  in  this  class. 

Dynamic  metamorphism  may  produce  remarkable  changes  in 
structure  and  mineral  association.  Replacement  of  various  rocks 
by  pyrite  has  played  an  important  part;  sometimes  this  process 
takes  place  in  shattered  zones  or  proceeds  from  fissures,  or  again 
it  may  be  caused  by  solutions  permeating  heated  limestone  masses 
without  fractures  at  igneous  contacts. 

The  ores,  while  consisting  mainly  of  pyrite  or  pyrrhotite,  derive 

1  A.  W.  Stickney,  Econ.  Geol,  vol.  10,  1915,  pp.  593-633. 

2  C.  H.  Clapp,  Mem.  13,  Canada  Geol.  Survey,  1912,  pp.  180-187. 
V.  Dolmage,  Econ.  Geol.,  vol.  11,  1916,  pp.  390-394. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  637 

their  value  from  a  small  percentage  of  chalcopyrite;  there  are 
usually  minute  quantities  of  gold  and  silver,  and  frequently  also 
zinc  blende  and  a  little  galena;  other  sulphides  are  rare.  In  all 
the  deposits  mentioned,  except  those  at  Rammelsberg,  their  con- 
nection with  rhyolite  porphyries,  alaskite  porphyry,  or  kerato- 
phyre  can  be  established,  and  deposition  by  hydrothermal  re- 
placement at  moderate  depth  and  temperature  seems  the  most 
reasonable  explanation. 

The  deposits  of  Class  1  are  described  in  Chapters  XXVII 
and  XXVIII.  Some  pyritic  deposits  of  Class  2  will  be  briefly 
characterized  in  the  following  paragraphs. 


Copper  Deposits  of  Shasta  County,  California1 

Copper  deposits  which  have  been  actively  mined  and  smelted 
since  1895  are  found  in  a  number  of  districts  in  Shasta  County, 
California;  among  the  more  prominent  mines  are  the  Iron 
Mountain,  Bully  Hill,  Mammoth,  and  Balaklala.  The  produc- 
tion of  copper  in  1917  was  26,700,000  pounds. 

The  sedimentary  rocks  consist  of  Devonian  and  Carboniferous 
closely  folded  slates  and  contain  intrusions  of  a  highly  siliceous 
and  sodic  alaskite  porphyry,  which  is  the  country  rock  of  almost 
all  the  important  copper  deposits.  Somewhat  later  than  the 
alaskite  porphyry,  but  belonging  to  the  same  (early  Cretaceous) 
period  of  intrusion,  is  a  quartz  diorite,  probably  equivalent  to  the 
granodiorite  of  the  Sierra  Nevada.  No  copper  deposits  occur  in 
the  quartz  diorite,  but  it  contains  workable  gold-bearing  quartz 
veins.  Deep  erosion  has  taken  place  since  the  period  of  intrusion ; 
Graton  estimates  the  depth  of  rocks  removed  as  not  less  than 
5,000  or  6,000  feet.  The  rocks  have  been  subjected  to  some 
shearing  and  brecciation,  but  little  extensive  dynamo-metamor- 
phism,  since  the  intrusion. 

The  copper  deposits  were  formed  during  the  interval  between 
the  two  epochs  of  intrusion.  The  ore-bodies  are  large,  irregular 
tabular  masses  of  pyrite  with  some  chalcopyrite  (Fig.  222); 

1  J.  S.  Diller,  Redding  Folio  138,  U.  S.  Geol.  Survey,  1906. 

L.  C.  Graton,  The  occurrence  of  copper  in  Shasta  County,  Cal.,  Bull. 
430,  U.  S.  Geol.  Survey,  1910,  pp.  71-111. 

A.  C.  Boyle,  The  geology  and  ore  deposits  of  the  Bully  Hill  mining 
district,  Trans.  Am.  Inst.  Min.  Eng.,  vol.  48,  1915,  pp.  67-117. 


638 


MINERAL  DEPOSITS 


single  ore  masses  have  dimensions  of  1,200  feet  in  length,  300 
feet  in  width,  and  nearly  300  feet  in  thickness,  and  some  of  them 
contain  many  million  tons  of  ore;  the  Iron  Mountain  mass  before 
a  great  part  of  it  was  converted  to  gossan  probably  contained 
20,000,000  tons  of  ore,  exclusive  of  the  large  amount  which  has 
been  removed  by  erosion.  Many  of  the  bodies  lie  flat  and  are 
easily  accessible  by  tunnels. 

The  ores  contain  chiefly  pyrite  with  about  3  per  cent,  of  copper 
as  chalcopyrite,  and  as  much  as  $2  per  ton  in  gold  and  silver, 
about  equally  divided  between  the  two  metals.  Zinc  blende  is 


Ore 


Metamorphic 
Slate 


FIG.  222. — Cross-section  of  ore-bodies  at  Balaklala,  California. 
After  W.  H.  Weed. 


present  in  varying  amounts,  and  the  ore  contains  a  little 
bismuth,  arsenic,  and  selenium.  The  gangue  minerals  in- 
clude quartz,  calcite,  barite,  gypsum  and  anhydrite;  the  suc- 
cession is  in  general  pyrite  (oldest),  zinc  blende,  chalcopyrite, 
quartz,  and  barite.  There  is  a  deep  oxidized  zone  with  sulphide 
enrichment. 

The  alaskite  porphyry  near  the  ore-bodies  is  more  or  less  al- 
tered and  contains  sericite  (probably  also  paragonite),  secondary 
quartz,  chlorite,  pyrite,  carbonates,  and  epidote.  Cogent 
evidence  is  cited  by  Graton  that  the  pyritic  ores  are  replacements 
of  the  surrounding  porphyry  in  sheared  and  brecciated  zones. 
This  replacement  is  believed  to  be  due  to  hot  solutions  emanating 
from  the  cooling  alaskite  porphyry.  The  action  of  surface  waters 
in  the  ore  deposition  is  probably  negligible,  for  at  that  time  the 
alaskite  porphyry  was  everywhere  covered  with  a  blanket  of 
impermeable  shales. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  639 

The  Pyritic  Deposit  of  Mount  Lyell,  Tasmania1 

At  Mount  Lyell,  on  the  west  coast  of  Tasmania,  is  one  of  the 
large  copper  deposits  of  the  world.  According  to  Gregory  the 
ore-bodies  are  contained  in  sericite  schists  (probably  with 
paragonite),  which  are  dynamo-metamorphic  forms  of  perhaps 
Paleozoic  acidic  porphyries.  Intrusive  in  these  schists  are  con- 
siderable masses  of  igneous  rocks  which  are  termed  "porphyrites," 
but  of  which  no  analyses  are  available.  Probably  of  later  age 
than  the  complexes  just  mentioned  are  Devonian  conglomerates. 

The  main  ore-bodies  are  lenticular  and  lie  in  part  in  the  seri- 
cite schist  and  in  part  at  its  contact  with  the  conglomerates 
which  have  been  brought  against  the  schist  by  faulting.  The 
largest  ore-body  is  that  of  the  Mount  Lyell  mine;  this  is  several 
hundred  feet  long  and  200  feet  wide,  but  appears  to  be  limited  in 
depth  by  a  fault. 

The  ore  consists  mainly  of  pyrite  with  but  little  gangue  of 
quartz  and  barite.  It  also  contains  5  to  6  per  cent,  of  copper  in 
the  form  of  chalcopyrite,  more  rarely  bornite  and  enargite; 
an  analysis  of  the  ore  given  by  Gregory  is  as  follows : 

Fe 40.30 

SiO2 4.42 

BaSO, 2.50 

Cu 2.35 

A12O3 2.04 

The  ore  yields  very  little  gold  and  1.33  ounces  of  silver  to 
the  ton.  The  output  of  copper  was  6,500  tons  in  1918.  Pyrite 
is  the  oldest  mineral;  it  was  followed  by  chalcopyrite,  bornite 
and  enargite. 

Enrichment  near  the  surface  and  in  the  footwall  of  the  deposit 
has  added  much  to  the  wealth  of  the  property.  The  croppings 
at  one  place  contained  quartz,  barite,  hematite,  and  about  15 
ounces  of  silver  and  $15  in  gold  per  ton.  The  presence  of  hema- 
tite in  the  croppings  would  suggest  that  at  some  time,  during  the 
weathering  of  the  deposits,  the  climate  was  warmer  than  now. 
The  secondary  sulphides  consisted  of  chalcopyrite,  bornite,  and 
tennantite,  with  stromeyerite. 

1  J.  W.  Gregory,  The  Mount  Lyell  mining  field,  Tasmania,  Trans.  Aust. 
Inst.  Min.  Eng.,  Melbourne,  vol.  10,  1905,  pp.  26-197. 

C.  G.  Gilbert  and  J.  E.  Pogue,  The  Mount  Lyell  (topper  district  of 
Tasmania,  Proc.  U.  S.  Nat.  Mus.,  vol.  45,  1913,  pp.  609-625. 


640  MINERAL  DEPOSITS 

Gregory  as  well  as  Gilbert  and  Pogue  find  that  the  ore  minerals 
replace  the  schists  and  the  latter  suggest  a  relationship  between 
the  "  porphyrites "  and  the  pyritic  deposits. 

The  Pyritic  Deposits  of  Rio  Tinto,   Spain1 

General  Features. — The  pyritic  ore-bodies  of  the  southern 
Spanish  province  of  Huelva,  more  generally  known  as  the  depos- 
its of  Rio  Tinto,  are  probably  the  greatest  in  the  world  and  have 
been  mined  since  Phoenician  and  Roman  times.  The  deposits 
are  in  the  main  lenticular;  there  are  at  least  50  of  these  pyritic 
lenses,  whose  length  varies  from  1,200  to  6,500  feet,  while  the 
width,  in  general  proportional  to  the  length,  reaches  a  maximum 
of  250  feet  and  the  depth  ranges  from  500  to  1,800  feet.  The 
vertical  range  of  deposition,  according  to  Finlayson,  probably  in 
no  case  exceeded  3,300  feet,  and  few  of  the  deposits  attain  a 
depth  of  1,000  feet.  In  the  slates  the  deposits  often  taper  down- 
ward to  a  point,  while  in  the  porphyry  a  flat  or  rounded  lower 
surface  is  not  uncommonly  observed.  On  the  whole  they  appear 
to  h'e  conformably  between  slates  and  porphyry  or  in  either  por- 
phyry or  slate. 

The  production  of  these  deposits  has  always  been  large,  but 
appears  now  to  be  diminishing;  in  1917  it  was  40,000  metric  tons 
of  copper,  all  sources  considered.  Besides  the  regular  copper 
ore  with  more  than  2  per  cent.  Cu,  large  quantities  of  pyrite, 
poor  in  copper,  are  shipped  for  sulphuric  acid  manufacture. 
A  part  of  the  copper  is  recovered  as  a  precipitated  cement  or  a 
sulphate. 

Geological  Formations. — The  rocks  consist  of  (1)  a  uniform 
series  of  folded  and  compressed  clay  slates  and  graywacke,  strik- 
ing east  and  west  and  believed  to  be  of  Devonian  and  Carbonifer- 

1  A.  Moncrieff  Finlayson,  The  pyritic  deposits  of  Huelva,  Spain,  Econ. 
Geol,  vol.  5,  1910,  pp.  357-372;  403-437. 

Bruno  Wetzig,  Beitrage  zur  Kenntniss  der  Huelvaner  Kieslagerstatten. 
Zeitschr.  prakt.  Geol,  vol.  14,  1905,  p.  173. 

H.  Preiswerk,  Die  Erzlagerstatten  von  Gala,  etc.,  Idem,  vol.  12,  p.  225. 

F.  Klockmann,  Ueber  das  Auftreten,  etc.,  der  Sudspanischen  Kieslager- 
statten, Idem,  vol.  10,  1901,  p.  113;  also  vol.  3,  1894,  p.  35. 

J.  H.  L.  Vogt,  Das  Huelva  Kiesfeld,  etc.,  Idem,  vol.  7,  1898,  p.  241. 

L.  de  Launay,  Memoire  sur  Findustrie,  etc.,  dans  la  region  d 'Huelva. 
Ann.  des  Mines,  Paris,  ser.  7,  vol.  16,  p.  407. 

J.  Gonzalo  y  Tarin,  Descripci6n,  etc.,  de  la  provincia  de  Huelva,  Mem. 
Com.  Mapa  Geol.  Espana,  Madrid,  vol.  1.  1886. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  641 


ous  age;  (2)  granites  and  granodiorites  intrusive  into  the.  Carbon- 
iferous rocks,  north  of  the  district;  (3)  several  varieties  of 
porphyry,  including  rhyolite  porphyry  "and  keratophyre,  some 
varieties  with  granophyric  structure;  in  places  the  porphyry  is 
affected  by  shearing  and  schistosity;  (4)  basic  dikes  and  sills, 
mainly  of  diabase,  but  including  also  camptonites  and  diorites. 
Some  authors,  including  Klockmann,  consider  the  porphyries 
as  effusive,  but  the  arguments  of  Finlayson  show  quite  conclu- 
sively that  they  are  intrusive  masses,  occurring  in  belts  and 
lenses  throughout  the  field  (Fig.  223).  The  basic  rocks  cut 
both  granites  and  porphyries. 


FIG.  223. — Cross-section  of  ore-bodies,  Rio  Tinto,  Spain. 
After  Gonzales  y  Tarin. 

Displacements  of  considerable  throw  occur  along  many  ore- 
bodies.  According  to  Finlayson,  the  last  event  in  this  series 
of  igneous  and  dynamic  disturbances  consisted  in  the  develop- 
ment of  the  mineral  deposits. 

The  lodes  that  occur  in  the  slate  are  in  the  main  conformable 
with  the  bedding,  but  the  ore  sometimes,  according  to  Finlayson, 
cuts  across  it;  the  lode  walls  are  usually  well  denned  and_smooth; 
the  deposits  occur,  as  a  rule,  along  contacts  or  other  lines  of 
weakness  and  crushing.  According  to  the  same  author  the 
adjoining  rock  shows  effects  of  hydrothermal  action  in  marked 
degree,  the  porphyry  being  transformed  into  an  aggregate  of 
chlorite,  sericite,  quartz,  carbonates,  and  pyrite.  Analyses 
show  extremely  well  marked  carbonatization  and  sericitization, 
entirely  similar  to  the  alteration  occurring  in  the  California 


642  MINERAL  DEPOSITS 

type  of  gold-quartz  veins,  and  undoubtedly  of  hydrothermal 
origin.  The  alteration  is  illustrated  by  the  following  analyses, 
quoted  from  Finlayson: 

ANALYSES  OF  FRESH  AND  ALTERED  PORPHYRY  FROM  THE 
SAN  DIONISIO  MINE,  RIO  TINTO,  SPAIN 

Fresh.  Altered. 


SiO2  

76.21 

70.68 

A12O3  

12.66 

11.45 

Fe2O3  

2.98 

1.31 

FeO  

1.46 

0.72 

MnO  

0.08 

0.05 

CaO  

1.15 

2.28 

MgO  

0.10 

0.17 

K2O  

3.27 

4.85 

Na20  

1  .  64 

0.65 

H20+  

0.18 

0.23 

H2O-  

0.35 

1.41 

CO2  

0.09 

5.08 

FeS,  

1.27 

100.17         100.15 

The  Ores. — The  ores  consist  of  almost  massive  pyrite,  with  but 
a  small  amount  of  quartz  and  few  other  gangue  minerals,  although 
barite  occurs  in  some  localities.  Banded  or  pressed  ore  is  rarely 
seen.  The  primary  ore  carries  from  48  to  50  per  cent,  of  sulphur. 
Chalcopyrite  is  present  in  minute  scattered  grains,  or  as  threads 
and  strings  traversing  the  granular  pyrite  and  filling  interstices 
in  the  ore.  Blende  and  galena  are  present  in  small  amounts,  and 
there  are  traces  of  bismuth,  selenium,  and  tellurium.  Arsenic 
varies  from  0.25  to  1  per  cent.  The  order  of  succession  is  pyrite 
(oldest),  chalcopyrite,  blende,  galena. 

Especially  interesting  are  the  changes  in  the  ore  produced  by 
weathering.  The  outcrops  are  gossans  of  hematite  carrying  10 
to  15  per  cent,  of  silica  and  alumina  and  little  or  no  copper. 
The  average  depth  of  this  gossan  is  100  feet.  The  lower  limit  of 
the  gossan  is  well  defined,  and  the  line  of  contact  between  it  and 
the  sulphide  ore  is  sometimes  marked  by  a  thin  earthy  material, 
which,  as  described  by  Vogt,  is  rich  in  gold  and  silver.  The  top 
portion  of  the  sulphide  zone,  for  a  thickness  of  3  feet  or  more,  is 
composed  of  leached  pyrite  with  traces  of  copper-  (Finlayson). 
Below  this  commences  the  zone  of  enriched  sulphides,  in  which 
the  ore  assays  from  3  to  12  per  cent,  of  copper.  This  enrichment 
is  effected  by  deposits  of  both  chalcopyrite  and  chalcocite,  and 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  643 

its  influence  may  be  traced  to  depths  of  300  feet  below  the  surface. 
In  the  San  Dionisio  lode  enrichment  was  noted  even  at  a  depth  of 
1,000  feet,  indicating,  according  to  Finlayson,  that  the  secondary 
changes  extend  far  down  into  what  is  usually  regarded  as  primary 
ore.  Wetzig  states  that  in  the  Cabeza  de  Pasto  mine  the  ore  at 
the  40-meter  level  contained  3.5  per  cent,  of  copper,  at  the  60- 
meter  level  3  per  cent.,  and  at  the  80-meter  level  2  per  cent.  The 
tenor  of  the  primary  ore  ranges  from  0.5  to  2  per  cent,  of  copper. 

Genesis. — The  origin  of  these  deposits  has  been  the  subject  of 
long  discussion  among  geologists.  The  earlier  geologists  believed 
in  a  sedimentary  origin,  and  this  view  is  still  held  by  some.  Its 
principal  defender  is  F.  Klockmann,  who  regards  the  pyrite 
bodies  as  concretionary  deposits  in  mud,  impregnated  by  copper 
from  the  supposedly  effusive  lavas.  Gonzalo  y  Tarin,  as  well  as 
De  Launay,  held  them  to  be  veins  or  lodes  deposited  in  open 
cavities  by  ascending  solutions.  Later,  Vogt  considered  the 
deposits  to  be  of  pneumatolytic  nature,  formed  as  an  after- 
effect of  the  extrusions  of  porphyry.  In  the  latest  contribution 
by  Finlayson  the  metasomatic  character  of  the  deposits,  which 
were  formed  by  the  hydrothermal  replacement  of  crushed  and 
sheared  zones,  appears  to  be  firmly  established.  He  believes, 
however,  that  the  deposits  were  formed  after  the  intrusion  of  the 
basic  dikes  and  sills,  which  are  considerably  later  than  the  por- 
phyry, and  thinks  that  the  concentration  of  the  ores  was  in  the 
first  place  due  to  a  process  of  magmatic  concentration  of  sul- 
phides, accompanying  the  differentiation  of  the  series  of  intrusive 
rocks  and  dependent,  with  the  latter,  on  the  Hercynian  tectonic 
movements. 

Expression  of  opinion  without  field  acquaintance  may  have 
little  value,  but  it  seems  to  me  that  it  has  not  been  definitely 
shown  that  the  deposits  are  later  than  the  basic  intrusions.  They 
are  clearly  of  hydrothermal  origin,  as  shown  by  the  character  of 
alteration,  and  the  replacement  origin  seems  definitely  proved. 
There  appears  to  be  good  reason  for  Vogt's  view  that  the  hydro- 
thermal  processes  of  deposition  followed  the  intrusion  of  acidic 
and,  in  part,  sodic  porphyries;  the  whole  giving  a  strong  im- 
pression of  similarity  to  the  pyritic  deposits  of  Shasta  County, 
California.  On  the  other  hand,  there  is  no  evidence  of  pneu- 
matolytic deposition.  On  the  contrary,  the  processes  of  re- 
placement probably  proceeded  at  moderate  temperature  and  at 
moderate  depth.  Magnetite  and  pyrrhotite  are  present  in  only 


644 


MINERAL  DEPOSITS 


a  few  deposits,  such  as  those  at  Gala,  and  opinions  differ  as  to 
whether  these  minerals  are  due  to  later  dynamometamorphic  or 
contact-metamorphic  processes  or  to  original  deposition. 

The  Pyritic  Deposit  of  Rammelsberg,  Germany1 

The  Rammelsberg  deposit  (Fig.  224),  lies  on  the  northern  slope 
of  the  Harz  Mountains,  in  Germany,  near  the  town  of  Goslar. 
As  is  well  known,  it  has  been  worked  for  copper  ores  since  ancient 
times,  the  first  records  dating  back  to  the  tenth  century.  Its 
geological  structure  has  been  investigated  by  a  number  of  authors, 
but  its  complete  and  detailed  description  is  as  yet  a  problem  of  the 
future.  The  most  diverse  explanations  have  been  offered  as  to 


FIG.  224. — Cross-section  of  Rammelsberg,  showing  overturned  anti- 
cline and  supposed  conformable  position  of  the  ore  deposit.  After  F. 
Klockmann. 

its  mode  of  origin.  By  some,  perhaps  by  a  majority,  including 
such  well-known  geologists  as  Bergeat  and  Klockmann,  it  has 
been  considered  as  sedimentary  deposit  contemporaneous  with 
the  surrounding  sedimentary  rocks.  Others,  like  Vogt,  following 
Freiesleben  and  Lessen,  explain  it  as  a  deposit  from  solutions 
immediately  derived  from  igneous  magmas.  Still  others,  like 
Beck,  are  non-committal. 

Geology  and  Structural  Features. — The  deposit  is  enclosed, 
apparently  conformably,  in  Devonian  rocks,  which  at  Goslar 
appear  as  an  overturned  anticlinal  and  dip  toward  the  north. 

1  F.  Klockmann,  Berg-  und  Htittenwesen  des  Oberharzes,  1895,  p.  57. 

J.  H.  L.  Vogt,  tJber  die  genesis  der  Kieslagerstatten  vom  Typus  Roros- 
Rammelsberg,  Zeitschr.  prakl.  Geol,  1894,  p.  173. 

W.  Lindgren  and  J.  D  Irving,  The  origin  of  the  Rammelsberg  ore  de- 
posit, Econ.  Geol.,  vol.  6,  1911,  pp.  303-313. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  645 

It  lies  in  the  so-called  Goslar  slates  of  the  Middle  Devonian; 
these  slates  are  overlain  by  a  thick  series  of  Lower  Devonian 
Spirifer  sandstone,  which  makes  up  the  summit  of  Rammelsberg 
Mountain,  at  the  foot  of  which  the  mine  is  located. 

The  slates  have  suffered  considerable  deformation  and  the  ore- 
body  apparently  follows  their  contortions  more  or  less  closely. 
The  underground  developments  extend  over  a  horizontal  dis- 
tance of  about  2,000  meters  and  have  attained  a  vertical  depth 
of  380  meters  from  the  level  of  the  Richtschacht.  It  will  be 
seen  from  this  that  mining  has  not  yet  penetrated  to  great  depths, 
in  spite  of  the  fact  that  the  deposit  has  been  worked  for  nearly 
1,000  years.  The  ore-body  is  divided  horizontally  into  two  parts, 
referred  to  as  the  old  and  the  new  beds;  they  are  connected  by  a 
narrow  and  contorted  seam,  showing,  however,  beyond  doubt 
that  the  two  are  really  parts  of  one  deposit. 

The  thickness  of  the  "ore  bed,"  as  it  is  generally  referred  to, 
varies  considerably;  in  places  it  swells  to  dimensions  of  as  much 
as  30  meters,  but  this  is  rather  due  to  folding  and  local  enlarge- 
ment. In  most  places  the  thickness  is  not  over  2  or  3  meters 
and  often  only  0.5  to  1  meter.  The  dip  is  uniformly  45°  to  the 
southeast. 

It  is  stated,  even  in  the  modern  descriptions  of  the  Rammels- 
berg, that  the  bedding  or  schistosity  conforms  exactly  with  the 
outline  of  the  ore  and  with  its  banded  structure.  While  true  in 
places,'  this  is  certainly  not  a  general  characteristic  of  the  deposit, 
which  in  part  is  absolutely  unconformable  to  the  stratification 
of  the  slate. 

The  ore  banding  everywhere  follows  with  great  faithfulness 
the  outlines  of  the  sulphide  mass,  whether  these  are  smooth  or 
irregular.  For  much  of  the  distance  above  the  third  level  the 
edge  of  the  new  ore-body  and  consequently  the  banding  of  the 
ore  also  are  indeed  parallel  to  the  lamination  of  the  enclosing 
rock.  On  the  third  level,  however,  the  ore  mass  flattens  out  and 
crosses  the  lamination  at  a  small  angle,  and  again  turns  down 
parallel  to  it  after  intersecting  the  laminae  for  a  very  considerable 
distance.  In  this  portion  of  the  ore-body  the  banding  of  the  ore 
follows  the  edge  of  the  sulphide  mass  and  therefore  makes  the 
same  angle  with  the  lamination  of  the  slates  as  the  outline  of 
the  ore  mass  itself. 

The  Ores. — The  principal  minerals  are  zinc  blende,  chalco- 
pyrite,  galena,  pyrite,  and  arsenopyrite,  which  are  abundant 


646  MINERAL  DEPOSITS 

approximately  in  the  order  enumerated.  The  gangue  is  almost 
entirely  barite,  but  it  rarely  occurs  in  large  quantities  and  often 
is  entirely  inconspicuous.  Masses  and  veinlets  of  calcite  are 
present  in  the  surrounding  slate,  but  rarely  contain  ore.  On  the 
whole  the  limits  of  the  "ore  bed"  are  sharply  denned  and  the 
ores  themselves  are  entirely  or  predominantly  composed  of  sul- 
phides. Alteration  of  the  enclosing  slates  is  rarely  observed. 
At  most  there  is  a  slight  impregnation  of  pyrite. 

The  texture  and  composition  of  the  ore  vary  with  the  locality, 
so  that  in  one  part  copper  will  predominate,  while  in  other  places 
the  ore  carries  mostly  zinc  blende  with  a  little  galena. 


FIG.  225. — Nodules  of  barite  and  calcite  (A)  in  banded  sulphide  ore  from 
Rammelsberg,  Germany.     After  Lindgren  and  Irving. 

The  intergrowths  of  these  minerals  are  fine  grained  and  inti- 
mate, which  adds  to  the  metallurgical  difficulties  of  treating  the 
ore.  The  most  common  texture  is  that  of  the  banded  ores 
consisting  of  dominant  zinc  blende  with  -narrow  and  gently 
curved  streaks  of  chalcopyrite  and  galena.  In  places  the  ore 
contains  rounded  nodules,  generally  of  pyrite,  around  which 
the  fine-grained  streaks  of  zinc  blende  and  ehalcopyrite  bend  in 
regular  curves.  Pyrite  shows  a  strong  resistance  to  such  defor- 
mation. Not  uncommonly  one  finds  rounded  nodules,  consisting 
of  zinc  blende  and  barite  in  granular  form,  coarser  than  that  of 
the  ordinary  ore.  Fig.  225  shows  how  the  streaks  of  other  sul- 
phides surround  and  envelop  these  nodules.  The  pyrite  nodules 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  647 

have  often  been  noted,  but  without  satisfactory  explanation. 
Some  observers  have  held  them  for  fossil  remains,  in  which  the 
pyrite  has  replaced  the  shell  of  the  organism.  A  goniatite  has 
actually  been  found  in  the  slates  which  form  the  easterly  continua- 
tion of  the  ore-body,  and,  according  to  K.  Andree,1  the  interior 
of  the  shell  contained  pyrite,  quartz,  barite,  and  calcite. 

Origin. — The  structural  relations  of  the  ore-body  indicate  that 
the  deposit  is  a  bedded  vein — that  is,  a  fissure  vein  lying  in  part, 
at  least,  conformable  to  the  surrounding  slates.  The  distribu- 
tion and  structure  of  the  ore  itself  are  inconsistent  with  the  theory 
of  sedimentary  deposition.  The  structure  is  unique  in  ore  de- 
posits, but  as  to  its  interpretation  there  can  be  no  reasonable 
doubt.  The  sulphides  do  not  occur  with  their  primary  texture. 
The  structure  is  that  of  a  dynamometamorphic  rock,  in  which  all 
the  constituents,  except  pyrite,  have  been  drawn  out  into  streaks 
which  are  intricately  mingled.  The  appearance  shown  in  the 
figure  could  be  easily  duplicated  from  any  area  of  fine-grained 
schist  resulting  by  pressure  from  an  original  granular  rock.  The 
different  constituents  have  recrystallized  and  flowed  under 
pressure. 

At  first  glance  it  seems  strange  that  the  pyrite  has  acted  so 
differently  from  the  other  constituents.  The  explanation  of  this 
behavior  is  found  in  some  interesting  experiments  recently 
undertaken  by  F.  D.  Adams,2  who  shows  that  rock  flowage  is 
a  function  of  hardness,  so  that  the  harder  minerals  are  the  less 
deformed.  He  finds  that  the  limit  of  easily  produced  flowage  lies 
in  the  vicinity  of  5  or  6  in  the  scale  of  hardness.  He  crushed  pyrite 
under  a  load  of  43,000  pounds  without  its  showing  any  trace  of 
deformation  by  recrystallization.  Minerals  of  lower  hardness 
presented  decided  evidence  of  flow.  The  pyrite  nodules  in 
the  Rammelsberg  ores  are  simply  residual  parts  of  the  original 
granular  deposit  which  have  been  less  deformed  than  the  other 
sulphides. 

The  Rammelsberg  deposit  is  then  probably  of  epigenetic  origin, 
but  the  structure  of  the  ore  has  been  profoundly  changed  by 
dynamometamorphism.  While  the  surrounding  slates  are  soft 
they  evidently  behaved  quite  differently  from  the  sulphide  mass. 

1  Zeilschr.  prakt.  Geol,  1908,  p.  166. 

2  F.  D.  Adams,  An  experimental  investigation  into  the  action  of  differen- 
tial pressure  on  certain  minerals  and  rocks,  Jour.  Geol.,  vol.   18,   1910, 
pp.  480-535. 


648  MINERAL  DEPOSITS 

The  association  of  minerals  with  barite  as  the  predominating 
constituent  of  the  gangue  tends  to  show  that  the  deposit  is  not 
of  the  deep-seated  type,  but  was  formed  at  a  medium  depth  be- 
low the  original  surface,  probably  within  a  few  thousand  feet  of  it. 

The  deposit  may  have  been  formed  by  ascending  solutions 
derived  from  the  neighboring  batholith  of  granite,  which  is  only 
3  kilometers  distant  from  the  mine. 

Similiar  phenomena  of  flowage  have  been  described  recently 
from  the  Slocan  district.1 

CADMIUM  ORES2 

Almost  the  only  cadmium  mineral  is  the  yellow  grenockite 
(CdS),  which  is  fairly  common  in  the  Joplin  region,  Missouri,  as 
a  yellow  coating  on  zinc  blende  and  disseminated  in  ozidized  zinc 
ores  coloring  them  yellow.  Cadmium,  probably  as  sulphide,  is 
almost  universally  present  in  zinc  blende.  In  the  Joplin  region 
the  metal  is  contained  to  the  extent  of  a  trace  up  to  1  per 
cent.,  averaging  0.3  per  cent.,  but  many  western  ores  also  contain 
this  metal  in  noteworthy  amounts.  It  is  found  in  hightempera- 
ture  deposits,  for  instance,  in  those  of  Bodenmais,  Bavaria,  and 
also,  as  just  stated,  in  zinc  ores  in  limestone  without  igneous 
affiliations.  The  zinc  deposits  of  Santander,  Spain  (p.  450), 
contain  0.4  per  cent.  Cd  or  more.  Traces  of  cadmium  have 
been  found  in  some  coals3  and  in  mine  waters.  The  allied  metals, 
indium,  thallium  and  gallium  generally,  accompany  cadmium  in 
small  amounts.  Germanium  occurs  in  the  tin  deposits  of  Bolivia 
(p.  672). 

Cadmium  is  produced  in  the  United  States  since  1907,  about 
100  tons  being  the  output  in  1917.  In  addition  25  tons  of  cad- 
mium sulphide  were  manufactured.  The  metal  being  more 
volatile  than  zinc  is  derived  from  the  dust  of  the  bag  houses  of 
lead  smelters  that  treat  zinc-bearing  lead  ores.  Much  more  could 
be  produced,  especially  from  the  electrolytic  zinc  plants  if  the 
demand  required  it.  The  German  output  of  40  to  50  tons  a  year 
is  a  by-product  in  the  distillation  of  zinc  in  Silesia,  the  ores  of 

1  W.  L.  Uglow,  Econ.  Geot.,  vol.  12,  1917,  pp.  643-662. 

2  E.  Jensch,  Das  Cadmium,  etc.,  Samml  chem.  u.  chem.  techn.  Vortrage, 
vol.  3.  1908,  pp.  201-232. 

.    C.  E.  Siebenthal,   Cadmium,  Mineral  Resources,  U.  S.  Geol.  Survey, 
pt.  1,  1917,  pp.  49-53. 

3  C.  Schnabel,  Handbook  of  Metallurgy,  London,  1907. 


DEPOSITS  FORMED  AT  INTERMEDIATE  DEPTHS  649 

that  district  containing  at  most  0.3  per  cent.  Cd.     The  price 
of  cadmium  ranges  from  $1  to  $1.50  per  pound. 

Cadmium  is  used  in  the  manufacture  of  silver  ware  and  for 
easily  fusible  alloys,  as  a  substitute  for  tin,  in  dental  amalgam,  etc. 
Cadmium  sulphide  is  a  brilliant  yellow  pigment. 

ARSENIC  DEPOSITS 

Arsenic  is  of  widespread  occurrence,  in  volcanic  exhalations, 
in  the  sea  water,  in  many  spring  waters  and  spring  deposits,  and 
in  some  products  of  organic  life.  Nevertheless  we  rarely  find 
pure  arsenical  ores  in  such  abundance  as  to  be  of  economic  impor- 
tance. About  the  only  plentiful  ore  mineral  of  arsenic  is  arseno- 
pyrite  (FeAsS),  which  is  found  in  many  veins  usually  associated 
with  quartz  and  gold.  More  rare  is  lollingite  (FeAs2),  smaltite 
(CoAs2)  and  niccolite  (NiAs)  and  other  cobalt  and  nickel  arse- 
nides; they  occur,  for  instance,  in  the  silver  veins  of  Cobalt, 
Ontario  (p.  627). 

The  minerals  of  arsenic  are  found  in  practically  all  classes  of 
sulphide  deposits  of  igneous  affiliations,  but  are  most  plentiful 
in  veins  of  the  intermediate  and  high-temperature  type. 

In  a  few  deposits  like  that  of  Hastings,  Ontario1  and  Brinton, 
Virginia,2  arsenic  us  oxide  (As203)  has  been  recovered  by  roasting 
of  arsenopyrite,  but  the  great  bulk  is  obtained  as  As20a  in  the 
flue  dust  of  smelters  using  mixed  ores  in  which  occur  the  above 
mentioned  arsenical  minerals  or  some  of  the  numerous  sulph- 
arsenides  as  tennantite  (Cu8As2S7),  pearcite  (Ag9AsS6),  enargite 
(CusAsS^  and  proustite  (AgsAsSs).  There  are  several  sulphar- 
senides  of  lead  but  they  are  rare. 

In  1917,  6,151  tons  of  arsenious  oxide  was  produced  from  flue 
dust.  The  price  was  from  10  to  16  cents  per  pound  and  the  use 
is  largely  for  killing  insect  pests  or  fungi,  etc. 

FLUORITE  DEPOSITS3 

Fluorite  (CaFl2)  is  the  only  simple  fluoride  occurring  in  nature 
It  is  a  persistent  mineral  (p.  67)  occurring  in  almost  all  deposits 
and  formed  at  all  temperatures.  It  rarely  occurs  in  great  abun- 

1  J.  W.  Wells,  Elevenlh  Rept.,  Ontario  Bur.  Mines,  1902,  pp.  101-122. 

2  F.  L.  Hess,  Bull.  470,  U.  S.  Geol.  Survey,  1911,  pp.  205-122. 

3  B.  F.  Burchard,  Fluorite,  Mineral  Resources,  U.  S.  Geol.  Survey,  pt.  2, 
annual  issues,  particularly  1916,  which  contains  a  good  description  of  the 
English  fluorite  deposits. 


650  MINERAL  DEPOSITS 

dance.  Most  of  the  fluorite  deposits  worked  are  veins  in  lime- 
stone with  igneous  affiliations  and  formed  at  shallow  or  inter- 
mediate depths;  among  the  associated  minerals  galena,  pyrite 
and  zinc  blende  are  most  common. 

Fluorite  is  chiefly  mined  in  the  United  States,  in  Illinois  and 
Kentucky1  from  wide  veins  in  Carboniferous  limestone,  shale  and 
sandstone.  The  deposits  contain  also  some  quartz,  barite,  pyrite 
and  galena.  They  are  believed  to  stand  in  genetic  connection 
with  dikes  of  peridotite. 

Fluorite  is  mined  from  veins  in  Colorado,  in  Boulder  and  Custer 
counties.  The  interesting  hot  spring  deposit  of  Wagon  Wheel 
Gap,  in  Mineral  County,  Colorado,  is  also  of  economic  importance. 

The  English  deposits  in  Derbyshire  and  Durham  are  likewise 
lead-bearing  veins. 

Fluorite  is  used  as  fluxing  material,  mainly  in  iron  furnaces 
and  for  various  industrial  uses.  Lenses  are  made  from  optically 
perfect  material  which  is  rare.  It  is  also  the  raw  material  for 
hydrofluoric  acid  and  other  chemicals.  In  1916,  155,735  short 
tons  of  fluorite  were  mined  in  the  United  States,  mainly  in  Illinois 
and  Kentucky,  and  brought  an  average  price  of  $5.34  per  ton. 
Regarding  cryolite  see  p.  774. 

SIDERITE  DEPOSITS 

Siderite  occurs  as  a  gangue  mineral  in  many  veins  (p.  595). 
When  it  predominates  and  the  veins  are  wide,  as  in  southern 
Westphalia,2  the  vein  filling  is  used  as  an  iron  ore.  No  such 
deposits  are  mined  in  the  United  States. 

Near  deposits  in  limestone  this  rock  is  often  replaced  by 
siderite  (p.  616).  Extensive  replacements,  probably  accom- 
panying intrusions,  may  result  in  deposits  of  siderite  of  eco- 
nomic value.  At  Eisenerz,3  in  Styria,  a  celebrated  deposit  of 
this  kind  is  worked.  Crushed  and  folded  Devonian  limestone 
form  the  material  for  the  replacement.  The  ore,  which  averages 
39  per  cent.  Fe,  is  worked  in  vast  open  cuts.  The  annual  out- 
put is  nearly  2,000,000  tons. 

1  E.  O.  Ulrich  and  W.  S.  T.  Smith,  Pro/,  Paper  36,  U.  S.  Geol.  Survey, 
1905. 

J.  F.  Fobs,  Bull.  9,  Kentucky  Geol.  Survey,  1907;  Econ.  Geol.,  vol.  5, 
1910,  p.  377. 

2  Beyschlag,   Krusch  and  Vogt,   Ore  deposits,   Translated  by  Truscott, 
vol.  2,  1916,  pp.  786-805. 

» Idem,  pp.  817-820. 


CHAPTER  XXVI 

VEINS   AND    REPLACEMENT   DEPOSITS    FORMED    AT 
HIGH  TEMPERATURE  AND  PRESSURE  AND  IN 
GENETIC  CONNECTION  WITH  INTRU- 
SIVE ROCKS 

GENERAL  FEATURES 

High-Temperature  Minerals. — In  the  ore  deposits  described  in 
previous  chapters  such  minerals  as  the  pyroxenes  and  amphi- 
boles,  the  garnets,  apatite,  ilmenite,  magnetite,  pyrrhotite,  tour- 
maline, topaz,  the  brown  and  green  micas,  the  spinels,  and  the 
soda-lime  feldspars  are  almost  entirely  absent.  In  the  veins 
and  replacement  deposits  formed  at  high  temperature  one  or 
more  of  these  minerals  are  commonly  present,  besides  many  other 
persistent  ore  and  gangue  minerals  which  are  formed  under  widely 
varying  conditions.  In  general,  simple  sulphides  and  arsen- 
ides prevail  and  are  in  many  deposits  associated  with  oxides, 
such  as  magnetite,  ilmenite,  and  cassiterite.  Some  of  the  min- 
erals enumerated  above  do  not  readily  crystallize  except  with  the 
aid  of  mineralizers  (p.  760)  for  instance,  certain  compounds  of 
boron,  chlorine,  or  fluorine,  which  effect  crystallization  without 
always  entering  into  the  final  compound. 

In  the  presence  of  such  mineralizers  crystallization  may  take 
place  at  a  much  lower  temperature  than  in  dry  fusion.  In  a 
magma  high  pressure  is  necessary  to  hold  these  substances  in  the 
fluid  melt,  which  then  is  really  a  magmatic  solution.  The  con- 
ceptions of  solvent  and  solute  are  inapplicable,  the  various  con- 
stituents of  the  magma  being  dissolved  in  one  another.  Under 
diminishing  pressure,  as  during  the  ascent  of  magmas  to  higher 
levels,  water  and  other  mineralizers  separate  from  the  magmatic 
solution  and  carry  with  them  certain  constituents  of  the  magma 
such  as  silica,  some  heavy  metals,  and  alkaline  metals.  This 
"magmatic  extract"  may  be  in  a  state  of  aqueo-igneous  fusion; 
or  when  the  temperature  is  lowered  the  crystallization  of  some 
constituents  may  convert  it  into  a  fluid.  In  general,  it  is  a  hot 

651 


652  MINERAL  DEPOSITS 

mixture  of  fluid  material  and  dissolved  gases;  many  substances 
are  doubtless  above  their  critical  points  and  would,  if  isolated,  be 
in  the  non-compressible  state  known  as  a  "perfect  gas."  This 
mixture  would  keep  fluid  far  below  the  ordinary  melting  points 
of  the  minerals  formed.  The  pegmatites,  with  their  wealth  of 
rare  minerals,  are  considered  as  the  product  of  consolidation  of 
such  aqueo-igneous  melts  and  they  also  contain  many  of  the 
minerals  of  the  list  given  above. 

Any  magma  always  contains  more  or  less  of  volatile  matter 
which  is  not  separated  from  it  until  the  act  of  crystallization  is 
in  progress.  Such  material  would  ascend  if  suitable  avenues 
of  escape  were  provided  and  would  probably  mix  with  the  earlier 
emanations  and  with  water  of  surface  origin. 

This  chemical  evidence  is  supported  by  field  evidence  of  the 
strongest  kind.  Practically  all  these  deposits  occur  in  or  near 
bodies  of  intrusive  rocks  and  have  been  exposed  by  deep  erosion. 
They  were,  therefore,  certainly  formed  at  considerable  depths 
below  the  surface.  For  some  of  them,  like  the  contact-meta- 
morphic  deposits,  cogent  proof  of  the  origin  of  the  metals  in  the 
adjacent  magmas  can  be  given.  On  the  other  hand,  the  high- 
temperature  veins  at  many  places  imperceptibly  grade  into  those 
in  which  the  magmatic  origin  is  far  less  clear,  thus  giving  in  such 
places  an  almost  complete  line  of  transition  from  the  rocks  con- 
gealed from  the  magma,  such  as  the  pegmatite  dikes,  to  the  metal- 
bearing  veins  of  the  ordinary  type. 

In  a  given  district  these  phenomena — the  pegmatitic  dikes, 
contact-metamorphic  deposits,  deep-seated  veins,  replacement 
deposits,  and  veins  of  the  common  type — all  developed  very  soon 
after  the  intrusive  activity  and  during  a  rather  short  and  sharply 
defined  epoch  of  metallization. 

Some  of  the  minerals  enumerated  on  the  previous  page  are 
dependent  upon  temperature  only;  such  are  the  pyroxenes,  spinel 
and  magnetite;  others  like  tourmaline,  topaz,  chondrodite,  the 
micas,  etc.,  contain  a  volatile  constituent  and  require  pressure 
and  the  presence  of  mineralizers  for  their  formation.  Some 
high-temperature  deposits  may,  therefore,  be  formed  compara- 
tively near  the  surface  and  even  in  lava  flows  like  certain  rare 
tin  deposits  in  rhyolite.  Similar  results  may  follow  in  case  of 
intrusions  near  the  surface  when  the  temperature  of  the  solutions 
are  raised  to  such  a  degree  that  the  vapor  tension  overcomes 
the  pressure  and  fumaroles  and  "soffioni"  result.  In  general, 


HIGH-TEMPERATURE  DEPOSITS  653 

however,  it  will  be  found  that  most  deposits  with  a  mineral  as- 
sociation indicating  high-temperature  have  been  formed  at  con- 
siderable or  great  depths. 

Fumarolic  action  might  result  if,  for  instance,  at  a  depth  of 
1,000  meters  and  consequent  hydrostatic  pressure  of  100  atmos- 
pheres there  existed  a  solution  temperature  of  330°  C.  The  vapor 
pressure  at  this  temperature  would  exceed  the  hydrostatic 
pressure.  It  is  probable,  however,  that  in  such  a  case  there 
could  be  no  delicate  banding  of  alternating  minerals  as  so  fre- 
quently characterizes  the  veins  of  shallow  depth  deposited  by 
ascending  hot  waters. 

The  term  pneumatolytic  has  often  been  used  to  indicate  de- 
position above  the  critical  temperature  but  it  is  also  employed 
for  any  mineral  formation  by  gases.  It  is  probably  best  to  dis- 
card a  term  of  such  an  indefinite  meaning. 

When  a  mineral  deposit  only  carries  persistent  minerals,  like 
quartz  and  pyrite,  there  is  no  mineralogical  criterion  present  to 
decide  whether  it  belongs  to  the  high  temperature  class.  In 
many  cases,  though  not  always,  mode  of  crystallization  and 
geological  criteria  may  solve  the  problem. 

Metasomatic  Processes. — The  minerals  enumerated  in  the  be- 
ginning of  this  chapter  generally  appear  in  the  metasomatically 
altered  country  rock  and,  to  a  less  extent,  in  the  fillings  of  the 
open  cavities.  The  metasomatic  action  is  often  intense  and  leads 
to  the  development  of  coarse-grained  aggregates.  Sericitization 
still  persists  in  some  of  these  deposits  though  the  foils  of  white 
mica  may  be  larger  and  usually  are  associated  with  biotite, 
tourmaline  and  similar  minerals.  The  total  changes  in  feld- 
spathic  rocks  are,  however,  often  less  pronounced  than  in  the  veins 
produced  at  lower  temperature. 

The  carbonate  rocks  are  always  peculiarly  susceptible  to  meta- 
somatic processes  and  usually  absorb  large  quantities  of  material 
from  adjacent  intrusives.  Silicates  rich  in  iron,  like  epidote, 
andradite  (garnet),  hedenbergite  (pyroxene),  cummingtonite 
(amphibole),  and  certain  varieties  of  biotite,  are  frequently 
found  in  these  deposits. 

Under  certain  rarely  occurring  conditions  andalusite  develops 
in  altered  igneous  rocks.  B.  S.  Butler1  describes  such  a  case  in 
the  Beaver  lake  mining  district,  Utah,  where  a  latite  has  been 
converted  to  quartz,  muscovite  and  andalusite. 

lProf.  Paper  80,  U.  S.  Geol.  Survey,  1913,  pp.  78-81. 


654  MINERAL  DEPOSITS 

Temperature  and  Pressure. — The  actual  temperature  during 
deposition  was  probably  rarely  above  575°  C.,  the  inversion 
point  of  crystallization  of  quartz.  It  has  been  shown  by  O. 
Miigge,  F.  E.  Wright,  and  E.  S.  Larsen1  that  the  pegmatite 
dikes  solidified  at  about  this  temperature.  Where  the  rarer 
minerals  have  formed,  the  temperature,  as  indicated  by  the 
crystallographic  behavior  of  the  accompanying  quartz,  was 
commonly  below  that  inversion  point.  The  quartz  veins  and 
other  deposits  that  by  their  mineral  content  show  a  relationship 
to  the  pegmatites  are,  as  a  rule,  later  than  these  and  present 
many  features  which  suggest  that  the  process  of  cooling  was 
further  advanced.  So,  in  a  rough  way,  the  temperature  of  deposi- 
tion of  this  class  must  have  been  lower  than  575°  C.,  but  in  all 
probability  higher  than  300°  C. 

In  the  formation  of  the  contact-metamorphic  deposits,  which 
were  developed  almost  immediately  after  the  actual  intrusion  of 
the  magma,  the  temperature  at  the  immediate  contact  may  have 
been  considerably  above  575°  C.,  attaining  in  acidic  magmas 
800°  or  900°  and  in  basic  magmas  1,200°  or  1,400°  C. 

It  has  been  assumed  that  the  heat  necessary  for  the  develop- 
ment of  these  deposits  is  derived  from  adjacent  bodies  of  igneous 
rocks.  The  possibility  cannot  be  denied  that  the  same  effect 
may  be  produced  simply  by  the  natural  increment  in  temperature 
due  to  increase  in  depth.  If  a  surface  temperature  of  +25°  C. 
and  an  increment  of  1°  C.  for  every  30  meters2  are  assumed,  a 
depth  of  10,200  meters,  or  about  33,600  feet,  will  be  required  for 
a  temperature  of  365°  C.,  the  critical  temperature  of  water.  Van 
Hise  has  shown  that  down  to  this  depth  even  the  hydrostatic 
pressure  is  sufficient  to  hold  the  water  in  the  form  of  a  liquid. 
Such  observations  as  have  been  made  in  the  Cordilleran  region 
show  that  contact-metamorphic  and  other  deposits  of  the  type 
here  called  deep-seated  have  been  formed  much  nearer  to  the 
surface,  some  of  them  at  depths  of  3,000  or  4,000  feet,  the  criterion 
being  a  rough  measurement  of  the  amount  of  erosion  on  the  basis 
of  known  thickness  of  strata.  It  may  be  true  for  some  problem- 
atical deposits  of  the  Archean  (for  instance,  the  zinc  deposit 
in  limestone  of  Franklin  Furnace,  in  New  Jersey)  that  the  rocks 
have  been  buried  to  a  depth  approximating  10,000  meters  and 

1  F.  E.  Wright  and  E.  S.  Larsen,  Quartz  as  a  geologic  thermometer.  Am. 
Jour.  Sri.,  4th  ser.,  vol.  27,  1909,  pp.  421-427. 

*  C.  R.  Van  Hise,  Mon.  47,  U.  S.  Geol.  Survey,  1904,  p.  567. 


HIGH-TEMPERATURE  DEPOSITS  655 

that,  at  that  depth,  they  have  been  exposed  to  the  metasomatic 
influence  of  magmatic  gases,  while  they  were  at  a  considerable 
distance  from  igneous  intrusions.  Such  deposits  would  be  con- 
necting links  between  igneous  and  regional  metamorphism,  and 
such  a  condition  would  explain  the  occasional  occurrence  of 
deposits  of  the  contact-metamorphic  type  at  a  distance  from 
known  igneous  bodies.  The  copper  deposits  of  Ducktown,  Ten- 
nessee, may  furnish  an  example  of  this  mode  of  formation,  for 
here,  although  the  ores  are  clearly  of  the  contact-metamorphic 
type,  there  are  no  adequate  igneous  masses  in  the  immediate 
vicinity  which  could  have  produced  the  metamorphism.  The 
Ducktown  district  is  one  of  intense  regional  metamorphism,  and 
it  is  possible  that  magmatic  gases  of  distant  origin  could  have 
searched  out  the  limestone  beds  and  transformed  the  calcareous 
rock  into  ore. 

As  to  the  pressures  actually  existing  our  knowledge  is  slight. 
The  hydrostatic  pressure  calculated  by  Van  Hise  would  have 
little  applicability,  for  at  a  relatively  short  distance  below  the 
surface  the  paths  of  underground  water  are  probably  effectively 
closed,  and  even  where  they  are  open  the  friction  would  be  a 
factor  of  no  mean  importance.  The  pressure,  therefore,  at  any 
considerable  depth  is  probably  far  higher  than  that  calculated 
from  the  weight  of  the  water  column.  At  a  depth  of  3,000  meters 
the  hydrostatic  pressure  would  be  300  atmospheres.  Under 
purely  static  conditions  the  greatest  pressure  at  any  given  point 
would  be  that  indicated  by  the  weight  of  the  overlying  rock  col- 
umn, or,  for  the  depth  just  mentioned,  equal  to  810  atmospheres. 
Arching  of  resistant  rocks  might  make  this  figure  smaller;  on 
the  other  hand,  if  the  conditions  are  those  of  lateral  stress  it 
is  possible  that  the  actual  pressure  might  be  considerably  higher 
and  would  then  be  measured  by  the  strength  of  the  buttress 
against  which  the  pressure  was  applied. 

If  magmas  and  their  differentiated  gases  invade  the  crust 
their  pressure  would  be  hydrostatic  and  could  not  exceed  that 
of  the  static  pressure  of  the  overlying  rock  column  without  rup- 
ture of  the  rock.  A  contact-metamorphic  deposit  developing  at 
a  depth  of  1,000  meters  under  a  covering  of  limestones  could, 
therefore,  have  been  formed  at  a  pressure  of  not  more  than  271 
atmospheres. 

Classes  of  Deposits. — The  veins  and  replacement  deposits 
formed  at  high  temperature  may  be  divided  as  follows: 


656  MINERAL  DEPOSITS 

A.  Veins;   replacement    deposits   not   adjacent   to    intrusive 
contacts : 

1.  Cassiterite,  wolframite,  and  molybdenite  veins. 

2.  Gold-bearing  veins  and  replacements. 

3.  Copper-tourmaline  deposits. 

4.  Lead-tourmaline  deposits. 

B.  Contact-metamorphic  deposits  (Chapter  XXVII). 

In  these  deposits  we  note  again  the  remarkable  connection 
of  certain  metals  with  certain  igneous  rocks.  The  tin,  tungsten, 
and  molybdenum  veins,  for  instance,  almost  always  appear  in  or 
near  intrusions  of  acidic  granites  and  porphyries. 

The  veins  and  replacement  deposits  carrying  gold,  copper,  and 
iron  are  mainly  connected  with  intrusive  rocks  of  monzonitic 
or  granodioritic  character.  In  general,  deposits  of  gold,  copper, 
iron,  tin,  tungsten  and  arsenic  are  much  more  common  than  those 
of  silver,  lead,  zinc  and  antimony. 

Mode  of  Fissuring  and  Filling. — The  question  whether  open 
spaces  exist  in  the  high-temperature  veins  has  been  discussed 
extensively.  Under  certain  conditions  at  least  it  would  seem 
improbable  that  open  spaces  could  have  existed  where  we  now 
find  cassiterite  veins  or  gold-bearing  quartz  veins,  for  instance. 
Many  investigators,  such  as  W.  O.  Crosby,  E.  J.  Dunn,  F.  B. 
Laney  and  Stephen  Taber  (p.  146)  hold  that  the  action  of 
crystallization  of  minerals  has  forced  the  walls  apart  and  thus 
provided  space  for  the  reception  of  ores.  But  aside  from  the 
problematic  intensity  of  this  force,  such  crystallization  could 
hardly  have  produced  perfect  crystals  or  drusy  structure.  L.  C. 
Graton,1  in  his  description  of  the  gold-quartz  veins  of  the  south- 
ern Appalachians,  has  suggested  that  the  vein-forming  solutions, 
representing  the  final  products  of  emanation  of  a  granitic  magma, 
were  injected  under  heavy  pressure  into  the  surrounding  rocks 
along  lines  of  weakness  and  so,  like  pegmatite  dikes, 'made  a 
space  for  themselves  by  opening  their  own  fissures.  The  crys- 
tallization would  be  effected  not  so  much  by  reduction  of  tempera- 
ture and  pressure,  but  rather  by  the  disturbance  of  a  nicely 
adjusted  equilibrium  of  solubility  and  concentration  by  accession 
of  substance  dissolved  from  the  wall  rocks.  This  reasoning, 
which  has  much  to  commend  it,  would  not  be  applicable  where 
earlier  fissures  had  established  connection  with  the  surface. 

1  Bull  293,  U,  S.  Geol.  Survey,  1906,  p.  59 


HIGH-TEMPERATURE  DEPOSITS  657 

The  texture  of  the  veins  is  generally  coarse  grained  and  irregu- 
lar. There  may  be  a  rude  banding  by  deposition  but  nothing 
to  equal  the  delicate  concentric  banding  of  the  veins  formed 
near  the  surface.  Compare,  for  instance,  Fig.  232  to  Figs.  160 
and  172.  The  structure  of  the  composite  veins  or  lodes  is  much 
like  that  of  the  deposits  formed  at  intermediate  depth.  Compare 
Figs.  40  and  230. 

THE  CASSITERITE  VEINS1 
Mineral  Association 

The  cassiterite  veins  form  a  rather  sharply  defined  group, 
connected  by  transitions  on  the  one  hand  with  the  copper- 
tourmaline  veins  and  on  the  other  hand  with  the  wolframite 
and  molybdenite  veins.  They  present  the  constant  association 
of  such  ore  minerals  as  cassiterite,  molybdenite,  arsenopyrite, 
wolframite  (also  scheelite),  bismuth,  and  bismuthinite,  with  less 
abundant  pyrite,  pyrrhotite,  chalcopyrite,  galena,  and  zinc  blende. 
Among  the  gangue  minerals  quartz  always  predominates  and  is 
accompanied  by  lithium  mica,  fluorite,  topaz,  tourmaline,  axinite, 
and  apatite;  more  rarely  beryl.  Specularite,  magnetite,  and 
ilmenite  are  sometimes  present.  Of  the  primary  carbonates, 
siderite  is  the  only  one  which  is  reported  from  the  cassiterite 
veins.  On  the  other  hand,  the  pyroxenes  and  amphiboles,  as  well 
as  magnesium  micas  and  garnets,  are  absent.  Orthoclase  is 
reported  from  several  localities  but  does  not  assume  the  form  of 
adularia.  Chlorite  is  occasionally  present.  Kaolin  and  allied 
hydrous  aluminum  silicates  are  often  recorded,  but  are  probably 
products  of  secondary  alteration  near  the  surface,  as  are  various 
hydrous  arsenates  and  phosphates. 

Cassiterite,  the  oxide  of  tin,  is  the  principal  ore  mineral.  The 
only  other  mineral  containing  tin  which  is  of  economic  impor- 
tance is  stannite  (Cu2FeSnS4),  which  is  seldom  found  in  pegma- 
tites and  in  the  cassiterite  veins  proper,  but  is  an  important  ore  in 
certain  Bolivian  veins. 

Small  quantities  of  tin,  probably  as  cassiterite,  are  sometimes 

1  H.  G.  Ferguson  and  A.  M.  Bateman,  Geologic  features  of  tin  deposits, 
Econ.  Geol,  vol.  7,  1912,  pp.  209-262. 

S.  Fawns,  Tin  deposits  of  the  world,  London,  1907. 
J.  T.  Singewald,  Jr.,  Some  genetic  relations  of  tin  deposits,  Econ.  Geol., 
vol.  7,  1912,  pp.  263-279. 


658  MINERAL  DEPOSITS 

contained  in  pyrite  or  zinc  blende  of  other  classes  of  veins — for 
instance,  in  those  of  Freiberg. 

Cassiterite  is  extremely  resistant  to  weathering,  as  shown,  for 
example,  by  its  occurrence  in  placers.  It  is  held  by  some  authors 
that  the  so-called  fibrous  tin  ore  or  "wood  tin"  which  is  often 
found  in  placers  is  a  product  of  alteration  of  cassiterite,  but  the 
question  does  not  seem  to  be  definitely  settled.  If  secondary,  it 
is  probably  derived  from  stannite. 

The  paragenesis  typical  of  many  cassiterite  veins  is  indicated 
in  the  following  table. 

SUCCESSION  OF  MINERALS  IN  THE  CASSITERITE  VEINS  OF  SAXONY 
(After  R.  Beck) 

Older  Younger 


Molybdenite.. 
Lithium  mica. 

Quartz... 

Topaz. 

Wolframite... 

Cassiterite 

Arsenopyrite . 

Fluorite 

Apatite 

Siderite 

Gilbertite 

Chlorite... 


The  tin-bearing  veins  appear  in  or  near  granites  (though  not 
all  granites  contain  them),  or  in  their  acidic  porphyries.  Only 
exceptionally,  as  in  Mexico,  are  they  connected  with  rhyolitic 
rocks.  The  tenor  of  the  ores  is  usually  low,  in  some  ores  as  low 
as  one-half  of  1  per  cent,  of  tin. 

Some  cassiterite  veins  contain  bismuth  and  tungsten  minerals 
in  commercial  quantities,  and  considerable  copper  is  often 
present.  A  little  silver  and  a  trace  of  gold  are  found  even  in  the 
vein  of  Cornwall,  while  in  the  Bolivian  veins  silver  minerals  occur 
in  important  amounts. 

In  1916  the  world's  output  of  tin  was  136,000  short  tons  most 
of  which  came  from  the  Malay  Peninsula  and  adjacent  regions. 
Bolivia  yielded  23,500  tons.  Only  140  tons  were  produced  from 
domestic  ores  in  the  United  States;  this  came  mainly  from  Alaska. 


HIGH-TEMPERATURE  DEPOSITS  659 

Metasomatic  Processes1 

General  Features. — -The  tin  ores  generally  appear  in  distinct 
fissure  veins  or  composite  lodes;  in  part  they  fill  open  cavities 
and  in  such  ores  a  banded  structure  may  be  emphasized  by  the 
deposition  of  lithium  mica  in  coarse  foils  along  the  walls.  Often, 
however,  the  fissures  are  merely  narrow  breaks  and  the  ore  is 
chiefly  disseminated  in  the  adjoining  altered  country  rock.  In 
ores  of  this  kind  also  a  rude  banding  may  result  from  the  accumu- 
lation of  tourmaline  or  cassiterite  along  certain  lines  parallel  to 
the  fissure. 

The    metasomatic    alteration    is    strong    and    characteristic, 


FIG.  226. — Thin  section  of  greisen  from  Banka,  derived  from  granite. 
g,  Lithium  mica;  q,  quartz;  z,  cassiterite;  t,  topaz;  stippled  spots  in  mica 
consist  of  zircon  and  rutile,  surrounded  by  pleochroic  rings.  Magnified  45 
diameters.  After  R.  Beck. 

resulting  in  coarse-grained  rocks  which  contain  muscovite,  quartz, 
and  topaz  or  tourmaline  and  to  which  the  name  greisen  is  usually 
applied  (Fig.  226).  Where  tourmaline  takes  the  place  of  topaz 
we  may  speak  of  tourmaline  greisen  or  luxullianite  (Cornwall). 
Granite,  granite  porphyry,  clay  slate,  calcareous  shale,  limestone, 
and  diabase  are  affected  by  this  mode  of  alteration  where  they 
form  the  country  rock  of  the  veins,  but  the  development  differs 

1  Besides  the  special  papers,  see  F.  Zirkel,  Lehrbuch  der  Petrographie, 
vol.  2,  1894,  pp.  118-127. 


660  MINERAL  DEPOSITS 

in  the  different  rocks.  Complete  silicification  of  the  wall  rocks 
is  a  phase  of  subordinate  importance. 

While  the  total  changes  in  composition  of  the  original  rock 
may  be  much  less  pronounced  than  in  other  veins,  the  metaso- 
matic  process  is  evidently  more  intense,  pointing  to  a  greater 
degree  of  heat  and  especially  energetic  action  of  the  mineralizers. 

In  granites  and  porphyries  adjoining  the  more  common  or 
Saxon  type  of  veins  the  feldspars  and  the  brown  mica  are  re- 
placed by  quartz,  topaz,  and  muscovite,  in  large  crystals  or  foils. 
Chlorite  is  sometimes  present.  Topaz  may  also  replace  quartz 
grains.  Sometimes  crystals  or  radial  aggregates  form.  Cassiter- 
ite,  wolframite,  and  more  rarely  sulphides  appear  as  accessories 
in  the  greisen,  which  spreads  out  from  the  fissure  plane  for  a  few 
inches  or  a  few  feet. 

The  quartz  porphyry  dikes  of  Mount  Bischoff,  in  Tasmania, 
contain  cassiterite,  with  much  topaz  and  subordinate  tourmaline. 
The  groundmass  is  changed  to  a  topaz-quartz  aggregate,  while 
the  feldspars  are  transformed  to  cassiterite,  pyrite,  pyrrhotite, 
arsenopyrite,  and  fluorite.  The  quartz  phenocrysts  remain  in- 
tact. In  the  final  product  quartz  and  minute  crystals  of  zircon 
are  the  only  constituents  which  have  withstood  the  altering  proc- 
esses. Siderite  appears  in  places  as  a  metasomatic  product. 

Metasomatic  Processes  in  the  Deposits  of  Cornwall. — The 
Cornwall  granites,1  which  consist  mainly  of  quartz,  acidic  feld- 
spars muscovite,  and  biotite,  also  carry  some  tourmaline,  topaz, 
and  fluorite,  which  Flett  considers  of  magmatic  origin.  The 
greisen  along  the  Cornwall  veins  consists  chiefly  of  granular 
quartz  and  muscovite,  often  with  aggregates  of  topaz  spreading 
through  the  partly  altered  feldspars.  Fluorite  is  occasionally 
present.  The  albite  is  more  resistant  than  orthoclase  or  perthite. 
Some  of  the  secondary  quartz  is  filled  with  liquid  inclusions  con- 
taining small  cubical  crystals  and  mobile  bubbles.  Excellent 
pseudomorphs  of  cassiterite  after  orthoclase  have  been  found  at 
Cornwall;  pseudomorphs  of  tourmaline  after  feldspar  and  of  topaz 
and  cassiterite  after  quartz  are  also  described  by  Flett. 

The  elvans,  or  quartz  porphyries,  are  also  altered  to  quartz, 
tourmaline,  topaz,  and  fluorite.  Kaolin  where  present  appears 
to  be  due  to  a  later  process. 

The  greisen  is  essentially  a  vein  formation  in  Cornwall  and  is 
not  known  to  occur  in  broad  masses  or  in  patches  either  within 

1  J.  S.  Flett,  Explan.  Sheet  374,  Geol.  Survey  England 


HIGH -TEMPERATURE  DEPOSITS  661 

the  granite  or  along  the  contact.  The  occurrence  along  the 
contact  is  typical  of  the  tourmaline  rock,  which,  however,  in 
places  also  appears  along  the  veins.  Tourmalinization  is  fre- 
quently superimposed  upon  the  normal  contact-metamorphic 
rocks,  of  which  hornfels  is  the  most  common.  There  is  not  much 
evidence  to  show  the  relative  age  of  the  two  processes,  but  the 
greisen  was  probably  developed  later  than  the  tourmaline  rocks." 
Both  are  held  to  have  been  formed  before  the  interior  of  the 
granite  was  fully  crystallized. 

Considerable  portions  of  the  granites  of  Cornwall  have  been 
altered  by  kaolinization,  but  Flett  believes,  with  good  reason, 
that  this  process  took  place  at  a  lower  temperature  than  the 
development  of  topaz,  tourmaline,  and  white  mica.  Kaolinized 
rocks  appear  mainly  in  the  central  parts  of  the  granite  masses, 
while  the  tourmaline  rock,  usually  called  "schorl,"  and  greisen 
lie  mostly  along  the  peripheral  parts.  The  kaolinized  portions 
often  form  pipes  having  rudely  circular  outlines;  the  granite  is 
altered  to  kaolin,  muscovite,  and  quartz,  and  the  product  rarely 
contains  cassiterite.  The  composition  differs  little  from  that  of 
the  granites.  It  seems  probable  that  these  kaolin  deposits  are 
due  to  the  oxidation  of  distinctly  later  pyritic  impregnations, 
by  the  action  of  the  liberated  sulphuric  acid  on  the  feldspar  and 
sericite.  It  should  also  be  recalled  that  topaz  is  rather  easily 
changed  to  products  allied  to  kaolin. 

Development  of  Greisen. — In  the  following  table  three  sets  of 
analyses  are  given  showing  the  development  of  greisen  in  Saxony, 
in  New  South  Wales,  and  in  Cornwall.  On  the  whole,  the  proc- 
esses are  identical.  If,  as  seems  probable,  there  has  been  but 
little  change  in  the  total  mass  of  the  alumina,  the  analyses  are 
roughly  comparable  in  their  present  form.  Silica  has  also 
remained  fairly  constant.  The  additions  consist  of  iron,  lithium, 
tin,  fluorine,  and  boron,  the  iron  evidently  entering  in  a  silicate. 
Calcium,  of  which  but  little  is  present,  is  strongly  leached  in  the 
rocks  represented  by  analyses  IV  and  VI;  the  evidence  as  to 
magnesia  is  less  conclusive.  Sodium  and  potassium  have  both 
been  abstracted,  the  former  more  than  the  latter,  but  neither  is 
wholly  removed.  In  this  feature  the  metasomatic  alteration  of 
tin  veins  differs  from  that  along  veins  formed  at  lower  tempera- 
ture, in  which  we  often  find  a  strong  concentration  of  potassium 
and  an  almost  complete  removal  of  sodium.  The  specific  gravi- 
ties of  the  rocks  are  not  determined. 


662  MINERAL  DEPOSITS 

ANALYSES  OF  GRANITES   AND   GUEISENS  DEVELOPED  FROM  THEM 


I 

11 

III 

IV 

V 

VI 

SiO2 

74  68 

70  41 

70  17 

69  42 

76  69 

78  47 

TiO2 

0  71 

0  49 

0.41 

Trace 

0  22 

SnO2 

0  09 

<*0.49 

0.08 

A1203  
Fe2O3  

12.73 

14.86 
1.42 

15.07 
0.88 

15.65 
1.25 

10.89 
0.76 

11.50 
2.64 

FeO  
CuO  
MnO...,  
CaO  
MgO  
K20  
Na2O  
Li,O  . 

3.00 
0.50 

0.09 
0.35 
4.64 
61.54 

5.09 

0.29 
0.21 
0.09 
3.01 
0.98 

1.79 

0.12 
1.13 
1.11 
5.73 
2.69 
0.11 

3.30 

0.39 
0.63 
1.02 
4.06 
0.27 
0  81 

0.39 

1.73 
0.18 
2.97 
5.35 

1.05 

Trace 
0.49 
1.17 
1.99 

H20+  \ 
H20-/ 
P,O. 

1.17 

/0.70 
10.18 
0  34 

0.54 
0.06 
0  40 

0.13 
0.37 

0.23 
1.17 

MoS, 

0  80 

Cl  . 

0.06 

Trace 

F 

3  10 

0.15 

3  36 

Present 

S  

0.04 

B203.. 

Strong 

0.59 

trace 

Less  O  for  F 
and  Cl 

100.68 
0.07 

101.75 
1  41 

Total  

99.50 

100.44 

100.61 

100.34 

99.46 

99.81 

a  As  cassiterite  0.43;  in  mica,  chemically  combined,  0.06. 
b  Including  lithia. 

.  I.  Fresh  granite,  Altenberg,  Saxony.     K.  Dalmer,  Explanations  to  the 
section  Altenberg-Zinnwald.     Geol.  map  Saxony. 

II.  Greisen,  Altenberg,  Saxony.     K.  Dalmer,  idem. 

III.  Fresh  Lamorna  granite,  Lands  End,  Cornwall.     W.  Pollard,  analyst. 

IV.  Greisen,  with  tourmaline  and  topaz.     St.  Michaels  Mount,  Lands 
End,  Cornwall.     W.  Pollard,  analyst.     Explan.  Sheets  351  and  358,  Geol. 
Survey  England. 

V.  Fresh  "acidic  granite,"   New  England,   New  South  Wales.     L.  A. 
Cotton,  analyst.     Proc.,  Linnean  Soc.  N.  S.  W.,  34,  pt.  2,  1909,  pp.  220-238. 

VI.  Greisen,  cassiterite  vein  near  Inverell,  same  locality.     L.  A.  Cotton, 
analyst.    Idem. 


HIGH-TEMPERATURE  DEPOSITS 


663 


The  composition  of  a  normal  greisen  from  the  Erzgebirge,  in 
Saxony,  is,  according  to  Dalmer,  as  follows:  Quartz,  50.28; 
topaz,  12.14;  lithium  mica,  36.80;  and  cassiterite,  0.43  per  cent. 

Alteration  of  Sedimentary  Rocks. — The  alteration  of  sedimen- 
tary rocks  proceeds  somewhat  differently.  In  Cornwall  the  ar- 
gillaceous slates  are  tourmalinized,  forming  "corriubianite"  (Fig. 
227),  the  biotite  and  muscovite  being  replaced  by  that  mineral. 
Andalusite  and  cordierite  also  disappear,  but  rutile,  ilmenite,  and 
magnetite  remain.  The  result  is  an  aggregate  of  quartz  and 
tourmaline,  which  well  preserves  the  original  structure  of  the 
rocks.  In  place  tourmaline-albite  rocks  are  formed. 


FIG.  227. — Vein  of  quartz,  cassiterite,  and  tourmaline  traversing  Paleozoic 
slates,  in  which  the  argillaceous  bands  are  replaced  by  tourmaline;  the 
siliceous  bands  in  the  slate  are  not  altered.  Belowda  Beacon,  Cornwall. 
Natural  size.  After  D.  A.  MacAlister. 

The  calcareous  rocks,  as  well  as  the  greenstones,  yield  mainly 
axinite  in  large  brown  crystals,  also  pyroxene,  actinolite,  epidote, 
garnet,  zinc  blende,  pyrite,  apatite,  specularite,  titanite,  and 
tourmaline,  but  no  topaz.  At  Mount  Bischoff,  Tasmania,  the 
probably  non-calcareous  schists  and  slates  are  in  part  changed 
to  tourmaline  fels,  containing  also  cassiterite,  pyrite,  arseno- 
pyrite,  pyrrhotite,  fluorite,  calcite,  siderite,  and  pyrophyllite. 

H.  Tronquoy1  describes  tin-bearing  veins  from  Villeder,  Morbi- 
han,  France,  which  are  accompanied  by  albitization  of  granite, 
without  tourmaline  or  topaz  while  tourmaline  develops  in  ad- 
jacent clay  slate.  .  ...• 

1  Comptu  Rendu,  154,  1912,  p.  899. 


664  MINERAL  DEPOSITS 

The  metasomatic  development  in  the  sedimentary  rocks  is  of 
great  interest,  for  it  connects  in  the  closest  manner  the  effects 
of  the  ore-bearing  solutions  with  those  in  contact-metamorphic 
deposits.  Regarding  the  cassiterite  deposits  of  Alaska  and  Pit- 
karanda  and  their  close  association  with  contact-metamorphism, 
see  p.  741. 

Origin  of  Tin-Bearing  Veins 

The  tin-bearing  veins  occupy  a  most  important  position  as 
connecting  links  between  the  deposits  of  igneous  and  aqueous 
origin. 

The  occurrence  of  cassiterite  as  a  primary  constituent  of 
granite  is  recalled,  as  well  as  its  occasional  appearance  in  the 
pegmatitic  druses  of  granite — for  instance,  those  in  Elba  de- 
scribed by  G.  vom  Rath. l  We  may  further  recall  the  appearance 
of  cassiterite  in  economic  quantities  in  pegmatite  dikes  (p.  768) 
and  its  occasional  occurrence  in  base-metal  veins — for  instance, 
in  those  of  Freiberg  and  in  the  lower  levels  of  the  Przibram  veins. 
The  occurrence  of  tourmaline  in  cassiterite  veins  is  also  important 
in  view  of  the  transitions  to  the  chalcopyrite-tourmaline  type. 

These  facts  were  realized  at  an  early  date  by  A.  Daubree  and 
E.  de  Beaumont.2  To  J.  H.  L.  Vogt3  belongs  much  credit  for 
his  resuscitation  of  these  meritorious  ideas  and  the  addition  of 
important  investigations.  The  extensive  literature  regarding 
the  occurrence  of  tin  deposits  has  been  summarized  by  F.  L.  Hess 
and  L.  C.  Graton.4 

Daubree  and  de  Beaumont  argued  from  the  close  association  of 
cassiterite  veins  and  acidic  granites  that  there  must  be  some 
genetic  connection  between  them  and  concluded  that  the  veins 
were  deposited  by  emanations  from  these  magmas.  This  con- 
clusion has  been  adopted  and  confirmed  by  almost  all  geologists 
who  have  studied  these  deposits. 

1  G.  vom  Rath,  Zeitschr.  Deutsch.  geol.  Gesell.,  1870,  p.  646. 

2  A.  Daubree,  Sur  le  gisement,  etc.,  des  amas  de  mineral  detain,  Annales 
des  Mines,  3d  ser.,  vol.   20,  1841,  and  other  later  papers. 

E.  de  Beaumont,  Notes  sur  les  emanations  volcaniques  et  metalliferes, 
Bull.  Soc.  Min.  France,  2d  ser.,  vol.  4,  1847,  p.  65. 

3  J.  H.  L.  Vogt,  Zinnstein-Ganggruppe,  Zeitschr.  prakt.  Geol.,  1895,  pp. 
145-156,  and  other  places. 

4  F.  L.  Hess  and  L.  C.  Graton,  The  occurrence  and  distribution  of  tin, 
Bull.  260,  U.  S.  Geol.  Survey,  1905. 

rF.  L.  Hess  and  Eva  Hess,  Bibliography  of  the  geology  and  mineralogy 
of  tin,  Smithsonian  Misc.  Coll.,  vol.  58,  No.  2,  1912. 


HIGH-TEMPERATURE  DEPOSITS 


665 


The  general  occurrence  of  cassiterite  in  well-defined  veins  shows 
clearly  that  the  granite  was  consolidated  when  these  fissures  were 
broken,  even  if  it  is  probable  that  the  whole  granitic  intrusion 
had  not  congealed  and  that  liquid  magma  still  existed  below  the 
vein.  The  veins  occur  usually  either  in  the  center  or  along 
the  contacts  of  the  granitic  masses,  but  some  of  them  extend  into 
the  adjoining  sedimentary  rocks.  Finally,  it  is  clear  that  the 
development  of  these  veins  cannot  be  identified  with  the  contact- 
metamorphism,  for  they  are  distinctly  later  and  their  metaso- 
matic  effects  are  superimposed  upon  the  products  of  contact - 


FIG.  228. — Ideal  section  of  granite  intrusion  showing  probable  develop- 
ment of  tin-bearing  veins  and  the  effect  of  successive  erosion  levels,  a,  b, 
and  c;  x,  metamorphic  aureole;  y,  inner  limit  of  mineralization.  After 
Ferguson  and  Bateman. 

metamorphism.  Granitic  magmas  apparently  do  not  easily 
part  with  their  volatile  constituents,  which  are  mainly  expelled 
during  consolidation. 

The  general  distribution  of  tin-bearing  veins  in  relation  to  an 
intrusive  mass  of  granite  gradually  eroded  is  shown  in  the  dia- 
gram, Fig.  228. 

The  constant  presence  of  boron  and  fluorine  compounds,  as 
well  as  those  of  phosphorus,  in  the  tin  veins  is,  of  course,  of  the 
highest  importance.  During  the  short  but  intense  epoch  of 
metallization  the  temperature  must  have  been  high,  probably 
above  the  critical  temperature  of  water,  and  therefore  the 
deposition  took  place  under  "pneumatolytic"  conditions.  In  the 
absence  of  exact  data  as  to  the  temperature,  pressure  and  con- 
stitution of  the  vein-forming  agents,  the  insistence  upon  pneu- 
matolysis  has,  however,  little  value.  Just  how  much  water  was 
present  is  problematical;  certainly  there  was  some,  as  shown  by 


666  MINERAL  DEPOSITS 

the  hydroxyl  radicle  in  muscovite  and  topaz,  and  by  the  presence 
of  aqueous  inclusions  in  quartz  crystals. 

DaubreVs  reaction,  inferred  as  probable  from  experiments 
with  the  chloride  is  as  follows: 

SnFl4+2H20  =  SnO4+4HFl. 

The  metasomatic  development  of  topaz  has  been  taken  to 
indicate  that  free  hydrofluoric  acid  was  present  in  the  solution, 
but  this  must  be  regarded  as  very  improbable.  We  may  better 
frankly  state  that  at  present  we  are  in  the  dark  as  to  the  exact 
formulas  of  the  reaction. 

Both  in  Saxony1  and  in  Cornwall  lead-silver  veins  occur  at 
some  distance  from  the  center  of  intrusions  and  the  cassiterite 
veins,  and  also  transitions  between  them.  This  would  tend  to 
show  that  these  metals  were  less  easily  precipitated  and  were 
carried  farther  away  from  their  sources  than  were  the  tin,  tung- 
sten, etc. 

A  similar  relation  exists  in  the  Cordilleran  region  between  the 
copper-bearing  contact-metamorphic  deposits  and  the  lead-silver 
replacement  deposits  in  limestone. 

The  Cassiterite  Veins  of  Cornwall,  England 

Literature. — An  extensive  literature  exists  on  the  subject  of 
the  tin  lodes  of  Cornwall,  for  they  have  been  repeatedly  studied 
by  geologists  since  they  were  first  described  in  1839  by  H.  T. 
de  la  Beche,  and  in  1843  by  W.  J.  Henwood.  Many  articles  by 
J.  H.  Collins  and  C.  Le  Neve  Foster  were  published  about  30 
to  35  years  ago.  The  deposits  have  been  studied  recently  by  the 
Geological  Survey  of  England,  and  the  results  are  published 
in  a  series  of  memoirs.2 

The  renowned  mining  region  of  Cornwall,  known  to  the  an- 
cients for  the  treasures  of  tin  which  it  contained,  furnishes  one  of 

1  K.  Dalmer,  Zeitschr.  prakt.  GeoL,  1894,  p.  321 . 

2  D.  A.  MacAlister,  Geological  aspects  of  the  lodes  of  Cornwall,  Econ. 
Geol,  vol.  3,  1908,  pp.  363-380. 

J.  B.  Hill,  D.  A.  MacAlister,  and  J.  S.  Flett,  Geology  of  Falmouth  and 
Truro,  Mem.  Geol.  Survey  England,  Explan.  Sheet  352,  1906. 

Ussher,  Flett,  et  al.,  The  geology  of  Bodmin  and  St.  Austell,  Idem, 
Explan.  Sheet  347,  1909. 

C.  Reid  and  J.  S.  Flett,  The  geology  of  the  Lands  End  district,  Idem, 
Explian.  Sheets  351  and  358,  1907. 

C.  Reid  and  J.  S.  Flett,  The  geology  of  Camborne  and  Redruth,  Idem. 


HIGH-TEMPERATURE  DEPOSITS 


667 


the  most  remarkable  instances  of  the  dependence  of  ore  deposits 
on  the  distribution  of  igneous  rocks.  The  folded  Paleozoic 
slates  and  sandstones  are  intruded  by  four  main  granitic  batho- 
liths  of  moderate  dimensions  (Fig.  229),  and  the  tin  deposits 
cluster  characteristically  in  the  marginal  zones  of  these  granitic 
intrusions,  both  in  slates  and  in  granites.  The  slates,  or  killas, 
as  they  are  locally  called,  in  part  overlie  the  granite,  forming  in 
places  the  roof  of  the  batholith.  The  intrusion  is  post-Carbon- 
iferous and  pre-Triassic  in  age,  and  the  veins  were  formed  shortly 
after  the  intrusive  activity,  probably  while  the  rock  still  remained 
hot.  Before  the  vein-forming  epoch  a  series  of  dikes  of  granite 


Copper  and  Tin  Deposits 
Granite 

3^  Contact  Metamorphic  Rocks 
Lower  Carboniferous 
Devonian 
Ordovician 
Cambrian 

Metamorphic(Dodman)8eries 
Greenstone 
Serpentine 


FIG.  229. — Geological  map  of  the  peninsula  of  Cornwall,  England. 
After  D.  A.  MacAlister. 

porphyry  (elvans)  intersected  granite  and  slate.  The  granite 
is  a  typical  rock  of  its  kind,  containing,  in  order  of  crystallization, 
zircon,  apatite,  magnetite,  biotite,  muscovite,  oligoclase,  albite, 
perthite,  and  quartz.  Tourmaline,  topaz,  and  fluorite  are  occa- 
sional accessories  in  the  crystallization  of  the  magma. 

The  tin  and  copper  veins  are  the  older  and  were  followed  by  a 
later  though  much  less  important  series  of  veins,  containing  lead, 
silver,  and  sometimes  also  cobalt,  nickel  and  uranium.  A  little 
gold  is  present,  both  in  the  lead-silver  veins  and  in  the  cassiterite 
veins.  Many  of  the  veins  are  of  complex  structure  and  may 
properly  be  called  lodes.  Some  of  them  are  traceable  for  2  or  3 
miles,  or  even  more.  The  width  of  infilled  fissures  and  altered 


668 


MINERAL  DEPOSITS 


rocks  averages  3.5  feet,  but  in  places,  especially  in  the  slates, 
reaches  50  feet.  The  general  direction  of  the  veins  is  northeast. 
Stockworks  of  irregular  veinlets  also  occur.  Faulting  has  taken 
place  along  many  of  the  fissures.  The  veins  are  simple  or  com- 
posite (Fig.  230) ;  reopening  of  fissures  was  evidently  a  common 
phenomenon.  The  filling  is  mainly  of  quartz,  frequently  with 
comb  structure  and,  in  the  upper  parts  of  the  lodes,  also  with 
drusy  cavities.  Chlorite,  fluorite,  scheelite,  zinc  blende,  molyb- 
denite and  bismuthinite  are  also  found  in  these  veins.  Mag- 
netite and  specularite  are  reported,  but  are  rare. 

The  alteration  spreading  into  the  country  rock  from  the  fissures 
is  characteristic  and,  when  effected  from  a  series  of  closely  spaced 
fissures,  may  produce  a  banded  appearance  (capel);  tourmalini- 


B     A      D  A 


B  D 


D 


Scale  of  Feet 

&  A  a  a  j 

FIG.  230. — Diagrammatic  section  of  the  main  lode  at  the  300-foot  level, 
Bunny  mine,  St.  Austell,  Cornwall.  A,  Kaolinized  granite;  B,  stanniferous 
greisen;  C,  silicified  granite;  D,  veins  of  quartz  and  cassiterite.  After 
Ussher,  Fktt,  et  al. 

zation  is  the  usual  mode  of  alteration  in  the  granite,  resulting  in 
an  aggregate  of  tourmaline  and  quartz.  In  calcareous  rocks  or 
greenstones  the  altered  zone  along  the  veins  contains  axinite, 
pyroxene,  garnet,  and  some  tourmaline. 

In  the  slates  the  lodes  often  contain  much  crushed  and  brecci- 
ated  material;  sometimes  cassiterite  and  tourmaline  following 
bedding  planes  impregnate  the  slates  for  some  distance  on  both 
sides  of  the  lode.  On  the  whole  the  copper  ores  are  confined  to 
the  lodes  in  the  slates.  Where  the  lodes  break  into  the  granite 
the  tin  ore  makes  its  appearance  (Fig.  231). 

The  main  lode  of  the  Dolcoath  and  Cam  Brea  contained  copper 
ores  down  to  a  depth  of  1,000  feet,  mainly  in  the  slates.  Below 
this  depth  the  tin  ore  predominated  and  is  now  worked  at  a  depth 


HIGH-TEMPERATURE  DEPOSITS  669 

of  3,000  feet.  According  to  Hill  and  MacAlister  the  lode  at  the 
bottom  of  the  mine  is  42  feet  wide  and  contains  1  to  3  per 
cent,  of  cassiterite.  The  lodes  of  Wheal  Vor  were  of  enormous 
value  in  the  killas,  but  proved  worthless  in  the  granite.  At  the 
Great  Work  mine,  not  far  distant,  these  relations  were  reversed. 
The  great  ore  shoots  of  both  mines  plunge  eastward  approxi- 
mately parallel  to  the  subterranean  contact  surface  of  the  granite. 


FIG.  231.— Section  from  Feltrick  to  New  North  Pool,  Cornwall,  showing 
relation  of  granite  and  slate  and  the  lodes  intersecting  them.  After  Hill 
and  MacAlister. 

Cassiterite  Veins  of  Saxony1 

Saxony  and  the  adjacent  part  of  Bohemia  contain  several  tin- 
producing  districts,  the  ores  occurring  in  or  near  granites  of  post- 
Carboniferous  age.  The  deposits  are  now  of  little  importance, 
but  have  an  interesting  place  in  the  history  of  the  study  of  ore 
deposits. 

At  Altenberg  the  ores  occur  in  a  stockwork,  about  3,000  feet 
in  diameter,  of  small  veins  cutting  across  the  granite  and  the 
adjacent,  older  granite  porphyry;  the  whole  mass  of  rock  is 
altered  to  a  greisen,  containing  a  little  cassiterite  and  arseno- 
pyrite.  The  characteristic  minerals  occurring  in  the  veinlets 
are  quartz,  bismuth,  bismuthinite,  pyrite,  chalcopyrite,  molyb- 
denite, zinc  blende,  wolframite,  fluorite,  tetrahedrite,  magnetite, 
and  specularite.  The  ore,  according  to  Dalmer,  continued  only 
to  a  depth  of  about  700  feet. 

1  The  older  literature  includes  the  papers  by  B.  v.  Cotta,  H.  Mtiller,  E. 
Reyer,  and  A.  W.  Stelzner. 

R.  Beck,  Einige  Beobachtungen  im  Gebiete  der  Altenberg-Zinnwalder 
Zinnerzlagerstatten,  Zeitschr.  prakt.  Geol,  1896,  pp.  148-150. 

K.  Dalmer,  Der  Altenberg-Graupener  Zinnerzlagerstattendistrikt. 
Zeitschr.  prakt.  Geol.,  1894,  pp.  313-322;  see  also  idem,  1895,  p.  228;  1896, 
p.  1;  1897,  p.  265;  1900,  p.  297. 

J.  T.  Singewald,  Jr.,  The  Erzgebirge  tin  deposits,  Econ.  Geol.,  vol.  5, 
1910,  pp.  166-177,  265-272 


670 


MINERAL  DEPOSITS 


At  Ziimwald  the  veins  are  likewise  in  granite,  which  with  flat 
contact  breaks  through  quartz  porphyry.  The  numerous  fis- 
sures are  approximately  parallel  to  the  contact,  and  the  veins 
are  formed  mainly  by  filling,  sometimes  with  excellent  banded 
structure  by  the  development  of  mica  next  to  the  wall;  they 
contain  a  considerable  amount  of  wolframite  (hiibnerite),  besides 
the  usual  minerals  accompanying  the  cassiterite  (Fig.  232). 


FIG.  232. — Diagrammatic  section  of  vein  at  Ziimwald,  Saxony,  g,  Granite; 
gr,  greisen ;  q,  quartz ;  I,  lithium  mica ;  z,  cassiterite ;  w,  wolframite ;  /,  fluorite  ; 
sch,  scheelite.  After  R.  Beck. 


Tin  Deposits  in  Other  Countries 

Cassiterite  veins  have  been  found  in  the  Transvaal,  New  South 
Wales,  Queensland,  Western  Australia,  Tasmania,  Bolivia, 
Mexico,  and  Alaska.  In  the  United  States  they  are  rare.  A  vein 
in  granite  has  recently  been  worked  near  El  Paso,  Texas,  on  the 
east  side  of  the  Franklin  Mountains.1  Some  low-grade  veins 
occur  in  the  Temescal  Mountains,  near  Riverside,  California.2 
The  tin-bearing  deposits  of  the  Appalachian  region  and  of  the 
Black  Hills  of  South  Dakota  are  mainly  pegmatite  dikes,  though 
a  few  quartz  veins  with  cassiterite  occur  in  the  Appalachian 
region.3  The  occurrences  in  Alaska  are  described  on  p.  741. 

1  W.  H.  Weed,  The  El  Paso  tin  deposits,  Bull  178,  U.  S.  Geol.  Surv.,  1901. 
G.  B.  Richardson,  Tin  in  the  Franklin  Mountains,  Bull.  235,  U.  S.  Geol. 
Survey,  1905,  pp.  146-149. 

z  H.  W.  Fairbanks,  The  tin  deposits  of  Temescal,  Am.  Jour.  Sci.,  4th  ser., 
vol.  4,  1897,  pp.  39-42. 

3L.  C.  Graton,  Reconnaissance  of  some  gold  and  tin  deposits  of  the 
southern  Appalachians,  Bull.  293,  U.  S.  Geol.  Survey,  1906 


HIGH-TEMPERATURE  DEPOSITS  671 

The  largest  part  of  the  tin  production  of  the  world  is  derived 
from  the  Malay  Peninsula  and  from  the  islands  of  Banka  and 
Billiton,  off  the  coast  of  Sumatra,  but  the  output  from  these 
localities  comes  mainly  from  placers.  In  the  Malay  States  granite 
invades  post-Triassic  limestone  and  schist,  and  tin-bearing  veins 
in  these  rocks  have  been  described  by  Penrose.1  On  the  islands 
granite  is  intruded  in  slate;  both  rocks  contain  tin  veins  which 
are  noteworthy  in  that  they  carry  magnetite. 

Interesting  tin  deposits  have  been  described  as  occurring  in  the 
Transvaal  and  New  South  Wales.  They  occur  in  granite  and  are 
roughly  cylindrical  "pipes"  consisting  of  highly  altered  granite 
with  disseminated  cassiterite  and  tourmaline.2  Veins  and  pipes 
of  cassiterite  accompanied  by  sulphides,  specularite,  tourmaline, 
and  ankerite  or  siderite  occur  in  the  quartzite  of  the  Rooiberg 
district3  in  the  Transvaal.  The  deposits  are  several  miles  dis- 
tant from  the  intrusive  contact. 

The  principal  tin  deposits  of  Tasmania  are  those  of  Mount 
Bischoff,  where  schists  are  intruded  by  dikes  of  granite  porphyry, 
both  rocks  being  rich  in  metasomatic  tourmaline  and  topaz.  The 
whole  forms  a  weathered  mass  of  rock  traversed  by  cassiterite 
veins — a  stockwork,  large  portions  of  which  averaged  2  to  3  per 
cent.  tin.  Magnetite,  fluorite,  pyrrhotite,  zinc  blende,  wolfram- 
ite, and  siderite  are  mentioned  as  accompanying  minerals.4 

1  R.  A.  F.  Penrose,  The  tin  deposits  of  the  Malay  Peninsula,  Jour.  Geology, 
vol.  2,  1903,  pp.  135-154. 

W.  Wolff,  J.  B.  Scrivenors  Arbeiten  iiber  die  Geologic  von  britisch 
Mala-ya,  Zeitschr.  prakt.  Geol,  1911,  pp.  152-158. 

J.  B.  Scrivenor,  The  topaz-bearing  rocks  of  Gunong-Bakau,  Quart. 
Jour.,  Geol.  Soc.  London,  vol.  70,  1914,  pp.  363-381. 

W.  R.  Jones,  The  origin  of  topaz  and  cassiterite  at  Gunong-Bakau, 
Geol.  Mag.,  vol.  3,  1916,  pp.  255-260. 

R.  D.  M.  Verbeek,  Ueber  die  Zinnerzlagerstatten  von  Bangka  und 
Billiton,  Zeitschr.  prakt.  Geol.,  1899,  pp.  134-136. 

C.  T.  Groothoff,  De  primaire  Tinertsafzettingen  van  Billiton,  Disser- 
tation, Delft,  1916. 

2  H.  Kynaston  and  E.  T.  Mellor,  The  geology  of  the  Waterbury  tin  field, 
Mem.  4,  Transvaal  Geol.  Survey,  1909. 

L.  A.  Cotton,  The  tin  deposits  of  New  England,  New  South  Wales,  Proc., 
Linnean  Soc.  N.  S.  W.,  vol.  34,  1909,  pp.  733-781. 

3  R.  Recknagel,  Trans.  Geol.  Soc.  S.  Africa,  vol.  11,  1908,  pp.  83-106. 

4  W.  von  Fircks,  Die  Zinnerzlagerstatten  des  Mount  Bischoff,  Zeitschr. 
Deutsch.  geol.  Gesell.,  Bd.  51,  Heft  3,  1899,  pp.  431-465. 

P.  Krusch,  Die  Zinnerzlagerstatten  des  Mount  Bischoff,  Zeitschr. 
prakt.  Geol.,  1900,  pp.  86-90. 


672  MINERAL  DEPOSITS 

The  Bolivian  veins,1  which  center  in  the  mining  districts  of 
Oruro  and  Tres  Cruces,  appear  in  Devonian  slates,  intruded  by 
dikes  of  granite  porphyry.  A.  W.  Stelzner,  who  first  described 
them,  drew  attention  to  certain  unusual  features  consisting  in 
the  association  with  silver  minerals  and  the  occurrence  of  much 
pyrite,  but  W.  R.  Rumbold  and  M.  Armas  have  shown  that  the 
veins  are  similar  to  the  normal  tin  veins  and  that  the  country 
rock  is  extensively  tourmalinized.  In  some  of  the  veins  stannite 
(FeCu2SnS4)  is  present,  as  well  as  ruby  silver,  stephanite,  tetra- 
hedrite,  zinc  blende,  arsenopyrite,  bournonite,  wolframite  and 
siderite.  Germanium  minerals  such  as  argyrodite  (3Ag2S.- 
GeS2)  was  found  at  Potosi,  and  canfieldite,  an  argyrodite  con- 
taining tin,  at  La  Paz.  The  tin  ores  of  Bolivia  are  in  part  of 
high  grade  and  yield  a  production  of  great  importance. 

The  tin-bearing  district  of  Zeehan,  Tasmania,2  contains  de- 
posits of  various  kinds  which  appear  to  show  an  unusually 
complete  series  of  transitions.  Silurian  sediments  are  intruded 
by  granite.  There  is  a  gradation  from  cassiterite  veins,  with 
tourmaline,  in  granite,  to  contact-metamorphic  deposits,  con- 
taining copper,  lead,  and  zinc,  in  which  cassiterite  has  been 
found,  and  from  these  to  normal  banded  veins  containing  pyrite, 
chalcopyrite,  galena,  and  stannite,  and  finally  to  galena-siderite 
veins.  In  other  words,  the  gradation  is  one  from  high-tempera- 
ture deposits  to  those  formed  in  the  cooler  zone  with  a  corre- 
sponding change  of  minerals  deposited. 

Minor  veins  of  cassiterite  are  sometimes  found  in  rhyolite 
flows,  in  which  they  were  evidently  formed  shortly  after  the  con- 
solidation of  the  rock.  We  have  here  then  high  temperature 
deposits  originating  near  the  surface.  It  may  be  recalled  that 
topaz  is  sometimes  found  in  lithophysse  in  rhyolite  and  these 
phenomena  indicate  a  retention  by  the  lavas  of  certain  of  their 
volatile  constituents  until  consolidation  of  the  rock.  Such  de- 
posits contain  concretionary  cassiterite  and  "wood  tin"  asso- 
ciated with  hematite,  chalcedony,  and  opal,  sometimes  also  with 

JA.  W.  Stelzner,  Die  Silber-Zinnerzlagerstatten  Boliviens,  Zeitschr 
Deutsch.  geol.  Gesell.,  vol.  49,  1897,  pp.  51-142. 

W.  R.  Rumbold,  The  origin  of  the  Bolivian  tin  deposits,  Econ.  Geol., 
vol.  4,  1909,  pp.  321-364. 

M.  Armas,  Ann.  des  Mines,  10th  ser.,  vol.  20,  1911,  pp.  149-213. 
2  W.  H.  Twelvetrees  and  L.  K.  Ward,  Butt.  8,  Dept  of  Mines,  Tasmania. 
1910. 


HIGH-TEMPERATURE  DEPOSITS  673 

wolframite  and  bismuth  minerals.     Occurrences  of  this  kind 
have  been  described  from  Mexico1  and  from  Nevada.2 

WOLFRAMITE  VEINS 

Wolframite,  including  the  tungstate  of  iron  (ferberite)  and  the 
tungstate  of  manganese  (hiibnerite),  has  a  field  of  occurrence 
similar  to  that  of  cassiterite.  It  appears  in  igneous  rocks,  in 
pegmatites,  in  cassiterite  veins,  and  sometimes  with  quartz  and 
bismuth  minerals  in  veins  which  are  evidently  of  the  deep-seated 
type  and  allied  to  the  tin  veins.  But,  unlike  cassiterite,  wol- 
framite also  appears  abundantly  in  veins  formed  under  much 
more  moderate  temperature  and  pressure — for  instance,  in  those 
of  Boulder  County,  Colorado  (p.  620).  Small  quantities  of 
hiibnerite  are  found  in  veins  formed  near  the  surface,  as  in  those 
of  Tonopah,  Nevada,  and  Cripple  Creek,  Colorado.  The  prin- 
cipal production  in  the  United  States  is  derived  from  Boulder 
County,  Colorado. 

Wolframite  veins  of  the  deep-seated  type  have  been  described 
from  the  Deer  Park  district,3  in  Washington  and  from  Sauce4  in 
the  Sierra  de  Cordova  in  Argentina.  Wolframite  lodes  of  great 
importance  have  lately  been  developed  in  the  Tavoy  district, 
Lower  Burma,5  where  they  occur  in  granite  and  metamorphic 
schist  and  contain  in  addition  quartz,  mica,  tourmaline,  colum- 
bite,  arsenopyrite,  pyrite,  bismuthinite  and  galena.  At  the  same 
place  wolframite  is  also  recovered  from  alluvial  deposits.  In 
1916  the  mines  of  Lower  Burma  produced  about  3,000  tons  of 
wolframite  concentrates,  which  is  about  one-third  of  the  annual 
output  of  the  world.  The  crude  ore  is  said  to  average  1.3  per 
cent.  WOs  per  ton. 

Regarding  molybdenite  veins   see  p.  777. 

1  W.  R.  Ingalls,  The  tin  deposits  of  Durango,  Mexico,  Trans.,  Am.  Inst 
Min.  Eng.,  vol.  25,  1896,  pp.  146-163;.  vol.  27,  1898. 

E.  Halse,  idem,  vol.  29,  pp.  1900,  502-511. 

E.  Wittich,  Zinnerze  in  der  Sierra  von  Guanajuato,  Zeitschr.  prakt. 
Geol.,  1910,  pp.  121-123. 

2  Adolph  Knopf,  Tin  ore  in  northern  Lander  Co.,  Nevada,  Bull.  640 
U.  S.  Geol.  Survey,  1916,  pp.  125-138. 

3  Rowland  Bancroft,  Bull.  430,  U.  S   Geol.   Survey,  1910,  pp.  214-216 

4  Bodenbender,  Zeitschr.  prakt.  Geol,  1894,  pp.  409-414. 

5  A.  W.  G.  Bleeck,  Records  Geol.  Survey  India,  vol.  43,  pt   1,  1913. 
E.  M.  Lefroy,  Min.  Mag.,  vol.  25,  1916,  pp.  83-100. 

H.  D.  Griffiths,  idem,  vol.  26,  1917,  pp.  60-65. 


674  MINERAL  DEPOSITS 

GOLD-QUARTZ  VEINS 

Gold-bearing  veins  of  a  deep-seated  type  are  found  in  many 
regions  in  the  pre-Cambrian  and  earliest  Paleozoic  rocks  of  the 
American  continents.  They  appear  in  the  gold  belt  of  the 
Appalachian  States,  mainly  from  Maryland  to  Alabama;  at 
various  places  in  the  Western  States,  particularly  in  South 
Dakota  and  New  Mexico;  in  Ontario  and  Quebec;  and  finally  in 
Brazil. 

The  Veins  of  the  Southern  Appalachians1 

The  placer  deposits  of  the  southern  Appalachians  have,  since 
their  discovery,  about  1800,  yielded  gold  estimated  at  $30,000,000. 
The  veins  from  which  the  placers  were  derived  proved  less 
productive,  though  they  have  been  profitably  worked  at  many 
places  in  North  and  South  Carolina,  at  Dahlonega  and  the 
Franklin  mine  in  Georgia,  and  at  the  Hogback  mine  in 
Alabama.  All  the  deposits  are  not  of  the  deep-seated  type; 
there  are  some  which  more  closely  approach  the  normal  gold- 
quartz  veins  similar  to  those  of  California,  but  even  in  these 
deposits  certain  features  indicate  deposition  at  higher  tempera- 
tures. Others,  like  those  described  by  Taber  from  Virginia, 
seem  to  be  related  to  pegmatite  dikes.  The  veins  are  contained 
in  crystalline  rocks,  usually  more  or  less  schistose,  which,  upon 
closer  examination,  prove  to  be  granites  and  quartz  monzonites, 
intrusive  into  mica  schists,  clay  slates,  altered  volcanic  tuffs,  and 
amphibolites.  The  age  of  the  veins  is  probably  early  Paleozoic. 

Structurally  the  deposits  may  be  classed  as  fissure  veins  and 
replacement  deposits  in  schists.  The  veins  are  in  general  of  the 
so-called  lenticular  type  illustrated  in  Fig.  43,  in  which  the  quartz 

1  G.  F.  Becker,  Gold  fields  of  the  southern  Appalachians,  Sixteenth  Ann. 
Rept.,  U.  S.  Geol  Survey.,  pt.  3,  1895,  pp.  250-331. 

H.  B.  C.  Nitze,  Bull.  10,  North  Carolina  Geol.  Survey,  1897. 

L.  C.  Graton  and  W.  Lindgren,  Reconnaissance  of  some  gold  and  tin 
deposits,  etc.,  BulL  293,  U.  S.  Geol.  Survey,  1906. 

H.  D.  McCaskey,  Gold,  etc.,  in  the  Eastern  States,  Mineral  Resources, 
U.  S.  Geol.  Survey,  1908,  pp.  645-681  (with  literature). 

H.  D.  McCaskey,  Notes  on  some  gold  deposits  of  Alabama,  Bull.  340, 
U.  S.  Geol.  Survey,  1908,  pp.  36-52. 

F.  B.  Laney,  The  Gold  Hill  mining  district,  Bull.  21,  North  Carolina 
Geol.  Survey,  1910. 

Stephen  Taber,  Geology  of  the  gold  belt  in  the  James  River  basin, 
Bull.  7,  Virginia  Geol.  Survey,  1913. 


HIGH-TEMPERATURE  DEPOSITS  675 

lenses,  which  collectively  constitute  the  veins,  lie  parallel  to  the 
foliated  structure.  In  detail  the  lenses  often  cut  across  the  schis- 
tosity  and  are  sometimes  of  irregular  form.  Sharply  defined 
veins  cutting  across  the  schistosity  also  occur.  The  quartz  is 
massive,  usually  without  banded  or  drusy  structure.  The  re- 
placement deposits  form  irregular  bodies  of  silicified  and  pyritic 
schist;  the  deposit  worked  by  the  Haile  gold  mine  is  the  most 
prominent  example  of  this  class.  The  ores  form  more  or  less 
regular  shoots,  often  also  pockets,  and  are  in  general  of  low 
grade;  pyritic  ore  containing  $2  per  ton  has  been  successfully 
treated  at  the  Haile  mine;  many  shoots,  however,  average  much 
higher,  sometimes  $15  or  $20  per  ton.  Free  gold  is  generally 
but  by  no  means  always  present  below  the  zone  of  oxidation. 

Quartz,  often  glassy  and  semi-transparent,  is  the  principal 
gangue  mineral  and  may  be  accompanied  by  calcite,  dolo- 
mite, apatite,  chlorite,  ilmenite,  magnetite,  tourmaline,  albite 
and  sometimes  zinc  spinel  (gahnite)  and  garnet.  The  ore  miner- 
als are  free  gold,  pyrite,  arsenppyrite,  pyrrhotite,  molybdenite, 
more  rarely  galena,  zinc  blende,  and  chalcopyrite.  Enargite, 
tetradymite,  altaite,  and  nagyagite  are  recorded,  but  are  rare. 
The  pyrite  is  always  the  oldest  sulphide  and  the  gold  fills  minute 
fractures  in  it,  or  in  the  quartz. 

The  metasomatic  alteration  of  the  wall  rock  shows  considerable 
variations.  The  most  intense  action  is  shown  by  some  quartz- 
tourmaline  veins;  the  adjoining  amphibolite  is  altered  to  garnet, 
tourmaline,  and  magnetite.1  In  some  of  the  Dahlonega  veins 
the  included  amphibolite,  as  well  as  the  adjacent  wall  rock,  is 
altered  to  well-developed  crystals  of  pale-red  garnet  and  a  dark- 
green  mica.  The  garnets  contain  visible  gold;  the  quartz  itself 
contains  pyrite,  pyrrhotite,  galena,  and  chalcopyrite.2  This 
mode  of  alteration  is  much  like  that  noted  in  the  rocks  adjacent 
to  the  pegmatite  dikes  of  the  same  region. 

In  other  veins  a  chestnut-brown  biotite  is  the  only  mineral 
resulting  from  metasomatic  alteration;  in  places  both  muscovite 
in  comparatively  large  foils  and  biotite  are  present,  sometimes 
with  calcite  or  dolomite,  besides  more  or  less  pyrite  or  pyrrhotite. 
Amphibolite  is  the  most  easily  attacked  of  the  various  kinds  of 
country  rock.  The  alteration  of  granite  is  usually  slight. 

The  replacement  bodies  are  generally  in  the  acidic  schist  de- 

1  L.  C.  Graton,  op.  cit.,  p.  47. 

2  W.  Lindgren,  op.  cit.,  pp.  126-127. 


676  MINERAL  DEPOSITS 

rived  from  volcanic  fragmental  rocks;  these  are  extensively 
silicified  and  contain  also  both  'sericite  and  biotite  as  products 
of  alteration. 

Genetically,  these  gold-bearing  veins  appear  to  be  connected 
with  granitic  intrusions,  representing  the  final  product  of  the 
most  volatile  part  of  the  magma.  They  are  considered  by 
Becker,  Graton,  and  Lindgren  to  be  the  deepest  parts  of  veins 
whose  upper  parts  have  been  carried  away  by  erosion. 

The  Quartz  Veins  of  Ontario1 

The  gold-bearing  quartz  veins  of  Ontario,  Canada,  are  widely 
distributed.  They  occur  at  Lake  of  the  Woods,  Rainy  Lake, 
Wahnapitse,  Abitibi,  Larder  Lake,  Kirkland  Lake  and  in  the  re- 
cently discovered  Porcupine  district.  They  also  extend  into 
Quebec  and  Manitoba. 

Until  recently  the  production  of  these  veins  has  been  dis- 
appointing but  they  have  yielded  heavily  in  the  last  few  years 
owing  to  the  wonderful  developments  in  the  Porcupine  district. 
The  gold  production  of  Ontario  was  $10,180,000  in  1916  which 
exceeds  the  combined  product  of  British  Columbia  and  Yukon. 
Almost  the  whole  of  this  came  from  the  Porcupine. 

In  some  districts,  Kirkland  Lake  for  instance,  the  veins  are 
similar  to  those  of  California  and  contain  no  distinctive  high 
temperature  minerals  while  at  many  other  places  tourmaline  and 
pyrrhotite  are  found  in  the  ores. 

The  veins  of  Ontario  are  found  in  the  Keewatin  greenstone  and 
allied  schist,  in  the  later  Timiskaming  (lower  Huronian)  conglom- 
erate and  greywacke  which  is  infolded  with  the  Keewatin  and 
finally  in  quartz  porphyry,  granite  and  syenite  of  post-Timis- 
kaming  (Algoman)  age.  The  mineralization  is  caused  by  these 
intrusions  according  to  the  Ontario  geologists.  The  deposits  are 

1  A.  P.  Coleman,  Reports  of  Bureau  of  Mines  of  Ontario,  Nos.  4,  5,  6,  and 
7,  1894-1896. 

W.  G.  Miller  and  C.  W.  Knight,  The  pre-Cambrian  geology  of  South- 
eastern Ontario,  Rept.  Ontario  Bur.  Mines,  vol.  22,  pt.  2,  1914. 

A.  G.  Burrows,  The  Porcupine  gold  area,  idem,  vol.  24,  pt.  3,  1915 

J.  Stansfield,  Microscopic  examination  of  Porcupine  rocks,  etc.,  Canadian 
Min.  Jour.,  Feb.  15,  1911. 

A.  G.  Burrows  and  P.  E.  Hopkins,  The  Kirkland  lake  and  Swastika 
gold  areas,  Rept.  Ont.  Bur.  Mines,  vol.  23,  pt.  1,  1914. 

A.  H.  Means,  Tourmaline  bearing  gold  quartz  veins  of  the  Michipicoten 
district,  Ontario,  Econ.  Geol,  vol.  9,  1914,  pp.  122-135. 


HIGH-TEMPERATURE  DEPOSITS  677 

lenticular  veins  in  schist  (Fig.  234),  fairly  regular  and  branching 
veins  in  massive  rocks  in  part  also  large  irregular  or  dome-shaped 
masses  of  quartz.  There  are  scant  amounts  of  simple  sulphides 
such  as  pyrite,  arsenopyrite,  pyrrhotite,  chalcopyrite,  galena, 
and  zinc  blende.  Molybdenite  and  tellurides  of  gold,  silver  and 
lead  occur  in  the  Kirkland  Lake  district;  scheelite  is  found  at 
Porcupine.  Besides  the  predominant  quartz  the  gangue  minerals 
include  ankerite,  dolomite,  tourmaline,  chlorite,  sericite  and  often 
also  albite.  The  gold  is  generally  free  and  often  coarse  averaging 
850  fine.  In  the  Hollinger  mine,  at  Porcupine,  the  gold  occurs 
in  quartz  and  schist,  the  latter  containing  about  5  per  cent, 
pyrite  besides  sericite,  dolomite  and  chlorite;  though  probably 


FIG.  233. — Drawing  of  thin  section  showing  native  gold  deposited  in 
crushed  gold-quartz,  Rea  vein,  Porcupine.  Black  spots  are  native  gold. 
Magnified  30  diameters.  After  A.  G.  Burrows. 

free,  it  is  so  fine  that  it  can  not  be  obtained  by  amalgamation  or 
panning.  Most  of  the  coarse  gold  appears  to  be  deposited  in 
crushed  and  fissured  quartz,  and  is  somewhat  later  than  the 
earliest  mineralization  (Fig.  233). 

At  Porcupine  the  principal  mines  have  attained  a  depth  of 
1,250  feet.  The  Hollinger  deposits,  which  in  1916  yielded  about 
$5,000,000,  consist  of  a  series  of  lenticular  veins  in  schist  (Fig. 
234).  In  1916,  600,000  tons  of  ore  were  treated.  In  1917  the 
output  decreased  somewhat. 

Next  to  the  veins  the  country  rock  is  altered  to  sericite  and 
carbonates,  and  carbonatization  of  basic  rocks  has  often  resulted 
in  large  masses  of  coarsely  crystalline  rock  consisting  of  ankerite 
and  bright-green  mariposite  (chromium  mica).  This  altered 
rock,  which  appears  to  occupy  considerable  areas,  is  often  cut  by 


678 


MINERAL  DEPOSITS 


quartz  veins  and  is  remarkably  similar  to  certain  rocks  along  the 
Mother  Lode  in  California,  which  have  resulted  from  the  altera- 
tion of  serpentine. 

In  places,  for  instance,  at  the  Rice  Lake  veins,  Manitoba, 
biotite  is  developed  in  the  country  rock. 

The  Pre-Cambrian  Gold  Veins  of  the  Cordilleran  Region 

It  has  already  been  explained  that  of  the  pre-Cambrian  veins 
many,  though  not  all,  suggest  formation  at  high  temperature. 
Brief  reference  suffices  to  the  gold-bearing  quartz  veins  of  Hope- 
well  and  Bromide,1  in  New  Mexico,  many  of  which  contain, 


A  B 

Fia.  234,  A  and  B. — Photographs  of  veins  in  Hollinger  mine,  Ontario. 
After  A.  G.  Burrows. 

as  metasomatic  products,  tourmaline,  garnet,  and  other  silicates 
and  also  the  characteristic  brown  or  green  mica.  Some  of  the 
veins  occurring  in  the  pre-Cambrian  rocks  of  southern  Wyoming,2 
at  Atlantic  and  South  Pass,  also  belong  to  this  type;  others 
give  evidence  of  deposition  at  lower  temperatures. 

1  L.  C.  Graton,  Prof.  Paper  68,  U.  S.  Geol.  Survey,  1910,  p.  126. 
*  A.  C.  Spencer,  Bull.  626,  U.  S.  Geol.  Survey,  1916,  pp.  9-45. 


HIGH-TEMPERATURE  DEPOSITS 


679 


The  Black  Hills  of  South  Dakota1  contain  many  gold-bearing 
deposits  in  the  pre-Cambrian  rocks.  They  occur,  as  a  rule,  in 
clay  slates  of  sedimentary  origin,  and,  while  some  of  them  are 
true  veins  with  glassy  quartz  and  free  gold,  others  are  lenticular 
bodies,  of  highly  altered  rock.  The  best  known  among  the  latter 
is  the  Homestake  lode  at  Lead.  Embedded  in  the  normal  clay 
slate,  which  contains  not  far  away  great  masses  of  intrusive  gran- 
ite, are  huge  lenticular  bodies  of  altered,  rock  with  quartz,  sul- 
phides, and  free  gold,  averaging  about  $4  per  ton.  The  ratio  of 
silver  to  gold  (by  weight)  is  about  1:5.  The  numerous  ore  lenses 


FIG.  235. — Thin  sections  of  Homestake  ores.  Left:  Gold  and  pyrrhotite 
(later),  in  arsenopyrite  (earlier).  Magnified  48  diameters.  Right:  Gold 
with  quartz,  iron-magnesium  carbonate,  pyrite,  and  cummingtonite.  Magni- 
fied 32  diameters.  After  W.  J.  Sharwood. 

are  in  places  several  hundred  feet  in  width  and  have  been  followed 
for  a  distance  of  about  1  mile  in  the  same  direction  as  the  strike 
of  the  clay  slates,  which  dip  steeply  to  the  northeast.  The  slates 
and  ore-bodies  are  intruded  by  rhyolite  porphyry,  which,  how- 
ever, seems  to  have  caused  little  if  any  additional  mineralization. 
At  the  depth  attained,  which  now  is  2,000  feet,  the  ore-bodies  are 

1  J.  D.  Irving  and  S.  F.  Emmons,  Prof.  Paper  26,  U.  S.  Geol.  Survey, 
1904. 

W.  J.  Sharwood,  Econ.  Geol.,  vol.  6,  1911,  pp.  729-786. 


680  MINERAL  DEPOSITS 

said  to  maintain  their  size  and  value.  In  a  broad  way  the  ore- 
bodies  pitch  to  the  southeast.  One  thousand  stamps  crush  the 
ore,  and  the  pulp  is  amalgamated  and  afterward  leached  with 
cyanide.  The  production  in  1916  had  a  value  of  $6,531,000. 
The  ores  differ  from  the  country  rock  in  containing  a  dissem- 
ination of  fine-grained  free  gold,  pyrrhotite,  pyrite,  arsenopyrite, 
and  a  little  chalcopyrite;  the  sulphurets1  are  not  rich  in  gold. 
The  ore-bodies  also  include  many  small  lenticular  masses  of 
coarse-grained  glassy  or  milky  quartz,  which  in  places  contains 
sulphides  but  rarely  free  gold.  The  larger  part  of  the  ore  is  also 
distinguished  by  the  appearance  of  much  light-brown  hornblende, 
often  with  radial  structure;  it  is  rich  in  iron  and  probably  belongs 
to  the  species  cummingtonite  (Fig.  235) .  There  is  also  in  places  a 
little  dolomitic  carbonate,  siderite,  and  garnet.  According  to 
J.  D.  Irving  the  silicates  are  older  than  the  gold,  but  they  surely 
belong  to  the  same  general  epoch  of  metallization.  The  gold 
often  accompanies  arsenopyrite.  Sidney  Paige1  has  suggested 
that  the  ore  replaces  lime  shale  along  a  main  fault. 

The  Gold-Bearing  Veins  of  Brazil2 

Some  of  the  provinces  in  the  great  pre-Cambrian  areas  of 
Brazil  contain  auriferous  lodes  of  great  value.  In  many  respects 
they  are  similar  to  those  of  the  Atlantic  coast  of  North  America, 
already  described,  and  may  be  classed  with  the  deep-seated  veins; 
the  mineral  association  suggests  deposition  at  high  temperatures. 

The  veins  occur  mainly  in  the  pre-Cambrian  clay  slates  or  cal- 
careous slates  of  Minas  Geraes,  in  southern  Brazil,  and  have  been 
worked  successfully  to  great  depths.  The  mine  of  St.  John  del 
Rey  (Fig.  236)  has  attained  6,300  feet  in  vertical  depth,  or  10,000 
feet  on  the  incline,  and  is  thus  the  deepest  gold  mine  in  the  world. 
At  a  depth  of  4,900  feet  the  lenticular  ore  body  is  1,028  feet  long 

1  Bull.  Geol.  Soc.  Am.,  vol.  24,  1913,  pp.  293-300. 

2Orville  Derby,  Notes  on  Brazilian  gold  ores,  Trans.,  Am.  Inst.  Min. 
Eng.,  vol.  33,  1903,  pp.  282-287;  Eng.  and  Min.  Journal,  vol.  74,  1902, 
pp.  142-143. 

Georg  Berg,  Beitrage  zur  Kenntniss  der  Goldlagerstatten  von  Raposos, 
Zeitschr.  prakt.  Geol.,  1902,  pp.  81-84. 

E.  Hussak,  Der  Quartzlagergang  von  Passagem,  Zeitschr.  prakt.  Geol., 
1898,  pp.  345-357. 

Orville  A.  Derby,  Gold-bearing  lode  of  Passagem,  Am.  Jour.  Sci.,  4th 
ser.,  vol.  32,  1911,  pp.  185-190. 


HIGH-TEMPERATURE  DEPOSITS 


681 


and  12£  feet  wide.  The  ore  has  a  value  of  about  $11  per  ton. 
The  mineral  association  consists  of  native  gold,  pyrrhotite,  pyrite, 
chalcopyrite,  arsenopyrite,  quartz,  and  siderite;  albite,  apatite, 
magnetite,  and  scheelite  are  also  known  to  occur.  The  ore  con- 
tains 28  per  cent,  pyrrhotite  and  25  per  cent,  quartz,  the  rest  being 
mainly  siderite.  Metasomatic  replacements  often  spread  lat- 
erally into  the  adjacent  schists. 


,    n\ 

XVII  •  4926 |_ 


FIG.  236. — Vertical  longitudinal  section  of  the  St.  John  del  Rey  Mining 
Company's  MOTTO  Velho  mine,  Brazil,    j 


The  Passagem  lode,  described  by  E.  Hussak  as  a  gold-bearing 
pegmatite  dike,  appears,  according  to  O.  Derby,  as  a  pegmatite 
dike  shattered  and  impregnated  by  gold,  arsenopyrite,  pyrrhotite, 
and  tourmaline,  with  a  little  siderite  and  calcite  (p.  776).  The 
gold  contains  some  bismuth.  The  appearance  of  cumming- 
tonite  allies  the  Passagem  vein  in  an  interesting  manner  to  the 
Homestake  deposit.  In  other  parts  of  the  Brazilian  gold-bearing 
area  palladium  occurs  in  alloy  with  gold. 


682  MINERAL  DEPOSITS 


The  Gold-Quartz  Deposits  of  Silver  Peak,  Nevada 

In  the  Cordilleran  region  of  North  America  gold-quartz  veins 
of  post-Cambrian  age  belonging  to  the  class  of  high-temperature 
veins  are  not  common.  Spurr,  however,  describes  such  deposits 
in  the  Silver  Peak  district,  in  western  Nevada,1  which  he  holds  to 
be  closely  allied  to  igneous  rocks.  Granites  with  transitions  to 
alaskite  and  quartz  monzonite  are  here  intrusive  into  Paleozoic 
limestone.  Gold-quartz  veins  of  irregular  form  and  typical 
alaskite  are  stated  to  "form  two  ends  of  a  rock  series  between 
which  every  gradation  is  represented."  The  gold  is  contained 
mainly  in  the  pure  quartz,  which  also  yields  sulphides  like  arseno- 
pyrite.  There  is  little  silver  present.  The  mines  closed  down 
in  1915. 

The  alaskite  is  a  granular  aplitic  rock  containing  quartz,  or- 
thoclase,  microcline,  albite,  anorthoclase,  and  oligoclase-albite. 
The  first  generation  of  feldspars  was  partly  altered  into  muscovite 
before  the  second  generation  of  feldspars  was  deposited.  The 
quartz  magma,  separated  by  differentiation,  collected  in  larger 
masses.  Later,  repeated  mineralization  filled  the  fractures  in  the 
magmatic  quartz  with  auriferous  pyrite  and  galena,  representing 
a  fresh  supply  of  ascending  waters.  The  hypothesis  is  advanced 
that  solutions  of  granitic  origin  have  deposited  gold  predomi- 
nantly in  the  granite  or  in  the  rocks  silicified  by  the  metamorphic 
effect  of  the  granite,  and  that  in  or  near  the  calcareous  rocks  more 
silver  and  copper  were  precipitated  from  the  same  solutions,  the 
difference  being  due  to  the  selective  influence  of  wall  rock. 
Garnet  and  epidote  are  mentioned  as  occurring  in  certain  of  the 
veins  in  calcareous  rocks. 

The  Gold-Quartz  Veins  of  Southeastern  Alaska2 

The  gold-bearing  veins  of  southern  Alaska  are  closely  allied 
to  those  of  the  Appalachian  region  and  to  those  of  Brazil, 
although  they  present  some  features  that  would  rather  connect 
them  with  the  gold-quartz  veins  of  California,  which  are  believed 

1  J.  E.  Spurr,  Ore  deposits  of  the  Silver  Peak  quadrangle,  Prof.  Paper 
55,  U.  S.  Geol.  Survey,  1906. 

2  A.  C.  Spencer,  The  Juneau  gold  belt,  Alaska,  Bull  278,  U.  S.  Geol. 
Survey,  1906. 

A.  Knopf,  The  Eagle  River  region,  Bull.  502,  U.  S.  Geol.  Survey,  1912. 


HIGH-TEMPERATURE  DEPOSITS  683 

to  be  formed  under  conditions  of  lower  temperature  or  more 
moderate  depth. 

The  veins  occur  mainly  in  the  narrow  strip  of  sharply  folded 
Paleozoic  slates  and  greenstones  which  form  the  western  margin 
of  the  great  batholithic  mass  of  granodiorite  of  late  Mesozoic  age, 
40  to  80  miles  wide  and  continuous  for  many  hundreds  of  miles 
parallel  to  the  coast.  The  conditions  are  therefore  essentially 
similar  to  those  of  the  California  gold  belt,  especially  as  the 
Paleozoic  sediments  farther  west  on  Admiralty  Island  are  ad- 
joined by  a  belt  of  slates  which  are  thought  to  correspond  in 
age  to  the  Mariposa  slate  of  California. 

In  the  long  strip  of  coast  country  extending  300  miles  there  are 
numerous  mining  districts,  among  which  are  Windham  Bay,  Port 
Snettishajn,  Sheep  Creek,  Gold  Creek,  Douglas  Island,  Eagle 
River,  and  Berners  Bay. 

The  gold  occurs  in  veins  and  lodes  of  various  kinds,  or  more 
rarely,  as  on  Douglas  Island,  in  altered  dikes  of  dioritic  character 
that  contain  disseminated  free  gold  and  sulphides.  The  individ- 
ual veins  are  rarely  continuous  for  more  than  a  few  hundred  feet, 
but  often  combine  to  form  more  extended  stringer  leads  or  lode 
systems.  As  the  veins  are  later  than  the  schistose  structure  of 
the  rocks  their  tendency  is  to  follow  foliation  planes,  and  in  places 
they  strongly  resemble  the  lenticular  veins  of  the  Appalachian 
region,  but  continuous  and  cross-cutting  veins  also  occur. 

The  gangue  minerals  are  mainly  milky  quartz  with  some 
calcite  or  dolomite;  tourmaline  is  occasionally  reported,  also 
magnetite.  The  ore  consists  of  free  gold  containing  more  or 
less  silver  and  associated  with  pyrite,  pyrrhotite,  zinc  blende, 
chalcopyrite,  galena,  and  arsenopyrite. 

Few  of  the  gold-quartz  veins  have  yet  been  followed  to  great 
depth.  Their  width  is  from  1  to  8  or  10  feet  at  most,  and  the 
metal  content  of  their  ore  must  necessarily  be  high,  for  the 
amount  that  can  be  taken  out  with  profit  is  small.  Their  ores 
would  probably  range  from  $5  to  $20  or  more  per  ton. 

The  Treadwell  ores,  which  are  mined  on  a  large  scale,  are  of 
low  grade,  containing  about  $3  in  gold  per  ton,  of  which  60  to 
75  per  cent,  is  free-milling,  the  concentrates  yielding  $30  to  $50 
per  ton.  The  Treadwell_deposits  consist  of  a  series  of  mineralized 
dikes  (Fig.  237)  of  albite  diorite  in  slates  near  the  east  shore  of 
Douglas  Island.  The  workings  extend  for  7,000  feet  along  the 
shore.  The  dikes  dip  about  50°  northeast.  The  dimensions 


684 


MINERAL  DEPOSITS 


of  the  dikes  are  variable,  the  larger  ones  having  a  maximum 
width  of  over  200  feet.  These  ore-bearing  dikes  have  been  fol- 
lowed to  a  depth  of  2,400  feet,  and  there  appears  to  be  no  diminu- 
tion of  the  average  tenor  of  the  ore  at  that  depth.  The  average 
annual  production  of  the  Treadwell  mines  was,  since  1910,  $4,000,- 
000.  In  1917,  an  invasion  of  sea  water  filled  most  of  the  mines. 


FIG.  237. — Horizontal  and  vertical  sections  of  ore-bodies  in  Alaska- 
Treadwell  mine,  Douglas  Island,  Alaska.     After  A.  C.  Spencer. 

The  ore-bodies  are  extensively  fractured  by  a  system  of  con- 
jugated joints,  along  which  there  are  irregular  veinlets  of  quartz 
and  calcite.  The  ore  minerals  are  chiefly  native  gold,  pyrite,  and 
pyrrhotite,  but  chalcopyrite,  galena,  zinc  blende,  and  molyb- 
denite are  also  found.  The  important  gangue  minerals  are 


HIGH-TEMPERATURE  DEPOSITS  685 

albite,  calcite,  and  quartz.  The  original  diorite  has  been  so 
thoroughly  altered  that  it  is  difficult  to  establish  its  exact  charac- 
ter. The  metasomatic  processes  of  the  Alaska  gold-quartz 
veins,  especially  the  Treadwell  dikes,  are  described  below  in  more 
detail. 

During  the  last  year  the  great  stringer-lodes  in  slate,  like  that 
of  the  Alaska  Gold  Mines  Company,  have  been  opened  on  a 
large  scale  on  the  mainland,  but  with  dubious  success  as  the 
ore  scarcely  averages  $1.25  per  ton. 

The  topographic  features  of  this  region  permit  the  generali- 
zation that  the  vertical  range  of  the  deposits  is  over  5,000  feet. 
They  have  been  followed  2,400  feet  below  sea  level,  and  typical 
veins  are  found  in  the  same  regions  at  elevations  of  3,000  feet 
or  more  above  sea  level.  They  were  formed  shortly  after  the 
great  intrusion  of  granodiorite,  and  the  vertical  range  now 
accessible  must  have  been  many  thousands  of  feet  below  the 
surface  of  the  earth  at  the  time  of  the  ore  deposition. 

Metasomatic  Processes  in  Veins  of  Southeastern  Alaska 

The  facts  that  the  Alaska  veins  contain  abundant  pyrrhotite 
and  some  tourmaline  and  magnetite  and  that  the  altered  country 
rock  contains  biotite  show  that  in  many  parts  of  the  region  the 
temperature  of  deposition  was  high.  Albitization  is  a  common 
process  and  appears  to  be  independent  of  the  amount  of  sodium 
in  the  country  rock.  It  takes  place  not  only  in  albite  diorite, 
where  it  might  be  interpreted  as  a  mass  reaction,  but  also  in  nor- 
mal diorite,  gabbro,  and  amphibolite. 

Adjacent  to  the  crosscutting  fissure  veins  of  the  Berners  Bay 
district  the  metasomatic  action  is,  as  shown  by  Knopf,  very 
similar  to  that  in  the  California  gold  belt.  Dolomite,  sericite, 
albite,  and  pyrite  are  the  principal  new  minerals  formed  in  the 
rock. 

The  Treadwell  mine  is  working  large  mineralized  dikes  of 
albite  diorite  in  slates  and  greenstones.  According  to  Spencer1 
the  original  rock  contained  albite-oligoclase,  microperthite,  horn- 
blende, and  biotite,  the  latter  two  minerals  in  small  amounts. 
The  altered  rock  contains  abundant  albite,  mostly  developed  by 

1  A.  C.  Spencer,  The  Juneau  gold  belt,  Alaska,  BuU.  287,  U.  S.  Geol. 
Survey,  1906,  p.  99. 


686  MINERAL  DEPOSITS 

the  replacement  of  microperthite,  also  quartz,  calcite,  muscovite, 
hornblende,  rutile,  epidote,  magnetite,  and  pyrite.  Albite  is  also 
formed  as  narrow  veinlets,  although  most  of  the  veinlets  con- 
sist of  calcite  and  quartz.  The  composition  of  the  altered  rock 
differs  considerably  from  place  to  place.  Spencer  holds  that 
sodium  has  been  added  to  the  rock,  together  with  carbon  dioxide 
and  sulphur.  Calcium  in  the  rock  has  been  fixed  by  the  carbon 
dioxide  and  suffered  little  leaching.  The  composition  of  one  of 
the  altered  rocks  is  calculated  as  follows: 

Quartz 2.34  Magnesite 0.11 

Albite 84.36  Siderite.... 0.57 

Anorthitc 1.11  Apatite 0.13 

Zoisite 0.91  Rutile 0.13 

Muscovite 3.03  Pyrite 2.10 

Calcite 3.80 

98.59 

Spencer  and  Knopf  have  shown  that  at  several  places  on  the 
mainland  near  Juneau  dioritic  rocks  near  the  veins  have  been 
altered  to  products  containing  brown  mica,  probably  biotite. 
Spencer  describes  the  alteration  in  the  Gold  Creek  district,  which 
results  in  the  development  of  biotite  (I  and  II  in  the  following 
table) .  The  mineral  composition  of  the  altered  rock  is  calculated 
as  follows:  Quartz,  45  per  cent.;  biotite,  22;  carbonates,  20; 
titaniferous  magnetite,  10.5;  and  sulphides,  2.5. 

Knopf  describes  an  altered  and  fresh  amphibolite  found  near 
the  Mendenhall  Glacier  (analyses  III,  IV,  V),  and  calculates  the 
mineral  composition  approximately  as  follows: 

Fresh  Altered 

amphibolite  amphibolite 

Orthoclase  (mol.) 6.7                          

Albite  (mol.) 18.3  39.3 

Actinolite 43 . 7  .... 

Biotite1 7.9  43.1 

Zoisite 4.5  16.4 

Epidote 18 . 3                          

Apatite 0.6  1.2 

100.0  100.0 
*By  difference. 


HIGH-TEMPERA  T URE  DEPOSI TS 


687 


ANALYSES   OF   FRESH    AND    ALTERED    ROCKS   FROM    QUARTZ    VEINS   OF 

SOUTHEASTERN  ALASKA 
(Analysts,  George  Steiger,  I,  II,  V;  J.  G.  Fairchild,  III,  IV) 


I 

II 

III 

IV 

V 

Si02  
TiO2  
Al  O 

47.76 
1.48 
13.98 
1.99 
8.72 
0.14 
9.07 
12.71 
Trace 
1.65 
0.20 
0.22 
2.06 

44.69 
2.25 
14.97 
0.60 
7.05 
0.14 
3.92 
10.07 
0.14 
2.36 
1.76 
0.36 
0.20 
0  02 

48 
1 
13 
3 
1  10 
0 

11 
2 

0 
2 

30 
01 
59 
12 
44 
25 
29 
09 

16 
55 
00 
06 

52.92 
0.99 
20.53 
Trace 
8.38 
0.28 
2.43 
4.76 

4.67 
2.96 
0.18 
1.58 

+    5 
-    0 
+  17 
-   9 
-   7 
+  0 
-12 
-20 

+  6 
+  3 
+  0 
-   2 

+  0 

10 
24 
70 
64 
so 
04 
33 
00 

92 
83 
34 
25 

86 

Fe2O3  

FeO  
MnO  ... 
MgO  
CaO  

BaO  
Na2O  
K2O  
H2O-  
H2O  + 

ZrO2 

CO2  
P,0.  

s 

None 
0.12 
0.04 

8.47 
0.26 

None      
0.26          0.57 

FeS2 

O  27 

Fe  S 

2  25 

LessO  0.02    

100.16 

99.78 

100 

12  |  100.25      -16 

80 

I.  Green  diorite,  Gold  Creek.     Contains  about  75  per  cent,  green  horn- 
blende; remainder  feldspar  with  some  quartz. 

II.  "Brown  diorite,"  Ebner  mine,  Gold  Creek. 

III.  Amphibolite,  Mendenhall  Glacier.     Sp.  gr.  3.084.     Dark  olive-green 
rock. 

IV.  Altered  amphibolite,   Mendenhall  Glacier.     Sp.   gr.   2.905.     Dark- 
brown  rock  with  pyrrhotite. 

V.  Gains  and  losses  in  grams  in  the  alteration  of  100  c.c.  of  amphibolite 
to  same  volume  of  altered  product. 

These  changes  differ  greatly  from  those  noted  along  fissure 
veins  of  the  more  ordinary  type.  In  the  first  place,  they  include 
actual  dehydration  and  distinct  additions  of  aluminum,  sodium, 
and  potassium,  the  alkalies  having  doubled  in  quantity.  In  both 
localities  ferric  oxide  is  almost  wholly  removed,  while  there  is 
some  decrease  in  the  ferrous  oxide.  Beyond  this  the  two  sets  of 
analyses  are  dissimiliar,  for,  while  one  indicates  20  per  cent,  of 


688  MINERAL  DEPOSITS 

carbonates,  the  other  is  entirely  without  carbon  dioxide.  As  a 
consequence  the  first  has  retained  much  more  calcium  and  mag- 
nesium than  the  second.  As  to  minerals  the  rock  rich  in  carbon- 
ate contains  biotite,  titaniferous  magnetite,  and  sulphides;  the 
one  without  carbonates  yields  albite,  biotite,  and  zoisite,  and 
Knopf  considers  that  apatite  has  been  formed  in  it. 

It  is  characteristic  of  the  deep-seated  veins  that  actinolite  is 
unstable,  while  biotite,  zoisite,  and  ilmenite  were  developed 
under  the  influence  of  the  vein-forming  solutions.  It  is  believed 
that  the  solutions  were  hot  and  ascending  and  that  they  carried 
both  sodium  and  potassium,  besides  phosphorus  and  sulphur. 

A  similiar  development  of  biotite  has  been  noted  in  the  Kolar 
gold  fields  in  Mysore,  India,1  which  are  very  productive  and  are 
worked  to  a  depth  of  4,000  feet.  The  veins,  which  are  prob- 
ably of  pre-Cambrian  age  are  contained  in  crystalline  schists. 
The  gangue  is  a  glassy  quartz  with  native  gold  and  a  small 
amount  of  pyrite,  pyrrhotite,  arsenopyrite,  etc.  Some  tourma- 
line is  present.  The  veins  contain  pitching  pay  shoots  in  which 
the  ore  is  5  feet  wide,  averaging  $20  to  $30  per  ton  in  the 
deepest  levels  (p.  189). 

The  Gold-Telluride  Veins  of  Western  Australia2 

Western  Australia  is  an  arid  tableland  of  moderate  elevation 
surmounted  by  short  and  low  ridges  (Fig.  238).  Crystalline 
schists  and  granites  are  the  principal  rocks.  The  schists  extend 
with  general  north-south  strike  and  vertical  or  steep  dip  across 
the  whole  central  part  of  the  state  and  consist  largely  of  amphibo- 
lites,  massive  or  foliated,  which  have  been  derived  by  meta- 
morphism  from  a  basic  rock — diorite,  gabbro,  or  diabase.  There 

1  F.  Hatch,  The  Kolar  gold  field,  Mem.  Geol.  Survey  India,  vol.  33,  pt. 
1,  1901. 

2  A.  Gibb  Maitland,  Bulls.  4,  15,  and  20,  Geol.  Survey  Western  Australia. 
C.  F.  V.  Jackson,  Bulls.  13  and  18,  Idem. 

E.  S.  Simpson,  Butt.  6,  Idem. 

T.  A.  Bickard,  The  telluride  ores  of  Cripple  Creek  and  Kalgoorlie,  Trans. 
Am.  Inst.  Min.  Eng.,  vol.  30,  1901,  pp.  708-718. 

P.  Krusch,  Zeitschr.  prakt.  Geol.,  1903,  pp.  321-331;  369-378. 

C.  O.  G.  Larcombe,  The  geology  of  Kalgoorlie,  Proc.  Austral.  Inst. 
Min.  Eng.,  vol.  5,  1910,  pp.  1-312. 

M.  Maclaren  and  J.  A.  Thomson,  Min.  and  Sci.  Press,  vol.  107,  1913, 
pp.  45,  95,  187,  228,  374. 


HIGH-TEMPERATURE  DEPOSITS 


689 


are  also  highly  altered  sedimentary  rocks  such  as  quartzites  and 
slates;  more  rarely  limestones.  At  Kalgoorlie,  for  example, 
slates  are  intimately  associated  with  amphibolites. 

Granitic  rocks,  in  part  gneissoid,  also  occur  extensively  in  the 
complex  of  crystalline  schists.    Lenticular  masses  of  amphibolite 


Illl    Devonian 

=    Pre  Cambrian 

'//,    Schists  of  uncertain  age 

+t  Granite 

A/C   Basaltic  Bocks 

•••    Permo  Carboniferous 

^   Cretaceous 


FIG.  238.— Geological  map  of  Western  Australia.     Scale  1  inch  =  330  miles. 

are  contained  in  the  granitic  rocks  and  vice  versa,  so  that  the 
sequence  of  the  rocks  is  not  always  clearly  apparent.     Maitland 
believes  that  many  of  the  granitic  rocks  are  intrusive  into  the 
amphibolites. 
The  age  of  the  rocks  is  not  definitely  known,  but  is  considered 


690  MINERAL  DEPOSITS 

pre-Cambrian.     Toward  the  northwest  coast  the  old  rocks  dis- 
appear beneath  transgressing  horizontal  Carboniferous  limestones. 
The  gold  deposits  are  contained  chiefly  in  the  amphibolites 
but  also,  though  less  commonly,  in  the  granitic  rocks.     Maitland 

says: 

All  the  important  auriferous  areas  occur  within  or  near  the  schistose 
rocks,  and  they  occupy  a  large  area  extending  from  the  south  coast  .  .  . 
to  the  northwest  coast,  over  about  14°  of  latitude.  The  auriferous  belts 
exceed  20  miles  hi  width  in  places. 

The  center  of  mining  activity  is  at  Kalgoorlie,  about  350  miles 
east  of  Perth.  From  the  mines  within  the  so-called  "Golden 
Mile"  at  that  place,  the  larger  part  of  the  output  of  Western 
Australia  has  been  derived,  although  other  mines  north  and 
northwest  of  Kalgoorlie  now  contribute  a  considerable  share. 
The  most  prominent  among  these  outside  mines  are  the  Westralia- 
Mt.  Morgan  and  Sons  of  Gwalia,  in  the  Mt.  Margaret  gold  field, 
and  the  Great  Fingall,  in  the  Murchison  gold  field,  300  miles 
northwest  of  Kalgoorlie.  The  total  gold  production  of  Western 
Australia  from  1886  to  1917,  inclusive,  is  about  $660,000,000, 
the  annual  yield,  which  is  gradually  decreasing,  being  now  about 
$22,000,000.  The  yield  of  the  Kalgoorlie  mines  since  discovery 
is  about  $330,000,000.  ' 

The  geologists  of  Western  Australia  distinguish  two  principal 
modes  of  occurrence  of  gold-bearing  lodes. 

1.  The  normal  quartz  veins  usually  occur  in  the  amphibolite 
or  along  the  contact  of  granitic  rocks  and  amphibolite.     Most  of 
the  veins  conform  in  strike  and  dip  with  the  steeply  dipping 
schists.     The  veins  are  usually  short  or  branched  and  curved, 
and  the  quartz  has  a  tendency  to  form  lenticular  ore-bodies. 
The  minerals  accompanying  the  native  gold  are  galena,  blende, 
pyrrhotite,    chalcopyrite,    arsenopyrite,    stibnite,    bismuthinite, 
pyrite,  scheelite,  chlorite,    calcite,    and    sericite.     In    addition 
tourmaline  is  reported  from  one  mine,  the  Sons  of  Gwalia. 

Most  of  the  deposits  of  this  class  have  probably  been  formed 
by  the  filling  of  open  cavities;  the  veins  are  often  bent,  corrugated, 
and  deformed.  At  Mt.  Morgan  the  quartz  bodies  form  solid 
pipes  of  lenticular  section,  the  main  axes  of  which  dip  45°  to  the 
south.  The  ore-shoots  within  these  lenses  also  have  a  southerly 
trend. 

2.  The  quartz  lenses  are  at  many  places  surrounded  by  altered 


HIGH-TEMPERATURE  DEPOSITS  691 

country  rock;  where  this  rock  prevails,  transitions  are  formed  to 
the  second  class  of  composite  replacement  deposits  or  sheared 
zones,  to  which  the  name  "lode  formations"  is  given.  Simpson 
describes  them  as  follows:1 

A  lode  formation  may  be  defined  as  a  more  or  less  vertical  zone  of  rock, 
usually  continuous  with  the  surrounding  rock  and  of  similar  origin,  but 
distinct  from  it  in  carrying  metallic  ores  disseminated  through  it  in  payable 
quantities  and,  as  a  rule,  characterized  by  strong  foliation.  The  typical 
lode  formations  probably  owe  their  origin  to  a  shearing  action  having  crushed 
and  foliated  portions  of  a  rock  mass  in  a  certain  definite  direction,  producing 
a  more  or  less  welt-defined  band  of  rock  through  which,  by  virtue  of  the  folia- 
tion, mineral-bearing  solutions  or  vapors  can  have  free  circulation.  In 
consequence  of  this,  mineral  deposits  are  formed  within  the  rock,  usually 
but  not  necessarily  extending  over  the  whole  of  the  foliated  zone,  but  seldom 
beyond  it,  and  having  no  definite  boundaries  horizontal!}'  or  vertically  other 
than  those  determined  by  the  decrease  of  the  assay  value  of  the  rock  in  any 
one  direction. 

In  the  southern  portion  of  the  Kalgoorlie  belt  the  rocks  consist  mainly 
of  amphibolites  (altered  in  various  ways,  but  largely  into  massive  chloritic 
rock  and  chlorite  schist,)  together  with  some  smaller  bodies  of  porphyrite, 
felsite,  graphitic  slate,  and  quartzite.  The  lode  formations  consist  almost 
entirely  of  vertical  or  steeply  inclined  zones  of  chlorite  schists  or  foliated 
greenstone,  often  passing  insensibly  into  unaltered  greenstone  on  either  side, 
but  sometimes  showing  an  irregular  boundary.  They  vary  in  width  from 
2  or  3  feet  up  to  80  feet. 

The  minerals  of  the  second  type  of  deposits  include  native  gold 
and  tellurides,  such  as  calaverite,  petzite,  hessite  and  coloradoite 
(telluride  of  mercury).  Pyrite  is  abundant,  but  is  almost  always 
finely  divided,  in  contrast  to  the  tellurides,  which  are  often 
massive.  Accessory  minerals  are  chalcopyrite,  zinc  blende, 
galena,  pyrargyrite,  enargite,  lollingite,  fluorite,  magnetite, 
rutile,  calcite,  dolomite,  siderite,  ankerite,  sericite,  chlorite,  and 
roscoelite.2  To  these  tourmaline  and  albite  should  be  added. 
The  ore-bodies,  as  shown  by  H.  C.  Hoover,  form  lenticular 
bodies.  They  have  usually  a  rich  core  from  which  the  gold 
content  decreases  outward,  and  the  lenses  are  of  large  dimensions. 
Mining  operations  have  attained  a  depth  of  3,600  feet,  and  at 
this  depth  some  of  the  mines  are  still  in  ore.  The  chief  mines  at 
Kalgoorlie  are  the  Great  Boulder,  Ivanhoe,  Horseshoe,  Persever- 
ance, Oroya-Brownhill,  Associated,  and  Lake  View  Consolidated. 

The  ore  varies  from  a  dark-green,  distinctly  chloritic  foliated 

1  E.  S.  Simpson,  Bull.  6,  Geol.  Survey  Western  Australia,  1902,  p.  22. 

2  Bull.  6,  Geol.  Survey  Western  Australia,  1902,  p.  21. 


692  MINERAL  DEPOSITS 

schist,  as  in  the  Oroya-Brownhill  mine,  to  pale-green  sericite 
schists  and  to  banded  or  massive  dark  rocks,  flinty  in  places  and 
ranging  from  dark  green  to  gray  or  brown.  Small  specks  of 
pyrite  are  distributed  through  the  ores,  which  contain  bright- 
yellow  gold  associated  with  much  pale-yellow  calaverite  and 
black,  lustrous  coloradoite  with  conchoidal  fracture.  In  places 
the  tellurides  and  gold  have  developed  as  seams  several  centi- 
meters thick,  in  joint  planes  crossing  the  schistosity.  The 
oxidized  zone  is  from  a  few  feet  to  200  feet  deep.  Some  geolo- 
gists believe  that  secondary  tellurides  and  native  gold  have  en- 
riched the  lodes  just  below  the  oxidized  zone.  At  greater  depths 
free  gold  is  rarely  visible  and  sulphides  tend  to  take  the  place  of 
the  rich  tellurides.  The  ores  have  gradually  declined  in  value 
from  $30-$40  per  ton  near  the  surface  to  $7-$10  in  the  deepest 
levels.  The  presence  of  sodium  chloride  in  the  mine  waters 
would  suggest  much  downward  transportation  of  gold. 

In  Western  Australia,1  as  in  Alaska,  there  have  been  local 
differences  in  the  processes  of  alteration.  In  the  Pilbara  gold 
field2  the  granite  next  to  the  vein  is  altered  to  a  greenish-gray 
rock,  the  calcium,  magnesium,  and  sodium  having  been  largely 
removed  and  the  potassium  considerably  increased.  It  contains 
no  carbonates.  The  course  of  alteration  is  entirely  similar  to 
that  of  many  deposits  found  at  intermediate  depths  in  the  Cor- 
dilleran  region  of  the  United  States. 

At  Kalgoorlie,  on  the  other  hand,  albite  and  carbonates  are  the 
principal  products.  The  unaltered  rock  consists  of  an  amphibo- 
lite  containing  amphibole,  chlorite,  zoisite,  and  albite.  The 
altered  rocks  forming  the  gold-bearing  lode  contain  chlorite, 
newly  formed  albite,  calcite,  dolomite,  siderite,  tourmaline,  seri- 
cite, roscoelite,  magnetite,  specularite,  and  nests  and  lenses  of 
fine-grained  quartz.  The  carbonate  grains  inclose  irregular 
masses  of  tellurides  and  coarse  gold,  but  the  larger  masses  of 
calaverite  also  contain  rhombohedrons  of  carbonates.  Crys- 
tals of  magnetite  embedded  in  tellurides  are  reported.  E.  S. 
Simpson  has  shown  that  the  ores  are  really  derived  from  am- 
phibolites  by  replacement.  The  replacement  is  irregular,  albite, 
quartz,  or  carbonates  alternately  predominating. 

The  character  of  alteration  is  shown  by  the  following  analyses : 

1  W.  Lindgren,  Metasomatic  processes  in  the  gold  deposits  of  Western 
Australia,  Econ.  Geol,  vol.  1,  1905,  pp.  530-544. 

2  A.  Gibb  Maitland,  BuU.  15,  Geol.  Survey  Western  Australia,  1904,  p.  12. 


HIGH-TEMPERATURE  DEPOSITS 

ANALYSES  OF  FRESH  AND  ALTERED  AMPHIBOLITES  FROM 
KALGOORLIE 


693 


3231 

1936 

206 

1753 

1751 

SiO 

48  86 

57  72 

51  27 

46  94 

42  01 

A1203  
Fe203  
FeO...  
MgO 

14.91 

11.13 

7  65 

9.68 
6.49 
9.17 
1  63 

13.85 
1.54 
2.63 
4  18 

12.49 
0.33 
9.20 
3  56 

8.42 
2.45 
15.76 
1  67 

CaO  
Na20  
K2O  
H2O- 

12.19 
2.58 
0.19 
0  04 

5.05 
3.92 
0.12 
0  16 

6.40 
1.78 
2.37 
0  40 

6.43 

1.84 
2.57 
0  09 

7.07 
2.62 
1.15 
0  23 

H2O+...  
TiO2 

1.51 
0  22 

1.51 
1   13 

0.22 
0  23 

0.30 
0  14 

0.67 
0  81 

CO2  

None 

1.84 

8.02 
Trace 

13.41 

15.65 

Te 

, 

Trace 

FeS2 

8  41 

2  25 

0  30 

MnO  

0.90 

0.09 

Trace 

0.32 

0.41 

100.18 

98.51 

101  .  30 

99.87 

99.22 

3231.  Star  of  Colac.  Bull.  6,  Geol.  Survey  Western  Australia,  1902, 
p.  67.  "Rock  consists  of  coarse-grained  mixture  of  feldspar  and  chlorite. 
It  contains  colorless  hornblende,  saussurite  with  clear  mosaic  of  albite,  also 
ilmenite  surrounded  by  leucoxene."  Analyst,  C.  C.  Williams.  .  . 

1936.  Hannans  main  shaft,  at  depth  of  600  feet.  "Rather  coarse 
grained  rock  containing  hornblende,  chlorite,  feldspar  (albite?),  ilmenite, 
and  secondary  quartz."  Analyst,  C.  C.  Williams. 

206.  300-foot  level,  Lake  View  Consols  mine.  Idem.,  pp.  23  and  67. 
"Strongly  foliated  gray  lode  stuff.  Assay,  9  oz.  12  dwt.  of  gold  and  6  oz. 
7  dwt.  of  silver  per  ton.  It  shows  chlorite  and  sericite  on  the  cleavage 
planes."  Analyst,  E.  S.  Simpson. 

1753.  A  foliated  greenish-gray  lode  stuff  from  the  400-foot  level,  Ivanhoe 
mine.  Idem.,  pp.  23  and  67.  "  Contains  chlorite,  carbonates,  a  feldspathic 
material,  ilmenite,  and  some  quartz.  Trace  of  gold  and  no  silver."  Analyst, 
C.  G.  Gibson. 

1751.  Siderite  rock,  West  crosscut,  400-foot  level,  Ivanhoe  mine.  Idem., 
p.  67.  "  Gray  compact  rock,  containing  carbonates,  grains  of  quartz  and 
of  black  iron  ores,  .  .  .  altered  feldspar,  and  some  scaly  green  chlorite." 
Analyst,  C.  G.  Gibson. 

Analysis  3231  is  probably  fairly  representative  of  many 
amphibolites  of  Western  Australia.  It  contains  no  free  quartz 


694 


MINERAL  DEPOSITS 


and  is  rich  in  lime  and  iron.     There  is  very  little  potash,  and  only 
2.58  per  cent,  of  soda. 

The  last  three  analyses  may  be  roughly  calculated  as  shown 
in  the  accompanying  table: 


PROBABLE  MINERAL  COMPOSITION  OF  ORES  FROM  KALGOORLIE 


206 

1753 

1751 

Remarks 

Quartz 

29  52 

25  20 

21  44 

Chlorite  
Albite  
Sericite  
CaC03  
MgC03  
FeCO3  
MnCO3  
Pyrite  
Magnetite  (?)  ... 

6.86 
15.12 
19.54 
11.42 
4.20 
2.32 

8.41 

2.76 
15.70 
21.52 
11.50 
5.38 
14.60 

2.25 
0.47 

2.94 
22.12 
9.58 
12.61 
1.76 
23.20 
0.67 
0.30 
3.53 

FeO.4MgO.  Al2O3.3SiO2.4H2O. 
K2O.3Al2O3.6SiO2.2H20, 

TiO2  
Fe,O, 

0.23 
1   54 

0.14 

0.81 

A1203  
Hygroscopic 
water 

2.14 

0.61 

0.23 

101.30 

100.13 

99.19 

At  first  glance  the  analyses  of  the  altered  rocks  do  not  show 
very  great  changes  so  far  as  bases  and  silica  are  concerned.  The 
principal  differences  are  in  the  added  carbon  dioxide  of  the  altered 
rocks,  ranging  from  8  to  nearly  16  per  cent. 

Magnesium,  calcium  and  iron  have  been  fixed  as  carbonates, 
the  latter  also  as  pyrite.  The  combined  water  has  decreased 
owing  to  replacement  of  chlorite  by  sericite.  The  silica  set  free 
has  been  deposited  as  quartz.  Soda  has  decreased  slightly  but 
potash  has  increased,  though  not  very  greatly.  The  abundant 
development  of  albite  and  carbonates  recalls  the  processes  of 
replacement  in  the  deposits  near  Angels  Camp  in  California, 
and  at  the  Alaska-Treadwell  mine. 


HIGH-TEMPER ATUER  DEPOSITS  695 

COPPER  DEPOSITS 
The  Copper-Tourmaline  Deposits 

In  several  parts  of  the  world  the  association  of  chalcopyrite 
with  tourmaline  is  fairly  common.  In  addition  the  ores  may  con- 
tain some  gold  and  silver.  Other  minerals  occasionally  present 
are  magnetite,  specularite,  rutile,  pyrite,  pyrrhotite,  molybdenite, 
bismuthinite,  wolframite,  scheelite,  tetrahedrite,  quartz,  siderite, 
fluorite,  and  biotite.  Anhydrite  is  often  present. 

The  copper-tourmaline  veins  of  Cornwall,  which  also  carry 
cassiterite,  establish  the  transition  from  these  deposits  to  the  tin- 
bearing  veins.  Other  occurrences,  like  those  of  Meadow  Lake, 
California,  and  the  Passagem,  Brazil,  emphasize  the  transition 
to  the  gold-bearing  quartz  veins. 

The  deposits  are  in  part  fissure  veins,  in  part  replacements  in 
brecciated  or  sheared  zones.  In  both  modes  of  occurrence  the 
country  rock  is  subject  to  intense  metasomatic  changes,  and  tour- 
maline is  developed  by  replacement,  often  for  a  considerable 
distance  from  the  solution  ducts.  The  normal  form  of  alteration 
in  feldspathic  rocks  is  sericitization,  sometimes  accompanied  near 
the  veins  by  silicification.  The  feldspars  are  replaced  by  sericite, 
and  both  feldspar  and  quartz  are  penetrated  by  acicular  tourma- 
line prisms,  usually  of  bluish-gray  color.  Chalcopyrite,  pyrite, 
and  sometimes  other  sulphides  also  develop  in  the  altered  rock. 
The  final  product,  unless  the  tourmalinization  is  unusually  intense, 
lacks  the  coarsely  crystalline  structure  of  the  typical  greisen; 
the  mica  foils  are  usually  small.  In  some  deposits  biotite 
develops. 

Chile. — 'Many  tourmaline-copper  deposits  are  found  in  Chile. 
They  appear  to  be  connected  with  basic  rocks — gabbro,  diabase, 
porphyrite,  diorites,  etc.1 

Von  Groddeck  has  described  the  formerly  important  Tamaya 
mines,  where  veins  containing  copper  ores  cut  diabase  and  por- 
phyrite. The  tourmaline  is  not  only  present  in  the  filling  of  the 
principal  vein,  which  dips  35°  and  is  from  3  to  6  feet  thick,  but 

*A.  von  Groddeck,  Zeitschr.  Deutsch.  geol.  Gesell.,  vol.  39,  1887,  pp. 
237-266. 

A.  W.  Stelzner,  Zeitschr.  prakt.  Geol.,  1897,  pp.  41-53. 

Moericke,  Tsch.  M.  u.  p.  M.,  vol.  12,  1891,  pp.  186-198. 

L.  Darapsky,  Das  Departement  Taltal,  Berlin,  1900,  pp.  167-172. 


696  MINERAL  DEPOSITS 

is  also  abundantly  developed  in  the  calcitic,  chloritic,  and  mica- 
ceous altered  country  rock.  Asbestos  and  tremolite  (cumming- 
tonite)  are  also  mentioned. 

Similar  veins  at  Las  Condes,  90  miles  east  of  Santiago,  have 
been  described  by  A.  W.  Stelzner.  The  rocks  are  granite  and 
altered  andesites.  The  vein  filling  consists  of  pyrite,  chalcopy- 
rite,  quartz,  and  a  loose  mass  of  tourmaline  needles  and  minute 
crystals  of  zircon,  octahedrite,  and  specularite.  The  country 
rock  is  bleached  and  impregnated  with  pyrite  and  tourmaline. 
At  Peralillo,  31  kilometers  from  Santiago,  a  similar  pyrite  - 
chalcopyrite  vein  in  diorite  carries  tourmaline,  molybdenite, 
scheelite,  and  cupro-scheelite. 

The  most  prominent  representative  of  this  type  in  the  world 
is  the  Teniente,  or  Braden,  deposit,  situated  in  the  Western 
Cordillera,  50  miles  S.S.E.  of  Santiago  at  an  elevation  of  8,000 
feet.  Very  large  ore-bodies  have  been  developed  by  tunnels  over 
a  vertical  interval  of  2,500  feet  and  the  production  is  now  about 
6,000  tons  per  day  of  2  to  4  per  cent.  ore. 

An  intrusive  mass  of  andesite  porphyry  and  monzonite  por- 
phyry is  injected  into  Tertiary  andesitic  lavas  and  has  been 
extensively  mineralized.  The  greatest  ore-bodies  surround  a 
volcanic  explosion  vent  about  3,600  feet  in  diameter.  This  is  filled 
with  rudely  stratified  tuff  and  the  tuff  is  again  intruded  by  masses 
of  andesitic  breccias  of  several  kinds.  The  period  of  greatest 
mineralization  followed  the  intrusion  of  the  breccias  and  resulted 
in  the  replacement  of  the  rocks  surrounding  the  "crater"  by 
quartz,  specularite,  tourmaline,  chalcopyrite,  pyrite  and  oc- 
casionally other  sulphides.  A  still  later  epoch  of  minerali- 
zation yielded  richer  ores  of  chalcopjTite,  bornite,  tennantite, 
siderite,  rhodochrosite  and  anhydrite  while  during  the  last 
commercially  unimportant  epoch  were  deposited  chalcopyrite, 
bornite,  quartz,  barite  and  gypsum.  Crystals  of  gypsum  27 
feet  long  and  2  feet  in  diameter  were  found  in  cavities  coated  by 
these  products  of  the  last  mineralization. 

The  deposit  has  been  somewhat  enriched  by  chalcocitization 
effected  by  descending  waters. 

The  mineralization  probably  took  place  at  a  depth  below  the 
surface  of  about  4,000  feet  and  was  effected  by  solutions 
containing  much  boric  acid.  They  were  probably  very  hot 
and  may  have  reached  the  surface  in  gaseous  form  like  the  "sof- 
fioni"  of  Tuscany.  The  abundance  of  boron  emanations  con- 


HIGH-TEMPERATURE  DEPOSITS  697 

nected  with  volcanic  action  in  the  southern  Andes  is  remarkable. 
The  numerous  borax  deposits  in  Chile,  Bolivia  and  the  Argentine 
bear  witness  that  some  of  these  emanations  reached  the  surface. 

United  States. — In  the  Cordilleran  region  of  the  United  States 
there  are  a  number  of  similar  smaller  deposits.  They  have  been 
described  from  Meadow  Lake,  California,1  from  the  Blue  Moun- 
tains of  Oregon2  and  from  the  upper  Pecos  River  in  New  Mexico.3 

The  most  productive  deposit  of  this  type  occurred  at  the  Cac- 
tus mine,4  in  the  San  Francisco  Range,  in  southern  Utah.  This 
deposit  is  contained  in  a  brecciated  zone  in  post-Paleozoic  mon- 
zonite;  it  is  in  places  200  feet  wide  and  at  least  900  feet  long. 


FIG.  239. — Replacement  veinlet,  War  Eagle  mine,  Rossland,  B.  C.  a, 
Granular  aggregate  of  orthoclase  with  a  little  sericite;  b,  biotite;  q,  quartz; 
c,  chlorite;  black,  pyrrhotite.  Magnified  40  diameters. 

The  ores  have  been  followed  to  a  depth  of  800  feet.  Brown  tour- 
maline, with  quartz,  pyrite,  and  chalcopyrite,  coats  the  fragments 
of  brecciated  rock,  which  is  sericitized  and  contains  some  meta- 
somatically  developed  tourmaline.  Other  minerals,  formed 
somewhat  later  than  the  tourmaline,  are  siderite,  anhydrite, 
specularite,  and  tetrahedrite;  these  also  are  associated  with 
chalcopyrite,  the  development  of  which  continued  during  the 

1  W.  Lindgren,  Am.  Jour.  Sci.,  3d  ser.,  vol.  46,  1893,  p.  201. 

2  W.  Lindgren,  Prof.  Paper  68,  U.  S.  Geol.  Survey,  1910,  p.  113. 

3  W.  Lindgren,  Twenty-second  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  2,  1901, 
pp.  551-776. 

4  W.  Lindgren,  Econ.  Geol,  vol.  5,  1910,  pp.  522-527. 

B.  S.  Butler,  Geology  and  ore  deposits  of  the  San  Francisco  and  adjacent 
districts,  Utah,  Prof.  Paper  80,  U.  S.  Geol.  Survey,  1913,  pp.  172-178. 


698  MINERAL  DEPOSITS 

whole  epoch  of  mineralization.  The  ores  are  of  low  grade.  In 
1908,  177,000  tons  of  ore  were  mined  which  yielded  copper,  2 
per  cent.;  silver,  0.2  ounce,  and  gold,  0.01  ounce  per  ton.  The 
ore  was  concentrated  and  smelted.  The  mine  is  now  closed, 
poorer  ores  having  been  found  in  depth. 

The  Gold-Copper  Deposits 

In  a  few  of  these  copper  deposits  tourmaline  is  absent  or  rare. 

The  Rossland  district1  is  situated  in  British  Columbia  near 
the  boundary  line  of  the  State  of  Washington.  It  has  been 
producing  smelting  ores  since  1890,  and  has  yielded  a  total  of 
$62,300,000  in  gold,  copper  and  silver.  The  ores  contain  about 
$5  to  $10  in  gold  and  0.3  ounce  silver  both  per  ton,  as  well  as 
0.5  to  3  per  cent,  copper.  The  mines  are  opened  to  a  greatest 
depth  of  2,200  feet.  The  deposits  are  steeply  dipping  replacement 
veins  (Fig.  239)  along  shear  zones  in  monzonite  and  augite 
porphyrite.  Granodiorite  appears  in  some  places  and  is  thought 
to  represent  upward  extensions  of  the  Trail  batholith,  the 
emanations  from  which  are  believed  to  have  formed  the  deposits. 
The  ore  minerals  are  chalcopyrite  and  pyrrhotite  with  some 
pyrite,  arsenopyrite,  molybdenite  and  bismuthinite.  The  gangue 
minerals  comprise  quartz,  magnetite,  calcite  and  biotite  with 
some  garnet,  arsenopyrite,  actinolite  and  wollastonite.  The 
metasomatic  action  on  the  country  rock  has  resulted  in  much 
secondary  biotite.  The  veins  were  formed  within  the  epoch  of 
intrusion  for  they  are  intersected  by  many  basic  dikes  related 
to  camptonite.  Apophyllite  and  other  zeolites  occur  in  druses. 
There  is  little  evidence  of  secondary  enrichment  as  perhaps  is 
natural  in  a  recently  glaciated  country. 

The  great  copper  mining  district  of  Cobar  in  western  New 
South  Wales2  contains  strong  lodes  in  a  deeply  eroded  desert 
range  of  older  Paleozoic  sediments;  they  are  replacement  veins 
from  10  to  120  feet  wide  cutting  sandstone  and  slate  at  acute  an- 
gles in  strike  and  dip.  The  lodes  show  typical  "hammock  struc- 
ture" (p.  152)  and  carry  chalcopyrite,  magnetite,  and  pyrrhotite 
in  big  lenses.  The  ores  gradually  fade  into  country  rock.  The 

1  W.  Lindgren,  Trans.  Am.  Inst.  Min.  Eng.,  vol.  30,  1901,  pp.  644-645. 
C.  W.  Drysdale,  Geology  and  ore  deposits  of  Rossland,  B.  C.,  Mem.  77, 

Canada  Geol.  Survey,  1915. 

2  E.  C.  Andrews,  Report  on  the  Cobar  copper  and  gold  field,  Mineral 
Resources  17,  Geol.  Survey,  N.  S.  W.,  1913. 


HIGH-TEMPERATURE  DEPOSITS  699 

average  contents  are  2.5  per  cent,  copper  with  $1  to  $2  in  gold 
and  2  to  3  ounces  of  silver  per  ton.  Among  the  gangue  minerals 
are  quartz  and  an  iron  silicate,  probably  ekmannite.  There 
is  no  arsenic  or  antimony  but  some  bismuth  is  present.  The 
greatest  depth  attained  is  1,600  feet.  Some  lodes  with  more 
quartz  and  less  sulphides  are  worked  as  gold  deposits. 

With  the  water  level  200  to  450  feet  below  the  surface  and 
strong,  salty  mine  waters  it  is  not  surprising  that  there  was 
strong  enrichment  of  copper  with  upper  levels  as  oxidized  ores. 
A  short  distance  below,  water  level  secondary  sulphides  were 
found,  comprising  according  to  Andrews,  both  chalcocite  and 
chalcopyrite.  There  was  little  enrichment  of  gold  and  silver. 

It  is  remarkable  that  no  intrusive  rocks  are  found  within  long 
distance  of  these  deposits.  They  were  doubtless  formed  at  very- 
great  depth  and  at  high  temperature. 

The    Copper-Bearing    Veins    Allied    to    Contact-Metamorphic 
Deposits  and  Pegmatites 

In  regions  containing  contact-metamorphic  copper  deposits 
it  is  not  altogether  unusual  to  find  pyritic  veins  which  exert  an 
alteration  on  adjoining  limestone  similar  to  contact  metamor- 
phism,  indicating  that  the  vein-forming  solutions  possessed  a 
high  temperature. 

The  veins  of  cupriferous  pyrite  at  Clifton,  Arizona,'1  which  in- 
tersect porphyry  and  contact-metamorphic  limestone,  are  prob- 
ably in  part  of  this  kind,  for  it  was  observed  in  many  places  that 
where  they  cut  across  limestone,  tremolite  and  magnetite  had 
developed  adjacent  to  the  vein.  The  primary  deposits  contain 
little  copper  but  are  enriched  by  surface  waters. 

Several  occurrences  of  this  kind  are  reported  from  New  Mexico, 
particularly  from  the  Sierra  Hachita  district. 

One  of  the  best  instances  is  that  of  Massa  Marittima,  in  Tus- 
cany, described  by  B.  Lotti2  and  V.  Novarese.  The  great  veins 
carry  chalcopyrite,  pyrite,  galena,  and  zinc  blende  and  cut  across 
Eocene  limestone  and  clay  shales.  The  limestone,  but  not  the 
shale,  is  replaced  near  the  vein  by  pyroxene,  epidote,  quartz, 
and  sulphides.  Some  bismuth  and  a  little  tin  are  present  in  the 

1  W.  Lindgren,  Prof.  Paper  43,  U.  S.  Geol.  Survey,  1905. 
-  B.  Lotti,   Descrizione,  etc.,  di  Massa  Marittima  in  Toscana,   Mem. 
descritt.  Carta  Geol  d' Italia,  vol.  8,  1893. 

K.  Ermisch,  Zeitschr.   prakt.  Geol,  1905,  pp.  206-241. 


700 


MINERAL  DEPOSITS 


ore.  The  mineralization  is  believed  to  be  due  to  the  intrusion 
of  a  granite  of  Tertiary  age  which  on  the  surface  does  not  come 
within  several  miles  of  the  deposit.  The  whole  region,  however, 
gives  evidence  of  strong  mineralization. 

Copper -Titanium  Veins. — The  small  but  interesting  group  of 
the  chalcopyrite  veins  associated  with  titanium  minerals  is  of 
uncertain  affiliations.  In  some  respects  they  are  very  closely 
allied  to  the  pegmatites. 


FIG.  240. — Stereogram  showing  relation  of  quartz  pipe  and  mineralized 
quartz  monzonite  in  the  O.K.  Mine,  Beaver  Lake  district,  Utah.  1,  quartz; 
2,  altered  monzonite;  3,  monzonite;  4,  high  grade  ore.  After  B.  S.  Butler, 
U.  S.  Geol.  Survey. 

Such  deposits  have  been  described  from  Hereroland  in  South 
Africa,1  at  Rehoboth  and  Otjizongati.  They  are  continuous 
quartz  veins  in  mica  schist  and  carry  pyrite,  chalcopyrite,  bor- 
nite  and  molybdenite  with  orthoclase,  albite-oligoclase,  rutile, 
ilmenite,  apatite  and  tourmaline.  They  contain  also  a  little 
gold. 

Copper-Molybdenum  Veins. — The  association  of  copper  [and 
molybdenum  is  not  uncommon  but  many  other  ore  minerals  are 

1  Eberhard  Rimann,  Zeitschr.  prakt.  Geol,  vol.  22,  1914,  pp.  223-226. 


HIGH-TEMPERATURE  DEPOSITS  701 

usually  also  present.  Butler1  describes  the  O.K.  deposit,  Beaver 
Lake  district,  Utah,  in  which  the  characteristics  of  pegmatites  are 
curiously  mixed  with  those  of  high  temperature  veins.  The  de- 
posit worked  to  a  depth  of  400  feet  consists  of  a  cylindrical  body 
of  extremely  coarse  and  drusy  pegmatitic  quartz  connected  with 
a  steep  fissure  and  surrounded  by  a  zone  of  sericitized  quartz 
monzonite.  The  quartz  has  many  offshoots  of  minor  veins, 
which  carry  quartz,  chalcopyrite  and  molybdenite  (Fig.  240). 
The  ore  in  the  upper  levels  is  oxidized  with  secondary  sulphides. 

THE  LEAD-SILVER-ZINC  DEPOSITS 

Veins  with  Tourmaline. — The  combination  of  galena  and  tour- 
maline is  rare,  galena  being  generally  found  in  deposits  formed 
at  lower  temperatures.  Recent  investigations  by  A.  Knopf,2 
for  the  U.  S.  Geological  Survey,  show  that  many  of  the  veins  in 
the  contact  zone  and  in  the  igneous  rock  of  the  Boulder  batho- 
lith  of  quartz  monzonite  in  Montana  belong  to  this  unusual 
group.  The  Alta  vein  is  the  best  known  and  the  richest  of  these 
deposits;  it  is  supposed  to  have  yielded  over  $32,000,000  in  lead 
and  silver  and  it  was  thus  one  of  the  greatest  lead-silver  deposits 
of  the  world.  The  monzonite  contains  a  little  tourmaline,  its 
aplite  dikes  somewhat  more,  and  the  quartz  veins  are  rich  in  this 
mineral.  In  the  same  district  H.  V.  and  A.  N.  Winchell3  ob- 
served a  pyrite-tourmaline  vein,  the  ore  of  which  contains  mainly 
silver  with  some  copper  and  lead  minerals.  P.  Billingsley  and 
J.  A.  Grimes4  have  also  examined  these  veins  and  conclude  that 
they  have  been  formed  in  or  near  the  flat  roof  of  that  batholith. 

Veins  with  Garnet. — The  great  Broken  Hill  lode5  in  the  desert 
region  of  western  New  South  Wales  is  representative  of  this 

1  B.  S.  Butler,  Prof.  Paper  80,  U.  S.  Geol.  Survey,  1913,  p.  125. 
7  The  tourmaline  silver-lead  type  of.  ore  deposit,  Econ.  Geol.,  vol.    8, 
1913,  pp.  105-118;  also  Bull.  627,  U.  S.  Geol.  Survey,  1913. 

3  Econ.  Geol,  vol.  7,  1912,  pp.  287-294. 

4  Trans.  Am.  Inst.  Min.  Eng.,  vol.  58,  1918,  pp.  284-368. 

5  E.  F.  Pittman,  Records,  Geol.  Survey  N.  S.  W.,  vol.  3,  pt.  2,  1892. 
I  .1.  B.  Jaquet,  Mem.  5,  Geol.  Survey  N.  S.  W.,  1894. 

D.  Mawson,  Memoris  Royal  Soc.  of  South  Australia,  1912. 
R.  Beck,  Zeitechr.  prakt.  Geol,  1899,  pp.  65-71. 

E.  S.  Moore,  Econ.  Geol,  vol.  11,  1916,  pp.  327-348. 

See  also  "Report  of  Subcommittee,"  Trans.  Australas.  Inst.  Min. 
Eng.,  vol.  15,  1911,  pp.  160-236. 

W.  E.  Wainright  and  P.  H.  Warren,  The  Broken  Hill  South  mine, 
Mining  Mag.,  Jan.,  1918,  pp.  12-19. 


702  MINERAL  DEPOSITS 

rare  class.  This  lode  which  has  yielded  lead,  silver  and  zinc  to 
the  value  of  about  $450,000,000  since  its  discovery  in  1883,  is 
contained  in  a  folded  complex  of  schists  of  sedimentary  and 
igneous  origin,  among  which  are  sillimanite  schist,  amphibolite, 
granite  gneiss  and  quartzite,  all  of  probable  pre-Cambrian  age. 
The  lode  has  been  opened  over  a  length  of  3  miles  and  the 


Scale  of  Feet 


FIG.  241. — Vertical  section  through  the  ore-body  of   Broken   Hill  South 
Mine,  N.  S.  W.    After  Wainright  and  Warren. 

deepest  shaft  has  attained  1,800  feet,  the  ore  continuing  to  that 
depth. 

A  fault  zone  6  to  10  feet  wide  occupies  the  footwall  of  the  deposit. 
On  the  hanging  side  the  ore  bulges  out  in  places  to  great  masses, 
which,  as  shown  in  Fig.  241,  seem  to  follow  the  folded  schists 
and  form  saddle-like  bodies  probably  formed  by  replacement 
of  the  schist.  According  to  F.  E.  Wright  the  quartz  in  the  de- 


HIGH-TEMPERATURE  DEPOSITS  703 

posit  has  been  formed  at  temperatures  below  575°  C.  The  ore 
contains  galena  and  zinc  blende,  with  subordinate  pyrite;  the 
gangue  includes  manganese,  garnet,  rhodonite,  quartz  and  calcite. 
The  ores  contain  from  3  to  14  ounces  of  silver  per  ton,  14  to  16 
per  cent,  lead  and  8  to  18  per  cent.  zinc. 

The  surface  gave  little  indication  of  the  character  of  the  deposit. 
Down  to  a  depth  of  300  feet  there  was  a  gossan,  20  to  100  feet 
wide  of  quartz,  limonite,  manganese  dioxide,  hematite  and  kaolin. 
Below  this  were  found  great  masses  of  cerussite,  anglesite, 
cuprite  and  malachite,  with  abundant  cerargyrite,  embolite  and 
iodyrite.  There  was  but  little  smithsonite,  the  zinc  having  been 
removed  by  leaching. 

Where  the  oxidized  ores  changed  to  primary  sulphides  there 
was  a  thin  deposit  of  sooty  chalcocite,  rich  in  silver  and  copper; 
the  slight  depth  of  these  secondary  sulphides  is  remarkable. 

The  important  deposits  in  the  Kootenay  district  of  British 
Columbia  must  have  been  formed  under  similar  conditions. 
According  to  S.  J.  Schofield1  they  may  be  considered  as  high 
temperature  equivalents  of  the  Coeur  d'Alene  lead  deposits. 

The  ores  form  replacement  deposits  of  argillaceous  quartzite 
of  pre-Cambrian  (Algonkian)  age.  These  rocks  probably  rest 
on  intrusive  granite. 

The  ore-bodies  conform  roughly  to  strike  and  dip  of  the  quart- 
zites;  the  greatest  dimensions  are  825  by  120  feet.  The  ore  is 
an  intimate  mixture  of  galena  and  zinc  blende  with  minor  amount 
of  pyrite,  pyrrhotite,  magnetite  and  jamesonite;  the  scant  gangue 
contains  red  garnet,  diopside,  actinolite  and  biotite  with  sub- 
ordinate calcite.  The  gangue  minerals  are  earlier  than  the 
sulphides. 

THE  COBALT-TOURMALINE  VEINS 

The  association  of  tourmaline  with  nickel  and  cobalt  minerals 
in  San  Juan,  Department  Freirina,  Chile,  has  been  described  by 
O.  Stutzer.2  In  the  same  paper  he  gives  a  general  review  of  the 
tourmaline  veins. 

1  Econ.  Geol,  vol.  7,  1912,  pp.  351-363. 

Kootenay  District,  B.  C.,  Summ.  RepL,  Canada  Geol.  Survey,  1915, 
pp.  93-94. 

2  Zeitschr.  prakt.  Geol.,  1906,  pp.  294-298. 


CHAPTER  XXVII 

DEPOSITS  FORMED  BY  PROCESSES  OF  IGNEOUS 
METAMORPHISM 

INTRODUCTION    " 

General  Features. — In  many  geological  provinces  and  during 
all  ages  molten  magmas  have  invaded  older  rocks  without  reach- 
ing the  surface.  The  intrusive  magma  cooled  slowly  and  crystal- 
lized either  as  rocks  with  coarsely  granular  texture,  such  as 
granite,  diorite,  syenite,  monzonite,  gabbro,  or  diabase,  or  as  the 
corresponding  porphyries  with  holo crystalline  groundmass.  By 
means  of  uplift  and  subsequent  erosion,  these  igneous  rocks 
become  exposed  at  the  surface.  If  the  rocks  bordering  the 
intrusives  are  crystalline  schists  or  older  igneous  rocks,  they 
seldom  show  much  alteration  along  the  contacts,  but  where 
they  are  of  sedimentary  origin,  like  sandstone,  shale,  and  lime- 
stone, considerable  metamorphism  is  effected  in  them  for  a 
varying  distance  from  the  contact.  In  many  places  deposits  of 
metallic  ores  or  other  useful  minerals  occur  at  these  contacts, 
particularly  where  the  older  rock  consists  of  limestone. 

The  form  of  such  deposits  is  irregular  and  bunchy,  but  many 
of  them  are  tabular  by  reason  of  following  the  contact  (Fig.  242) 
or  certain  strata  in  the  intruded  rocks  favorable  for  deposition 
(Fig.  243).  Their  mode  of  occurrence  is  almost  wholly  meta- 
somatic — that  is,  they  are  formed  by  replacement  of  the  enclosing 
rock. 

The  mineral  association  is  characteristic:  Chalcopyrite,  pyrite, 
pyrrhotite,  zinc  blende,  and  molybdenite  are  the  most  common 
sulphides;  magnetite  and  specularite  the  most  common  oxides. 
The  most  prominent  gangue  minerals  are  various  silicates  of 
calcium,  magnesium,  iron,  and  aluminum,  which  have  been  par- 
tially furnished  by  the  carbonate  rocks  and  shales.  Among 
these  so-called  contact-metamorphic  silicates  are  garnet,  epidote, 
vesuvianite,  diopside,  tremolite,  and  wollastonite.  Recrystal- 
lized,  sometimes  exceedingly  coarse  calcite  is  abundant;  quartz 
is  rarely  present  in  large  amounts.  The  ore  minerals  are  usually 
later  than  the  silicates. 

704 


IGNEOUS  METAMORPHISM 


705 


Some  of  these  deposits  contain  valuable  non-metallic  minerals, 
like  graphite  or  corundum,  but  ordinarily  they  are  mined  for  the 
base  metals.  In  the  main  the  ore  and  minerals  are  of  simple 
composition  and  formulas.  The  association  indicates  an  origin 
at  high  temperature,  perhaps  from  300°  to  600°  C.  Close  to  the 
igneous  rock  the  temperature  may  have  been  materially  higher. 


IlilllP^^ 
^^l/G™tek  '^% 


Granodiorite 


Gray  Marbleized  Limestone 
N 


arnet  Rock 

th  Areas  of 

Copper 
Sulphides 


300  Feet 


FIG.  242. — Sketch  showing  relation  of  ore  zone  to  granodiorite  and  lime- 
stone, Bullion  district,  Nevada.     After  W.  H.  Emmons. 

The  igneous  rock  itself  may  be  wholly  fresh  or  it  may  contain 
minerals  closely  allied  to  those  in  the  deposit  itself,  such  as  garnet 
and  epidote,  or  it  may  show  sericitization  with  veins  somewhat 
later  than  the  alteration  at  the  contact. 

History. — In  1865  Bernard  von  Cotta  described  the  iron 
deposits  of  the  Banat  province  of  Hungary  and  expressed  the 
opinion  that  they  were  due  to  the  action  of  intrusive  rocks  on 
the  adjoining  Mesozoic  limestone.  He  also  correlated  these 
ores  with  those  of  Bogoslowsk,  in  the  Ural  Mountains,  of  Kris- 


706 


MINERAL  DEPOSITS 


tiania,  in  Norway,  and  of  other  districts.  Von  Groddeck,1 
however,  first  recognized  them  as  a  definite  group  to  which  he 
gave  the  name  "Kristiania  type."  He  stated  that  they  were 
produced  by  contact  metamorphism  and  called  them  briefly 
"contact  deposits."  Some  of  the  examples  mentioned  by  von 
Groddeck  and  others  are  doubtful,  and  in  later  text-books  the 
type  was  rather  neglected. 

In  the  last  years  of  the  nineteenth  century  Vogt2  revived  the 
interest  in  this  class  by  describing  the  contact-metamorphic 
deposits  of  Kristiania.  A  little  later  the  deposits  at  Seven 
Devils,  Idaho3 — the  first  of  this  type  to  be  noted  in  the  United 


FIG.  243. — Diagram  of  contact-metamorphic  deposit  in.  vertical  section. 
Ore  shown  in  black.     Contact-metamorphic  rocks  beyond  ore  stippled. 

States — were  described,  and  in  a  paper  on  the  character  and 
genesis  of  certain  contact  deposits4  the  type  was  redefined  and  a 
number  of  examples  from  the  United  States  were  cited.  W.  P. 
Blake5  mentioned  the  frequent  occurrence  of  this  type  in  Arizona. 
W.  H.  Weed6  described  a  number  of  additional  sub-types;  a 

1  A.  von  Groddeck,  Die  Lehre  von  den  Lagerstatten  der  Erze,  Leipzig, 
1879,  p.  260. 

2  J.  H.  L.  Vogt,  Zeitschr.  prakt.  Geol,  1894,  pp.  177,  464;  1895,  p.  154. 

3  W.  Lindgren,  Min.  and  Sci.  Press,  vol.  78,  1899,  p.  125. 

W.  Lindgren,  Twentieth  Ann.  Kept.,  U.  S.  Geol.  Survey,  1900,  pt.  3,  pp. 
249,  253. 

4  W.  Lindgren,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  31,  1901,  p.  23. 

5  W.  P.  Blake,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  34,  1904,  pp.  886-890. 

6  W.  H.  Weed,  Ore  deposits  near  igneous  contacts,  Trans.,  Am.  Inst.  Min. 
Eng.,  vol.  33,  1903,  pp.  719  et  seq. 


IGNEOUS  METAMORPHISM  707 

little  later  J.  F.  Kemp,1  O.  Stutzer2  and  J.  E.  Spurr3  discussed 
the  subject  again  from  a  general  standpoint;  W  Lindgren, 
J.  F.  Kemp,  S.  F.  Emmons,  J.  E.  Spurr,  and  others  described  in 
detail  similar  deposits  in  Arizona,  New  Mexico,  and  Mexico, 
and  it  became  evident  that  this  type  was  far  more  common  than 
had  been  suspected  It  was  found  that  in  many  regions  intrusive 
masses  were  normally  accompanied  by  contact-metamorphic 
deposits  which  in  some  cases  were  connected  by  transitions  with 
the  swarm  of  veins  that  usually  surround  these  igneous  bodies 
as  an  aureole  of  metallic  treasure.  The  great  importance  of  this 
type  for  the  solution  of  problems  related  to  the  genesis  of  ore 
deposits  became  clear  to  the  minds  of  many  investigators.  In 
Europe  many  geologists  have  of  late  made  detailed  studies 
of  contact-metamorphic  deposits — among  them  B.  Lotti,  R. 
Beck,  Loewinson-Lessing,  E.  Weinschenk,  A.  Bergeat,  and  V.  M. 
Goldschmidt. 

The  views  expressed  in  the  above-mentioned  papers,  involving 
accession  of  material  from  the  magma  have  not  been  allowed  to 
pass  unchallenged.  F.  Klockmann4  expressed  the  opinion  that 
these  deposits  were  older  accumulations  of  iron  ore,  altered  at 
the  intrusive  contact;  W.  O.  Crosby5  and  A.  C.  Lawson6  held 
that  such  bodies  were  simply  the  result  of  the  ordinary  circulation 
of  meteoric  character.  As  a  general  explanation  none  of  these 
views  appear  to  be  tenable. 

In  1914  the  discussion7  regarding  the  origin  of  the  garnet  zones 
flared  up  again  and  was  participated  in  by  W.  L.  Uglow,  W.  Lind- 
gren, J.  F.  Kemp,  C.  K.  Leith,  A.C.  Lawson,  andC.  A.  Stewart. 

CONTACT  METAMORPHISM 

General  Features/ — It  will  first  be  necessary  to  enter  a  little 
more  deeply  into  the  problem  of  contact  metamorphism.  This 
peculiar  action  of  intrusive  igneous  bodies  upon  adjacent  sedi- 

1  J.  F.  Kemp,  Ore  deposits  at  the  contact  of  intrusive  rocks  and  limestone, 
Econ.  Geol,  vol.  2,  1907,  pp.  1-13. 

2  O.  Stutzer,  Kontaktmetamorphe  Erzlagerstatten,  Zeitschr.  prakt.Geol, 
vol.  17,  1909,  pp.  145-155. 

3  A  theory  of  ore  deposition,  Econ.  Geol.,  vol.  7,  1912,  pp.  485-492. 

4  F.  Klockmann,  Zeitschr.  prakt.  Geol,  1904,  pp.  73-85. 

6  W.  O.  Crosby,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  36,  1906,  pp.  626-646. 

6  Min.  and  Sci.  Press,  Feb.  3,  1912. 

7  See  Econ.  Geol.,  vols.  8  and  9;  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  48, 
1915,  and  Min.  and  Sci.  Press,  Oct.  17,  1914. 


708  MINERAL  DEPOSITS 

mentary  rocks  has  been  a  well-known  fact  in  geology  since  the 
days  of  Durocher  (1846),  and  the  processes  have  been  described 
in  much  detail.  Effusive  rocks — that  is,  lava  flows — rarely 
exert  intense  metamorphism  beyond  a  baking  or  hardening  of  the 
sediments  at  the  contact  or  an  alteration  of  included  rock  frag- 
ments. The  magmas  intruded  in  sedimentary  rocks,  on  the 
other  hand,  are  in  most  cases  surrounded  by  a  halo  of  gradually 
fading  metamorphism  which  may  extend  over  a  width  of  1  or  2 
miles,  although  usually  much  narrower.  The  immediate  contact  is 
ordinarily  sharp,  with  no  evidence  of  melting;  only  at  contacts 
which  were  deeply  submerged  is  there  evidence  of  assimilation 
and  extensive  injection  or  feldspathization  of  the  sediments. 
Slates  and  shales  are  ordinarily  converted  to  hard,  compact 
"hornfels" — that  is,  fine-grained  ho lo crystalline  rocks  containing 
biotite,  andalusite,  staurolite,  scapolite,  garnet,  and  feldspar; 
in  extreme  cases  gneissoid  rocks  result.  This  metamorphism 
gradually  diminishes  and  at  some  distance  the  only  evidence  of 
change  is  a  knotty  texture  of  the  rocks.  Sandstones  change  to 
quartzite  at  the  contact.  Calcareous  rocks  become  highly 
crystalline  marbles  and  usually  develop  the  contact-metamorphic 
minerals  garnet,  epidote,  diopside,  tremolite,  vesuvianite,  etc. 
According  to  the  older  view  expressed  by  Rosenbusch,  Zirkel, 
Brogger,  and  others,  there  is  little  change  in  composition  aside 
from  the  expulsion  of  carbon  dioxide  from  the  limestones.  The 
silicates  are  held  to  be  formed  under  the  influence  of  the  heat 
of  the  magma  from  the  impurities  contained  in  the  limestone. 

It  is  well  known  that  Rosenbusch1  proposed  a  way  of  calcu- 
lating the  original  character  of  a  metamorphic  rock  from  its  present 
composition,  and  in  regional  metamorphism  this  is  undoubtedly 
often  justified.  More  recently  this  thought  has  been  followed  by 
J.  Barrell2  in  more  direct  application  to  contact  metamorphism. 

G.  W.  Hawes,3  however,  many  years  ago  pointed  out  that 
emanations  of  boron  and  silica  entered  the  sediments  from  the 
magma,  and  the  introduction  of  tourmaline,  for  instance,  has 
often  been  proved  since  then;  it  is  in  fact  admitted  even  in  the 
older  text-books  of  petrography.4 

1  Elemente  der  Gesteinslehre,  2d  ed.,  1901,  p.  484. 

2  J.  Barrell,  The  physical  effects  of  contact  metamorphism,  Am.  Jour. 
Sci.,  4th  ser.,  vol.  13J  1902,  pp.  279-296. 

3  G.  W.  Hawes,  Am.  Jour.  Sci.,  3d  ser.,  vol.  21,  1881,  p.  21. 

4  F.  Zirkel,  Lehrbuch  der  Petrographie,  Leipzig,  vol.  2,  1894,  p.  97. 


IGNEOUS  METAMORPHISM  709 

The  development  of  greisen  by  the  action  of  fluorine  vapors 
on  granite  is  in  some  of  these  books  regarded  as  a  contact-meta- 
morphie  process,  but  in  almost  all  cases  it  is  really  a  distinctly 
later  stage.  J.  S.  Flett,1  for  instance,  has  shown  that  in  Cornwall 
both  tourmaline  and  topaz,  where  present  on  a  large  scale,  have 
been  introduced  into  rocks  already  contact-metamorphosed. 

Petrographers  of  the  present  day,  represented,  for  instance,  by 
Alfred  Barker  and  A.  Lacroix,  lean  strongly  toward  the  view 
that  "it  is  safe  to  assume  that  the  water  and  other  volatile  con- 
stituents actually  found  in  igneous  rocks,  whether  chemically 
combined  or  mechanically  enclosed,  represent  only  a  fraction  of 
what  was  originally  contained  in  the  parent  rock  magmas.  The 
rest  has  been  lost  and  in  the  case  of  intrusive  rocks  must  have 
passed  into  or  through  the  surrounding  country  rocks.  Probably 
some  leakage  goes  on  throughout  the  process  of  consolidation, 
but  not  with  equal  freedom-  at  different  stages  .  .  .  and  it 
is  likely  that  a  large  part  of  the  volatile  constituents  is  in  general 
retained  down  to  a  late  stage.  Nevertheless,  more  or  less  of  the 
water  and  other  gases  must  pass  into  the  neighboring  rocks  while 
these  are  still  heated  by  the  intrusion."2 

The  surrounding  rocks  of  some  intrusive  bodies  were,  for  in- 
stance, permeated  for  long  distances  by  chlorine  solutions,  this 
action  resulting  in  the  development  of  scapolite  (NaiAlaSigC^Cl). 
Just  how  far  the  substances  liberated  from  the  magma  are  in  the 
state  of  perfect  gases — that  is,  above  the  critical  temperature — 
or  in  liquid  form,  is  not  easy  to  decide.  At  the  contact  the  car- 
bonate rocks  appear  to  have  been  permeable  to  gases  to  a  remark- 
able degree. 

If  we  admit  that  the  original  magma  contained  in  solution 
various  volatile  substances,  such  as  water,  carbon  dioxide, 
sulphur,  boron,  chlorine,  and  fluorine,  it  follows  that  the  decrease 
of  pressure  caused  by  its  ascent  will  result  in  the  escape  of  a  large 
part  of  these  volatile  compounds,  which  will  entrain  with  them 
various  metals  also  held  in  solution.  The  higher  the  rise  of  the 
magma  the  more  complete  will  be  the  liberation  of  these  sub- 
stances. In  what  form,  time,  and  quantity  they  will  escape 
depends  upon  pressure,  chemical  affinities,  and  miscibility. 

The  escape  of  these  substances,  as  pointed  out  by  Harker,  may 

1  The  geology  of  Bodmin  and  St.  Austell,  Mem.,  Geol.  Survey  England, 
Explan.  Sheet  347,  1909,  p.  58. 

2  Alfred  Harker,  The  natural  history  of  igneous  rocks,  1909,  pp.  302-303. 


710  MINERAL  DEPOSITS 

not  have  been  uniform.  A  large  part  was  doubtless  given  off 
while  the  magma  was  still  fluid.  Another  part  may  have  been 
liberated  at  the  time  of  consolidation;  still  another  part  may  have 
been  retained  until  cooling  had  advanced  considerably.  Finally 
fissures  and  shattering  of  the  partly  consolidated  or  congealed 
mass  may  have  permitted  gases  from  the  still  fluid  interior  or 
basal  part  to  reach  the  outside  of  the  intrusions. 

It  does  not  seem  possible  that  atmospheric  waters  could  have 
gained  access  to  the  contacts  during  intrusion.  Both  the  heat  of 
the  magma  and  the  pressure  of  the  volatile  compounds  striving 
to  free  themselves  would  prevent  such  access. 

The  magmatic  solutions  enter  into  the  rocks  adjacent  to  the 
magma  and  produce  a  series  of  metasomatic  changes  the  charac- 
ter of  which  will  depend  upon  the  composition  of  the  solu- 
tions and  their  temperature,  as  well  as  upon  the  kind  of  country 
rock  exposed  to  these  hot  extracts. 

Ore  deposits  of  this  kind  are  rarely  formed  in  argillaceous 
shales,  sandstones  and  quajtzite.  Limestone,  dolomite  and  cal- 
careous shale  on  the  other  hand  are  easily  permeable  and  appear 
to  soak  up  the  solutions  like  a  sponge.  It  will  be  recalled  that 
even  at  ordinary  temperature  limestone  will  absorb  oil  and 
similar  substances.  This  permeation  is  facilitated  by  the  fissur- 
ing  and  crushing  along  the  contact.  If  the  solutions  are  dilute 
little  replacement  occurs  but  the  limestone  usually  becomes 
coarsely  crystalline  or  scattered  silicates  will  be  produced  by 
reaction  between  the  calcite  and  the  silica  usually  contained  in 
most  limestones.  When  the  solutions  were  more  concentrated 
and  contained  much  silica,  sulphur,  iron  and  other  metallic 
constituents  the  replacement  will  proceed  with  energy  and  the 
calcareous  rock  may  be  converted  to  a  mass  of  ore  and  gangue 
minerals.  In  places  whole  beds  of  pure  limestone  or  dolomite 
may  be  converted  to  garnet,  diopside  and  other  silicates  associated 
with  recrystallized  calcite  and  with  magnetite,  specularite  and 
simple  metallic  sulphides. 

Form  and  Texture. — The  frequency  of  rudely  tabular  bodies 
dependent  upon  parallelism  with  contacts  or  beds  has  already 
been  emphasized.  Sometimes  the  outlines  are  entirely  irregular, 
but  a  selective  action  is  very  common  by  which  the  development 
of  the  best  ore  takes  place  along  certain  horizons  in  which  the 
limestone  is  especially  susceptible  to  metamorphism.  This 
is  doubtless  caused  by  the  physical  condition  of  a  particular 


IGNEOUS  METAMORPHISM  711 

bed  which. may  be  a  pure  or  an  impure  limestone,  but  the  reason 
for  this  selective  action  is  not  always  clearly  apparent.  While 
the  deposits  rarely  extend  far  from  the  contacts  cases  are  known 
where  they  reach  out  along  certain  beds  for  a  distance  of  2,000 
feet  or  more.  The  contact-metamorphic  minerals  in  limestone 
often  cease  suddenly  and  form  sharp  contacts  with  the  unaltered 
rock.  Fissures  may  guide  the  solutions  and  transitions  to  veins 
may  be  formed  by  deposition  along  them. 

Favorite  points  of  maximum  ore  development  are  limestone 
fragments  included  in  the  igneous  rock  or  points  where  the  cal- 
careous rock  projects  into  the  intrusive.  In  most  places  the 
ore-bodies  are  of  comparatively  small  size,  but  occasionally  we 
find  ore-bodies  containing  millions  of  tons.  In  case  of  copper 
deposits,  subsequent  oxidation  and  enrichment  has  often,  as  at 
Bisbee,  Arizona,  greatly  enlarged  the  available  ore-bodies. 

The  texture  of  the  ore  is  generally  coarse,  characteristic  of 
replacement  at  high  temperature.  Fine-grained  ores  like  those 
of  Bisbee  are  not  common.  The  absence  of  banding  by  crusti- 
fication  or  deposition  in  open  space  is  notable.  But  replace- 
ment processes  sometimes  result  in  a  rude  banding  by  a  rhythmic 
metasomatic  action  or  in  orbicular  or  pipe-like  arrangement 
of  minerals.  Such  orbicular  structures  have  been  described  by 
Triistedt1  in  tin  deposits  at  Pitkaranta,  Finland  and  by  Knopf2 
in  similar  deposits  in  Alaska. 

The  calcite  is  often  recrystallized  to  extremely  coarse  masses. 
Garnet  and  other  silicates  frequently  show  crystal  outlines  but 
the  sulphides,  excepting  pyrite,  are  rarely  crystallized  in  distinc- 
tive form.  Drusy  cavities  into  which  quartz  crystals,  garnets, 
etc.,  project  are  found  in  some  deposits. 

Mineralogy. — Among  the  ore  minerals  those  of  simple  composi- 
tion prevail.  In  order  of  abundance  we  have  pyrite,  chalcopy- 
rite,  bornite,  pyrrhotite,  zinc  blende,  molybdenite,  arsenopyrite 
and  galena.  Tetrahedrite,  jamesonite  and  other  complex  sulpho- 
salts  are  rare;  most  of  those  common  in  deposits  formed  at  lower 
temperature  are  absent.  Tellurides  are  rare  though  tetradymite 
is  reported  from  a  few  localities.  Native  gold  is  seldom  present 
though  most  of  the  ores  contain  a  trace  of  that  metal.  Graphite 
is  not  uncommon  and  platinum  has  been  reported  from  one  or 

1  O.  Trustedt,   Die  Erzlagerstatten  von  Pitkaranta,  Bull.  Com.   Geol. 
Finlande,  vol.  19,  1907. 

2  Adolph  Knopf,  Bull.  358,  U.  S.  Geol.  Survey,  1908. 


712  MINERAL  DEPOSITS 

two  localities.  The  oxides  are  represented  by  magnetite,  ilmen- 
ite,  hematite,  quartz,  corundum,  cassiterite  and  spinels  of  various 
kinds. 

The  silicates,  especially  those  containing  lime,  magnesia  and 
iron,  are  abundant.  We  enumerate  garnet  (grossularite  and  and- 
radite)  epidote,  zoisite,  diopside,  hedenbergite,  tremolite,  vesuv- 
ianite,  forsterite,  anorthite,  ilvaite  and  wollastonite.  Some 
of  these  contain  the  hydroxyl  molecule. 

Among  the  minerals  containing  chlorine,  fluorine  or  boron  we 
mention  scapolite,  chondrodite,  axinite,  ludwigite,  topaz  and 
tourmaline.  The  latter  two  are  not  abundant. 

Other  minerals  occasionally  present  are  orthoclase5  albite, 
muscovite,  biotite,  wolframite,  scheelite  and  fluorite  while  barite 
and  anhydrite  are  absent.  Andalusite,  cyanite  and  staurolite, 
though  common  in  contact-metamorphosed  shales,  are  not  usually 
connected  with  ore  deposits.  Chlorite  is  sometimes  found  in 
large  crystals;  serpentine  is  a  product  of  decomposition  of  other 
silicates.  Calcite,  ankerite  and  dolomite  are  usually  present. 

Many  zeolites  are  also  found  but  always  crystallize  during  the 
last  phase  of  mineralization. 

On  the  whole,  iron  and  copper  deposits  are  the  most  common. 
Zinc,  lead,  gold,  silver  and  tin  are  much  less  abundantly 
represented. 

Intensity  of  Metamorphism. — There  are  all  degrees  of  meta- 
morphism  at  the  contacts  of  intrusive  masses.  Some  magmas 
are  evidently  poor  in  volatile  constituents  and  may  exert  only 
a  slight  recrystallization  on  adjacent  limestone,  while  along 
the  contact  of  others  large  bodies  of  ore  may  form.  Along  the 
same  contact  there  are  often  great  irregularities  in  mineralization. 
The  degree  of  alteration  of  non-calcareous  shales  is  a  good  in- 
dication of  the  intensity  of  the  metamorphism,  though  these 
rocks  rarely  contain  ore  deposits. 

Influence  of  Composition  of  Igneous  Rock. — Highly  acidic 
rocks,  such  as  normal  granites,  are  not  usually  accompanied  by 
ore  deposits  of  the  contact-metamorphic  type,  although  they 
may  produce  widespread  effects  of  metamorphism  and  a  later 
mineralization  of  quartz  veins.  The  rocks  accompanying  the 
contact-metamorphic  deposits  of  the  Cordilleran  type  are  gen- 
erally monzonites  or  quartz  monzonites  or  granodiorites. 

Many  examples  show,  however,  that  more  basic  rocks  also  may 
produce  metallization  of  adjoining  limestones — diabases,  for 


IGNEOUS  METAMORPHISM  713 

instance,  at  Cornwall,  Pennsylvania,  and  gabbro  at  the  Nickel 
Plate  mine,  British  Columbia. 

Alteration  of  the  Intrusive  Rock. — Fresh  granitic  rock  often 
adjoins  the  contact  and  is  then  separated  sharply  from  the 
altered  limestone.  In  many  cases  the  intrusive  rock  is  more 
or  less  altered;  the  alteration  may  have  taken  place  simultane- 
ously with  the  metamorphism  of  the  sedimentary  rock,  or 
afterward. 

The  hot  intrusive,  whether  molten  or  just  consolidated,  and 
the  adjacent  cooler  limestone,  form  a  system  in  which  by  means 
of  gases,  there  will  usually  take  place  a  vigorous  exchange  of 
material.  Most  of  the  changes  will  take  place  at  the  cooler 
side  but  the  intrusive  will  also  receive  material  perhaps  mainly 
carbon  dioxide,  and  oxides  of  calcium  and  magnesium  from  the 
sediments.  Thus  we  find  frequently  epidote  in  large  masses, 
more  rarely  garnet,  or  diopside  developing  in  the  igneous  rock 
by  replacement  through  the  mass  or  in  replacement  veinlets. 
Such  phenomena  have  been  described  frequently  by  Spurr  and 
Garrey  from  Dolores1  and  Velardena,2  by  Bergeat3  from  Concep- 
cion  del  Oro,  all  in  Mexico,  and  by  Umpleby4  from  White  Knob, 
Idaho.  At  the  latter  place  it  is  often  impossible  to  define  the 
contact  between  garnetized  limestone  and  granite-porphyry 
(Fig.  244). 

In  other  cases  the  alteration  observed  consists  in  sericitization 
and  impregnation  with  pyrite.  It  is  believed  that  this  is  largely 
caused  by  hot  waters  similar  to  those  which  form  ordinary  fissure 
veins;  these  waters  may  ascend  from  deeper  portions  of  the  in- 
trusive from  which  emanations  continue  to  be  given  off  for  con- 
siderable time  after  the  irruption. 

In  the  ordinary  course  of  events  an  intrusive  suffers  various 
changes  as  to  composition  and  texture  at  a  contact.  Basic 
facies  may  appear,  or  fine-grained  texture,  or  a  laminated  or  schis- 
tose structure. 

The  history  of  any  igneous  rock  is  marked  by  a  successive 
development  of  minerals.  Flett5  states  that  in  the  Cornwall 

1  J.  E.  Spurr  and  G.  H.  Garrey,  Econ.  Geol.,  vol.  3,  1908,  pp.  688-725. 

2  J.  E.  Spurr,  G.  H.  Garrey  and  C.  N.  Fenner,  Econ.  Geol,  vol.  7,    1912, 
pp.  444-492. 

3  A.  Bergeat,  Boletin  27,  Inst.  Geologico  de  Mexico,  1910. 

4  J.  B.  Umpleby,  Prof.  Paper  97,  U.  S.  Geol.  Survey,  1917. 

5  J.  S.  Flett,  Mem.,  Geol.  Survey  England,  Explan.  Sheet  347,  Bodwin 
and  St.  Austell,  1909,  p.  58. 


714  MINERAL  DEPOSITS 

granites  tourmaline,  white  mica  and  topaz  may  be  primary 
minerals  and  have  a  long  period  of  formation  continuing  to 
develop  until  the  quartz  separates  out,  and  even  long  afterward. 

Spurr  and  Garrey  state  that  at  Dolores  and  Velardena  many 
changes  took  place  after  consolidation  in  the  small  stocks  of 
diorite,  alaskite  and  monzonite  breaking  through  Cretaceous 
limestone,  and  that  a  transition  between  contact-metamorphic 
deposits  and  normal  veins  are  found  at  the  former  place.  Peg- 
matitic  vuggy  segregations  are  observed  and  the  older  minerals 
have  been  altered  to  an  early  phase  characterized  by  orthoclase, 
apatite,  titanite,  chlorite,  quartz,  diopside  and  pyrite.  At  Vel- 
ardena grossularite,  garnet  and  vesuvianite  also  appear. 

This  is  followed  at  Dolores  by  iron  silicates,  such  as  heden- 
bergite  (iron  pyroxene)  and  andradite,  both  replacing  earlier 
minerals.  After  this  deposition  of  garnet,  but  overlapping  it, 
were  formed  pyrite,  chalcopyrite,  actinolite,  fluorite  and  quartz, 
and  this  association  also  occurs  in  the  fissure  veins.  Zoisite, 
prehnite  and  apophyllite  occur  in  the  later  phases,  which  like 
the  former  were  formed  by  solutions  rising  through  the  already 
congealed  monzonite. 

In  the  surrounding  limestone  the  succession  of  minerals  is 
garnet  (oldest),  magnetite,  hematite,  pyrite,  chalcopyrite,  zinc 
blende  and  galena. 

Succession  of  Events. — The  idea  of  successive  epochs  during 
the  intrusion  is  frequently  advanced.  The  general  metamor- 
phism  by  heat  and  dilute  vapors  is  generally  considered  to  have 
taken  place  first  while  foreign  substances  were  added  later. 
However,  if  we  conceive  magma  rising  to  regions  of  lessening 
pressure  it  is  difficult  to  see  why  the  escape  of  gases  should  not 
have  begun  at  once  when  the  melt  was  brought  into  contact 
with  the  cooler,  surrounding  sediments.  The  frequency  of  con- 
tact-metamorphic deposits  in  roof  segments  of  intrusive  masses 
and  along  dikes  in  the  roof  shows  that  the  volatile  substances 
were  concentrated  in  the  upper  parts  or  cupolas  of  the  intrusives 
and  in  dikes  radiating  from  these  "gassed"  parts  of  the  magma. 
We  observe  also  the  scant  metamorphism  along  flat  intrusions, 
like  laccoliths  and  its  relative  strength  in  crosscutting  bodies 
where  gases  from  below  could  more  easily  gain  access. 

The  description  of  a  few  typical  localities  will  show  that  the 
contact  phenomena  may  vary  greatly  in  different  places.  At 
times  the  general  metamorphism  is  very  weak  while  the  meta- 


IGNEOUS  METAMORPHISM  715 

somatic  phases  are  strong  and  vice  versa.  Undoubtedly  emana- 
tions along  fissures  continued  after  the  general  consolidation  in 
many  places  and  if  they  had  a  high  temperature  the  later  altera- 
tion would  be  of  the  same  type  as  that  produced  during  the  in- 
trusive act. 

In  an  admirable  study  of  contact  metamorphism  at  Marysville, 
Montana,  Barrell1  describes  the  metamorphic  zone,  %  to  1 
mile  wide  surrounding  a  stock  of  quartz  monzonite  which  on 
the  surface  occupies  an  area  of  only  2  square  miles,  but  which 
widens  below.  The  intensity  of  the  metamorphism  is  thus 
increased. 

Barrell  distinguishes  between  (1)  contact  metamorphism, 
which  results  in  recrystallization  of  the  sediments  to  hornfels, 
marble  and  lime  silicate  rock,  and  this  was  produced  by  the  first 
wave  of  metamorphism;  (2)  contact  metasomatism  in  which 
magmatic  emanations  added  some  constituents  to  the  altered 
rocks.  This  latter  zone  is  at  most  1,000  feet  wide.  '  Silica,  iron 
and  sulphur  were  added.  Diopside  and  hornblende,  with  a  little 
apatite,  tourmaline,  garnet  and  pyrite  were  the  minerals  formed. 
There  are  no  contact-metamorphic  deposits  of  economic 
importance. 

A  very  different  state  of  affairs  is  described  by  Calkins2  from 
Philipsburg,  Montana,  where  a  larger  batholith  invades  Algon- 
kian  sediments  of  all  kinds.  The  contact  zones  are  half  a  mile 
or  more  in  width.  The  author  draws  no  sharp  line  between 
metamorphism  and  metasomatism,  and  evidently  considers  both 
to  progress  at  the  same  time.  The  metamorphism  is  strong; 
close  to  the  contacts  are  masses  of  magnetite,  in  part  carrying 
gold,  with  garnet,  vesuvianite,  humite  and  forsterite.  On  the 
other  hand  scapolite  and  tourmaline  are  distributed  in  the  sedi- 
ments for  a  distance  of  a  mile  or  more  from  the  contacts  showing 
a  widespread  diffusion  of  magmatic  chlorine  and  boron.  Fluor- 
ine has  also  been  introduced,  further  sodium,  silica  and  iron, 
the  latter  only  close  to  the  contact. 

A  monograph  by  V.  M.  Goldschmidt3  describes  in  great  detail 
the  type  locality  of  the  Norwegian  contact-metamorphic  deposits 
near  Kristiania.  At  present  they  are  of  little  economic  impor- 

1  Joseph  Barrell,  Prof.  Paper  57,  U.  S.  Geol.  Survey,  1907. 

2  W.  H.  Emmons  and  F.  C.  Calkins,  Prof.  Paper  78,  U.  S.  Geol.  Survey 
1913. 

3  Die  Contactmetamorphose  im  Kristianiagebiet,  Kristiania,  1911. 


716  MINERAL  DEPOSITS 

tance.  On  a  basement  of  Archean  rocks  rest  Paleozoic  sedi- 
ments; these  are  broken  by  laccoliths  of  gradually  more  acidic 
composition,  beginning  with  essexite,  which  is  followed  by  sye- 
nitic  rocks.  Near  the  contacts  of  the  essexite  the  metamor- 
phism  is  exceedingly  strong  but  takes  place  without  addition  of 
substance. 

Along  the  syenite  contacts  Goldschmidt  observed  both  an 
older  normal  contact  metamorphism  by  recrystallization  without 
addition  and  a  younger  "  pneumatolytic "  metamorphism  by 
recrystallization  under  the  addition  of  magmatic  gases,  resulting 
in  "skarn  rocks/'1  in  which  the  copper  deposits  are  contained. 
The  characteristic  minerals  of  the  hornfels  or  altered  slates  were 
formed  before  the  consolidation  of  the  magma  and  probably 
without  the  aid  of  other  water  than  that  normally  contained  in 
the  rock.  Though  the  ' '  skarn  rocks ' '  and  their  metallic  sulphides 
are  later  than  the  general  metamorphism  they  were  formed 
shortly  before  the  crystallization  of  the  magma,  though  the 
immediate  contact  may  have  been  congealed.  Pyroxene  occurs 
in  the  inner  and  amphibole  in  the  outer  contact  zone;  according 
to  Becke  the  transition  point  between  the  stability  fields  of  the 
two  minerals  is  about  550°  C.  at  200  atmospheres. 

The  "skarn  rocks"  are  coarsely  crystalline  and  consist  of 
andradite,  hedenbergite,  wollastonite,  scapolite,  axinite,  adularia, 
albite,  calcite,  fluorite,  zeolites,  specularite,  magnetite,  bis- 
muthinite,  galena,  chalcopyrite,  primary  chalcocite,  primary 
willemite,  zinc  blende,  pyrrhotite,  molybdenite,  and  bornite. 
Many  of  these  also  occur  as  primary  minerals  in  miarolitic  cavities 
in  the  syenite.  Magnetite  forms  nearer  to  the  igneous  rock 
than  specularite.  The  scapolite  becomes  unstable  at  lower 
temperatures  and  is  transformed  to  albite,  epidote,  microper- 
thite  and  zeolites.  The  metallic  ores  are  somewhat  later  than 
the  skarn  minerals. 

From  Ely,  Nevada,  Spencer2  describes  contact  zones  along 
small  stocks  of  quartz  monzonite.  These  zones  extend  a  few 
hundred  feet  to  half  a  mile  from  the  contact.  The  monzonite 
porphyry  is  greatly  altered  with  development  of  sericite,  biotite 
and  pyrite  but  no  garnet  or  diopside.  The  process  involved  loss  of 
sodium,  calcium  and  some  alumina;  gain  of  potassium  and  sulphur. 

1  An  old  Swedish  mining  term  signifying  the  garnet-pyroxene-epidote 
rocks  accompanying  many  Scandinavian  magnetite  deposits. 

2  A.  C.  Spencer,  Prof.  Paper  96,  U.  S.  Geol.  Survey,  1917. 


IGNEOUS  METAMORPHISM 


717 


The  shales  are  converted  to  strongly  pyritic  hornfels  involving 
addition  of  sulphur  and  iron.  The  limestones  have  developed 
white  and  brown  mica,  tremolite,  pyroxene,  garnet,  epidote, 
scapolite,  pyrite,  pyrrhotite,  chalcopyrite,  galena,  zinc  bjende, 
molybdenite,  magnetite  and  hematite.  In  places  very  heavy 
masses  of  j asperoid  have  been  formed  from  limestone.  In  general, 
silicon,  sulphur,  potassium  iron,  copper  and  other  metals  have 
been  added. 

Spencer  again  draws  no  sharp  line  between  metamorphism 
and  metasomatism,  conceiving  that  the  whole  process  took  place 
by  emanations  from  deeper  parts  of  the  magma  after  the  parts 
now  at  .the  surface  had  consolidated.  The  areas  exposed  he  con- 
siders too  small  to  have  effected  the  changes  shown.  It  seems 


White  marble' 


FIG.  244. — Longitudinal  section,  Empire  mine,  White  Knob,  Idaho. 
After  J.  B.  Umpleby,  U.  S.  Geol.  Survey. 

probable  that  in  this  case  the  alteration  of  the  porphyry  and  the 
silicification  of  the  limestone  resulted  from  a  somewhat  later 
mineralization  though  practically  continuous  with  the  earlier 
phase. 

Very  different  conditions  exist  at  White  Knob,  Idaho,  de- 
scribed by  Umpleby.1  Here  the  Carboniferous  limestone  is  very 
little  altered  in  contact  with  fresh  granite  porphyry.  Marble 
has  developed  close  to  the  contact  and  more  extensively  in  a 
roof  segment  of  the  batholith  with  scattered  crystals  of  wollas- 
tonite,  tremolite  and  diopside.  Engulfed  blocks  of  limestone  are, 
however,  more  extensively  garnetized  with  development  of  con- 
siderable ore  shoots  (Fig.  244).  The  process  shows  great 
additions  of  iron,  alumina  and  silica.  To  some  extent  the  granite 

i  J.  B.  Umpleby,  Prof.  Paper  97,  U.  S.  Geol.  Survey,  1917. 


718  MINERAL  DEPOSITS 

porphyry  is  also  garnetized  so  that  in  places  the  contacts  are 
indistinct,  and  it  contains  also  diopside  which  replaces  biotite 
and  hornblende.  The  feldspars  are  the  last  minerals  of  the  in- 
trusive to  be  affected.  Two  stages  of  metamorphism  are  recog- 
nized: (1)  Contact  metamorphism  at  time  of  intrusion;  (2)  contact 
metasomatism  after  the  consolidation  of  the  magma.  The  garnet 
and  the  ore  was  developed  during  the  last  stage.  That  the  last 
process  took  place  after  the  intrusive  had  solidified  is  proved  by 
garnetization  following  joint  planes  in  the  rock. 

In  the  San  Francisco  district,  Utah,1  Butler  finds  silicate  zones 
one-fourth  mile  wide  and  recrystallization  of  limestone  over 
much  wider  areas.  The  contacts  of  garnetized  limestone  and 
fresh  monzonite  are  often  exceedingly  sharp,  the  transition 
zone  being  less  than  one  inch  in  width. 

All  this  shows  that  the  process  varies  considerably  in  different 
magmas,  and  that  there  are  considerable  differences  of  opinion 
as  to  the  various  stages  of  the  process. 

Succession  of  Minerals. — The  introduction  of  sulphides  and 
other  metallic  ores  into  the  limestone  is  too  obvious  to  be  dis- 
puted. Many  observers  have  noted  a  certain  succession  of 
developments.  Generally,  the  silicates  and  the  magnetite  are 
earlier  than  the  sulphides,  but  the  periods  of  deposition  overlap. 

In  the  deposits  at  White  Horse,  Northwest  Territory,  Stutzer2 
reports  the  succession :  wollastonite,  pyroxene,  magnetite,  garnet, 
calcite  and  sulphides;  at  Berggieshiibel,  Saxony,  pyroxene,  gar- 
net, pyritic  ores,  zinc  blende,  arsenopyrite.  At  the  Holgol 
Mine,  Korea,  Koto3  found  the  succession  to  be  ilvaite,4  diopside, 
garnet,  while  the  sulphides  fill  the  interstices  between  the  diop- 
sides  (Fig.  251).  Frequently  chalcopyrite  replacing  calcite 
cements  the  garnet  and  other  silicates  (Fig.  245). 

In  the  Boundary  district,5  British  Columbia,  magnetite,  gar- 
net and  epidote  were  formed  together  while  pyrite  followed  by 
chalcopyrite  came  later  though  partly  overlapping  in  sequence. 

Other  authors  admit  that  the  sulphides  are  in  part  later  but 
point  out  that  to  a  considerable  extent  they  have  crystallized 

1  B.  S.  Butler,  Prof.  Paper  78,  U.  S.  Geol.  Survey,  1913. 

2  Zeitschr.  prakt.  Geol.,  vol.  17,  1909,  pp.  116-120. 

3  Jour.  Coll.  Sci.,  Imp.  Univ.,  Tokyo,  May  28,  1910. 

4  Hedenbergite  according  to  D.  F.  Higgins,  Econ.  Geol.,  vol.  13,  1918, 
p.  19. 

5  O.  E.  LeRoy,  Mem.  21,  Canada  Geol.  Survey,  1912. 


IGNEOUS  METAMORPHISM  719 

together  with  the  silicates.  This  seems  to  be  the  condition  in  the 
White  Knob,  San  Francisco,  Ely,  Clifton1  and  Camp  Hedley2 
districts.  At  the  Imperial  Mine,  Utah,  Butler  finds  magnetite 
in  part  later  than  chalcopyrite. 

Takeo  Kato3  found  that  at  the  Okufo  Mine,  Japan,  the  suc- 
cession began  by  wollastonite,  which  is  replaced  by  andradite. 
The  sulphides  are  deposited  contemporaneously  with  the  andra- 
dite or  at  the  very  close  of  its  deposition. 

C.  H.  Clapp4  describes  contact-metamorphic  deposits  on  Van- 
couver Island,  in  which  the  order  is  diopside,  epidote,  garnet, 
magnetite,  pyrrhotite,  pyrite  and  chalcopyrite  but  the  silicates 
continued  to  form  after  some  metallization  had  taken  place. 


FIG.  245. — Garnet  crystals  in  matrix  of  chalcopyrite  replacing  residual 
calcite.     After  J.  B.  Umpleby,  U.  S.  Geol.  Survey. 

The  succession  of  the  sulphides  generally  begins  with  arsenopy- 
rite  and  pyrite;  then  follows  pyrrhotite,  chalcopyrite,  galena,  zinc 
blende;  the  rarer  sulphosalts  were  formed  last. 

The  early  appearance  of  pyrite  is  explained  by  Spencer5  by 
the  accession  of  additional  iron  in  the  solutions  from  the  sedi- 
ments and  igneous  rock  which  would  bring  about  precipitation 
of  iron  compounds  before  those  of  copper.  He  regards  the 

1  W.  Lindgren,  Prof.  Paper  43,  U.  S.  Geol.  Survey,  1905. 

2  C.  Camsell,  Mem.  2,  Canada  Geol.  Survey,  1910. 

3  Jour.  Geol.  Soc.,  Tokyo,  vol.  20,  1913,  pp.  13-32. 

4  Mem.  13,  Canada  Geol.  Survey,  1912,  p.  158. 

5  A.  C.  Spencer,  Prof.  Paper  96,  U.  S.  Geol,  Survey,  1917,  pp.  64-72. 


720  MINERAL  DEPOSITS 

magmatic  solutions  to  contain  much  Si02,  H2S,  KSH,  CO2, 
HC03,  F,  Fe,  and  Cu.  The  relative  abundance  of  metals  in 
the  deposits  at  Ely  is  Fe,  Cu,  Pb,  Zn,  Mo  and  in  the  main  that  is 
also  the  sequence  of  deposition.  This  leads  to  the  suggestion 
that  the  principal  control  in  determining  the  order  of  deposition 
of  the  sulphides  was  the  relative  concentration  of  metal  radicles 
in  the  mineralizing  solutions. 

Volume  Relations. — The  replacement  of  limestone  by  sulphides, 
oxides,  and  silicates  liberated  a  large  volume  of  carbon  dioxide, 
and  this  at  first  probably  was  above  the  critical  temperature; 
possibly  a  portion  may  have  been  resorbed  in  the  magma,  but  a 
larger  part  was  doubtless  dissipated  in  the  fractures  surrounding 
the  intrusive  mass  and  gradually  escaped  or  mingled  with  the 
escaping  magmatic  water  and  some  distance  away  with  the 
ground  waters,  thus  adding  to  the  load  of  ascending  thermal 
springs. 

In  the  Morenci  district,  Arizona,  the  clearest  evidence  is 
given  by  the  transformation  effected  along  a  dike  of  unaltered 
quartz  monzonite  porphyry,  20  to  50  feet  wide,  which  crosses  the 
successive  Paleozoic  formations  with  no  evidence  of  fractures' 
that  could  have  admitted  solutions  from  the  depths  after  the 
consolidation.  In  the  lower  limestones  the  contact  zones  are 
only  a  few  feet  wide,  consisting  of  epidote  next  to  the  intrusive 
rock,  followed  by  garnet,  which  adjoins  the  unaltered  limestone. 
The  addition  of  iron  and  silica  to  this  narrow  zone,  which  shows 
no  evidence  whatever  of  contraction  of  volume,  is  so  clear  that 
it  hardly  admits  of  discussion.  Farther  up  the  same  dike  cuts 
across  a  pure  limestone  about  80  feet  in  thickness.  This  has  been 
changed  to  massive  andradite  garnet,  with  some  epidote,  for  a 
distance  of  about  100  feet  from  the  dike.  Stains  of  malachite 
are  present,  but  this  particular  rock  is  poor  in  copper. 

If  all  of  the  lime  has  been  used  in  the  garnetization  and  only 
CO2  has  escaped,  the  volume  of  the  rock  would  have  increased 
about  one-half.  If,  on  the  other  hand,  as  seems  probable,  the 
volume  has  remained  approximately  constant,  then  460  kilo- 
grams CaO  and  1,190  kilograms  CO2  per  cubic  meter  have  been 
carried  away,  while  1,330  kilograms  Si02  and  1,180  kilograms 
Fe203  have  been  added.  These  are  astonishing  figures  and  give 
an  idea  of  the  vigorous  transfer  of  material  which  took  place 
during  metamorphism.  The  Modoc  limestone  contains  94  to 
96  per  cent.  CaC03,  less  than  1  per  cent.  MgC03,  1  per  cent. 


IGNEOUS  METAMORPHISM  721 

Si02,  and  1  to  3  per  cent.  A12O3  and  Fe2O3.  The  andradite 
garnet  contains  1.53  per  cent,  alumina,  31.41  Fe203,  42.63  Si02, 
and  23.37  CaO.  The  transfer  has  been  mutual,  for  at  some 
places  the  intrusive  rock  next  to  the  contact  has  been  strongly 
epidotized  by  lime  derived  from  the  calcareous  rock. 

That  the  volume  has  remained  practically  constant  even  in  the 
most  intense  metasomatism  may  be  considered  proved  and 
confirmed  by  the  observations  in  the  Ely,  San  Francisco  and 
White  Knob  district  referred  to  above.  Preservation  of  struc- 
ture like  stratification  planes,  joints  and  fossils  has  repeatedly 
been  observed  in  the  silicate  rock  and  in  the  sulphides.1 

C.  K.  Leith  and  E.  C.  Harder2  have  attempted  to  account  for 
the  silicate  rock  at  the  contact  of  the  Iron  Springs  intrusive  in 
southern  Utah  by  assuming  that  the  silica  and  alumina  have 
remained  constant  while  the  volume  of  the  rock  has  been  de- 
creased as  much  as  60  or  80  per  cent,  by  reason  of  abstraction  of 
lime.  No  definite  field  evidence,  however,  was  found  in  favor 
of  this  view. 

Calkins3  thinks  that  at  Philipsburg  a  part  but  not  the  whole  of 
the  shrinkage  was  offset  by  accessions  chiefly  of  silica  and 
alkalies,  and  that  the  shrinkage  may  have  been  obliterated  by 
pressure. 

Mode  of  Transfer. — During  the  alteration  of  the  carbonate 
rock  much  of  the  carbon  dioxide,  lime  and  magnesia4  was  carried 
away.  In  various  proportions  this  has  been  compensated  by 
additions  from  magmatic  emanations  of  silica,  iron,  alumina, 
sodium  and  perhaps  potassium,  and  a  number  of  other  useful 
metals.  Mineralizers  like  sulphur,  chlorine,  boron,  fluorine  and 
arsenic  have  also  been  introduced.  Many  writers,  like  Leith 
and  Harder,  Calkins  and  Goldschmidt,  believe  that  the  mag- 
matic gases  consisted  largely  of  chlorides,  fluorides,  etc.,  and 
this  view  is  very  likely  correct. 

The  equations  roughly  representing  these  transformations  in 
case  of  iron  would  be: 

1  Adolph  Knopf,  Bull.  580,  U.  S.  Geol.  Survey,  1915,  pp.  1-18. 
W.  Lindgren,  Prof.  Paper  68,  U.  S.  Geol.  Survey,  1910,  p.  294. 

2  Bull.  338,  U.  S.  Geol.  Survey,  1908.     See  also  review  by  Kemp,  Econ. 
Geol,  vol.  4,  1909,  p.  782,  and  answer  by  Leith,  Econ.  Geol,  vol.  5,  1910, 
p.  188. 

3  Prof.  Paper  78,  U.  S.  Geol.  Survey,  1913,  p.  132. 

4  J.  M.  Boutwell  observed  a  concentration  of  magnesia  in  the  altered 
limestone  at  Bingham,  Utah,  Prof.  Paper  48,  U.  S.  Geol.  Survey,  1905. 


722  MINERAL  DEPOSITS 

2FeF3+3CaC03  =  Fe203+3CaF2-r-3CO2 
2FeCl3+3CaCO3  =  Fe203+3CaCl2+3C02 

Physical  Conditions  at  the  Contact. — The  temperature  at  the 
contact,  according  to  the  composition  of  the  magma,  may  have 
been  as  much  as  1,500°  C.,  the  siliceous  rocks  consolidating  at  a 
temperature  of  500°  to  1,100°  C.  When  there  is  no  chemical 
action  involved  calcium  carbonate  begins  to  lose  carbon  dioxide 
at  550°  C.,  but  the  reaction  would  begin  at  a  much  lower  tem- 
perature under  the  influence  of  magmatic  gases  acting  chemically 
on  the  calcite.  Under  abyssal  conditions  no  opportunity  would 
be  afforded  for  the  liberated  carbon  dioxide  to  escape  through 
the  fissured  rocks. 

Even  where  the  carbon  dioxide  can  not  escape  there  may  be 
intense  action  between  the  igneous  rock  and  the  limestone.  The 
two  rocks  will  form  a  chemical  system  with  great  difference  of 
temperature  and  it  may  be  assumed  that  there  will  be  intense 
transfer  of  material  between  the  two.  Possibly  lime  and  carbon 
dioxide  will  be  absorbed  by  the  magma  in  exchange  for  metallic 
constituents  exhaled  from  the  igneous  rock.  At  great  depths 
the  action  will  be  sustained  over  a  longer  period  and  the  results 
may  be  somewhat  different  from  those  obtained  within  the 
cooler  and  brittle  upper  zone. 

A  gradual  lowering  of  the  temperature  from  the  maximum 
obtaining  at  the  contact  will  take  place  1.  during  the  diffusion 
of  the  gases  outward  into  cooler  sediments;  2.  during  the 
gradual  cooling  of  the  intrusive  itself.  The  presence  of  wollas- 
tonite  indicates  that  the  temperature  at  this  particular  place 
could  not  have  exceeded  1,300°  C.  above  which  point  this 
mineral  is  unstable. 

Lindgren  and  Whitehead1  attempted  to  determine  the  tem- 
perature by  the  solubility  curve  of  sodium  chloride,  which  salt 
is  present  in  sharp  cubes  in  fluid  inclusions  in  quartz  in  a  contact- 
metamorphic  deposit  at  Zimapan,  Mexico.  They  concluded 
that  the  temperature  of  formation  was  about  400°  to  500°  C. 

Wright  and  Larsen2  have  shown  that  the  quartz  in  contact- 
metamorphic  deposits  was  formed  below  575°  C.  Spencer3 
examined  fluid  inclusions  in  jasperoid  at  Ely,  Nevada,  and  by 

1  Econ.  Geol.,  vol.  9,  1914,  pp.  435-462. 

2  Am.  Jour.  Sci.,  4th  ser.,  vol.  27,  1909,  pp.  421-447. 

3  Prof.  Paper  96,  U.  S-  Geol.  Survey,  1917,  p.  63. 


IGNEOUS  METAMORPHISM  723 

calculations  based  on  relative  volume  of  gas  and  liquid  found  prob- 
able temperatures  of  200  °  to  350°C.  The  jasperoids  were,  how- 
ever, probably  formed  at  lower  temperatures  than  the  contact- 
metamorphic  silicates. 

Depth  of  Formation. — In  many  cases  it  is,  of  course,  difficult 
to  ascertain  the  depth  below  the  surface  at  which  contact-meta- 
morphic  deposits  were  formed.  In  the  province  which  contains 
the  most  abundant  and  charcateristic  examples  of  this  type, 
however,  namely,  the  Cordilleran  region  of  America,  the  condi- 
tions of  sedimentation  and  intrusion  were  such  that  approxi- 
mately correct  measurements  are  feasible.  Brogger,  many  years 
ago,  pointed  out  that  granular  intrusive  rocks  by  no  means 
always  crystallized  at  abyssal  depths  and  that  some  intrusions 
in  the  Kristiania  region  had  congealed  much  less  than  1,000  feet 
below  the  surface. 

In  the  central  Cordilleran  region  sedimentation  was  almost 
continuous  from  the  Cambrian  to  the  late  Cretaceous;  the  intru- 
sive rocks  now  exposed  were  injected  into  these  sediments  dur- 
ing Cretaceous  or  earlier  Tertiary  time,  well  up  in  the  zone  of 
fracture  and  far  above  normal  "anamorphic"  conditions.  In 
1907  Barrell1  showed  that  at  Marysville,  Montana,  the  batholith 
reached  within  4,000  feet  of  the  surface,  and  Leith  and  Harder 
gave  the  same  figure  for  the  iron  deposit  at  Iron  Springs,  Utah. 
In  New  Mexico2  similar  conditions  existed.  At  the  close  of 
the  Cretaceous  period  practically  the  whole  State  was  covered 
by  a  mantle  of  sedimentary  rocks  from  6,000  to  9,000  feet  thick. 
The  Cretaceous  part  of  this  section,  into  which  most  of  the  nu- 
merous intrusive  masses  reached,  was  between  3,000  and  5,000 
feet  thick;  much  of  it  consisted  of  tough  but  pliable  shales  not 
easily  broken  through  by  the  intrusions.  At  many  places 
contact-metamorphic  deposits  were  formed  less  than  3,000  feet 
below  the  surface.  C.  R.  Keyes3  arrived  at  similar  conclusions. 

The  intrusive  "trap"  sheets  of  Triassic  age  in  Connecticut, 
New  Jersey,  and  Pennsylvania  have  exerted  some  contact-meta- 
morphic action  and  produced  small  copper  deposits;  at  Cornwall, 
Pennsylvania,4  important  magnetite  deposits  were  formed  in 

1  Prof.  Paper  57,  U.  S.  Geol  Survey,  1907. 

2  W.  Lindgren,  L.  C.  Graton,  and  C.  H.  Gordon,  Prof.  Paper  68,  U.  S. 
Geol.  Survey,  1910,  p.  41. 

3  Econ.  Geol.,  vol.  4,  1909,  pp.  365-372. 

4  A.  C.  Spencer,  Bull.  359,  U.  S.  Geol.  Survey,  1908. 


724  MINERAL  DEPOSITS 

calcareous  Carboniferous  rocks.  The  depth  below  the  surface 
was  probably  less  than  1,000  feet. 

For  contact-metamorphic  deposits  in  pre-Cambrian  areas  and 
in  general  where  periods  of  dynamic  metamorphism  have  inter- 
vened exact  data  of  this  kind  can  rarely  be  obtained.  Some 
deposits  of  this  class  were  formed  at  great  depth  and  under 
distinctly  abyssal  conditions. 

Piezo-Metamorphism.  —  Where  the  intrusion  and  metamor- 
phism took  place  under  dynamic  conditions  —  that  is,  under 
strong  pressure  from  one  direction  —  the  results  may  be  expected 
to  differ  from  those  already  described.  Such  dynamic  condi- 


t  above 

6000 


100        300        500        700        900 

Carboniferous  Devonian?  Ordovidan  Cambrian       Cretaceous  or  Tertiary  Pre-Cambrian 

ED  '  ^  MM  .      Hi! 

Modoc  Limestone        Morenci  Shale     Longfellow  Limestone         Coronado        Granite  Porphyry  Granite 

(Altered  to  Garnet  with    ( Partly  Altered)      (Partly  Altered)  Quartzite 

Iron  and  Copper  Ores )       Shannon  Mountain  [~~~~| 

™\  SSZ2& 

5400 
5300 
5'JOO 
5100 
5000 
4UOO 


FIG.  246. — Vertical  section  showing  flat  ore-bodies  at  Shannon  mine,  Clifton, 
Arizona.     Ore-body  in  porphyry  dike  consists  of  secondary  chalcocite. 

tions  did  not  exist  in  the  Cordilleran  region  during  the  Cretaceous 
and  Tertiary  intrusions,  but  would  be  more  likely  to  occur  in 
the  abyssal  zone.  Probably'many  enigmatic  deposits  of  the  pre- 
Cambrian  have  been  formed  in  this  manner. 

E.  Weinschenk1  has  studied  this  kind  of  alteration  in  the 
Alpine  region  and  names  it  piezo-metamorphism.  According  to 
him  many  of  the  gneisses  of  the  central  Alps  are  post-Carbon- 
iferous intrusives,  pressed  during  metamorphism.  Adjoining 
limestones  have  been  made  crystalline  and  contain  character- 
istically rounded  crystals  of  quartz,  corundum,  and  micaceous 
and  chloritic  minerals. 

In  the  northern  part  of  the  Cordilleran  region  the  deposits 
seem  to  be  less  abundant,  though  several  representatives  may  be 

1 E    Weinschenk    Allgemeine  Gesteinskunde,   1902 


IGNEOUS  METAMORPHISM  725 

found  in  Canada,  Alaska,  Montana,  Idaho,  Nevada,  and  Utah. 
There  are  relatively  few  of  them  in  Colorado,  but  they  occur  in 
much  greater  abundance  in  New  Mexico,  Arizona,  and  Mexico. 

A  recent  reconnaissance  of  the  metal  deposits  in  New  Mexico 
permits  a  good  review  of  the  frequency  and  relationships  of 
these  ores.  Along  a  belt  extending  from  the  northern  boundary 
down  to  the  southwestern  part  of  the  State  the  Paleozoic  and 
Mesozoic  strata  are  intruded  by  at  least  20  stocks  .of  early  Terti- 
ary quartz  monzonite  or  monzonite,  usually  of  moderate  size. 
The  major  part  of  the  commercial  mineral  deposits  cluster 
around  these  intrusions.  Contact-metamorphic  deposits  were 
found  in  14  districts  and  at  least  25  mines  have  been  worked  on 
a  commercial  scale.  At  San  Pedro  and  Jarilla  primary  chalco- 
pyrite  ores  are  smelted;  at  Magdalena  the  deposits  yield  zinc, 
copper,  and  lead;  at  Hanover,  magnetite  and  chalcopyrite.  In 
the  minor  deposits  the  ores  may  simply  form  irregular  masses 
at  the  contact,  rarely  extending  more  than  200  feet  away  from  it. 

In  southern  Arizona  the  deposits  are  equally  common.  Among 
them  are  the  copper  deposits  of  Clifton,  Bisbee,  Saddle  Mountain, 
Twin  Buttes,  Washington,  Silver  Bell,  A  jo,  Casa  Grande,  and 
Vekol.  At  Clifton  and  Bisbee  the  ores  have  been  greatly  en- 
riched by  oxidation;  at  Saddle  Mountain,  Twin  Buttes,  Wash- 
ington, and  Silver  Bell  primary  chalcopyrite  ores  are  worked. 

In  eastern  Mexico  cupriferous  contact-metamorphic  deposits 
are  common  where  monzonites  break  through  Mesozoic  lime- 
stones, as  first  mentioned  by  Ordonez  and  Aguilera.  Since 
then  detailed  descriptions  have  been  given  of  the  districts  of 
Santa  Fe,  in  Chiapas;  Velardefia,  in  Durango;  San  Jose,  in  Tam- 
aulipas;  Concepcion  del  Oro,  in  Zacatecas;  Dolores,  in  San  Luis 
Potosi;  and  Cananea,  in  Sonora. 

PRINCIPAL  TYPES  OF  CONTACT-METAMORPHIC  DEPOSITS 

The  contact-metamorphic  deposits  may  be  classified  as  follows : 

1.  Magnetite  deposits. 

2.  Chalcopyrite  deposits.     Principal  ore  minerals  are  chalco- 
pyrite, pyrite,  pyrrhotite,  zinc  blende,  molybdenite,  magnetite, 
and  specularite. 

3.  Galena  and  zinc  blende  deposits. 

4.  Arsenopyrite-gold  deposits.     Principal  minerals  are  arseno- 
pyrite  and  pyrrhotite. 


726  MINERAL  DEPOSITS 

5.  Gold  deposits. 

6.  Cassiterite  deposits. 

7.  Titanium  deposits. 

8.  Seheelite  deposits. 

9.  Graphite  deposits. 

The  chalcopyrite  deposits  present  the  most  common  type;  the 
magnetite  deposits  are  fairly  abundant,  while  the  ores  containing 
galena,  arsenopyrite,  gold,  or  cassiterite  are  distinctly  rare. 


Magnetite  Deposits 

General  Character.- — The  magnetite  deposits  of  this  class  are 
of  common  occurrence,  though  rarely  very  large.  Associated 
with  the  magnetite  is  more  or  less  specularite,  almost  always  a 
little  pyrite  and  chalcopyrite,  and  the  contact  silicates  andra- 
dite,  ilvaite,  olivine,  and  hedenbergite — all  four  rich  in  iron 
(Fig.  247).  The  magnetite  is  sometimes  crystallized  and  often 
developed  in  coarsely  granular  masses  (Fig.  248).  The  ore- 
bodies  are  of  irregular  form,  unless,  as  often  happens,  they  follow 
the  stratification  for  some  distance. 

Foreign  Occurrences. — Among  the  European  deposits  those 
of  Berggiesshiibel,  in  Saxony;  Schmiedeberg,  in  Silesia;  and 
Gora  Magnitnaja  and  Wyssokaja  Gora,  in  the  Ural  Mountains, 
are  usually  described  in  the  text-books.  Regarding  the  latter 
two  occurrences,  the  opinions  seem  to  be  somewhat  divided. 

The  classical  deposits  of  the  Banat  province,  in  southeastern 
Hungary,  first  described  by  von  Cotta,  deserve  special  mention. 
In  this  region  early  Tertiary  intrusive  rocks,  designated  as  diorite, 
syenite,  and  their  porphyries,  break  through  Mesozoic  limestones. 
Along  the  contacts  the  limestones  become  coarsely  crystalline, 
and  the  usual  metamorphic  silicates,1  together  with  irregular 
masses  of  magnetite  and  some  sulphides,  develop  in  them.  A 
banded  structure,  sometimes  apparent,  is  caused  by  alternating 
layers  of  garnet  and  magnetite  of  contemporaneous  origin. 
Masses  of  garnet  from  70  to  several  hundred  feet  thick  occur. 
The  best-known  mines  of  this  region  are  at  Moravicza,  Do- 
gnacska,  and  Oravicza.  The  present  annual  production  is  only 
about  150,000  short  tons.  According  to  Bergeat,  there  can  be 
no  doubt  that  the  ores  are  of  contact-metamorphic  origin. 

1  A  ferromagnesian  borate,  ludwigite,  is  recorded  from  Moravicza 


IGNEOUS  METAMORPHISM 


727 


FIG.  247. — Thin  section  showing  magnetite  replacing  limestone  in  con- 
tact-metamorphic  zone,  Philipsburg,  Montana.  Intermediate  zone  rich  in 
olivine.  After  F.  C.  Calkins.  U.  S.  Geol.  Survey. 


FIG.  248. — Magnetite  replacing  limestone  in  contact-metamorphic  zone, 
Cable  mine,  Philipsburg,  Montana.     Natural  size.     After  F.  C.  Calkins. 


728  MINERAL  DEPOSITS 

The  celebrated  mineral  deposits  of  Elba,1  Italy,  with  their 
beautifully  crystallized  hematite,  are  likewise  of  contact-meta- 
morphic  origin  and  were  formed  under  the  influence  of  post- 
Eocene  granite. 

Fierro,  New  Mexico. — Many  magnetite  deposits  of  this  kind 
are  known  in  the  United  States,  particularly  in  the  Western 
States,  but  most  of  them  are  comparatively  small.  A  deposit 
at  Fierro,2  in  southwestern  New  Mexico,  is  actively  worked  at 
present,  the  ore  being  shipped  to  Pueblo,  Colorado.  The  ore 
occurs  at  the  contact  of  quartz  monzonite  porphyry,  probably 
of  early  Tertiary  age,  with  Paleozoic  limestone;  it  outcrops  in 
bold  masses  and  is  mined  in  open  cuts.  The  ore-bodies  are 
mainly  irregular,  lenticular  masses  of  magnetite  with  a  little 
chalcopyrite;  in  part  they  are  pure  magnetite  containing  from 
60  to  70  per  cent,  of  iron.  Those  parts  which  contain  a  notable 
quantity  of  chalcopyrite  are  left  as  pillars.  Small  bunches  of 
garnet  and  epidote  are  present  in  the  ore,  and  in  places  there  are 
horses  of  more  or  less  metamorphosed  limestones;  the  phos- 
phorus is  rarely  above  0.07  per  cent.;  the  sulphur  averages  0.02 
per  cent.  Similar  deposits,  richer  in  copper,  have  been  mined 
for  flux,  the  ores  being  used  in  the  copper  furnaces  at  Douglas, 
Arizona. 

Heroult,  California. — Another  deposit  is  situated  in  Shasta 
County,  California.3  The  ore  is  smelted  locally  at  Heroult,  in 
an  electric  furnace. 

The  ore-bodies  are  found  mainly  at  the  contacts  of  diorite  and 
Triassic  limestone,  and  also  to  a  minor  extent  at  the  contacts 
of  the  same  diorite  with  Permian  shale  and  with  granodiorite. 
The  order  of  crystallization  appears  to  have  been  as  follows: 
1.  Magnetite;  2.  garnet  and  hedenbergite;  3.  ilvaite  and  quartz ; 
4.  pyrite  and  chalcopyrite.  The  limestone  is  practically  pure, 
and  that  material  has  been  transferred  from  the  intrusive  seems 
to  be  the  unavoidable  conclusion.  The  ore  is  low  in  phosphorus 
and  sulphur. 

Iron  Springs,  Utah. — The  important,  yet  unworked  deposits 
of  Iron  Springs,  in  southern  Utah,  have  been  described  by 

1  B.  Lotti,  Zeitschr.  prakt.  Geol,  1905,  p.  141. 

2  L.  C.  Graton,  Prof.  Paper  68,  U.  S.  Geol.  Survey,  1910,  p.  313. 
Sidney  Paige,  Bull.  380,  U.  S.  Geol.  Survey,  1909,  pp.  199-214. 
Sidney  Paige,  Folio  199,  U.  S.  Geol.  Survey,  1916. 

3  Basil  Prescott,  Econ.  Geol.,  vol.  3,  1908,  pp.  465-480. 


IGNEOUS  METAMORPHISM 


729 


C.  K.  Leith  and  E.  C.  Harder.1  A  laccolith  of  quartz  syenite 
porphyry  (andesite  according  to  the  nomenclature  of  the  authors) 
breaks  through  sediments  of  Carboniferous,  Cretaceous,  and 
Tertiary  age  (Fig.  249).  The  magnetite  appears  in  fissure  de- 
posits and  replacements  along  the  contact  with  the  Carboniferous 
limestone.  Quartz,  garnet,  diopside,  apatite,  and  hornblende 
are  minor  constituents  of  the  ore.  According  to  the  authors 
only  a  part  of  the  ore  is  associated  with  contact  metamorphism, 
for  the  probable  gaseous  emanation  of  iron  compounds  continued 
after  the  consolidation,  and  the  resulting  magnetite,  sometimes 
associated  with  apatite,  garnet,  etc.,  filled  contraction  fissures 
in  the  intrusive  and  replaced  the  limestone  near  the  contact. 


^.•S^daifo^:;^ 


Scale  of  Feet 
600  IQpO 


FIG.  249. — Plan  showing  magnetite  and  limestone  in  projecting  point  of 
limestone.  Iron  Mountain,  Iron  Springs,  Utah.    After  Leith  and  Harder. 


But  this  by  no  means  proves  that  magnetite  was  not  also  gen- 
erally introduced  on  a  large  scale  during  the  early  metamorphic 
action.  In  fact,  most  observers  of  contact  metamorphism 
agree  that  magnetite  is  introduced  at  an  early  stage,  generally 
before  the  sulphides. 

Harder2  describes  certain  vein-like  masses  of  magnetite  and 
hematite,  associated  with  garnet  and  epidote,  in  granite  of  San 
Bernardino  County,  California. 


1  Bull.  338,  U.  S.  Geol.  Survey,  1908. 

*  Bull.  430,  U.  S.  Geol.  Survey,  1910,  pp.  228-239. 


730  MINERAL  DEPOSITS 

Cornwall, Pennsylvania. l — The  Carboniferous  consists  at  Corn- 
wall of  shales  and  sandstone,  with  beds  of  impure  limestone, 
and  is  intruded  by  sheets  of  diabase  probably  consolidated  at 
slight  depth  below  the  surface.  Near  the  contact  the  shales  are 
somewhat  baked,  and  in  the  calcareous  sandstone  some  garnet, 
pyroxene,  quartz,  and  pyrite  have  developed.  The  limestone, 
near  the  contact,  is  replaced  by  irregular  masses  of  magnetite, 
accompanied  by  garnet,  pyroxene,  epidote,  albite,  pyrite,  and 
chalcopyrite.  In  some  places  the  ores,  which  are  mined  on  a 
fairly  extensive  scale,  continue  along  the  slight  dip  of  the  beds 
for  several  hundred  feet  from  the  contact.  The  minerals  were 
formed  in  the  following  order:  Garnet  (not  abundant),  pyrite, 
pyroxene,  magnetite,  feldspar,  epidote.  Some  chalcopyrite  is 
present  and  is  later  than  the  magnetite.  Both  Spencer  and 
Harder  believe  that  the  -iron  in  the  magnetite  was  derived  from 
the  molten  diabase  magma. 

Chalcopyrite  Deposits 

General  Character. — The  contact-metamorphic  deposits  that 
carry  chalcopyrite  as  the  predominating  ore  mineral  are  not 
abundantly  represented  in  Europe,  but  are  the  most  common 
type  in  North  America,  particularly  in  New  Mexico,  Arizona, 
and  Mexico.  Similar  deposits  have  been  found  in  Australia, 
Japan  and  Korea.  They  occur  as  a  rule  at  the  contacts  'of 
smaller  intrusive  masses  of  monzonite  or  quartz  monzonite 
against  limestone.  Their  form  is  irregular  or  tabular.  The 
tabular  deposits  follow  certain  beds  in  the  limestone  formations, 
and  their  hanging  and  foot  walls  may  consist  of  little-altered  or 
unaltered  limestone.  The  structure  of  the  ore  is  massive  and 
coarse  granular  (Figs.  250  and  251).  The  ore  minerals  consist 
of  chalcopyrite,  bornite,  pyrite,  more  rarely  pyrrhotite,  and  zinc 
blende,  often  also  molybdenite  and  other  sulphides;  galena  is 
on  the  whole  rare.  The  ore  contains  also  more  or  less  magnetite 
and  specularite.  The  gangue  minerals  are  andradite,  grossu- 
larite,  epidote,  diopside,  tremolite,  ilvaite,  and  calcite.  The 
deposits  are  poor  in  gold  and  silver  and  are  frequently  enriched 

1  A.  C.  Spencer,  Magnetite  deposits  of  the  Cornwall  type  in  Pennsylvania. 
Bull.  359,  U.  S.  Geol.  Survey,  1908,  pp.  74-76.  Idem,  Bull.  430,  1910,  pp. 
247-249. 

E.  C.  Harder,  Structure  and  origin  of  the  magnetite  deposits  near  Dills- 
burg,  Pennsylvania,  Econ.  Geol.,  vol.  5,  1910,  pp.  599-622. 


IGNEOUS  METAMORPHISM 


731 


FIG.  250. — Thin  section  showing  contact-metamorphic  ore,  Clifton, 
Arizona,  c,  Calcite;  g,  garnet;  q,  quartz;  cu,  chalcopyrite.  Magnified  15 
diameters. 


A  B 

FIG.  251. — Thin  sections  showing  contact-metamorphic  ores  at  Holgol, 
Korea.  A,  Radiating  crystals  of  ilvaite  or  hedenbergite  (black)  in  granular 
limestone.  Ilvaite  incloses  crystals  of  diopside.  B,  Chalcopyrite  (black)  in 
diopside.  Magnified  30  diameters.  After  B.  Koto. 


732  MINERAL  DEPOSITS 

in  copper  by  oxidation,  but  in  many  occurrences  the  primary 
ore  is  rich  enough  to  be  utilized. 

New  Mexico.1 — In  the  fourteen  districts  of  New  Mexico  dis- 
tinguished by  contact-metamorphic  deposits,  the  copper  ores  are 
by  far  the  most  common'. 

The  most  important  of  these  deposits,  economically,  is  that  of 
the  San  Pedro  mine,  in  the  laccolithic  mountain  group  of  the 
same  name.  Beds  of  upper  Carboniferous  rocks  over  700  feet 
thick  have  been  metamorphosed  by  the  underlying  laccolith 
of  granodiorite  porphyry  and  by  dikes  extending  upward  from 
it.  The  lower  200  feet  of  shaly  limestone  is  only  partly  altered, 
with  local  development  of  garnet  and  tremolite  and  a  little 
chalcopyrite  and  pyrrhotite,  but  along  a  certain  bed  of  purer 
limestone  garnetization  has  taken  place  for  half  a  mile,  the  thick- 
ness of  this  strongly  metamorphosed  stratum  being  about  50 
feet.  Bunches  of  chalcopyrite  are  irregularly  distributed  in  it. 
Within  this  zone  beds  of  pure  crystalline  limestone  adjoin  wholly 
garnetized  beds.  In  places  the  rock  consists  of  a  mixture  of 
garnet  and  coarsely  crystalline  limestone.  On  the  dip  the 
gently  inclined  ore  beds  have  been  followed  for  300  feet.  The 
upper  beds  of  the  series  consist  mainly  of  somewhat  metamor- 
phosed and  baked  shale  and  sandstone. 

Clifton,  Arizona. — In  Arizona  almost  all  the  contact-meta- 
morphic deposits  yield  copper  as  the  principal  metal.  Near 
Clifton2  a  stock  of  granite  porphyry  and  quartz  monzonite 
porphyry  breaks  across  pre-Cambrian  granite,  a  Paleozoic  series 
about  1,000  feet  thick,  and  Cretaceous  sediments  about  400  feet 
thick.  The  Paleozoic  limestones  and  shales,  as  well  as  the 
Cretaceous  sandstones,  are  contact  metamorphosed.  The  ore 
deposits  lie  mainly  near  Morenci  and  Metcalf;  at  both  places 
the  beds  are  cut  by  an  unusual  number  of  dikes,  which  have 
exerted  a  specially  strong  contact-metamorphic  action  on  the 
sediments. 

The  ore  deposits  form  a  complicated  series  very  similar  to 
those  observed  at  Cananea,  Mexico,  at  Ely,  Nevada,  and  at 
Bingham,  Utah.  The  oldest  ores  are  contact-metamorphosed 
limestones;  these,  as  well  as  the  adjoining  porphyry,  are  cut  by 

1  Lindgren,  Graton,  and  Gordon,  Prof.  Paper  68,  U.  S.  Geol.  Survey,  1910. 

2  W.  Lindgren,  Prof.  Paper  43,  U.  S.  Geol.  Survey,  1905. 

'  L.  E.  Reber,  Jr.,  The  mineralization  at  Clifton-Morenci,  Econ.  Geol, 
vol.  11,  1916,  pp.  528-573. 


IGNEOUS  METAMORPHISM  733 

a  series  of  pyritic  veins,  poor  in  copper,  which  in  the  sericitized 
porphyry  spread  out  into  disseminations  of  pyrite.  "Wide- 
spread oxidation  has  altered  all  the  deposits  and  enriched  them; 
well-defined  chalcocite  zones  (p.  837)  have  formed  "by  replace- 
ment of  the  pyrite  by  descending  cupric  sulphate  solutions,  and 
the  present  importance  of  the  district  is  due  wholly  to  the  ex- 
ploitation of  these  chalcocite  ores,  which  contain  from  2  to  4 
per  cent,  of  copper. 

The  primary  contact-metamorphic  deposits  lie  in  limestone 
and  form  irregular  bunches  or  tabular  deposits  parallel  to  dikes 
or  following  the  stratification.  Wherever  the  character  is  not 
masked  by  oxidation  these  primary  ores  consist  of  garnet,  epi- 
dote,  diopside,  calcite,  chalcopyrite,  pyrite,  magnetite,  and 
zinc  blende,  occasionally  also  molybdenite.  In  the  earlier  days 
of  the  district,  from  1875  to  1900,  these  oxidized  ore-bodies  were 
mined;  they  were  easily  reduced  and  comparatively  rich  in  cop 
per,  containing  mainly  malachite,  azurite,  and  limonite.  The 
celebrated  Longfellow  ore-body  formed  a  funnel-shaped  mass  in 
Ordovician  limestone,  between  two  porphyry  dikes. 

Farther  west,  in  the  Manganese  Blue  and  Detroit  mines, 
were  several  tabular  ore-bodies,  following  the  stratification  in 
the  Ordovician,  Devonian,  and  Carboniferous  limestones;  these 
also  owed  their  richness  to  several  porphyry  dikes,  a  few  hundred 
feet  from  the  main  contact.  Along  the  main  contact  were  many 
irregular  bunches  of  oxidized  contact-metamorphic  ores.  At 
Metcalf  the  Shannon  Mountain  contained  several  similar  ore- 
bodies  (Fig.  246),  lying  in  an  isolated  mass  of  Paleozoic  lime- 
stones extensively  cut  by  porphyry  dikes. 

Bisbee,  Arizona.1 — At  Bisbee,  Arizona,  pre-Cambrian  rocks 
are  overlain  by  about  5,000  feet  of  Paleozoic  limestones.  After 
their  deposition  they  were  deformed  by  folding  and  faulting 
and  were  cut  by  intrusions  of  granitic  porphyry  of  probably 
Jurassic  Age,  which  is  intimately  connected  with  the  origin  of 
the  copper  deposits.  The  principal  mass,  of  which  the  most 
prominent  point  is  Sacramento  Hill,  close  to  Bisbee,  is  about 

1  F.  L.  Ransome,  Pro/.  Paper  21,  U.  S.  Geol.  Survey,  1904. 

W.  L.  Tovote,  Min.  and  Sci.  Press,  Feb.  4,  1911. 

Arthur  Notman,  The  Copper  Queen  mine  and  works,  Trans.,  Inst.  Min. 
and  Met.  (London),  vol.  22,  1913,  pp.  550-562. 

Y.  S.  Bonillas,  J.  B.  Tenney  and  L.  Feuchere,  Geology  of  the  Warren 
mining  district,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  55,  1917,  pp.  284-355. 


734 


MINERAL  DEPOSITS 


1  mile  long  and  l£  miles  wide.  After  a  period  of  erosion  a 
transgression  of  the  Cretaceous  sea  deposited  a  thick  series  of 
beds  on  the  older  series  (Fig.  252). 

The  copper  deposits  lie  in  the  limestones,  surrounding  crescent- 
like  the  east  side  of  the  intrusive  mass,  and  appear  as  irregular  and 
ill-defined  or  rudely  tabular  masses,  which  on  the  whole  follow 
the  dip  of  the  stratification.  They  are  almost  entirely  oxidized, 


Surface 


Porphyry 


<  Secondary  li'i'i'i'i'i'i'i'll  Primary 

^^2  Sulphide  Ore  tili  !.•.!.:  n'tgnlphide 


FIG.  252. — Generalized  vertical  section  showing  relations  of  primary  ore, 
secondary  sulphides  and  oxidized  ore  at  Bisbee,  Arizona.  Ce,  Escabrosa 
limestone;  Dm,  Martin  limestone.  After  J.  Douglas  and  A.  Notman. 


even  down  to  depths,  of  1,400  feet  below  the  surface.  The 
oxidized  ore,  containing  copper  carbonates,  cuprite,  and  some- 
times also  chalcocite,  with  much  limonite,  passes  gradually  on  its 
peripheries  into  "ledge  matter"  or  limonitic  clays,  which  in  turn 
grade  into  altered  and  contact-metamorphosed  limestones.  The 
contact  metamorphism  is  unusually  inconspicuous,  but  the  lime- 
stone surrounding  the  intrusive  mass  contains  fine-grained  tremo- 


IGNEOUS  METAMORPHISM  735 

lite,  diopside,  garnet,  vesuvianite,  and  quartz,  associated  witli 
magnetite,  pyrite,  bornite,  and  a  little  chalcopyrite  and  galena 
and  sphalerite.  In  the  Oliver  shaft,  close  to  the  contact,  on  the 
1,200-foot  level,  the  limestone  is  more  intensely  metamorphosed 
than  elsewhere  and  contains  bodies  of  pyrite,  mixed  with  chal- 
copyrite and  bornite. 

The  porphyry  dips  underneath  the  limestones  and  the  contact 
is  thus  found  farther  west  in  the  mines  than  on  the  surface. 
The  limestones  are  also  cut  by  a  considerable  number  of  dikes. 
A  heavy  mantle  of  pyritic  quartzose  ore,  with  some  chalcopyrite 
and  chalcocite,  surrounds  the  porphyry,  pitching  parallel  to  the 
contact  underneath  the  limestone  of  the  surface. 

The  deep  oxidization  of  the  ores  was  practically  completed 
before  the  Cretaceous  and  under  conditions  of  water  level  dif- 
ferent from  those  of  today.  The  sericitized  and  chloritized  por- 
phrry  contains  some  large  bodies  of  chalcocite  ore.  Here,  the 
mineral  replaces  pyrite. 

The  annual  ore  production  of  the  Bisbee  (Warren)  district  is 
now  about  2,000,000  tons  of  5  to  6  per  cent.,  ore  yielding,  in  1916. 
about  200,000,000  pounds  of  copper  with  some  lead,  gold  and 
silver;  the  total  metallic  value  was  $52,000,000.  The  principal 
production  is  derived  from  the  Copper  Queen  and  Calumet  & 
Arizona  mines. 

Silver  Bell,  Arizona. — At  Silver  Bell,1  southwest  of  Tucson, 
extensive  primary  chalcopyrite  deposits  have  been  worked. 
The  mines  are  near  the  summit  of  one  of  the  numerous  desert 
ranges  of  that  region;  the  ores  were  smelted,  without  concentra- 
tion, at  the  Sasco  plant.  Several  small  masses  of  Paleozoic  lime- 
stone are  engulfed  in  a  large  mass  of  granite  porphyry  and  along 
their  contacts  metamorphism  is  irregularly  developed — in  part 
by  marmorization,  in  part  by  garnetization.  The  ore  consists 
of  chalcopyrite  and  light-brown  garnet,  said  to  be  andradite, 
with  a  little  magnetite,  zinc  blende,  galena,  and  molybdenite. 
Much  of  the  ore  averaged  7  per  cent,  copper,  but  that  smelted 
would  average  about  4  per  cent.  About  800  tons  were  mined 
per  day  in  1909.  A  trace  of  gold  and  1  to  2  ounces  of  silver  per 
ton  are  present.  The  oxidation  is  shallow,  wholly  fresh  rock  being 
encountered  at  the  200-foot  level.  The  porphyry  is  locally 
silicified,  but  otherwise  not  greatly  altered,  except  for  some 

1  C.  F.  Tolman,  Min.  and  Sci.  Press,  Nov.  27,  1909. 
C.  A.  Stewart,  Trans.,  Am.  Inst.  Min.  Bag.,  vol.  43,  1913,  pp.  240-290. 


736  MINERAL  DEPOSITS 

disseminated  pyrite  and  chalcopyrite.  No  extensive  chalcocite 
zone  has  been  found  in  the  porphyry. 

Cananea,  Mexico.- — The  mines  at  Cananea  lie  a  short  distance 
south  of  the  Arizona-Sonora  boundary  line,  in  one  of  the  short 
ranges  that  rise  out  of  the  gently  sloping  desert  plains.  Since 
1900  these  deposits  have  yielded  large  quantities  of  copper  from 
ores  enriched  by  oxidation  and  development  of  secondary 
chalcocite.  The  district  was  described  by  S.  F.  Emmons.1 

The  deposits  show  some  similarity  to  those  of  Clifton,  Arizona, 
but  the  geological  history  is  much  more  complicated.  Three 
successive  irruptions,  termed  diorite  porphyry,  granodiorite,  and 
quartz  porphyry,  have  caused  contact  metamorphism  in  rela- 
tively small  areas  of  Paleozoic  limestone.  Among  the  primary 
minerals  are  chalcopyrite,  bornite,  zinc  blende,  magnetite,  and 
specularite;  the  limestones  are  garnetized,  marmorized,  and 
silicified. 

A  second  epoch  of  mineralization  by  aqueous  solutions  re- 
sulted in  veins  and  disseminations  of  pyrite,  chalcopyrite,  and 
quartz.  Both  classes  have  been  enriched  by  oxidizing  solutions. 

Bingham,  Utah. — The  Bingham  district,  near  Salt  Lake, Utah, 
is  now  most  widely  known  by  the  extensive  mining  operations 
of  the  Utah  Copper  Co.  in  chalcocitized  porphyry  (p.  866). 
According  to  J.  M.  Boutwell,2  laccoliths  and  stocks  of  monzonite, 
as  well  as  sills  and  dikes  of  diorite  porphyry,  invade  the  Carbon- 
iferous quartzite  and  limestone.  Some  of  the  ore-bodies  are 
irregular  replacements  of  contact-metamorphic  type,  others  are 
later  quartz  veins  containing  lead,  copper  and  zinc;  both  are 
altered  and  enriched  by  descending  waters.  Near  the  contacts 
the  limestone  is  extensively  marmorized  and  replaced  by  pyrite 
and  chalcopyrite  with  local  retention  of  its  bedded  structure, 
but  the  development  of  garnet  and  other  silicates  is  unusually 
scant. 

Ketchikan,  Alaska. — Several  contact-metamorphic  copper  de- 
posits in  southeastern  Alaska  are  described  by  F.  E.  and  C.  W. 
Wright.3  Those  of  Copper  Mountain,  Prince  of  Wales  Island, 
present  an  unusually  excellent  illustration  of  deposits  occurring 
at  intervals  along  the  contacts  of  an  isolated  granite  intrusion. 

1  S.  F.  Emmons,  Earn.  Geol,  vol.  4,  1910,  pp.  312-356. 

2  J.  M.  Boutwell,  Prof.  Paper  38,  U.  S.  Geol.  Survey,  1905. 

3  Econ.  Geol.,  vol.  3,  1908,  pp.  410-417.     Bull.  347,  U.  S.  Geol.  Survey, 
1908. 


IGNEOUS  METAMORPHISM  737 

On  the  Kasaan  Peninsula  are  several  magnetite-chalcopyrite 
deposits,  also  containing  pyrrhotite  and  pyrite,  in  a  gangue  of 
amphibole,  epidote,  orthoclase,  garnet,  and  calcite.  Wright 
believes  that  the  ores  were  formed  after  the  consolidation  of 
the  last  intrusions  of  syenite.  At  both  places  shear  zones  and 
vein  deposits  containing  copper  accompany  the  contact  deposits. 

Zinc  and  Lead  Deposits 

Almost  all  contact-metamorphic  sulphide  deposits  contain 
some  zinc  blende,  and  often  also  a  little  galena,  but  only  a  few 
deposits  are  known  in  which  these"*metals  constitute  the  prin- 
cipal value  of  the  ore.  Where  they  occur  the  amphiboles  and 
epidote  appear  to  be  more  prominent  than  garnet. 

One  of  the  best  examples  is  furnished  by  the  Magdalena  mines,1 
in  New  Mexico,  which  in  the  oxidized  zone,  200  to  300  feet  deep, 
were  worked  for  their  lead,  silver,  and  zinc.  In  depth  large 
bodies  of  zinc  blende  were  found,  together  with  a  little  galena 
and  chalcopyrite.  According  to  Gordon  the  Magdalena  Range 
consists  of  faulted  blocks  of  Paleozoic  (Mississippian  and  Pennsyl- 
vanian)  limestone,  resting  on  pre-Cambrian  crystalline  rocks. 
The  limestones  are  cut  by  dikes  of  granite  porphyry,  which  are 
exposed  near  the  Graphic  mine  and  which  are  believed  to  have 
caused  the  mineralization.  In  the  limestones,  which  dip  west- 
ward, toward  a  hidden  contact  with  the  granite  porphyry,  min- 
eralization has  taken  place  at  five  horizons,  of  which  only  one, 
just  below  the  "Silver  Pipe"  limestone,  is  of  great  importance. 
The  ore-bodies  are  roughly  lenticular  and  may  be  as  much  as 
40  feet  in  thickness.  They  occur  at  irregular  intervals  along 
the  bedding  planes,  the  principal  bodies  lying  apparently  at  the 
crests  of  low  arches  transverse  to  the  strike  of  the  beds.  Besides 
the  sulphides  mentioned  they  contain  magnetite  and  specularite, 
with  much  epidote,  pyroxene,  and  tremolite,  but  little  if  any 
garnet.  The  distance  along  the  dip  of  the  strata  to  the  intrusive 
rock  is  probably  not  less  than  2,000  feet. 

Knopf2  describes  lead  deposits  from  Darwin,  Inyo  County, 
California,  which  present  an  interesting  succession  ranging  from 
contact-metamorphic  types  to  fissure  veins.  Over  large  areas 
the  Carboniferous  calcareous  rocks  are  altered  to  wollastonite, 

1  C.  H.  Gordon,  Prof.  Paper  68,  U.  S.  Geol.  Survey,  1910,  pp.  241-258. 
*  Adolph  Knopf,  Bull.  580,  U.  S.  Geol.  Survey,  1915,  pp.  1-18. 


738  MINERAL  DEPOSITS 

diopside  and  grossularite  rocks,  with  perfect  preservation  of 
structure.  An  enormous  quantity  of  material,  chiefly  silica 
has  been  added  during  metamorphism.  The  contact-metamor- 
phic  ores  lie  between  quartz-diorite  and  lime  silicate  rocks.  The 
minerals  are  galena,  andradite,  calcite  and  fluorite.  Orthoclase 
and  apatite  are  present  in  other,  similar  deposits.  Numerous 
veins  of  galena  and  fluorite,  break  through  lime  silicate  rock 
in  other  parts  of  the  district.  The  contact-metamorphic  de- 
posits are  considered  by  Knopf  to  be  later  than  the  general 
metamorphism,  but  the  argument  is  not  wholly  convincing. 

Contact-metamorphic  deposits  have  been  described  from  east- 
ern Mexico,  where  the  thick  Cretaceous  limestone  is  broken 
through  by  many  small  intrusives.  Most  of  these  are  copper 
deposits  but  sometimes  they  contain  lead  as  at  La  Sirena  Mine, : 
near  Zimapan,  Hidalgo,  where  dikes  and  masses  of  monzonitic 
character  intruded  into  the  pure  limestone  have  caused  minerali- 
zation along  the  contacts.  The  ore  formation  began  by  develop- 
ment of  quartz,  albite,  danburite  (CaB2Si208),  apatite,  garnet, 
actinolite  and  fluorite  in  the  order  given.  Then  followed  de- 
position of  arsenopyrite  and  pyrrhotite,  and  lastly  galena,  zinc 
blende  and  jamesonite. 

Finally  there  is  to  be  mentioned  the  great  Santa  Eulalia2 
lead  deposits  near  Chihuahua.  These  wonderful  deposits 
which  have  yielded  during  the  last  two  hundred  years,  lead  and 
silver  to  the  estimated  value  of  $300,000,000  to  $500,000,000, 
carry  mainly  oxidized  ores  and  their  geological  affiliations  have 
not  been  known  until  recently.  They  form  tabular  or  irregular 
bodies  or  pipes  in  Cretaceous  limestone  and  have  been  opened  to 
a  depth  of  2,000  feet.  These  replacement  ores  are  connected 
with  N.-S.  fissures.  One  type  contains  pyrite  (replaced  by 
pyrrhotite  in  depth),  galena  and  zinc  blende.  The  other  type 
appears  closely  allied  to  the  contact-metamorphic  ores,  and  forms 
irregular  masses.  They  are  described  as  argentiferous  pyritic 
ores  and  contain  pyrrhotite  with  a  little  pyrite,  galena,  sphalerite, 
arsenopyrite,  associated  with  iron  manganese  silicates  such  as 
ilvaite,  knebelite,  hedenbergite,  fayalite  and  chlorite.  The 
presence  of  intrusive  bodies  in  depth  is  inferred,  but  so  far  none 
have  been  disclosed. 

1W.  Lindgren  and  W.  L.   Whitehead,  Econ.  Geol,  vol.   9,    1914,    pp 
435-462. 
2  Basil  Prescott,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  51,  1916,  pp.  57-99. 


IGNEOUS  METAMORPHISM  739 

Gold  Deposits 

Gold  is  present  in  traces  in  almost  all  sulphide  deposits  of  the 
contact-metamorphic  type,  and  a  few  ounces  of  silver  to  the 
ton  is  likewise  not  unusual,  but  it  is  rare  to  find  in  such  deposits 
ores  which  are  valuable  chiefly  on  account  of  their  precious  metals. 

At  the  Santa  Fe  mine,1  Chiapas,  Mexico,  limestone  close  to 
an  intrusive  contact  is  changed  to  garnet  and  wollastonite;  this 
rock  contains  in  disseminated  form,  or  concentrated  in  ore 
channels,  auriferous  and  argentiferous  bornite.  The  gold  is  in 
part  free.  The  average  ore  consists  of  90  per  cent,  garnet  and 
10  per  cent,  quartz  (and  chalcedony)  carrying  from  3  to  4  per 
cent,  copper,  with  6  to  8  ounces  in  silver  and  $6  to  $20  in  gold  per 
ton. 

One  of  the  best  examples  of  a  gold-bearing  contact-metamor- 
phic deposit  is  that  of  the  Cable  mine,  Montana,  described  by 
W.  H.  Emmons.2  The  ores  are  included  in  a  long,  thin  block  of 
limestone,  in  contact  on  both  sides  with  quartz  monzonite. 
The  principal  minerals  are  calcite,  quartz,  pyrrhotite,  pyrite, 
magnetite,  and  chalcopyrite,  with  actinolite  garnet,  and  green 
mica.  The  gold,  in  part  coarse,  is  disseminated  in  calcite,  quartz, 
and  sulphides.  This  deposit  has  yielded  about  $3,000,000. 

Platinum  is  rarely  found.  One  occurrence  in  Sumatra  is 
mentioned  by  L.  Hundeshagen;3  the  metal  occurs  in  wollastonite. 

Gold-Arsenopyrite  Type. — The  best  example  of  this  rare  type, 
lately  described  by  C.  Camsell,4  is  the  deposit  worked  by  the 
Nickel  Plate  mine,  British  Columbia.  Gently  folded  Carbon- 
iferous limestones  associated  with  shale,  quartzite,  and  volcanic 
tuffs  are  intruded  by  sheets  of  gabbro  and  diorite.  Along  the 
contacts  of  these  sheets,  and  particularly  of  their  apophyses,  the 
impure  limestones  are  converted  into  contact-metamorphic  min- 
erals with  arsenopyrite.  The  commercial  ore-bodies,  which  have 
yielded  several  million  dollars  in  gold,  are  tabular  and  follow  the 
dipping  contacts  of  the  basic  rock,  which  are  not  exactly  parallel 

1  E.  T.  McCarty,  Trans.,  Inst.  Min.  and  Met.  (London),  vol.  4,  1895-96, 
pp.  169-189. 

Also  H.  L.  Collins,  idem,  February,  1900,  and  Trans.,  Am.  Inst.  Min. 
Eng.,  vol.  31,  1901,  p.  446. 

'Pro/.  Paper  78,  U.  S.  Geol.  Survey,  1913,  pp.  221-231. 

3  L.  Hundeshagen,  Trans.,  Inst.  Min.  and  Met.  (London),  1904,  pp.  1-3. 

4  Geology  and  ore  deposits  of  the  Hedley  district,  B.  C.,  Mem.  2,  Canada 
Dept.  Mines,  Geol.  Survey  Branch,  1910. 


740  MINERAL  DEPOSITS 

with  the  inclination  of  the  strata.  The  outside  of  the  ore-body  is 
irregular  and  gradually  fading,  reaching  somewhat  farther  away 
from  the  contact  in  some  beds  than  in  others.  The  principal 
ore-body  has  been  followed  350  feet  along  the  dip  and  has  a 
width  parallel  to  the  contact  of  125  feet. 

The  ore  minerals  are,  named  in  order  of  quantity,  arseno- 
pyrite,  pyrrhotite,  chalcopyrite,  pyrite,  zinc  blende,  tetradymite 
(Bi2Te3)  and  molybdenite.  The  depth  of  oxidation  is  slight, 
but  in  the  upper  levels  free  gold  occurred  associated  with  tetra- 
dymite, while  at'the  greater  depth  now  attained  it'seems  to  be 
intimately  bound  up  with  the  arsenopyrite  and  is  not  amenable 
to  amalgamation.  The  pure  arsenopyrite  may  contain  as  much 
as  12  ounces  of  gold  per  ton.  The  gold  tenor  varies  from  $6  to 
$14  per  ton,  but  beyond  the  ore-body  minor  quantities  of  gold  are 
widely  disseminated  in  the  contact-metamorphic  rocks.  There 
is  very  little  silver;  traces  of  platinum  (as  sperrylite?)  and  nickel 
are  present.  The  gangue  minerals  are  andradite,  pyroxene, 
epidote,  calcite,  and  axinite,  and  the  sulphides  are  closely  inter- 
grown  with  them,  pointing  to  contemporaneous  deposition;  the 
chalcopyrite  is  somewhat  later  than  the  arsenopyrite.  Quartz 
is  distinctly  later  than  the  other  gangue  minerals.  Orthoclase 
appears  in  gabbro  dikes  replacing  pyroxene  by  a  process  of 
endomorphic  contact  metamorphism.  Although  the  rocks  are 
faulted  and  fissured  by  post-intrusive  stresses,  these  fractures 
contain  practically  no  valuable  ores. 

The  only  similar  deposit  described  in  the  literature  is  that  of 
Reichenstein,  Silesia,  the  auriferous  leucopyrite  and  arsenopyrite 
of  which  have  been  worked  on  a  small  scale,  probably  since  the 
thirteenth  century.  According  to  C.  Wienecke1  and  A.  Bergeat2 
the  ore-producing  intrusive  is  probably  a  neighboring  granite, 
and  the  altered  rock  a  dolomitic  limestone. 

Telluride  Type. — Contact-metamorphic  deposits  carrying  tel- 
luride  ores  are  rare.  W.  H.  Weed3  describes  such  an  occurrence 
at  the  Dolcoath  mine  at  Elkhorn,  Montana,  where  auriferous 
tetradymite  is  found  in  a  15-  to  18-inch  bed  of  garnet,  diop- 
side,  and  calcite.  Weed4  also  mentions  a  deposit  at  Bannock, 

1  C.  Wienecke,  Zeitschr.  prakt.  Geol,  1907,  p.  273. 

2  Die  Erzlagerstatten,  vol.  2,  1906,  p.  1137. 

3  W.  H.  Weed  and  J.  Barrell,  Elkhorn  mining  district,   Twenty-second 
Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  2,  1901,  p.  506. 

*  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  33,  1903,  p.  732. 


IGNEOUS  METAMORPHISM  741 

Montana,  where  tellurides  occur  with  specularite,  garnet,  pyrite, 
and  free  gold  at  a  contact  between  diorite  and  limestone. 

The  occurrence  of  altaite  in  the  Cable  mine,  Montana,  is 
mentioned  by  W.  J.  Sharwood.1 

It  seems  well  established,  then,  that  tellurides  may  crystallize 
at  high  temperatures.  They  are  not  known  as  products  of 
igneous  consolidation. 

Cassiterite  Deposits 

Contact-metamorphic  deposits  with  the  assemblage  of  fluorine 
and  boron  minerals  characteristic  of  cassiterite  veins  are  rare. 
The  tin-bearing  magmas,  which  are  generally  normal  granites, 
seem  to  retain  the  tin  and  associated  substances  until  a  later 
stage,  after  consolidation  of  the  rock. 

Cassiterite  occurs,  in  part  in  connection  with  fissures,  in  the 
contact-metamorphic  deposits  of  Pitkaranta,  in  Finland;  of 
Schwarzenberg  and  Berggiesshiibel,  in  Saxony;  and  of  Campiglia 
Marittima,  in  Tuscany.  Other  examples  of  more  typical  char- 
acter have  been  noted  at  Dartmoor,  in  Devonshire,  England,2 
and  lately  by  A.  Knopf3  on  Lost  River,  Seward  Peninsula, 
Alaska.  At  the  latter  place  the  granite  has  produced  a  narrow 
but  long  contact  zone  of  lime-silicate  rocks  rich  in  tourmaline, 
axinite,  ludwigite,  hulsite  and  paigeite  (both  ferromagnesian 
stannoborates) ,  vesuvianite,  fluorite,  scapolite,  chondrodite, 
galena,  sphalerite,  arsenopyrite,  pyrrhotite,  scheelite,  magnetite, 
pyroxene,  and  cassiterite.  In  the  banded  limestone  the  ar- 
gillaceous layers  are  converted  to  tourmaline,  with  tremolite  and 
vesuvianite,  while  the  purer  calcareous  layers  are  marmorized. 
The  orbicular  structure  of  the  contact  minerals  of  this  district 
has  been  mentioned  on  p.  711.  The  deposit  is  said  to  be  of  little 
economic  importance. 

At  this  interesting  locality  cassiterite  also  occurs  in  tourma- 
linized  granitic  masses  or  dikes,  in  quartz  veins  cutting  granite 
and  developing  greisen,  in  quartz  porphyry  dikes,  and  in  quartz 
stringers  cutting  limestone  and  slate. 

The  dikes  of  quartz  porphyry,  which  pierce  the  limestone, 
contain  cassiterite,  pyrite,  arsenopyrite,  wolframite,  and  fluorite, 

1  Econ.  Geol,  vol.  6,  1911,  pp.  22-36. 

2  K.  Busz,  Neues  Jahrb.,  Beil.  Bd.  13,  1899,  p.  100. 

3  Bull.  358,  U.  S.  Geol.  Survey,  1908. 


742  MINERAL  DEPOSITS 

with  mica  and  topaz.  The  adjoining  limestone  is  reticulated  by 
veins  which,  carry  cassiterite,  and  around  these  veinlets  horn- 
blende, vesuvianite,  fluorite,  plagioclase,  mica,  and  topaz  have 
formed. 

In  the  so-called  Dolcoath  lode  a  narrow  dike  is  transformed 
into  danburite  (borosilicate  of  calcium)  and  tourmaline,  with 
some  arsenopyrite  and  cassiterite. 

In  the  offshoots  from  the  main  granite  mass  are  found  fluorite, 
cassiterite,  muscovite,  tourmaline,  and  topaz,  the  last  two  having 
crystallized  after  the  feldspar  and  quartz. 

These  interesting  observations  clearly  show  the  intimate  con- 
nection and  in  fact  the  transition  between  contact-metamorphic 
deposits  and  veins. 

TITANIUM  DEPOSITS 

Titanium  minerals  are  rare  in  contact-metamorphic  deposits. 
Rutile  is  occasionally  present.  Singewald1  has  lately  described 
ores  of  this  type  from  Cebolla,  Colorado,  which  contain  ilmenite 
and  magnetite  together  with  garnet,  augite  and  calcite.  From 
later  information  it  appears  that  perowskite  (CaTiO3)  and 
titanium  garnet  are  also  present. 

SCHEELITE  DEPOSITS 

Scheelite  (CaWO4),  a  heavy  white  mineral  of  non-metallic 
luster,  occurs  in  many  contact-metamorphic  deposits.  Recently 
such  scheelite  deposits  have  been  discovered  in  Mono,  Inyo 
County,2  California,  which  apparently  are  of  economic  impor- 
tance. The  scheelite  is  associated  with  garnet,  calcite,  hornblende 
and  pyroxene.  Metallic  minerals  are  rare.  In  1917  and  1918 
several  such  deposits  were  discovered  and  worked  in  the  Hum- 
boldt  Range,  Nevada,  particularly  near  Mill  City.  The  associa- 
tion here  is  calcite,  scheelite,  garnet,  epidote  and  pyrite,  and 
the  deposit  occurs  in  limestone  close  to  granite  and  is  intersected 
by  a  dike  of  aplite.  Possibly  many  such  deposits  have  been 
overlooked. 

1  J.  T.  Singewald,  Jr.,  Econ.  Geol.,  vol.  7,  1912,  pp.  560-573. 
'Adolph  Knopf,  Bull.  640,  TJ.  S.  Geol.  Survey,  1917,  pp.  229-249. 


IGNEOUS  METAMORPHISM  743 

Graphite1 

Properties. — Graphite  is  a  form  of  carbon  crystallizing  in 
the  rhombohedral  system;  it  is  soft,  is  steel-gray  to  black,  and 
has  a  greasy  feel.  Even  in  its  purest  form  it  contains  a  little 
volatile  matter  and  ash,  usually  less  than  1  per  cent.  Many 
varieties  are  impure,  and  for  some  purposes,  like  paint-making, 
material  with  as  little  as  35  per  cent,  graphitic  carbon  is  utilized. 
Analyses  quoted  by  Cirkel  show  that  the  commercial  grades  of 
graphite,  even  those  considered  as  of  high  quality,  contain 
several  per  cent,  of  volatile  matter  and  may  be  high  in  ash.  One 
analysis  of  graphite  from  Ceylon  shows  5.20  per  cent,  volatile 
matter  and  22.15  per  cent.  ash. 

The  question  whether  graphite  really  exists  in  some  of  the 
varieties  of  "graphitic  slate"  yielding  "amorphous  graphite"  is 
debatable;  the  minuteness  of  the  particles  renders  it  difficult  to 
determine  whether  they  are  crystalline  or  not.  The  term 
"graphitoid"  has  been  proposed  for  such  substances,  but  is 
not  accepted  by  all  authors.  The  best  test  for  graphite  is  said 
to  be  its  characteristic  property  of  yielding  "  graphitic  acid " 
(CnH^s)  with  strong  oxidizing  reagents  such  as  nitric  acid. 
The  amorphous  carbons  do  not  respond  to  this  test. 

According  to  H.  Moissan  graphite  begins  to  oxidize  at  650°  to 
700°  C.  In  texture  graphite  is  flaky  or  scaly  or,  when  in  veins, 
is  often  fibrous  perpendicular  to  the  walls;  these  varieties  are 
called  "crystalline"  in  the  trade.  "Plumbago"  and  "black 
lead"  are  trade  names  for  the  mineral. 

General  Occurrence  and  Origin. — Graphite  appears  mainly  in 
rocks  which  have  suffered  intense  regional  or  igneous  meta- 
morphism.  The  literature  on  its  occurrence  and  origin  is  very 
extensive  and  shows  plainly  that  the  mineral  may  have  origin- 
ated in  several  ways: 

1 F.  Cirkel,  Graphite,  its  properties,  occurrence,  refining,  and  uses, 
Dept.  Mines,  Ontario,  Canada,  1907,  p.  307. 

J.  H.  Pratt,  G.  O.  Smith,  E.  S.  Bastin  and  H.  G.  Ferguson  in  Mineral 
Resources,  U.  S.  Geol.  Survey,  Annual  issues,  American  Bibliog.  in  1913, 
vol.  2,  p.  245. 

B.  L.  Miller,  Mineral  Industry,  Annual  issues. 

E.  Donath,  Der  Graphit,  Leipzig  and  Vienna,  1904. 

E.  Weinschenk,  Der  Graphit,  etc.,  Leipzig,  1904. 

E.  Weinschenk,  Weitere  Beobachtungen,  etc.,  Zeilschr.  prakt.  Geol. 
1903,  pp.  16-24. 


744  MINERAL  DEPOSITS 

1.  It  may  form  an  integral  part  of  rock  magmas  and  crystal- 
lize together  with  the  rock.     This  possibility  is  indicated  by  its 
presence  in  meteorites,  in  the  terrestrial  iron  of  Ovifak,  Green- 
land, in  nepheline  syenite,1  and  in  pegmatites.2     In  some  of  the 
occurrences  in  pegmatite  dikes  the  graphite  has,  however,  clearly 
been  absorbed  from  the  surrounding  crystalline  limestone.     This 
is  the  origin  of  one  of  the  occurrences  described  by  George  Otis 
Smith,  as  well  as  of  the  graphite  in  a  dike  near  Franklin  Furnace, 
New  Jersey,    described   by  A.    C.    Spencer.3    These   deposits 
are  rarely  of  economic  importance. 

2.  Graphite  forms  by  the  recrystallization  of  carbonaceous 
matter  in  metamorphic  sedimentary  rocks  resulting  from  sand- 
stone, shale,  limestone,  or  coal.     This  transformation  is  well 
established  and  can  evidently  be  effected  under  conditions  of 
intense  regional  or  igneous  metamorphism,  but  it  probably  re- 
quires a  relatively  high  degree  of  heat,   perhaps  well  above 
200°  C.     The  development  of  graphite  in  the  zone  of  contact 
metamorphism  is  assumed  by  some  authors,  like  E.  Weinschenk, 
to  mean  that  the  carbon  has  been  supplied  by  emanations  from 
the  magma.     Weinschenk  also  applies  this  theory  to  its  occur- 
rence in  many  areas  of  regional  metamorphism,  but  this  view  is 
probably  not  justified. 

In  studying  the  contact-metamorphic  graphite  from  Ticon- 
deroga,  New  York,  E.  S.  Bastin  showed  by  experiments  that  the 
contemporaneous  quartz  crystals  had  not  been  exposed  to  a 
temperature  of  575°  C.  While  a  very  high  temperature  is  neces- 
sary for  the  manufacture  of  artificial  graphite,  the  transformation 
can  evidently  be  effected  in  nature  at  a  much  lower  degree  of 
heat. 

3.  Lastly,  graphite  occurs  in  veins,  sometimes  2  or  3  feet 
wide,  having  the  appearance  of  resulting  from  the  filling  of  open 
fissures,  and  in  this  form  the  mineral  usually  possesses  a  marked 
transverse  fibrous  structure.     Such  veins  are  found  in  igneous 
rocks  like  pegmatites  and  granites,  and  also  in  the  surrounding 
metamorphosed    sediments.     Fine    examples    are   seen    in    the 
graphite  regions  of  New  York,  Canada,  and  Ceylon. 

The  origin  of  this  type  is  less  easy  to  explain.  As  the  veins 
are  usually  found  near  intrusive  contacts  where  high  heat  pre- 

1  T.  H.  Holland,  Mem.,  Geol.  Survey  India,  vol.  30,  1901,  p.  201. 

2  G.  O.  Smith,  Bull.  285,  U.  S.  Geol.  Survey,  1906,  pp.  480-483. 

3  Geologic  Folio  161,  U.  S.  Geol.  Survey,  1908. 


IGNEOUS  METAMORPHISM  745 

vailed,  it  may  be  conjectured  that  they  were  formed  by  deposi- 
tion from  gaseous  carbon  compounds,  such  as  carbon  monoxide 
or  cyanogen  compounds,  perhaps  with  metals;  in  some  of  these 
graphites  the  ash  contains  much  iron.  The  prevailing  opinion 
is  that  the  carbon  is  derived  from  surrounding  sediments  and 
was  deposited  shortly  after  the  intrusion,  but  E.  Weinschenk1 
and  others  consider  it  as  originating  from  exhalations  of  igneous 
origin.  The  Ceylon  veins,  described  by  the  same  author,  contain, 
in  addition  to  graphite,  quartz,  rutile,  orthoclase,  apatite,  pyrox- 
ene, and  pyrite.  Calcite  is  contemporaneous  and  intergrown 
with  the  graphite.  Finally  there  are,  both  here  and  at  other 
places  described  by  Weinschenk,  kaolin  and  the  corresponding 
iron  compound,  nontronite,  and  these  occurrences  are  held  to 
support  the  theory  of  igneous  derivation.  This  view  is  assuredly 
not  justified,  as  the  possibility  that  such  highly  hydrated 
compounds  can  be  formed  by  igneous  exhalations  is  decidedly 
remote  (p.  328).  Types  2  and  3  form  many  valuable  graphite 
deposits. 

Occurrences.- — Deposits  of  graphite  have  been  found  in  a 
number  of  States  in  the  Union,  but  few  are  of  economic  impor- 
tance; many  of  them  are  graphite  slates  or  clays  which  are  util- 
ized as  pigments  or  as  lubricants. 

A  part  of  the  domestic  supply  of  "crystalline"  graphite  is  ob- 
tained from  New  York;  the  mines  are  located  in  Essex,  Warren, 
Washington,  and  Saratoga  counties,  in  the  Adirondack  region,2 
and  the  largest  mine,  that  of  the  American  Graphite  Company, 
has  been  worked  for  30  years.  The  rocks  are  pre-Cambrian 
crystalline  schists  of  sedimentary  origin.  The  principal  bed 
worked  is  a  dark  silver-gray  quartz-graphite  schist  and  is  said 
to  average  about  6  per  cent,  graphitic  carbon.  Elongated 
quartz  grains,  muscovite,  apatite,  pyrite,  and  graphite,  the  latter 
in  thin  and  ragged  flakes,  averaging  about  1  millimeter  in  length, 
are  the  constituents.  Two  beds  are  known,  one  about  8  feet 
thick,  the  other  from  3  to  20  feet.  Excavations  have  extended 
for  2,000  feet  along  the  gentle  dip  of  the  thicker  bed,  the  greatest 

1  Abhandl.  Bayer.  Akad.  d.  Wissensch.,  vol.  21,  1901,  pp.  218-335. 

*  E.  S.  Bastin,  Econ.  Geol,  vol.  5,  1910,  pp.  134-157. 
D.  H.  Newland,  New  York  State  Museum,  Butts.,  1905  to  1916.     Annual 
reports  of  the  graphite  industry. 

Ida  H.  Ogilvie,  Bull.  96,  idem.     (Geological  map.) 
H.  L.  Ailing,  The  Adirondack  Graphite  Deposits,  Bull.  199,  New  York 
State  Museum,  1918. 


746  MINERAL  DEPOSITS 

depth  below  the  surface  being  250  feet.  The  associated  rocks 
are  garnetiferous  gneisses  and  limestones  of  the  Grenville  series. 
The  sediments  are  metamorphosed  by  intrusion  and  injection 
of  granite  and  gabbro  of  Laurentian  and  possibly  Algoman  age. 

Three  miles  northwest  of  Ticonderoga,  in  the  same  region, 
coarse  graphite  plates  are  irregularly  distributed  throughout  the 
contact  zone  between  pegmatite  and  pegmatitic  granite  and  the 
schists  and  limestones  which  these  rocks  intrude.  Contact- 
metamorphic  minerals,  like  scapolite,  pyroxene,  and  vesuvian- 
ite,  occur  in  this  zone.  The  graphite  also  forms  veins,  1  to  2 
inches  wide,  which  cut  across  both  the  schist  and  granite.  The 
deposits  at  this  locality  have  been  worked  for  a  number  of 
years. 

In  the  last  years  the  production  of  flake  graphite  from  a  belt 
of  Paleozoic  mica  schist  in  Clay  county,  Alabama,  and  adjacent 
region  has  acquired  considerable  importance.  The  ore  contains 
about  3  per  cent,  graphite. 

A  deposit  containing  graphite  in  veins  similar  to  those  of 
Ceylon  has  recently  been  found  near  Dillon,  Montana.1  The 
veins  occur  along  a  contact  zone  of  granites  and  pegmatites, 
intrusive  in  pre-Cambrian  schists  and  calcareous  rocks  which 
have  been  contact-metamorphosed. 

At  several  places  in  New  Mexico2  intrusions  of  basic  igneous 
rocks  have  altered  the  coal-beds  of  the  Tertiary  or  Cretaceous 
formations.  At  Madrid  the  coal  was  converted  to  anthracite. 
Near  Raton  the  intrusions  have  turned  the  coal  into  a  coke-like 
material,  but  at  one  place  7  miles  southwest  of  Raton  a  number 
of  sills  produced  exceptionally  intense  metamorphism,  convert- 
ing the  coal  to  graphite.  Graphite  also  occurs  in  irregular 
masses  in  the  diabase  and  has  a  more  or  less  columnar  texture 
normal  to  the  faces  of  the  igneous  rock. 

Similar  conditions  produced  the  important  deposit  of  amor- 
phous graphite  of  Santa  Maria,  in  central  Sonora,  Mexico.  Ac- 
cording to  F.  L.  Hess3  several  coal-beds,  attaining  a  maximum 
thickness  of  24  feet,  have  been  subjected  to  contact  metamor- 
phism and  folding  by  intruding  granite  and  are  converted  into 
amorphous  graphite.  The  mam  vein  averages  86  per  cent. 

JA.  N.  Winchell,  Econ.  Geol,  vol.  6,  1911,  p.  218.  E.  S.  Bastin,  Econ. 
Geol,  vol.  7,  1912,  p.  435. 

2  W.  T.  Lee,  Mineral  Resources,  U.  S.  Geol.  Survey,  pt.  2,  1908,  p.  733. 

3  Idem,  p.  734. 


IGNEOUS  METAMORPHISM 


747 


graphitic  carbon  and  furnishes  a  good  material  for  the  manu- 
facture of  lead  pencils. 

The  graphite  deposits  of  Ceylon1  are  among  the  most  pro- 
ductive in  the  world,  yielding  about  38,000  short  tons  a  year 
of  high  grade  product.  The  mineral  is  said  to  occur  as  veins, 
varying  in  width  from  12  to  22  centimeters.  The  mines  are  from 
100  to  500  feet  deep.  The  rough  material  often  contains  up  to 
50  per  cent,  impurities  and  is  hand  picked  and  sorted. 

According  to  Bastin  the  veins  are  found  in  a  fine-grained  acidic 
or  basic  gneiss  to  which  he  applies  the  name  granulite.  The 
rock  contains  quartz,  feldspar,  garnet,  pyroxene,  biotite,  etc. 


FIG.  253. — Vertical    section    of    graphite    veins,    Buckingham    Township, 
Quebec.     After  A.    Osann. 

Some  crystalline  limestone  is  also  present.  The  gneisses  are 
intruded  by  granites  and  pegmatites.  In  the  last  few  years 
Madagascar  is  rivaling  Ceylon.  In  1917,  35,000  tons  are  said 
to  have  been  produced,  the  quality  of  flake  graphite  being  about 
the  same  as  that  of  the  domestic  output. 

The  Siberian  deposits,  in  the  Batagol  Mountains  near  Irkutsk, 
yield  material  of  great  purity,  which  formerly  supplied  the  lead- 
pencil  industry.  L.  Jaczewski,2  describing  the  Alibert  mines  in 
this  region,  states  that  the  graphite  occurs  in  a  nepheline  syenite 

1  J.  Walther,  Zeitschr.  Deutsch.  geol.  Gesell.,  vol.  41,  1889,  p.  359. 

E.  Weinschenk,  Op.  cit. 

E.  S.  Bastin,  Econ.  Geol.,  vol.  7,  1912,  pp.  419-443  (with  literature). 
2Neues  Jahrb.,  1901,  2,  ref.  p.  74.     (Original  in  Russian.) 


748 


MINERAL  DEPOSITS 


close  to  the  contact  of  a  schist  that  also  contains  graphite,  the 
latter,  as  well  as  the  inclusions  in  the  igneous  rock,  being  con- 
sidered of  organic  origin.  This  conclusion  is  vigorously  at- 
tacked by  E.  Weinschenk.1 

The  deposits  at  Passau,  in  Bavaria,  comprise  few  veins;  the 
graphite  occurs  in  a  crushed,  schistose  rock  and  Weinschenk 
regards  the  deposits  as  caused  by  volcanic  emanations.  The 
occurrences  hi  Moravia  are  apparently  similar. 

The  graphite  deposits  of  Canada  are  contained  chiefly  in  Buck- 
ingham and  Grenville  townships,  Quebec,  near  Ottawa.  The  pro- 
duction in  1917  was  about  3,700  long  tons.  These  deposits, 


FIG.  254 — Vertical  section  of  graphite  vein  in  limestone,  Grenville  district, 
Quebec.     After  A.  Osann. 

which  have  been  described  by  A.  Osann,2  show  particularly  clear 
relations  to  contact-metamorphism.  The  rocks  are  largely 
gneiss,  quartzite,  and  crystalline  limestone  of  Grenville  age 
cut  by  granite,  pegmatite,  and  diorite.  Graphite  is  widely 
distributed  in  fissure  veins  or  lenticular  masses  in  these  intrusions 
or  near  their  contacts,  also  as  disseminated  flakes  in  limestone 
or  gneiss  (Figs.  253  and  254).  Associated  minerals  are  apatite 
and  scapolite,  often  appearing  in  the  wall  rocks  of  the  veins, 

XE.  Weinschenk,  Op.  cit. 

E.  Weinschenk,  Zur  Kenntniss  der  Graphitlagerstatten.,  Abhandl.  Bayer. 
Akad.  d.  Wiss.,  vol.  19,  1899,  pp.  511-563. 

2  Ann.  Rept.,  Canada  Geol.  Survey,  1899,  pp.  660-820.     See  also  Cirkel's 
report  quoted  above. 


IGNEOUS  METAMORPHISM  749 

also  biotite,  titanite,  wollastonite,  and  pyrite.  The  analogy  of 
these  deposits  with  the  apatite  veins  is  striking  and  the  conclusion 
seems  justified  that  they  were  developed  by  igneous  emanations 
shortly  after  the  close  of  the  intrusive  activity. 

Production  and  Uses. — The  production  of  natural  graphite  in 
the  United  States  has  varied  considerably,  owing  to  the  large 
quantities  of  low-grade  material  used  for  paints  and  fertilizers. 
The  output  of  flake  graphite  from  New  York  State  is  about 
1,500  tons  per  annum.  Alabama,  in  1917,  produced  3,100  tons; 
the  total  domestic  output  of  flake  graphite  in  1917  was  7,000 
tons.  Much  larger  is  the  production  of  artificial  graphite  now 
manufactured  in  electric  furnaces  at  Niagara  Falls  at  the  rate  of 
5,000  tons  per  annum,  from  anthracite  coal  mixed  with  a  small 
percentage  of  ash.  In  addition  about  20,000  to  42,000  tons  of 
graphite  are  imported  from  the  .highly  productive  mines  in 
Ceylon,  Mexico,  Korea  and  Madagascar.  Ceylon  graphite  sold 
in  New  York  (1911)  for  7  to  9  cents  a  pound,  but  during  the  war 
the  price  rose  to  30  cents.  Domestic  No.  1  Flake  brought  13 
to  18  cents  per  pound  in  1917.  It  should  contain  90  per  cent, 
graphitic  carbon. 

There  is  a  great  demand  for  graphite  from  many  branches  of 
industry.  The  inert  and  heat-resisting  nature  of  the  "crystal- 
line" graphite  makes  it  particularly  valuable  for  crucibles,  the 
fibrous  Ceylon  product  being  most  suitable  for  this  purpose. 

Graphite  is  extensively  used  as  a  lubricant,  with  oil,  and  for 
this  purpose  the  artificial  mineral,  which  is  "deflocculated," 
causing  it  to  remain  indefinitely  in  suspension  in  oil,  is  especially 
employed.  Other  uses  are  for  pencils,  foundry  facings,  polish- 
ing powder,  paint,  electrodes,  and,  strange  to  say,  as  an  adul- 
terant for  fertilizers;  it  is  claimed  that  it  prevents  absorption  of 
moisture  and  caking. 

The  low-grade  material  from  New  York  State  is  concentrated 
at  the  mines  by  crushing,  washing  on  buddies  or  other  appliances, 
and  settling,  but  the  details  of  the  process  have  not  been  made 
public.  Present  practice  in  Clay  County,  Alabama,  includes 
dry  crushing,  drying  and  water  flotation.1 

Garnet 

Some  varieties  of  garnet,  especially  almandite,  are  mined  and 
used  as  abrasive  material.     In  the  State  of  New  York  there  are 
1  Irving  Herr,  Eng.  and  Min.  Jour,,  April  11,  1917. 


750  MINERAL  DEPOSITS 

several  deposits  of  this  kind.1  The  garnets  occur  in  highly 
altered  rocks  of  somewhat  uncertain  history  but  are  probably 
the  result  of  contact  metamorphism. 

DEPOSITS  DUE  TO  IGNEOUS  METASOMATISM  NOT  DISTINCTLY 
RELATED  TO  CONTACTS 

General  Features. — The  deposits  thus  far  described  lie  close 
to  the  well-defined  contact  of  an  intrusive  rock  with  a  sedimen- 
tary series.  There  are  deposits,  however,  in  which  the  mineral 
association  points  to  the  same  mode  of  origin,  but  which  are  not 
clearly  related  to  any  given  contact.  This  may  result  from  a 
sloping  or  irregular  contact  of  a  large  batholith,  so  that  a  point 
on  the  surface  that  is  several  miles  from  the  contact  horizontally 
may  be  only  a  few  thousand  feet  from  it  in  a  vertical  direction. 
General  metamorphism,  without  special  development  of  mineral 
deposits,  appears  to  have  been  effected  by  such  conditions  at  the 
northern  end  of  the  great  batholith  of  Idaho  between  the  Clear- 
water  and  St.  Joe  rivers.2  During  a  long  and  deep  immersion 
into  the  abyssal  zone,  metallic  gases  given  off  by  magmas  may 
have  penetrated  farther  from  the  intrusion  than  they  have  near 
the  surface.  It  is  also  possible  that  erosion  may  have  cut  away 
the  metallizing  dike  or  mass,  so  that  its  relation  to  the  deposit 
is  no  longer  apparent. 

At  any  rate  such  ore-bodies  are  termed  deposits  due  to  igneous 
metamorphism,  rather  than  contact-metamorphic  deposits. 

Ores  of  copper,  zinc,  lead,  and  iron  are  included  in  this  class. 
Many  representatives  are  found  among  the  obscure  deposits  in 
the  pre-Cambrian  of  Scandinavia. 

Boundary  District. — At  Phoenix3  and  Greenwood,  in  British 
Columbia  near  the  international  boundary,  are  a  number  of  ore- 
bodies  which  in  the  last  decade  have  yielded  about  125,000  tons 
of  copper.  The  geology  of  the  region  is  complex.  A  thick  series 
of  volcanic  rocks  (porphyrites) ,  both  clastic  and  massive,  crystal- 
line limestones,  and  argillites,  all  of  upper  Paleozoic  age,  is 
intruded  by  a  granitic  batholith  of  probable  Jurassic  age  and 
smaller  masses  of  syenite. 

1  W.  J.  Miller,  Garnet  deposits  of  Warren  County,  N.  Y.,  Econ.  Geol, 
vol.  7,  1912,  pp.  493-501. 

2  F.  C.  Calkins"and  E.  L.  Jones,  Bull.  530,  U.  S.  Geol.  Survey,  1913, 
pp.  75-86. 

3  O.  E.  LeRoy,  Mem.  21,  Canada  Geol.  Survey,  1912. 


IGNEOUS  METAMORPHISM  751 

The  large  ore  deposit  of  the  Granby  Company  lies  in  a  miner- 
alized zone  which  represents  a  part  of  the  limestone  replaced  by 
epidote,  garnet,  etc.  The  ore-bodies  are  lenses  or  large  masses 
one  of  which  is  2,500  feet  long  and  900  feet  wide  and  has  a  maxi- 
mum thickness  of  125  feet:  The  dip  becomes  flat  in  depth  and 
the  ore  ceases  at  a  vertical  depth  of  675  feet.  Fig.  255  represents 
the  somewhat  clearer  condition  at  the  adjoining  Brooklyn  Mine. 
The  ore  consists  of  chalcopyrite,  pyrite,  hematite,  and  magnetite, 
with  andradite,  actinolite,  and  epidote.  Calcite  and  quartz 
fill  the  interstices  between  the  lime-iron  silicates.  The  ore  as 
smelted  contains  from  1.2  to  1.6  per  cent,  copper  with  0.04  ounce 
of  gold  and  0.3  ounce  of  silver  per  ton.  The  original  limestone 
which  appears  in  some  remnants  near  the  ore-body  is  compara- 


FIG.  255. — Generalized  section  of  Brooklyn  ore-body,   Phoenix,  B.  C.    o, 
ore-body;  Is,  limestone;  g,  gangue;  j,  jasperoid.    Scale  400  feet  =1  inch. 

tively  pure  and  contains  from  1  to  10  per  cent,  of  silica  and  little 
or  no  iron.  Magnetite,  epidote,  and  garnet  formed  contempora- 
neously; somewhat  later  but  partly  overlapping  came  the  de- 
velopment of  chalcopyrite,  pyrite,  and  hematite.  The  limestone 
is  in  large  part  converted  to  jasperoid,  the  alteration  appearing 
to  have  taken  place  before  the  development  of  the  ore. 

No  large  bodies  of  igneous  rocks  appear  in  or  near  the  deposits, 
and  the  nearest  small  outcrops  of  granodiorite  are  1  to  2  miles 
away;  one  of  these  outcrops  has  been  locally  replaced  by  garnet, 
epidote,  and  actinolite.  Deep  drilling  below  the  deposits  failed 
to  disclose  intrusive  rocks.  It  is  held  that  the  ores  were  formed 
by  igneous  emanations  of  iron,  silica,  and  copper  which  traversed 
the  limestone  laterally  from  some  unit  of  the  intrusive  series 
that  is  now  eroded. 

Ducktown,  Tennessee. — The  copper  ores  at  Ducktown  have 
been  worked  since  about  1848  and  still  maintain  an  output  of 
8,000  tons  of  copper  a  year.  In  addition,  about  700  tons  of 
sulphuric  acid  is  now  obtained  daily  from  these  ores.  The 
district,  which  lies  in  the  mountainous  area  of  the  southern 


752 


MINERAL  DEPOSITS 


Appalachians,  has  been  the  subject  of  repeated  geologic  investi- 
gation by  C.  Heinrich,  J.  F.  Kemp,  and  W.  H.  Weed.  Lately, 
W.  H.  Emmons  and  F.  B.  Laney1  have  examined  the  deposits. 
According  to  Emmons  and  Laney  the  deposits  are  contained  in 
a  highly  compressed  metamorphosed  and  schistose  series  of 
arkose  sediments  of  Cambrian  age,  consisting  of  poorly  sorted 
conglomerates,  grits,  sandstone,  and  shale.  Garnet  and  stauro- 
lite  have  developed  abundantly  in  the  rocks,  the  staurolite 
following  certain  horizons  persistently.  Thin  lenses  of  limestone 


Schist       Ore  Zone      Gossan     Chalcocite 
Ore 

FIG.  256. — Cross-section  of  Mary  mine,  Ducktown,  Tennessee. 
After  W.  H.  Emmons,  U.  S.  Geol.  Survey. 

were  contained  in  the  series  and  are  exposed  in  some  places  in 
the  mines;  they  are  now  crystalline  and  contain  layers  of  biotite 
and  muscovite.  Here  and  there  are  small  ill-defined  lenses  of  a 
highly  metamorphic  rock  looking  like  a  diorite-pegmatite  and 
consisting  of  quartz,  feldspar,  hornblende,  and  garnet.  These 
peculiar  phases  are  now  believed  to  be  the  result  of  strong  meta- 
morphism  of  the  arkose  sediments.  Dikes  of  gabbro,  later  than 
the  mineralization,  are  intruded  in  the  sediments. 

The  deposits  are  elongated,  roughly  tabular  masses,  some  of 
them  curved,  lens-shaped,  or  folded,  striking  northeast  and  mostly 
dipping  southeast  (Fig.  256).  The  ore  beds  are  parallel  to  the 

1  Preliminary  report  in  Bull.  470,  U.  S.  Geol.  Survey,  1911,  pp.  151-172. 


IGNEOUS  METAMORPHISM  753 

strike  of  the  schists  and  average  60  feet  in  width.  The  primary 
ore  is  a  coarsely  crystalline  mass  of  pyrrhotite,  pyrite,  chalcopyrite, 
zinc  blende,  specularite,  magnetite,  actinolite,  calcite,  tremolite, 
quartz,  pyroxene,  garnet,  zoisite,  chlorite,  mica,  graphite,  titanite, 
and  feldspars,  all  of  practically  contemporaneous  crystallization. 

Much  of  the  ore  is  almost  massive  pyrrhotite  and  pyrite. 
Along  the  strike  and  dip  the  ore  may  grade  into  a  lime  silicate 
rock  of  gangue  minerals  and  these  in  places  grade  into  crystalline 
limestone.  The  contact  between  schist  and  ore  is  sharp  or 
gradational  within  a  few  inches.  The  beds  have  been  worked 
to  a  maximum  depth  of  1,000  feet.  A  thin  but  rich  chalcocite 
zone  due  to  enrichment  by  surface  waters  was  found  at  a  depth 
of  50  feet  (p.  853),  but  below  this  the  ores  contain  1.5  to  3.0 
per  cent,  copper,  a  small  amount  of  silver,  and  a  trace  of  gold. 
The  ores  from  the  Mary  mine  now  average  2.5  per  cent.  It  is 
held  that  the  ores  are  formed  by  the  replacement  of  thin  limestone 
beds;  all  the  abundant  gangue  minerals  are  in  fact  rich  in  lime. 
The  replacement  is  believed  to  have  been  effected  by  igneous 
emanations,  as  a  general  association  of  minerals  is  typical  of 
normal  contact  deposits.  At  the  time  of  ore  formation  the  rocks 
were  at  a  high  temperature  and  deeply  buried,  and  it  is  thought 
probable  that  the  emanations  from  some  intrusion  far  below  the 
surface,  which  had  little  effect  on  the  schist,  caused  mineralization 
in  the  limestone  beds.  The  mineralization  fell  within  the  epoch 
of  dynamometamorphism;  some  deformation  of  the  ore  has  taken 
place  since  its  deposition. 

Franklin  Furnace,  New  Jersey. l- — The  great  zinc-manganese 
deposits  of  northern  New  Jersey  are  of  exceptional  complexity 
and  interest.  Known  since  1650  and  actively  worked  since 
1860,  they  now  yield  annually  about  700,000  short  tons  of  ore 
containing  about  120,000  tons  of  zinc  The  treatment  of  the 
crude  ore  by  magnetic  concentration  yields  franklinite,  "half 
and  half,"  and  willemite;  the  first  is  used  for  the  manufacture 
of  zinc  oxide  for  paints  and  leaves  a  manganiferous  residue 
which  goes  to  the  blast  furnace  to  make  spiegeleisen;  the  second 
is  also  used  for  zinc  white;  and  the  third  after  further  concen- 
tration yields  a  product  of  willemite  from  which  a  high-grade 
spelter  (zinc)  is  made. 

*A.  C.  Spencer,  H.  B.  Ktimmel,  J.  E.  Wolff,  and  Charles  Palache, 
Geologic  Folio  161,  1908. 

See  also  review  by  C.  K.  Leith,  Econ.  Geol,  vol.  4,  1909,  p.  265. 


754 


MINERAL  DEPOSITS 


The  two  ore  deposits  of  Mine  Hill  and  Sterling  Hill,  3  miles 
apart,  are  situated  along  a  belt  of  pre-Cambrian  crystalline 
limestone  adjoined  on  the  west  by  coarse  gneisses  of  igneous 
origin.  Cambrian  limestone  covers  these  rocks  to  the  east  and 
west.  Both  deposits  form  canoe-shaped  beds  in  the  pre-Cam- 
brian limestone.  The  Mine  Hill  ore  bed  (Fig.  257)  is  closely 
adjoined  along  its  west  flank  by  the  gneiss,  the  contact  of  which 
is  parallel  to  the  ore-body.  The  ore  mass  is  thus  a  layer  varying 
from  12  to  100  feet  in  thickness  and,  bent  upon  itself,  forms  a 
long  trough  or  one-half  of  a  canoe  with  sides  of  unequal  height, 
the  keel  pitching  north  at  a  gentle  angle. 


Plan 


FIG.  257. — Plan  of  outcrop  and  levels  and  vertical  section  of  Mine  Hill  ore- 
body,  Franklin  Furnace,  New  Jersey.   After  A .  C.  Spencer,  U.  S.  Geol.  Survey. 

The  mines  are  opened  by  a  vertical  shaft  965  feet  deep  and  an 
incline  1,500  feet  long.  The  ore  forms  transitions  into  the 
limestone  and  at  Sterling  Hill  the  limestone  between  the  flanks 
also  contains  lean  ore.  Pegmatite  dikes  cut  ore,  limestone,  and 
gneiss.  The  ore  is  a  coarse  aggregate  of  franklinite,  50  per  cent.; 
willemite,  20  to  30  per  cent. ;  zincite,  2  to  6  per  cent. ;  and  calcite, 
3  to  11  per  cent.  Franklinite,  (Fe,Mn,Zn)O.(Fe,Mn)2O3,  con- 
tains about  42  per  cent,  iron,  15  per  cent,  manganese  and  12  per 
cent,  zinc;  willemite,  Zn2SiOi,  58  per  cent,  zinc;  zincite,  ZnO,  77 
per  cent.  zinc.  The  four  minerals  mentioned  are  held  to  consti- 
tute the  original  ore.  Besides,  there  are  a  great  number  of  rarer 
minerals  such  as  tephroite  (Mn2SiC)4),  zinc  pyroxene  (schefferite), 
zinc  amphibole,  zinc  spinel  (gahnite),  manganese  garnet  (poly- 


IGNEOUS  METAMORPHISM  755 

adelphite),  axinite  (borosilicate  of  Al,  Ca,  Fe,  Mn),  apatite  and 
scapolite  (containing  chlorine),  rhodochrosite,  fluorite,  zinc 
blende,  galena,  arsenopyrite,  chalcopyrite,  and  lollingite.  Most 
of  these  minerals  are  regarded  as  products  of  secondary  meta- 
morphism  due  to  the  pegmatite  dikes.  Many  veins  cut  the 
deposits,  some  of  them  containing  the  normal  recrystallized  ore 
minerals,  others  distinctly  later  with  sulphides  associated  with 
calcite,  albite,  bornite,  quartz,  dolomite,  etc. 

In  the  older  literature  the  deposits  were  considered  of  sedi- 
mentary origin.  The  question  of  genesis  was  reopened  in  1889 
by  F.  L.  Nason,  who  admitted  the  possibility  of  igneous  origin. 
Spencer  believes  that  the  original  deposit  was  formed  by  the 
injection  of  magmatic  emanations  from  the  gneiss  intrusions  into 
the  limestone.  Participation  in  the  general  deep  metamorphism 
which  affected  this  region  in  pre-Cambrian  time  has  further 
complicated  the  relations.  It  is  certain  that  the  texture  of  the 
ore  and  the  universal  rounding  or  corroding  of  the  ore  minerals 
point  distinctly  to  igneous  metasomatic  action.  The  abundance 
of  the  spinel  minerals  is  indicative  of  high  temperature. 

Metasomatic  Magnetite  Deposits  of  Sweden.1 — Many  of  the 
earliest  known  and  longest  worked  of  the  Swedish  iron  deposits 

1  Hj.  Sjogren,  The  genesis  of  our  iron  ores  (Swedish),  Geol.  For.  Forhandl, 
vol.  28,  1906,  pp.  314-344.  With  discussion  by  Tornebohm,  Hogbom, 
Holmquist,  Backstrom,  etc. 

Hj.  Sjogren,  The  geological  relations  of  the  Scandinavian  iron  ores, 
Trans.,  Am.  Inst.  Min.  Eng.,  vol.  38,  1908,  pp.  766-835. 

Hj.  Sjogren,  The  question  of  the  origin  of  the  iron  ores  in  the  older  pre- 
Cambrian  series  of  Sweden,  Geol  For.  Forhandl.,  vol.  30,  1908,  pp.  115-155. 

H.  Johansson,  The  question  of  the  origin  of  the  middle-Swedish  iron  ores 
(Swedish),  Geol.  For.  Forhandl.,  vol.  28,  1906,  pp.  516-538;  vol.  29,  1907, 
pp.  143-186,  232-255;  vol.  30,  1908,  pp.  232-235. 

Review,  Econ.  Geol.,  vol.  5,  1910,  pp.  494-498. 

L.  de  Launay,  L'origine  et  les  caracteres  des  gisements  de  fer  scandinaves. 
Ann.  des  Mines  (10),  vol.  4,  1903,  pp.  49-211. 

See  also  a  summary  of  recent  literature  by  A.  Bergeat  in  Fortschritte  der 
Mineralogie,  etc.,  vol.  2:  Jena,  1911,  pp.  43^4. 

Excellent  descriptions  of  individual  districts  are  found  in  the  guide  to 
the  excursions  of  the  Internat.  Geol.  Congress,  Stockholm,  1910. 

P.  Geijer,  Some  problems  in  iron  ore  geology  in  Sweden  and  America, 
Econ.  Geol,  vol.  10,  1915,  pp.  209-239. 

P.  J.  Holmquist,  Swedish  archaean  structures  and  their  meaning,  Bull, 
Geol.  Inst.  Upsala,  vol.  15,  1916,  pp.  125-148. 

P.  J.  Holmquist,  Structure  and  metamorphism  of  Swedish  iron  ores. 
(Swedish)  Geol  For.  Forhandl,  April,  1913,  pp.  233-272. 


756 


MINERAL  DEPOSITS 


form  irregular  masses  or  lenses  in  rocks  of  upper  Archean  age. 
They  are  either  directly  associated  with  crystalline  limestone, 
or  they  occur  near  limestone  but  surrounded  by  masses  of  silicates 
like  pyroxene,  garnet,  and  epidote,  to  which  the  term  "skarn" 
is  usually  applied.  Though  not  as  large  as  some  of  the  more 
recently  discovered  deposits  of  certain  or  probable  magmatic 


Limestone  |*»^»%|  Iron -ore  |>.'->.'v  VJ  Ciun«ue 

Uiorite  I  I  Or 

FIG.  258.— Plan  of  the  Persberg  mines,  Sweden.     After  Hj.  Sjogren. 

origin,  the  deposits  have  in  the  aggregate  furnished  much  ore  of 
exceptional  purity  and  as  yet  are  far  from  being  exhausted. 
Until  about  ten  years  ago  these  deposits  were  considered  by  the 
Swedish  geologists  as  of  sedimentary  origin,  like  bog  iron  ores, 
but  subsequently  metamorphosed.  In  modified  form  this 
opinion  was  expressed  by  de  Launay  in  1903.  At  present  few 
observers  hold  to  this  view.  There  is,  for  instance,  a  strong 


IGNEOUS  METAMORPHISM 


757 


similarity  between  the  Swedish  ores  and  those  of  the  Banat 
province  of  Hungary,  first  described  by  von  Cotta,  and  the  latter 
are  generally  accepted  as  of  contact-metamorphic  origin.  Strik- 
ing and  unmistakable  also  is  their  similarity  to  the  metasomatic 
contact  deposits  of  North  America,  many  of  which  contain 
much  magnetite  and  which  at  some  places  are  worked  for  iron. 
The  Swedish  deposits  are,  however,  not  so  simply  explained, 
for  while  in  the  districts  mentioned  the  ores  unquestionably 
adjoin  igneous  intrusions,  the  granitic  rocks  of  Sweden  are  gener- 


FIG.  259. — Vertical  sections  of  the  Kran  mine,  Persberg,  Sweden.     Shaded 
areas  indicate  stopes.     After  Hj.  Sjogren. 

ally  later  than  the  deposits,  which  normally  are  contained  in  a 
peculiar  fine-grained  rock  with  gneissoid  texture  that  is  widely 
distributed  in  the  iron  region  and  that  has  been  variously  desig- 
nated "halleflinta,"  eurite,  leptite,  or  granulite.  These  rocks, 
which  form  wide  zones  in  the  pre-Cambrian  of  Sweden  and  are 
locally  associated  or  interbedded  with  amphibolites  and  smaller 
masses  of  more  distinctly  sedimentary  quartz-mica  slates  and  also 
with  lenses  of  crystalline  limestone  or  dolomite,  are  salic  rocks, 
generally  with  at  least  67  per  cent,  silica,  and  consist  largely  of 
albite,  orthoclase,  and  quartz.  Johansson  has  shown  that  they 
are  in  part  potassic,  in  part  sodic,  and  that  intermediate  composi- 


758  MINERAL  DEPOSITS 

tion  is  rare.  He  therefore  interprets  them  as  highly  differentiated 
intrusives.  The  striped  structure  is  interpreted  by  him  as  the 
result  of  a  mechanical  churning  of  the  magma  during  differentia- 
tion. The  most  prevalent  opinion  is  that  these  rocks  are  in  part 
effusive,  perhaps  originally  tuffaceous,  and  in  part  intrusive, 
and  that  the  limestone  and  mica  schist  are  of  sedimentary  origin. 

The  bodies  of  magnetite  are  in  general  associated  with  masses 
of  crystalline  limestone  in  this  leptite  formation,  and  it  appears 
as  if  the  mineralization  were  caused  by  the  action  of  the  granulite 
on  the  limestone.  The  ores  form  stock-like  masses  with  greatest 
extension  in  a  vertical  direction  and  border  directly  against 
granulite,  limestone,  or  "skarn."  The  bodies  have  been  followed 
to  a  depth  of  about  1,000  feet;  some  of  them  cease  distinctly 
at  various  depths  but  other  stocks  still  continue  below  the  great- 
est depth  reached.  Many  of  them,  but  not  all,  conform  with 
the  banding  of  the  leptite  (Figs.  258  and  259). 

The  "limestone  ores"  are  more  directly  embedded  in  limestone, 
but  here  too  skarn  minerals  may  be  present.  In  such  an  ore- 
body  at  Klackberg  a  narrow  zone  of  dark-brown  garnet  was  noted 
along  the  contact  of  limestone  and  ore  and  in  the  limestone 
itself  was  disseminated  a  dark-brown  amphibole.  The  limestone 
ores  often  carry  manganese  and  some  of  them  constitute  manga- 
nese deposits  like  that  of  Langbanshyttan,  at  which  an  unusually 
great  number  of  rare  minerals  are  found.  Stretching  and 
schistosity  were  superimposed  upon  the  deposits  in  places  and 
sometimes  the  direction  of  the  stretching  indicates  the  pitch  of 
the  ore-body.  The  magnetite  is  fine-grained ;  it  contains  in  places 
a  little  specularite.  Some  deposits  contain  small  quantities  of 
pyrite,  pyrrhotite,  chalcopyrite,  and  arsenopyrite. 

The  composition  of. one  of  the  Persberg  ores  is  as  follows: 

Fe3O4 71.56  CaO 4.85 

FeO 5.11  A12O3 0.77 

Fe 55.79  SiO 12.76 

MnO 0.17  P2OS 0.005 

MgO 4.18  S 0.031 

Secondary  changes  have  resulted  in  crushing  along  certain  zones 
(skolar)  and  a  great  development  of  chlorite  and  other  minerals 
of  dynamometamorphic  affiliations.  Among  the  celebrated  de- 
posits of  this  type  should  be  mentioned  those  of  Persberg, 
Taberg  (in  Wermland),  Nordmark,  Norberg,  and  Dannemora. 


IGNEOUS  METAMORPHISM  759 

The  field  relations  indicate  beyond  doubt  that  the  ores  and 
skarn  are  metasomatic  replacements  of  limestone  or  dolomite 
similar  to  contact-metamorphic  deposits,  probably  effected  by 
very  hot  solutions  containing  iron,  manganese,  silica,  etc.,  de- 
rived from  intrusive  magmas. 

Holmquist  holds  that  the  bedded,  supracrustal  leptites  with 
accompanying  sedimentary  iron  ores  (p.  825)  subsided  into  under- 
lying granitic  magma,  which  effected  igneous  metamorphism  of 
the  bedded  ores  and  developed  magnetite  and  lime  silicates  in 
the  limestone.  The  later  events  included  a  regional  meta- 
morphism which  affected  the  ores  to  some  degree,  and  the  final 
intrusions  of  granite,  pegmatite  and  diabase  which  have  exerted 
very  slight  influence  on  the  deposits. 

Magnetite  Deposits  in  the  United  States. — Deposits  of  magne- 
tite which  are  similar  to  the  Swedish  ores  just  described  are 
found  in  the  United  States  at  few  places.  The  Tilly  Foster 
mine,1  in  New  York  State,  contained  a  steep  lenticular  body  of 
ore  embedded  in  gneiss;  the  magnetite  was  associated  with  cal- 
cite,  dolomite,  chondrodite,  enstatite,  epidote,  chlorite,  garnet, 
and  scant  sulphides.  The  ore-body  was  followed  to  a  depth  of 
about  600  feet. 

Another  locality  is  in  the  Cranberry  district,  in  North  Carolina, 
described  by  A.  Keith,2  where  low-grade  ores  have  been  mined 
and  concentrated.  The  ore  here  occurs  as  a  series  of  lenses  of 
magnetite  in  a  gangue  of  hornblende,  pyroxene,  and  epidote; 
the  lenses  dip  southwest  at  angles  of  45°,  about  parallel  to  the 
schistosity  of  the  surrounding  gneiss.  The  ore  is  pure,  with  little 
phosphorus.  It  is  not  certain  whether  it  represents  replaced 
limestone. 

1  F.  R.  Koeberlin,  The  Brewster  iron-bearing  district  of  New  York,  Econ. 
Geol,  vol.  4,  1909,  pp.  713-754. 

'  Butt.  213,  U.  S.  Geol.  Survey,  1903,  pp.  243-246;  also  in  Folio  90, 
U.  S.  Geol.  Survey. 


CHAPTER  XXVIII 
MINERAL  DEPOSITS  OF  THE  PEGMATITE  DIKES 

INTRODUCTION 

Each  large  intrusive  mass  is  usually  accompanied  by  a  series 
of  later  dikes.  These  "complementary"  dikes  have,  as  a  rule, 
a  composition  similar  to  that  of  the  prevailing  rock,  but  differ 
from  it  in  showing  an  enrichment  of  certain  constituents  and  a 
reduction  of  others.  They  are  generally  regarded  as  products 
of  magmatic  differentiation,  forming  residual  parts  of  the  domi- 
nant magma  after  its  consolidation  has  begun.  Some  of  them 
are  basic,  like  kersantite,  minette,  or  camptonite;  others  are 
acidic,  like  granite  porphyry,  aplite,  or  pegmatite. 

Under  the  name  of  pegmatite  are  grouped  the  coarse  granular 
dike  rocks,  often  with  well-developed  idiomorphic  texture,  which 
accompany  intrusive  rocks,  each  group  being  characterized  by 
pegmatites  of  special  types. 

Gabbros  are  sometimes  accompanied  by  basic  pegmatites  of 
feldspar  and  pyroxene,  and  diorite  by  similar  dikes  of  a  basic 
feldspar  and  hornblende.  The  anorthosites  are  followed  by 
pegmatitic  dikes  containing  labradorite,  hypersthene,  and 
ilmenite;  the  nepheline  syenites  by  pegmatites  of  soda  feldspars, 
nephelite,  sodalite,  lepidomelane  mica,  aegirine,  arfvedsonite, 
and  minerals  containing  zirconium  and  titanium. 

Most  abundant  are  the  granitic  pegmatites,  which  consist 
mainly  of  coarsely  crystallized  orthoclase  and  quartz  with 
muscovite;  they  often  contain  tourmaline,  cassiterite,  monazite, 
orthite,  topaz,  and  a  host  of  other  rare  minerals. 

MINERALIZERS  AND  THE  NATURE  OF  THEIR  ACTION1 

The  processes  of  intrusion  and  crystallization  bring  about  an 
increasing  concentration  of  the  volatile  constituents  of  the  rock 
magma,  if  no  other  avenue  of  escape  is  open  to  such  substances. 

1  In  part  after  A.  Harker,  Natural  history  of  igneous  rocks,  1909,  pp.  282- 
302. 

W.  O.  Crosby  and  M.  L.  Fuller,  Origin  of  pegmatite,  Am.  Geologist, 
vol.  19,  1897,  pp.  147-180. 

760; 


THE  PEGMATITE  DIKES  761 

In  subaerial  eruptions  they  are  given  off  into  the  atmosphere. 
These  volatile  substances,  which  of  course  formed  an  integral  part 
of  the  original  magma,  consist  of  water  and  compounds  of  boron, 
fluorine,  chlorine,  phosphorus,  sulphur,  carbon,  arsenic,  tellurium, 
selenium  and  also  other  rarer  elements.  They  exert  a  peculiarly 
favorable  action  upon  the  crystallization  of  magmas  and  miner- 
als by  decreasing  their  viscosity,  lowering  their  freezing  point, 
and  furthering  the  development  of  minerals  which  otherwise  do 
not  crystallize  from  dry  magmas.  Harker  says: 

The  action  is  doubtless  partly  physical,  partly  chemical.  The  nature 
of  the  chemical  effect,  where  the  agent  does  not  enter  the  crystallized 
product,  is  sometimes  designated  as  a  catalytic  action,  signifying  a  peculiar 
property  possessed  by  certain  bodies  of  inducing  chemical  changes  in  other 
bodies  without  themselves  entering  into  the  composition  of  the  final  product. 
In  other  instances  the  "mineralizer"  forms  part  of  the  rrystalli.ied  material. 

French  investigators  from  the  days  of  filie  de  Beaumont1  have 
justly  laid  stress  on  the  part  played  by  mineralizers  in  mag- 
matic  differentiation  and  in  the  formation  of  mineral  deposits. 
In  the  acidic  rocks,  which  are  known  to  contain  fluorine  and 
boron,  the  action  of  mineralizers  is  particularly  clear,  but  they 
are  doubtless  present  also  in  basic  rocks,  in  which  chlorine, 
phosphorus,  and  sulphur  take  the  place  of  fluorine  and  boron. 
Some  water  is  probably  always  present,  although  the  tendency  of 
some  investigators  is  to  minimize  its  importance.2 

The  presence  of  inclusions  containing  water  in  quartz  crystals 
of  acidic  intrusive  rocks  shows  plainly  enough  that  the  magma 
contained  some  water,  as  do  also  the  transitions  from  pegmatite 
dikes  to  deep-seated  ore-bearing  veins. 

The  residual  magma  contains,  besides  these  volatile  mineral- 
izers, the  principal  elements  of  the  igneous  rock  crystallizing  as 
quartz,  feldspar,  ferromagnesian  minerals,  and  muscovite,  and  a 
number  of  rarer  elements,  such  as  tin,  tungsten,  zircon,  tanta- 
lum, columbium,  cerium,  beryllium,  molybdenum,  lead,  copper, 
lithium,  and  caBsium.  These  rare  elements  appear  to  have  been 
carried  along,  in  the  process  of  differentiation  by  the  mineral- 
izers, which  in  many  cases  have  also  carried  large  quantities  of 
iron  differentiated  from  the  main  igneous  body. 

1  Note  sur  les  emanations  volcaniques  et  metalliferes,  Bull.  Soc.  geol. 
France  (2),  vol.  4,  1847,  pp.  1249-1333. 

2  A.  Brun,  Recherches  sur  1'exhalaison  volcanique,  Geneva,  1911. 


762  MINERAL  DEPOSITS 

The  mineralizing  agents  do  not  confine  their  action  to  the 
later  stages  of  differentiation,  but  doubtless  play  a  part  in  the 
crystallization  of  the  main  body  of  every  magma.  This  is  shown 
by  the  occurrence  of  molybdenite,  pyrite,  bismuthinite,  zinc  blende 
titanite,  and  zeolites  in  the  druses  of  granitic  rocks;  among  such 
are  the  occurrences  of  Striegau,  described  by  A.  Schwantke,1  and 
those  in  the  syenitic  rocks  in  the  vicinity  of  Kristiania,  mentioned 
by  Goldschmidt.2  The  granites  of  the  Island  of  Elba  contain  in 
druses  such  minerals  as  albite,  tourmaline,  beryl,  garnet,  pyrite, 
arsenopyrite,  cassiterite,  and  zeolites.3 

Many  of  the  silicate  minerals,  formed  by  the  aid  of  mineralizers 
as  the  last  stage  of  intrusive  action,  are  of  remarkably  complex 
chemical  nature.  To  many  minerals  of  this  class  no  formula  can 
confidently  be  assigned.  Other  minerals,  especially  the  sul- 
phides, are  characteristically  of  simple  formula  and  composition. 

A  distinct  paragenesis  or  succession  of  minerals  is  noted  in 
many  pegmatites.  With  successively  lower  temperatures  new 
sets  of  minerals  were  formed  and  many  of  those  stable  at  a  higher 
degree  of  heat  became  subject  to  alteration  as  the  temperature 
became  lower.  Thus  in  the  Norwegian  pegmatite  dikes  Brogger 
distinguishes  three  epochs  of  crystallization  ending  with  the 
zeolites. 

TEMPERATURE  OF  CONSOLIDATION 

The  residual  magma,  then,  contains  an  increased  quantity  of 
mineralizers  and  their  accompanying  metals  and  has  also  a  lower 
temperature  than  the  original  magma,  in  some  cases  doubtless 
lower  than  500°  C.  It  is  injected  into  the  earlier  consolidated 
magmas  and  also  into  the  encasing  rocks;  its  fluidity  and  low 
melting-point  are  factors  of  great  importance,  allowing  it  to  com- 
pletely soak  and  penetrate  schistose  and  fissile  rocks  encountered 
in  its  way.  The  pegmatites  are  essentially  residual  magmas, 
but  they  may  become  so  admixed  with  water  and  dissolved 
gases  that  we  may  speak  of  them  as  in.  aqueo-igneous  fusion  at 
a  temperature  of  300°  or  400°  C.  Even  at  this  point  with  pres- 
sures over  200  atmospheres  the  critical  point  of  water  is  exceeded. 

1  Drusenmineralien  des  Striegauer  Granits,  Leipzig,  1890. 

2  V.  M.  Goldschmidt,  Die  Contactmetamorphose  im  Kristiania  Gebiet, 
Kristiania,  1911. 

3  G.  Vom  Rath,  Die  Insel  Elba,  Zeilschr.  Deutsch.  geol.  Gesell.,  vol.  22, 
1870,  p.  466. 


THE  PEGMATITE  DIKES  763 


OCCURRENCE  AND  GENERAL  CHARACTER 

The  pegmatites  form  dikes,  sheets,  pipes,  and  irregular  masses; 
where  appearing  as  dikes  or  sheets  no  great  regularity  or  extended 
continuation  in  depth  can  be  counted  upon,  and  this  is  important 
to  consider  in  the  exploitation  of  such  bodies.  Probably  this 
irregularity  is  explained  by  the  sudden  and  explosive  action  by 
which  they  make  room  for  themselves  and  hold  the  cavities 
open  until  their  substance  is  crystallized.  Very  different  is  this 
action  from  the  slowly  applied  compressive  stresses  by  which  the 
fissures  of  most  veins  are  opened. 

The  pegmatites  are  essentially  coarsely  crystalline  rocks. 
Under  some  circumstances  the  dimensions  of  the  crystals  may  be 
enormous.  In  the  Ural  Mountains  a  quarry  was  opened  in  a 
single  orthoclase  crystal;  in  India  muscovite  plates  10  feet  in 
diameter  have  been  found;  at  the  Etta  mine,  in  the  Black  Hills 
of  South  Dakota,  spodumene  occurs  in  crystals  resembling  tree 
trunks  and  as  much  as  42  feet  in  length;  quartz  crystals  several 
feet  in  length  are  not  uncommon.  Often  the  minerals  crystallize 
together,  as  feldspar  and  quartz  in  graphic  granite,  but  in  other 
pegmatites  there  is  a  distinct  succession,  with  muscovite,  for 
instance,  at  the  walls  and  quartz  and  feldspar  in  the  center,  or 
with  feldspar  crystals  along  the  walls  and  a  central  filling  of 
quartz.  The  rarer  minerals  usually  form  the  later  generations 
and  probably  crystallized  below  575°  C.,  the  crystallographic 
inversion  point  for  quartz.  The  pegmatites  are  evidently  not 
eutectics.  They  crystallized  under  the  same  general  pressure 
and  temperature  as  the  magma  itself.  The  rarer  minerals  are 
accessory,  as  a  rule,  for  there  are  enormous  masses  of  pegmatites 
which  contain  little  but  quartz  and  feldspar.1 

In  their  present  condition  there  is  little  evidence  of  water  as  a 
constituent  of  their  magma,  but  facts  already  referred  to  force 
us  to  the  belief  that  some  water  was  present  as  well  as  some 
carbon  dioxide. 

Liquid  inclusions  in  pegmatitic  quartz  from  Branchville,  Con- 
necticut,2 were  found  to  consist  of  98.33  per  cent.  CO2,  1.67  per 

1  E.  S.  Bastin,  Origin  of  the  pegmatites  of  Maine,  Jour.  Geology,  vol.  18, 
1910,  pp.  297-320.     Bull.  445,  U.  S.  Geol.  Survey,  1911. 

W.  T.  Schaller,  Southern  California  pegmatites,  Science,  April  1,  1910. 

2  G.  W.  Hawes  and  A.  W.  Wright,  Am.  Jour.  Sci.,  3d  ser.,  vol.  21,  1881, 
pp.  203  and  209. 


764  MINERAL  DEPOSITS 

cent,  nitrogen,  and  traces  of  hydrogen  sulphide,  ammonia,  fluo- 
rine, and  possibly  chlorine. 

A  marked  contact-metamorphic  action,  sometimes  stronger 
than  that  of  the  original  magma,  characterizes  many  pegmatites. 
Here,  too,  as  in  the  case  of  normal  igneous  rocks,  it  is  well  to 
distinguish  between  the  ordinary  contact  metamorphism  without 
additions  of  material,  and  metasomatic  contact  metamorphism, 
in  which  substances  contained  in  the  pegmatite  penetrate  into 
the  surrounding  rock  and  replace  some  of  its  minerals.  H.  B. 
Patton1  describes,  for  instance,  a  pegmatite  dike  in  Colorado 
which  is  10  feet  wide  and  which  contains  but  little  tourmaline, 
but  which  strongly  impregnates  the  surrounding  rock  with  this 
mineral  for  a  distance  of  2  or  3  feet  from  the  contact.  It  is, 
however,  worthy  of  note  that  no  sulphide  impregnations  analogous 
to  the  normal  contact-metamorphic  deposits  have  been  found  at 
the  contacts  of  pegmatite  and  limestone.  The  quartz  monzonites 
of  the  Western  States,  along  whose  contacts  most  of  the  deposits 
mentioned  occur,  are  rarely  accompanied  by  pegmatitic  dikes. 

The  pegmatites  often  absorb  material  from  their  walls,  and 
near  them  minerals  otherwise  foreign  are  likely  to  appear;  anda- 
lusite,  garnet,  and  staurolite  are  among  these  minerals. 

TYPES  OF  PEGMATITES 

Acidic  Pegmatites. — The  most  common  type  consists  of  the 
granitic  pegmatites,  which  always  contain  orthoclase,  albite,  and 
quartz,  usually  also  microcline  and  rnuscovite.  Among  the 
accessory  minerals  magnetite,  often  in  crystals,  is  perhaps  most 
common.  Other  rarer  minerals  are  tourmaline,  topaz,  fluorite, 
cassiterite,  apatite,  ilmenite,  rutile,  orthite,  monazite,  beryl, 
samarskite,  spodumene,  amblygonite,  and  many  more.  The 
typical  mineralizers  are  boron  and  fluorine,  together  with  a  little 
phosphorus  and  sulphur.  Lithium  and  the  metals  of  the  cerium 
and  thorium  groups  are  also  characteristic.  Among  sulphides 
molybdenite  and  bismuthinite  are  the  most  common,  but  pyrite, 
arsenopyrite,  pyrrhotite,  chalcopyrite,  bornite,  and  sphalerite 
are  also  found.  In  the  south  Norwegian  granitic  pegmatites 
lithium  and  tin  are  absent.2 

1  Bull,  Geol.  Soc.  Am.,  vol.  10,  1898,  p.  21. 

1  W.  C.  Brogger,  Die  Mineralien  der  siidnorwegischen  Granit-pegmatit 
Gange,  Videnselskabets  Skrifter,  Math-naturv.  Klasse,  Kristiania,  1906, 
No.  6,  pp.  159. 


THE  PEGMATITE  DIKES  765 

A  second  group  is  formed  by  the  syenitic  pegmatites,  rich  in 
alkalies  and  especially  in  sodium.  These  contain  soda  ortho- 
clase,  aegirine,  acmite,  arfvedsonite,  biotite,  nephelite,  sodalite, 
lavenite,  and  a  number  of  rare  titanium  and  zirconium  minerals, 
also  fluosilicates.  There  is  little  or  no  quartz.  The  character- 
istic mineralizers  are  fluorine  and  chlorine.  Here,  as  elsewhere, 
the  sulphides  belong  to  a  rather  late  stage  of  consolidation. 
Brogger1  distinguishes  in  south  Norwegian  syenite  pegmatites 
four  phases  of  crystallization.  In  the  first  phase  (of  earliest 
development)  he  places  feldspars,  nephelite,  sodalite,  segirine, 
lepidomelane,  barkevikite,  and  magnetite,  followed  by  fluorite, 
rosenbuschite,  lavenite,  and  woehlerite  (containing  fluorine); 
by  sodalite  (containing  chlorine);  by  helvite  (containing  sul- 
phur) ;  by  lollingite  (containing  arsenic) ;  and  by  homilite  and 
melanocerite  (containing  boron).  There  is  no  tourmaline, 
topaz,  or  quartz. 

The  second  phase  consists  in  the  filling  of  drusy  cavities  in  part 
by  destruction  of  the  older  minerals;  these  druse  minerals  consist 
of  leucophane  and  fluorite,  representing  the  fluorine  group;  of 
homilite  and  datolite,  representing  the  boron  group;  and  of  the 
simple  sulphides,  such  as  molybdenite,  sphalerite,  and  galena. 

The  third  phase,  at  a  considerably  lower  temperature  but 
still  probably  above  100°  C.,  comprises  the  zeolites,  which  are 
followed  by  a  fourth  phase  of  low-temperature  carbonates  and 
fluocarbonates. 

Interesting  pegmatite  pipes  in  the  riebeckite  granite  of  Quincy, 
Massachusetts,  have  been  described  by  Warren  and  Palache.2 
A  zone  of  a  coarse  granitic  aggregate  of  quartz,  feldspar,  riebeck- 
ite, and  aegirine  graduates  into  a  central  mass  of  almost  pure 
massive  quartz,  sometimes  containing  molybdenite,  sphalerite 
galena,  and  chalcopyrite  and  in  its  miarolitic  cavities  fluorite, 
octahedrite,  ilmenite,  and  parisite,  the  last  a  fluocarbonate  of 
the  cerium  metals. 

Basic  Pegmatites. — The  basic  pegmatites  are  less  common. 
Boron  and  fluorine  are  not  usually  present,  but  phosphorus  and 
chlorine,  probably  also  sulphur,  play  important  parts.  Soda- 
lime  feldspars,  amphibole,  pyroxene,  quartz,  apatite,  rutile, 
and  brown  mica  are  the  most  abundant  minerals. 

1  W.  C.  Brogger,  Zeitschr.  Kryst.  Min.,  Bd.  16,  1900. 
*  Bull,  Geol.  Soc.  Am.,  vol.  21,  1910,  p.  784. 


766  MINERAL  DEPOSITS 

ECONOMIC  FEATURES  OF  PEGMATITE  DIKES 

The  pegmatites,  on  one  hand,  contain  many  of  the  common 
minerals  in  exceptional  size  of  grain  and  purity,  and,  on  the 
other  hand,  they  are  a  storehouse  for  a  great  number  of  the 
rarest  minerals,  many  of  which  are  not  found  elsewhere.  These 
deposits  are  therefore  of  considerable  economic  importance  and 
their  valuable  products  are  of  manifold  kind. 

Feldspar  and  Quartz. — The  orthoclase  and  quartz  of  granitic 
pegmatites  are  mined  or  quarried  at  numerous  places,  particu- 
larly in  Maine,  Connecticut,  and  Pennsylvania,  in  Norway,  and 
in  many  other  countries.  The  total  value  of  the  quartz  and 
feldspar  obtained  from  pegmatite  dikes  in  the  United  States 
amounts  annually  to  several  hundred  thousand  dollars.  The 
minerals  are  used  for  pottery  and  for  many  other  industrial 
purposes;  the  quartz  in  particular  is  also  utilized  as  an  abrasive, 
in  paints,  and  for  the  coating  of  tarred  roofing.  A  minor  quan- 
tity of  quartz  is  also  cut  as  a  semi-precious  stone  under  the  names 
rock  crystal,  smoky  quartz,  rose  quartz,  and  rutilated  quartz.1 

Mica. — White  mica,  more  commonly  known  as  muscovite,  is 
also  an  important  product  of  the  granitic  pegmatites.  It  occurs 
as  irregularly  disseminated  bunches  of  foils,  or  "books,"  in 
pegmatite  dikes,  some  tunes  crystallizing  along  the  walls.  The 
mica-bearing  pegmatites  are  worked  in  three  belts  in  the  North 
Carolina  mountain  region,  where  they  break  into  the  pre-Cam- 
brian  crystalline  schists  and  gneisses.2  The  dikes,  which  also 
carry  orthoclase,  perthite,  oligoclase,  and  quartz,  are  of  vary- 
ing thickness  and  persistence,  at  some  places  lenticular  and 

1  D.  B.  Sterrett,  Gems  and  Precious  Stones,  Mineral  Resources,  U.  S. 
Geol.  Survey,  pt.  2,  1910,  pp.  963-975;  also  in  later  issues. 

2  D.  B.  Sterrett  and  W.  T.  Schaller,  Mineral  Resources,  U.  S.  Geol.  Survey, 
1908-1917. 

D.  B.  Sterrett,  Mica  deposits  of  South  Dakota,  Bull.  380,  U.  S.  Geol, 
Survey,  1909,  pp.  382-397. 

D.  B.  Sterrett,  Mica  deposits  of  North  Carolina,  Bull.  315,  U.  S.  Geol. 
Survey,  1907,  pp.  400-422. 

J.  A.  Holmes,  Twentieth  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  6,  1899,  pp. 
691-707. 

R.  W.  Ells,  Mica  deposits  of  Canada,  Mineral  deposits  of  Canada,  1904. 

F.  Cirkel,  Mica,  its  occurrence,  exploitation,  and  uses,  Dept.  of  Interior, 
Mines  Branch,  Canada,  1905. 

A.  Osann,  Notes  on  certain  Archean  rocks  of  the  Ottawa  valley,  Ann. 
Rept.,  Canada  Geol.  Survey,  vol.  12,  1899. 


THE  PEGMATITE  DIKES 


767 


following  the  schistosity,  at  others  cutting  across  the  country 
rock.  Accessory  minerals  are  biotite  and  several  rare  or  gem 
minerals,  among  them  beryl  and  its  variety  aquamarine.  Some 
of  the  occurrences  constitute  transitions  to  quartz  veins,  which 
are  assumed  to  have  been  formed  by  more  distinctly  aqueous 
solutions. 

Muscovite  is  also  mined  in  New  Hampshire,  Virginia,  South 
Dakota,  Alabama  and  Georgia. 

Muscovite  owes  its  usefulness  to  its  transparency,  elasticity, 
great  resistance  to  heat  and  weathering,  and  applicability  as  a 


Mica  pockets 


granular 
m quartz 


FIG.  260. — Vertical  section  across  pegmatite  dike,  Thorn  Mountain  mine, 
N.  C.    After  D.  B.  Sterrett,  U.  S.  Geol.  Survey. 

non-conductor  of  electricity.  Its  crystals  are  sometimes  several 
feet  in  diameter,  but  this  is  exceptional,  sheets  of  1  foot  in 
diameter  being  considered  large.  Smaller  sheets  a  few  inches 
square  find  ready  use  and  are  split  into  thin  lamellae  and  cut  into 
proper  shapes  for  stove  doors  and  for  various  electrical  insulating 
purposes.  The  scrap  from  the  trimming  is  often  ground  and 
compressed  into  mica  board  or  "micanite"  for  use  in  insulating. 
It  is  also  used  for  the  manufacture  of  wall  papers  and  roofing 
materials.  The  quality  of  mica  is  best  judged  by  the  trans- 
parency of  sheets  about  2  millimeters  thick;  it  is  graded  as 
"wine"  or  "rum"  or  smoky  and  spotted  mica,  the  latter  being 
undesirable  for  insulation.  The  price  paid  for  sheet  mica 
varies  greatly  according  to  the  size  of  the  sheets.  The  average 


768  MINERAL  DEPOSITS 

price  ranges  up  to  $1  per  pound  for  sheets  2  by  3  inches;  larger 
sheets  are  worth  several  dollars  per  pound.  In  1917  the  domestic 
output  was  1,217,000  pounds  of  sheet  mica  and  3,250  tons  of 
scrap.  Considerable  quantities  are  imported,  mainly  from 
Canada  and  from  India. 

The  Indian  mica,  which  is  mined  on  a  large  but  primitive 
scale,  is  generally  muscovite  contained  in  pegmatite  dikes  cut- 
ting gneissoid  rocks.  The  Canadian  mica,  of  which  much  is 
also  exported,  is  mainly  a  phlogopite  or  brown  magnesium  mica 
and  is  better  adapted  to  electrical  uses  than  the  muscovite.  It 
occurs  with  apatite,  in  dikes  or  veins  of  pyroxene  in  gneiss  or 
limestone,  the  principal  localities  being  north  of  Ottawa.  Asso- 
ciated minerals  are  calcite,  scapolite,  titanite,  various  metallic 
sulphides,  among  which  molybdenite  is  mentioned,  and  one  or 
two  zeolites.  These  peculiar  deposits  are  undoubtedly  analogous 
to  the  Norwegian  apatite-scapolite  veins;  in  part  they  are  cer- 
tainly derived  from  limestone  by  contact  metamorphism.  Many 
of  these  dikes  have  been  worked  to  a  depth  of  several  hundred 
feet.  The  quantity  of  trimmed  mica  obtained  from  the  rock 
mined  is  small,  often  less  than  1  per  cent.  Occasionally  plates 
5  feet  in  diameter  are  found. 

Oxide  Ores.— Specularite,  magnetite,  and  ilmenite  are  of 
common  occurrence  in  pegmatites,  but  scarcely  ever  of  economic 
importance.  Cassiterite,  or  oxide  of  tin,  is  also  very  common 
in  many  granitic  pegmatites.  Cassiterite  in  many  places  forms 
an  integral  constituent  of  granites,  having  unquestionably  been 
consolidated  with  the  other  magmatic  minerals.  Its  most  com- 
mon occurrence  is  in  quartz  veins  which  were  formed  at  high 
temperatures,  as  indicated  by  the  mineral  association,  but  which 
differ  from  the  pegmatitic  dikes  and  assuredly  were  formed  at 
somewhat  lower  temperatures  than  the  dikes.  These  veins,  too, 
stand  in  closest  areal  connection  with  the  acidic  intrusive  rocks. 

Pegmatites  containing  cassiterite,  with  phosphates  and  lithium 
minerals,  have  been  mined  near  Gaffney,  South  Carolina,  and 
about  50  tons  of  tin  have  been  obtained  from  the  detrital  de- 
posits.1 The  average  tenor  is  low,  but  the  mineral  is  concen- 
trated along  certain  lines  in  the  dike  not  unlike  a  shoot  in  a  met- 
alliferous vein.  Tin-bearing  pegmatites  occur  also  in  the  Black 
Hills  of  South  Dakota,  where  attempts  to  mine  them  have  showed 

1 L.  C.  Graton,  Gold  and  tin  deposits  of  the  southern  Appalachians 
Bull.  293,  U.  S.  Geol.  Survey,  1906. 


THE  PEGMATITE  DIKES  769 

that  they  carried  a  very  low  percentage  of  the  metal.1  AtTinton, 
in  the  northern  Black  Hills,  mining  operations  have  been  carried 
on  and  some  cassiterite  recovered.  At  the  Etta  or  Harney  Peak 
mine,  in  the  southern  Black  Hills,  the  percentage  of  tin  appears 
to  be  too  small  for  successful  recovery,  but  other  minerals,  par- 
ticularly those  of  lithium,  have  been  mined  (p.  773). 

In  the  New  England  district  in  New  South  Wales,  as  described 
by  E.  C.  Andrews  and  L.  A.  Cotton,2  there  are  pipes  of  greisen 
with  transitions  into  pegmatite  containing  cassiterite  associated 
with  wolframite,  molybdenite,  bismuth,  arsenopyrite,  tourmaline, 
fluorspar,  and  beryl. 

Remarkable  rutile  deposits  have  been  discovered  recently  in 
Virginia,  in  Amherst  and  Nelson  counties.3  They  are  probably 
pegmatitic  developments  of  probably  pre-Cambrian  gabbro 
magmas,  which,  in  other  parts  of  the  world,  are  also  character- 
ized by  the  concentration  of  titanium  and  phosphorus. 

It  is  apparently  a  case  where  it  is  difficult  to  draw  the  line 
between  ordinary  rock  differentiation  and  pegmatization,  but 
the  features  of  the  deposits  clearly  recall  the  latter  process.  The 
districts  mentioned  contain  a  predominant  rock  of  quartz 
monzonite  gneiss  with  an  unusually  large  percentage  of  titanium 
and  phosphorus.  Besides  there  are  dikes  of  gabbro  still  richer 
in  titanium.  The  pegmatitic  facies  consist  essentially  of  a  bluish 
quartz  with  plagioclase,  orthoclase,  and  pyroxene,  the  last  con- 
verted into  hornblende  with  much  rutile  and  accessory  apatite 
and  ilmenite.  The  rock  has  an  even  granular  texture  and  con- 
tains as  much  as  59  per  cent,  titanium  dioxide  and  12  per  cent, 
phosphoric  pentoxide.  Fluorine  is  present  in  quantities  of  1 
per  cent,  or  more  and  sulphur  to  the  same  amount,  but  there  is 
very  little  chlorine.  The  rutile  as  well  as  the  ilmenite  is  re- 
covered by  concentration  and  is  used  mainly  for  the  manufacture 
of  arc-lamp  electrodes. 

1  F.  L.  Heos,  Tin,  tungsten,  and  tantalum  deposits  of  South  Dakota,  Bull. 
380,  U.  S.  Geol.  Survey,  1909,  pp.  131-163. 

2  E.  C.  Andrews,  The  geology  of  the  New  England  Plateau,  Records, 
Geol,  Survey,  N.  S.  W.,  vol.  8,  pt.  2,  1905,  pp.  131-136. 

L.  A.  Cotton,  The  tin  deposits  of  New  England,  Proc.,  Linnean  Soc. 
N.  S.  W.,  vol.  34,  pt.  4,  November  24,  1909. 

3  T.  L.  Watson  and  S.  Taber,  The  Virginia  rutile  deposits,  Butt.   3-A, 
Virginia  Geol.  Survey,   1913. 

T.  L.  Watson,  Occurrence  of  rutile  in  Virginia,  Econ.  Geol,  vol.  2,  1907 
pp.  493  504. 


770  MINERAL  DEPOSITS 

Wolframite. — As  noted  above,  wolframite  usually  accompanies 
cassiterite  in  pegmatites,  but  only  a  small  amount  of  the  world's 
supply  of  tungsten  is  derived  from  these  sources. 

Columbite  and  Tantalite.1 — These  minerals  are  columbates 
and  tantalates  of  iron  and  manganese  ((Fe,  Mn)  (Cb,  Ta)206). 
Their  home  is  in  the  granitic  pegmatites,  from  which  the  small 
quantities  needed  for  incandescent  lamps,  electrodes  and  surgical 
instruments  are  derived.  Large  masses  of  columbite  in  black 
tabular  crystals  have  been  found  in  the  pegmatites  of  the  Black 
Hills,  especially  at  the  Etta  mine.  Mangano-tantalite,  richer  in 
tantalum  is  mined  from  similar  sources  in  Western  Australia. 
Columbite  is  not  uncommon  in  many  regions  characterized  by 
pegmatite  dikes,  such  as  Connecticut  and  Virginia.  Striiverite 
(FeO  (TaCb)2O5.  6Ti02),  isomorphous  with  rutile  has  been  found 
in  abundance  at  the  Etta  mine,  South  Dakota  and,  in  places, 
in  the  Federated  Malay  States.  The  price  of  tantalum  is  about 
$18  per  ounce. 

Yttrium,  Thorium,  and  Cerium  Minerals. — Among  the  many 
rare  earth  minerals  the  following  are  the  more  important :  thorite, 
(ThO2);  monazite  (Ce,  La,  Y,  Th)  PO4;  gadolinite  (beryllium- 
iron-yttrium  silicate);  allanite  (cerium  epidote);  yttrialite 
(silicate  of  yttrium  and  thorium);  euxenite  (columbate  and 
titanate  of  cerium  metals  and  uranium) ;  samarskite  (columbate 
and  tantalate  of  cerium  metals  and  uranium).  Some  of  these 
minerals,  mainly  monazite,  are  used  extensively  as  a  source 
of  thorium  salts  in  the  manufacture  of  incandescent  mantles; 
the  yttrium  minerals,  like  fergusonite  and  gadolinite,  were  used 
in  the  manufacture  of  Nernst  lamps  in  which  the  incandescent 
part  consisted  of  25  per  cent,  yttria  and  75  per  cent,  zirconia. 
The  cerium  minerals  have  a  limited  use  for  chemicals,  etc.,  as 
well  as  for  the  manufacture  of  ferrocerium,  an  alloy  emitting 
sparks  when  rubbed  by  a  hard  substance.  The  Welsbach 
incandescent  mantles  are  coated  by  a  substance  containing  60 
per  cent,  zirconia,  20  per  cent,  yttria,  and  20  per  cent,  oxide  of 
lanthanum. 

At  the  present  time  incandescent  mantles  are  said  to  contain  99 
per  cent.  ThO2  and  1  per  cent.  CeCO3.  The  principal  com- 
pound manufactured  from  the  thorium  minerals  is  thorium 
nitrate  which  formerly  was  worth  about  $2  per  pound;  the  present 

i  F.  L.  Hess,  Bull,  380,  U.  S.  Geol.  Survey,  1909,  pp.  157-161;  Mineral 
Resources,  pt.  1,  U.  S.  Geol.  Survey,  1912,  pp.  977-979. 


THE  PEGMATITE  DIKES  771 

price  is  about  $8  per  pound.  Most  of  the  thorium  nitrate  used 
in  the  United  States  was  imported  from  Germany  where  it  was 
manufactured  from  Brazilian  monazite.  In  1913,  119,000 
pounds  were  imported.  The  small  amount  of  cerium  used  is 
derived  from  monazite.  Mesothorium,  an  element  similar  to 
radium  and  used  for  curative  purposes  and  luminous  paint,  is 
also  contained  in  thorium  minerals  and  is  extracted  as  a  by- 
product. 

All  these  minerals  find  their  home  in  the  granitic  pegmatites 
and  in  the  placers  derived  from  them.  To  some  extent  they  are 
also  primary  constituents  of  igneous  rocks.  In  Scandinavia 
there  are  some  celebrated  occurrences,  like  those  of  Hittero, 
in  southern  Norway,  and  of  Ytterby,  Korarfvet,  Brodbo,  and 
Finbo,  in  Sweden.  One  of  the  most  renowned  localities  in  the 
United  States  is  Baringer  Hill,  100  miles  northwest  of  Austin, 
Texas;  few  other  localities  have  yielded  as  large  amounts  of 
rare-earth  minerals  as  this  place.1 

Baringer  Hill  is  a  low  mound,  about  100  feet  wide  and  250 
feet  long,  preserved  from  erosion  by  its  relative  hardness.  The 
country  rock  is  a  coarse,  porphyritic  granite  of  pre-Cambrian 
age,  and  the  dike  itself  an  unsymmetrical  body  or  pipe.  At  the 
edge  of  the  dike  is  pegmatite  of  the  "graphic"  variety  1  to  6 
feet  wide.  The  central  part  is  made  up  of  large  individuals  of 
quartz  and  feldspar,  the  latter  being  microcline  and  albite.  In 
the  center  of  the  dike  the  quartz  appears  to  be  concentrated. 
Some  of  the  feldspar  crystals  are  several  feet  long.  Vugs  are 
lined  with  smoky  quartz.  The  rarer  minerals,  some  of  which 
occur  in  large  amounts,  are  fluorite,  ilmenite,  gadolinite,  allanite, 
fergusonite,  and  polycrase — in  short,  a  series  of  silicates,  colum- 
bates,  titanates,  and  uranates  of  cerium,  yttrium,  and  other  rare 
metals.  There  are  also  sulphides,  particularly  chalcopyrite, 
pyrite,  sphalerite,  and  molybdenite,  the  last  named  being  the 
most  abundant.  The  rock  contains  no  tourmaline,  beryl,  zircon, 

1  W.  E.  Hidden  and  C.  H.  Warren,  Am.  Jour.  Sti.,  4th  ser.,  vol.  22,  1906. 
p.  515. 

F.  L.  Hess,  Minerals  of  the  rare-earth  metals  at  Baringer  Hill,  Llano 
County,  Texas,  Bull.  340,  U.  S.  Geol.  Survey,  1908,  pp.  286-294.  ' 

K.  L.  Kithil,  Monazite,  thorium  and  mesothorium,  Tech.  Paper  110,  U.  S. 
Bureau  of  Mines,  1915. 

W.  T.  Schaller,  Thorium  minerals  in  1916,  Min.  Res.,  U.  S.  Geol.  Survey. 

W.  T.  Schaller,  Zirconium  and  rare-earth  minerals  in  1916,  Idem. 

C.  R.  Bohm,  Die  Verwendung  der  seltenen  Erden,  Leipzig,  1913. 


772  MINERAL  DEPOSITS 

garnet,  or  cassiterite.  The  deposit  is  worked  intermittently 
for  the  yttrium  which  its  minerals  contain.  Some  of  the  minerals 
show  a  marked  radioactivity. 

Monazite  and  Zircon.1 — Both  these  minerals  form  accessories 
of  granitic  and  monzonitic  rocks;  they  also  occur  in  pegmatites 
and  apparently  are  formed  in  some  veins  developed  at  com- 
paratively high  temperatures.  On  the  other  hand,  they  are 
absent  from  veins  formed  nearer  the  surface  or  under  conditions 
of  lessened  temperature  and  pressure. 

Zircon  (ZrSiO^  occurs  in  considerable  amounts  in  many  placer 
deposits  derived  from  the  disintegration  of  granitic  and  pegma- 
titic  rocks.  In  the  miner's  pan  it  is  concentrated,  with  the  gold, 
as  a  string  of  minute  crystals  of  brilliant  white,  almost  metallic 
luster.  The  best-known  deposits  are  at  Zirconia,  near  Green 
River,  in  Henderson  County,  North  Carolina;  from  the  decom- 
posed croppings  of  a  pegmatite  dike  at  this  locality  many  tons  of 
zircon  have  been  obtained.  The  value  of  the  concentrated  zircon 
sand  is  about  20  cents  per  pound. 

A  zirconium  mineral,2  at  first  thought  to  be  baddeleyite  (Zr02) 
has  recently  been  found  in  Brazil  in  the  Caldas  District  on  the 
border  of  the  States  of  Minas  Geraes  and  Sao  Paulo.  It  occurs 
in  large  quantities  and  is  probably  connected  with  pegmatite 
dikes  in  nephelite-syenite,  and  appears  to  consist  of  brazilite, 
a  fibrous  variety  of  ZrO2,  zircon  and  an  unknown  zirconium  min- 
eral with  75  per  cent.  ZrO2,  possibly  a  silicate  but  not  identical 
with  zircon.  Much  of  this  material  is  imported. 

The  principal  use  of  zircon  is  as  an  excellent  refractory  ma- 
terial; it  has  also  a  very  low  coefficient  of  expansion.  Zirconia 
is  also  used  for  incandescent  mantles  and  zirconium  as  a  steel 
hardening  metal.  Nickel-zirconium  (cooperite)  is  another  alloy 
used  for  high-speed  tools.  The  value  of  zircon  varies:  $100  to 
$400  per  ton  have  been  paid. 

Monazite,  a  honey-yellow  to  brown  phosphate  of  cerium  and 
cerium  metals,  with  a  varying  percentage  of  thoria3  and  silica, 
is  almost  wholly  recovered  from  placers  where  it  often  occurs  with 

1  D.  B.  Sterrett,  Mineral  Resources,  U.  S.  Geol.  Survey,  1908-1911. 

J.  H.  Pratt,  Zircon,  monazite,  etc.,  Butt.  25,  North  Carolina  Geol.  & 
Econ.  Survey,  1916. 

2  W.  T.  Schaller,  Mineral  Resources.  U.  S.  Geol.  Survey,  1916,  pp.  377-379. 

3  For  commercial  purposes  the  mineral  should  contain  from  3  to  9  per 
cent,  of  thoria.     See  footnotes  on  p.  244.     See  also  p.  770. 


THE  PEGMATITE  DIKES  773 

gold.  The  best-known  occurrences  are  in  South  and  North  Caro- 
lina and  in  the  Boise  Basin  of  Idaho.  The  primary  home  of  the 
mineral  in  these  districts  is  in  the  granitic  rocks  and  in  pegmatized 
schist.  The  concentrates  obtained  in  the  sluices  are  cleaned 
in  electro-magnetic  separators.  Large  deposits  of  monazite, 
in  part  marine  shore  deposits  of  sand,  are  worked  in  Brazil  and 
India,  and  the  mineral  is  exported  to  Europe  and  the  United 
States.  The  production  in  the  United  States  varies  considerably. 
In  1910  the  output  of  concentrated  monazite  sand  was  99,000 
pounds,  for  which  about  12  cents  per  pound  was  paid.  From 
1911  to  1915  there  was  no  domestic  production.  In  1916,  1,200 
tons  of  monazite  sand  was  imported  chiefly  from  Brazil. 

Apatite. — Apatite  associated  with  pyroxene  (malacolite) , 
hornblende,  phogopite,  titanite  and  much  calcite  occurs  in 
many  deposits  in  southern  Ontario,  near  Kingston.1  The  de- 
posits are  said  to  be  dikes  of  pyroxenite  with  segregations  of 
apatite  and  calcite  intruded  in  gneiss  and  limestone  of  pre- 
Cambrian  (Grenville)  age.  It  seems  certain  now  that  some  of 
these  pyroxene  rocks  are  contact  metamorphosed  limestone  (Fig. 
254),  but  there  are  also  dikes,  though  with  what  intrusions  these 
dikes  are  associated  is  not  certain.  In  part  the  apatite  may  be 
regarded  as  of  pegmatitic  origin.  The  apatite  contains  fluorine, 
instead  of  the  usual  chlorine.  The  mineral  is  well  crystallized 
and  of  greenish  color.  In  small  quantities  apatite  also  occurs 
here  in  veins  in  gneiss,  and  is  then  associated  with  pyroxene  and 
scapolite.  The  output  of  Canadian  apatite  has  ceased. 

The  Norwegian  apatite  veins2  described  in  detail  by  Vogt 
now  yield  a  small  production.  In  some  features  they  stand 
between  the  high  temperature  veins  and  the  pegmatites.  The 
minerals  are  chlorine,  apatite,  rutile,  ilmenite,  pyrrhotite,  horn- 
blende, enstatite,  malacolite  and  specularite.  The  feldspar 
in  the  rocks  adjacent  to  the  veins  are  altered  to  scapolite  indicat- 
ing the  presence  of  chlorine  in  the  emanations. 

Lithium  Minerals.3 — Among  the  alkaline  metals  lithium  ac- 
companies potassium  in  the  pegmatites  and  appears  in  a  series  of 

1  W.  H.  McNairn,  Trans.  Canad.  Min.  Inst.,  vol.  8,  1910,  pp.  495-514. 

F.  D.  Adams  and  A.  E.  Barlow,  Geology  of  the  Haliburton  and  Ban- 
croft areas,  Mem.  6,  Geol.  Survey  Canada,  1910,  p.  383. 

2  J.  H.  L.  Vogt,  ZeUschr.  prakt.  Geol.,  1895,  pp.  367-370,  444-459,  465- 
740. 

3  F.  L.  Hess,  Min.  Res.,  U.  S.  Geol.  Survey  for  1909,  pp.  649-653. 
W.  T.  Schaller,  Min.  Res.,  U.  S.  Geol.  Survey  for  1916,  pp.  7-17. 


774 


MINERAL  DEPOSITS 


minerals,  the  most  common  of  which  are  lepidolite,  or  lithium 
mica  (4  per  cent,  lithia);  spodumene,  or  lithium-aluminum  sili- 
cate allied  to  pyroxene  (8  per  cent,  lithia);  petalite,  lithium- 
aluminum  disilicate  (5  per  cent,  lithia) ;  triphylite,  a  lithium-iron- 
manganese  phosphate  (9  per  cent,  lithia);  and  amblygonite, 
a  fluophosphate  of  aluminum  and  lithium  (10  per  cent,  lithia). 
Spodumene  and  particularly  amblygonite  are  the  principal  raw 
materials  from  which  lithia  salts  are  manufactured.  These 
minerals  have  been  mined  in  the  Peerless  and  Etta  pegmatite 
dikes,  near  Keystone,  South  Dakota.  In  1916,  619  tons  of  this 
mineral  were  mined.  Amblygonite  also  occurs  in  pegmatites 
near  Pala,  San  Diego  County,  California.  The  use  of  lithium 
salts  is  small,  chiefly  for  fireworks,  medicine  and  certain  storage 
batteries. 

At  the  Etta  mine  attempts  have  also  been  made  to  mine  the 
pegmatite  for  tin  and  columbite.  The  Etta  deposit  is  a  roughly 
circular  mass  of  coarse  pegmatite  about  150  by  200  feet  in  ex- 
tent. Spodumene  crystals  as  much  as  42  feet  in  length  and  hav- 
ing a  cross  section  of  3  by  6  feet  are  found  here.  The  list  of 
primary  minerals  found  at  this  remarkable  locality  is  given 
below.1  No  topaz  or  axinite  is  present. 

MINERALS  FOUND  AT  THE  ETTA  MINE 


Orthoclase 

Lepidolite 

Quartz 

Columbite 

Molybdenite 

Albite 

Petalite 

Zircon 

Tantalite 

Arsenopyrite 

Microcline 

Spodumene 

Rutile 

Wolframite 

Lollingite 

Almandite 

Tourmaline 

Spinel 

Monazite 

Leucopyrite 

Grossularite 

Epidote 

Cassiterite 

Amblygonite 

Bismuth 

Andalusite 

Beryl 

Corundum 

Apatite 

Galena 

Muscovite 

Titanite 

Ilmenite 

Triplite 

Stannite 

Biotite 

Triphylite 

Cryolite.2 — Cryolite  (3NaF.AlF3,  with  12.8  per  cent,  aluminum) 
is  a  white  to  brown  or  even  black  mineral  of  which  only  one  large 
deposit  is  known.  The  locality  is  Ivigtut,  in  west  Greenland, 
close  to  the  sea,  where  it  occurs  as  a  large  mass  having  surface 
dimensions  of  200  by  600  feet;  it  has  been  worked  to  a  depth 

1  F.  L.  Hess,  Bull.  380,  U.  S.  Geol.  Survey,  1909,  p.  149. 

2  N.  V.  Ussing,  Denmark  Geol.  Undersog.,  2d  ser.,  No.  12,  pp.  97-102. 
R.  Baldauf  (and  R.  Beck),  Ueber  das  Kryolith-Vorkommen  in  Gron- 

land;  Zeitschr.  prakt.  Geol,  vol.  18,  1910,  pp.  432-446. 

A.  S.  Halland,  Cryolite  and  its  industrial  applications,  Jour.  Industr. 
and  Eng.  Chemistry,  Feb.,  1911,  pp.  63-66. 


THE  PEGMATITE  DIKES  775 

of  150  feet.  The  cryolite  occurs  in  a  coarse  granite  and  is  un- 
doubtedly to  be  classed  as  an  unusual  pegmatite  mass.  The 
coarsely  crystalline  mineral  is  associated  with  some  crystallized 
siderite,  galena,  chalcopyrite,  pyrite,  fluorite,  topaz,  and  ivigtite. 
The  sulphides  are  said  to  contain  a  little  gold.  About  17,000 
tons  are  produced  annually,  4,000  tons  being  imported  in  the 
United  States. 

A  pegmatite  mass  adjoining  the  cryolite  contains  the  same 
minerals  and  also  cassiterite  in  a  coarse-grained  aggregate  of 
microcline,  albite,  and  quartz.  The  cryolite  is  said  to  be  intru- 
sive into  the  granite  and  to  effect  many  changes  in  it.  The 
deposit  is  thus  an  unusually  large  magmatic  concentration  of 
fluorides. 

Bauxite,  the  hydroxide  of  aluminum,  is  now  used  for  the  manu- 
facture of  the  metal.  Before  the  present  methods  of  smelting 
aluminum  were  introduced  the  easily  fusible  cryolite  was  used 
for  this  purpose,  and  even  now  it  is  added  to  the  charge  to 
promote  the  melting.  It  is  also  used  for  enameling  iron  ware, 
and  in  making  white  Portland  cement. 

Precious  Stones.1 — The  pegmatite  dikes  have  always  been 
famous  as  the  source  of  gem  minerals,  which  are  valued  for 
ornaments  on  account  of  their  color,  hardness,  and  brilliancy. 
Many  of  these  beautiful  crystals  appear  to  belong  to  one  of  the 
later  magmatic  stages  of  consolidation  and  usually  occur  in 
druses  of  the  rock.  Among  the  most  productive  American 
regions  are  North  Carolina,  Maine,  and  San  Diego  County, 
California.  The  pink  tourmaline  of  Pala  and  other  places  in 
San  Diego  County  are  famous  and  the  crude  output  has  an  annual 
value  of  over  $100,000.  Accompanying  this  mineral  are  hidden- 
ite  and  kunzite,  the  lilac  colored  gem  varieties  of  spodumene. 

Green  tourmaline  comes  from  Maine;  emerald  and  aqua- 
marine (Be3Al2(Si03)6,  both  varieties  of  beryl,  are  found  in  peg- 
matites in  North  Carolina,2  accompanied  by  quartz,  albite,  and 
tourmaline.  Aquamarine  of  gem  quality  with  much  greenish 
beryl  is  found  in  pegmatite  quartz  of  southern  New  Hampshire. 
The  famous  emeralds  of  Columbia,3  at  Muso,  occur  in  carbo- 

1D.  B.  Sterrett,  "Gems,"  Mineral  Resources,  U.  S.  Geol.  Survey, 
1908-1915. 

2  D.  B.  Sterrett,  Mineral  Resources,  U.  S.  Geol.  Survey,  1910,  pt.  2,  p.  865; 
1911,  pt.  2,  pp.  1051-1052. 

3J.  E.  Pogiie.  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  55,  1917,  pp.  910-934. 


776  MINERAL  DEPOSITS 

naceous,  Cretaceous  limestone,  in  calcite  veins  which  seem  to 
belong  to  the  high  temperature  deposits  rather  than  to  the 
pegmatites. 

Rubies  are  also  found  in  pegmatite  dikes.  Most  of  the  sup- 
ply is  derived  from  Burma,  but  some  good  stones  have  been  ob- 
tained from  gravels  near  pegmatite  dikes  in  Cowee  Valley, 
North  Carolina. 

Topaz  is  found  in  pegmatite  as  well  as  in  lithophyse  or  rhyolite 
and  in  high  temperature  veins.  Quartz  of  the  clear,  smoky  and 
rutilated  varieties  is  common  in  pegmatite. 

Native  Metals,  Sulphides,  and  Arsenides. — The  sulphides  and 
allied  minerals  so  abundant  in  fissure  veins  play  a  very  subordi- 
nate part  in  the  pegmatite  dikes;  nevertheless  their  occurrence 
is  of  great  scientific  interest,  for  the  pegmatites  form  a  transition 
between  the  magmas  and  many  ore  deposits  connected  with 
igneous  rocks. 

Gold  in  visible  form  is  exceedingly  rare;  some  instances  are 
mentioned  in  Chapter  I;  additional  and  exact  information  is  greatly 
desired.  Spurr1  states  that  gold  occurs  in  pegmatite  dikes  or  in 
quartz  veins  closely  connected  with  them  in  the  Yukon  districts  in 
Alaska  and  at  Silver  Peak,  Nevada,  but  the  quantities  reported 
are,  as  a  rule,  small — at  most  0.05  ounce  per  ton — and  the  assays 
are  made  without  special  precautions.  I  have  no  personal 
knowledge  of  any  pegmatite  nor  of  any  quartz  veins  directly 
traceable  into  pegmatites  that  contain  enough  gold  for  profitable 
extraction.  Spurr,  however,  describes  important  gold-bearing 
quartz  veins  at  Silver  Peak,  Nevada,  which  are  said  to  form 
transitions  into  alaskite  or  aplitic  quartz-orthoclase  rock. 

In  1898  E.  Hussak2  described  the  Passagem  lode,  in  Brazil, 
and  regarded  it  as  a  gold-bearing  pegmatite  dike.  Orville  A. 
Derby3  has  lately  reviewed  the  evidence  and  arrived  at  the 
following  conclusions,  which  are  quoted  because  they  very  likely 
apply  to  many  similar  occurrences: 

The  Passagem  lode  presents  evidence  of  three  distinct  processes  of 

filling (1)  An  extensive  fissure  opened  by  stress 

was  closed  by  an  invasion  of  pegmatite  running  off  into  clear  quartz. 
At  this  stage  the  lode  contained  only  the  minerals  characteristic  of  a 

1 J.  E.  Spurr,  Ore  deposits  of  the  Silver  Peak  quadrangle,  Nevada,  Prof, 
Paper  55,  U.  S.  Geol.  Survey,  1906,  p.  49. 

2  E.  Hussak,  Zeitschr.  prakt.  Geol.,  October,  1898,  pp.  345-357. 

3  Am.  Jour.  Sci.,  4th  ser.,  vol.  32,  1911,  pp.  185-190. 


THE  PEGMATITE  DIKES 


777 


granitic    magma (2)     A    subsequent    stress  .    .    .,  .    . 

fractured  this  pegmatitic  quartz,  rendering  it  accessible  to  a  pneumato- 

lytic  action which  filled  its  fissures  with  tourmaline  and  seri- 

citized  the  feldspar  of  the  pegmatite.  (3)  A  third  stress  coming  near  the 
end  of  the  second  phase  of  the  lode  fractured  the  tourmaline  filling  and 
gave  access  to  a  pneumatolytic  action  characterized  by  sulphur,  arsenic, 
metallic  oxides,  and  metals  (gold  and  silver),  which  filled  the  fissures  of 
the  lode,  invading  to  some  extent  its  pre-existing  portions  and  prob- 
ably also  some  of  the  adjacent  and  enclosed  country  rock. 

Arsenopyrite,  lollingite,  galena,  zinc  blende,  pyrite,  and  pyr- 
rhotite  have  been  reported  from  numerous  localities  in  the 
granitic  and  syenitic  pegmatites;  there  is  not  the  slightest  reason 
to  doubt  that  they  are  here  primary  minerals,  even  if  they  belong 
to  one  of  the  later  phases  of  magmatic  consolidation.  There  are, 
however,  no  deposits  known  in  which  they  are  abundant  enough 
to  be  mined. 

Bismuth  and  bismuthinite  are  reported  from  many  places,  and 
they  are  said  to  be  so  abundant  in  certain  pipes  of  pegmatite 
in  the  New  England  district  of  New  South  Wales  as  to  have 
some  economic  value. 

Molybdenite.* — Molybdenite  is  an  accessory  mineral  in  certain 
granites.  It  is  common  in  many  veins  of  the  deep-seated  class, 


FIG.  261. — Molybdenite   (black)  along  the  borders  of  pegmatite  dike  in 
gneiss,  Romaine.  Quebec.     One-half  natural  size.     After  T.  L.  Walker. 

more  or  less  closely  connected  with  pegmatites.  It  is  also  of  fre- 
quent occurrence  in  contact-metamorphic  deposits  and  in  ordi- 
nary fissure  veins,  both  in  those  formed  at  greater  depth  and  in 
those  deposited  near  the  surface,  but  in  the  main  is  confined  to 
deposits  genetically  allied  to  igneous  rocks.  In  the  pegmatite 

1F.  W.  Horton,  Molybdenum,  its  ores  and  their  concentration,  Bull.  Ill, 
U.  S.  Bureau  of  Mines,  1916. 


778  MINERAL  DEPOSITS 

and  the  abyssal  veins  the  individual  particles  of  molybdenite 
are  often  larger  and  sometimes  well  crystallized.  In  the  deposits 
formed  under  conditions  of  less  intense  heat  and  pressure  mol- 
ybdenite usually  appears  as  small  or  microscopic  scales. 

In  Canada,  in  Quebec  and  Cape  Breton  provinces,  some  peg- 
matitic  dikes  contain  enough  molybdenite  to  be  of  economic 
importance  (Fig.  261). l  In  Ontario,  near  Kingston,2  molybden- 
ite occurs  with  pyrrhotite  in  contact-metamorphic  or  pegmatitic 
deposits,  similar  to  those  which  carry  pyroxene,  apatite,  and 
brown  mica  (Fig.  254). 

Pegmatites  containing  molybdenite  occur  in  Washington  and 
Hancock  counties,  Maine.3  Some  of  them  may  have  economic 
importance.  One  of  the  principal  deposits  occurs  at  Cooper, 
22  miles  southwest  of  Calais;  here  the  molybdenite  is  especially 
associated  with  the  more  quartzose  phases  of  the  pegmatite. 
Fluorite  in  places  accompanies  the  molybdenite. 

Similar  deposits  are  found  in  many  parts  of  the  United  States 
but  difficulties  of  concentration  long  confined  the  product  to  hand 
picked  material.  By  aid  of  the  oil  flotation  process  the  concen- 
tration of  molybdenite  has  become  possible  and  the  mineral 
promises  to  become  of  great  importance.  So  far  most  of  the 
product  has  come  from  Ontario  and  British  Columbia;  in  1916, 
84  tons  of  pure  sulphide  was  produced  from  concentrates  con- 
taining 60  to  70  per  cent,  of  molybdenite.  Pyrite,  if  present, 
must  be  eliminated.  The  concentrate  is  converted  to  ammonium 
molybdate,  which  is  charged  with  iron  in  the  electric  furnace, 
yielding  ferromolybdenum  with  70  to  76  per  cent.  Mo,  0.1  per  cent. 
S.  and  3  per  cent,  carbon.4  The  price  of  concentrate  with  90 
per  cent.  MoS2  was  $0.75  to  $1  per  pound  in  January,  1919. 
Ores  containing  1  per  cent.  MoS2  can  evidently  be  worked  with 
profit. 

At  several  places,  for  instance  at  Climax,  Summit  Co.,  Colorado, 
preparations  are  made  to  recover  molybdenite,  generally  from 

1  T.  L.  Walker,  Report  on  the  molybdenum  ores  of  Canada,  Canada 
Dept.  of  Mines,  1911,  p.  64. 

A.  L.  Parsons,  Molybdenite  deposits  of  Ontario,  Twenty-sixth  Ann. 
Rept.,  Ontario  Bur.  Mines,  1917,  pp.  275-313. 

2  E.  Thomson,  A  pegmatitic  origin  for  molybdenite  ores,  Econ.  Geol., 
vol.  13,  1918,  pp.  302-313. 

3  Geo.  O.  Smith,  Bull.  260,  U.  S.  Geol.  Survey,  1905,  pp.  197-199. 
F.  L.  Hess,  Bull.  340,  U.  S.  Geol.  Survey,  1908,  pp.  231-238. 

4  H.  H.  Claudet,  Trans.,  Canad.  Mining  Inst.,  vol.  20,  1917,  pp.  121-134. 


THE  PEGMATITE  DIKES  779 

aplite  or  pegmatite,  sometimes  from  veins.  New  South  Wales, 
Queensland  and  Norway  also  yield  molybdenite. 

Wulfenite  (CaMoCh),  a  yellow  tabular  mineral,  is  not  un- 
common in  many  oxidized  deposits,  the  primary  ore  of  which 
contained  galena  and  molybdenite.  Most  of  the  small  output 
of  molybdenum  in  the  United  States  has  been  derived  from 
this  mineral,  though  it  is  less  desirable  than  molybdenite. 

Aside  from  many  chemical  and  industrial  uses,  the  principal 
value  of  molybdenum  lies  in  its  steel-hardening  qualities.  Molyb- 
denum steel,  containing  a  very  small  amount  of  the  metal,  is 
used  for  rifle  barrels,  propeller  shafts,  and  especially  for  high- 
speed steel-cutting  tools. 


CHAPTER  XXIX 

MINERAL  DEPOSITS  FORMED  BY  CONCENTRATION 
IN  MOLTEN  MAGMAS 

CONSTITUTION    OF    MAGMAS    AND    THEIR    DIFFERENTIATION 
AND  CONSOLIDATION 

General  Features 

Certain  kinds  of  mineral  deposits  form  integral  parts  of  igneous 
rock  masses  and  permit  the  inference  that  they  have  originated, 
in  their  present  form,  by  processes  of  differentiation  and  cooling 
in  molten  magmas.  The  minerals  are  of  simple  composition 
and  few  in  number;  most  prominent  among  them  are  magnetite, 
ilmenite,  spinel  minerals,  cassiterite,  pyrrhotite,  chalcopyrite, 
molybdenite,  lollingite  (FeAs2),  arsenopyrite,  corundum,  plati- 
num, and  diamond.  At  some  places  the  resulting  deposits  are 
large  and  rich,  but  as  a  whole  they  are  of  much  less  importance 
than  those  formed  by  aqueous  solutions. 

The  characteristic  feature  of  a  deposit  of  this  class  is  that  it  is 
a  part  of  a  body  of  igneous  rock;  the  crystals  of  its  minerals 
formed  in  the  magma  solution  from  which  the  rock  crystal- 
lized, or  in  one  similar  to  it.  The  associated  gangue  minerals 
are  those  which  make  up  igneous  rocks.  Structures  other  than 
those  of  purely  igneous  origin  should  be  absent.  If  there  is 
evidence  of  metamorphism  or  metasomatic  replacement,  with  the 
development  of  minerals  like  sericite,  carbonates,  chlorite,  ura- 
lite,  garnets,1  or  epidote,  or  bleaching  or  kaolinization,  we  must 
conclude  that  processes  other  than  those  of  purely  igneous  origin 
have  been  active.  Many  igneous  deposits  have,  at  a  later  period, 
been  subjected  to  influences  producing  alteration  and  their  origi- 
nal characteristics  may  then  have  become  veiled. 

Some  igneous  deposits  are  simply  parts  of  the  rock,  which  con- 
tains disseminations  of  the  useful  mineral,  like  diamond  in  cer- 
tain peridotites,  and  have  then  the  form  of  that  rock  mass 
itself — a  dike  or  a  volcanic  neck,  for  instance.  In  other  deposits 

1  Garnets  are,  however,  also  occasionally  found  in  normal  igneous  rocks, 
but  their  primary  or  secondary  nature  is  not  difficult  to  establish. 

780 


CONCENTRATION  IN  MOLTEN  MAGMAS       781 

the  massive  ore  forms  a  dike,  as  in  certain  titaniferous  magnet- 
ites. Or  again  the  ore  minerals  may  have  become  concentrated 
in  parts  of  the  igneous  rock  and  form  rudely  tabular  or  wholly 
irregular,  usually  ill-defined  masses  in  the  rock.  Unless  the 
deposit  is  of  large  cross-section  it  can  rarely  be  followed  to  great 
depth  like  a  fissure  vein,  for  the  movements  in  a  viscous  magma 
facilitated  the  formation  of  irregular  streaky  or  pasty  masses' — 
often  termed  "schlieren,"  after  the  German  usage — rather  than 
bodies  persistent  for  long  distances  in  a  given  direction. 

These  deposits  represent  extreme  conditions  in  mineral  forma- 
tion; the  temperature  of  basic  surface  lavas  is  considered  to  have 
been  about  1,000°  to  1,250°  C.,  but  the  deep-seated  granular 
rocks  in  which  most  of  the  igneous  mineral  bodies  occur  crystallize 
more  slowly  than  lavas  and  in  general  at  lower  temperatures 
probably  from  about  575°  to  1,000°  C.  The  temperature  before 
emission  and  consolidation  may  have  been  hundreds  of  degrees 
higher  than  the  various  figures  given  above. 

Constitution  of  Magmas 

In  order  to  explain  the  genesis  of  the  igneous  deposits  it  is 
necessary  to  inquire  into  the  nature  of  igneous  magmas.1  The 
magmas  are  not  haphazard  aggregates  of  elements  and  com- 
pounds. They  are  probably  solutions  of  definite  silicate  com- 
pounds in  one  another  (after  the  manner  of  a  mixture  of  water 
and  alcohol) ;  certain  oxides  like  silica,  alumina,  ferric  oxide,  and 
water  may  also  be  present,  at  least  in  a  magma  approaching  the 
point  of  crystallization,  and  these  silicates  and  oxides  are  freely 
miscible  in  any  proportion.  Diabases,  leucite  basalts,  and 
similar  rocks  may  be  reproduced  by  dry  fusion,  but  water  is 
present  in  almost  all  magmas  and  is  in  fact  necessary  for  the 
crystallization  of  a  great  number  of  rocks.  That  the  magma  is 
a  solution  is  inferred  from  the  lowering  of  the  freezing-point  as 
shown  by  the  order  of  crystallization,  and  from  the  fact  that  some 

1 J.  H.  L.  Vogt,  Bildung  von  Erzlagerstatten  durch  Differentiations 
Processe,  Zeitschr.  prakt.  Geol,  1893. 

A.  C.  Lane,  Wet  and  dry  differentiation  of  igneous  rocks,  Tufts  College 
Studies,  vol.  3,  No.  1,  1910. 

Compare  chapters  on  magmas  and  differentiation  in  "Natural  history  of 
igneous  rocks,"  by  Alfred  Marker;  "Igneous  rocks,"  by  J.  P.  Iddings; 
"Igneous  rocks  and  their  origin,"  by  R.  A.  Daly;  and  "Geochemistry," 
by  F.  W.  Clarke,  all  of  which  have  been  freely  consulted  in  the  preparation 
of  this  summary. 


782  MINERAL  DEPOSITS 

of  the  last  residues  of  crystallization  have  the  character  of  eutectic 
mixtures.  Dissociation  takes  place  to  some  extent  and  the 
magmas  are  electrolytes.  Arrhenius  and  Konigsberger  believe 
that  at  high  temperatures  water  must  be  a  stronger  acid  than 
silica  and  that  the  latter  exists  as  hydrates  and  basic  silicates. 
At  the  surface  lavas  emit  water  and  other  volatile  substances 
and  it  is  therefore  concluded  that  before  reaching  the  surface 
the  magmas  must  be  more  or  less  heavily  charged  with  such 
gases.  When  the  magmas  are  forced  to  a  higher  level  in  the 
crust  the  pressure  is  diminished  and  a  part  of  the  volatile  sub- 
stances are  liberated  just  as  carbon  dioxide  escapes  from  soda 
water  when  the  bottle  is  opened.  Another  part  is  still  held,  but 
most  of  that  is  doubtless  expelled  when  crystallization  takes  place. 
The  presence  of  water  greatly  affects  the  physical  properties  of 
the  magma  and  especially  increases  the  fluidity.  Barus,  for 
instance,  obtained  mixtures  of  various  glasses  with  much  water 
and  these  congealed  at  low  temperatures  as  "solid  solutions." 
Upon  heating  in  the  air,  water  is  expelled  and  a  pumice-like  mass 
results  which  has  a  much  higher  point  of  fusion.  Many  pitch 
stones  and  obsidians,  which  contain  much  water,  act  in  the  same 
way. 

Crystallization  of  Magmas 

As  in  an  aqueous  solution,  the  successive  crystallization  of 
given  minerals  in  these  deposits  is  dependent  upon  their  solubility 
in  the  rest  of  the  magma  and  does  not  follow  their  temperature 
of  fusion.  When  a  salt  dissolves  in  water  the  temperature  of 
solidification  is  changed.  Water  freezes  at  0°  C.,  but  an  addition 
of  sodium  chloride  to  it  depresses  its  melting  or  solidifying  point 
many  degrees.  Alloys  show  the  same  behavior' — for  example, 
those  with  extraordinary  low  temperature  of  fusion,  sometimes 
below  100°  C.  In  the  same  way  an  igneous  rock  may  become 
fluid  at  a  temperature  far  below  the  average  melting  point  of  its 
constituent  minerals,  or  even  lower  than  the  lowest  of  these. 

On  the  other  hand,  no  mineral  can  separate  if  the  temperature, 
for  a  given  pressure,  is  higher  than  the  point  of  fusion  of  this 
mineral.  Below  this  point  crystallization  takes  place  whenever 
the  point  of  saturation  of  the  solution  for  this  mineral  is  ex- 
ceeded. Some  of  its  components  will  form  isomorphous  mixtures, 
but  a  part  of  it  will  remain  in  eutectic  proportions,  which  differ 
according  to  the  composition  of  the  rock. 


CONCENTRATION  IN  MOLTEN  MAGMAS       783 

According  to  the  empirical  rule  of  Rosenbusch  the  separation 
of  crystals  in  a  silicate  magma  follows  an  order  of  decreasing 
basicity,  so  that  at  every  stage  the  residual  magma  is  more 
acidic  than  the  aggregate  of  the  crystals  already  separated  out. 
This  rule  is  subject  to  important  exceptions,  especially  in  basic 
magmas,  but  in  the  granitic  and  dioritic  rocks  the  basic  and 
difficultly  fusible  minerals,  such  as  zircon,  magnetite,  apatite, 
ilmenite,  and  rutile,  crystallize  first.  Then  follow  biotite, 
hornblende,  and  augite,  or  in  general  the  magnesium  and  iron 
silicates,  then  the  soda-lime  feldspars,  later  orthoclase,  and  finally 
the  residual  quartz,  which  probably  separates  at  about  800°  C. 
The  "mother  liquor"  of  a  granite  thus  becomes  successively 
richer  in  silica.  The  "mineralizers,"  or  the  volatile  substances, 
like  boron,  fluorine,  and  tin,  follow  the  acidic  rather  than  the 
basic  constituents.  The  residue,  in  granitic  rocks,  is  a  solution 
rich  in  alkalies  and  silica,  probably  with  water,  which  under 
certain  circumstances  may  be  a.eutectic  and  may  be  pressed 
out  of  the  partly  consolidated  magmas  as  if  from  a  sponge  and 
crystallize  as  pegmatites  in  fissures  held  open  by  the  hydrostatic 
pressure  of  the  fluids. 

The  order  of  crystallization  of  substances  in  a  magma  prob- 
ably depends  upon  their  relative  abundance  and  upon  their 
solubility  in  the  eutectic. 

Near  the  surface  the  order  of  crystallization  is  not  entirely 
like  that  just  outlined;  there  are  usually  two  generations  of 
crystals,  and  sometimes  an  older  generation,  of  hornblende, 
for  instance,  may  be  resorbed  and  almost  obliterated.  In 
rock-forming  minerals  the  volume  of  the  crystallized  substances 
is  smaller  than  that  of  the  corresponding  fluid  substance;  their 
fusibility  and  also  their  solubility  diminish  with  increasing 
pressure.  A  sudden  release  of  pressure  may  then  act  as  an 
increase  of  temperature  and  newly  formed  crystals  may  be 
remelted. 

Much  time  has  been  given  of  late  to  the  study  of  eutectic 
mixtures  in  rock  magmas,  especially  by  J.  H.  L.  Vogt,1  of  Kris- 
tiania.  In  comparatively  few  magmas,  however,  does  the 
residual  part  closely  approach  well-defined  eutectic  composition. 

Melts  of  certain  proportions  of  miscible  salts  will  solidify  to- 
gether at  a  temperature  lower  than  the  point  of  congealing  of 
each  constituent.  These  are  called  eutectic  mixtures,  and  their 

1  J.  H.  L.  Vogt,  Die  Silikatschmelzlosungen,  Kristiania,  1903  and  1904. 


784  MINERAL  DEPOSITS 

minimum  temperature,  with  its  definite  corresponding  propor- 
tions, is  called  the  eutectic  point.  The  salts  must  be  miscible; 
if  not,  they  separate  in  layers.  The  salts  must  not  act  chemically 
upon  one  another,  for  if  they  do  new  compounds  are  formed. 
Finally,  the  salts  must  not  be  isomorphous,  for  then  no  eutectic 
point  is  possible;  albite  and  anorthite,  for  example,  crystallize 
together  in  all  proportions  and  the  melting  points  of  the  mixed 
crystals  form  a  series  with  no  eutectic  depression. 

The  assumption  of  free  miscibility  is  probably  subject  to  some 
exceptions.  Vogt,  for  instance,  has  brought  evidence  to  show 
that  sulphides  are  more  soluble  in  basic  than  in  acidic  magmas 
and  that  the  solubility  increases  at  higher  temperatures.  This 
is,  then,  probably  a  case  of  limited  miscibility,  and  Barker 
believes  that  the  same  may  be  true  of  alumina  in  the  case  of  the 
association  of  corundum  with  peridotite  magmas,  and  of  the 
spinel  minerals  (like  chromite)  and  the  silicates. 

Differentiation  in  Magmas 

Differentiation,  according  to  Iddings,  means  the  separation 
of  a  homogeneous  rock  magma  into  chemically  unlike  portions. 
Modern  views,  based  on  field  work  and  petrologic  studies,  include 
the  belief  that  for  each  region,  in  each  separate  "magma  basin," 
there  is  one  essentially  homogeneous  magma  from  which  by 
some  process  of  differentiation  the  various  rock  types  have  been 
derived.  In  general  it  is  thought  that  the  primary  magma  was 
of  intermediate  composition  and  has  been  separated  into  basic 
and  acidic  forms,  like  basalts,  latites,  and  rhyolites. 

Lagorio,1  in  1887,  began  the  investigations  on  differentiation  in 
his  memoir  on  the  nature  of  the  glass-base  or  groundmass  by 
calling  attention  to  "Soret's  principle,"  which  states  that  when 
two  parts  of  a  solution  are  at  different  temperatures,  the  dissolved 
substance  will  be  concentrated  in  the  cooler  portion. 

This  unequal  cooling,  it  was  thought,  produced  the  hetero- 
geneity in  an  originally  homogeneous  magma.  The  substances 
with  which  the  magma  was  most  nearly  saturated  tended  to 
accumulate  at  the  cooler  points,  leaving  the  warmer  portions 
with  an  excess  of  the  solvent  material.  There  are  many  objec- 
tions to  this  view.  G.  F.  Becker  showed  that  molecular  diffu- 
sion would  in  a  viscous  magma  require  almost  unlimited  time. 
H.  Backstrom  has  pointed  out  that  although  the  action  assumed 

1  Tsch.  Min.  u.  pet.  Mitt.,  vol.  8,  1887,  p.  421. 


CONCENTRATION  IN  MOLTEN  MAGMAS       785 

by  Soret's  principle  might  cause  changes  in  the  absolute  con- 
centration, it  would  be  powerless  to  alter  the  relative  proportions 
of  the  dissolved  substances.  Absorption  and  assimilation  of 
the  substances  contained  in  the  surrounding  rocks  might  alter 
the  composition  of  the  magma,  and  sometimes  this  undoubtedly 
takes  place,  although  most  intrusive  contacts  show  little  evidence 
of  such  assimilation.  But  such  absorption  would  not,  for 
instance,  account  for  the  occurrence  of  separated  portions  of 
titanic  iron  ore. 

"  Gravitative  adjustment,"  advocated  by  J.  Morozewicz 
and  R.  A.  Daly,  may  play  a  considerable  part  in  differentiation. 
According  to  this  theory  a  great  mass  of  magma,  like  a  high 
column  of  salt  solution,  would  separate  into  a  denser  substratum 
and  a  lighter  upper  part.  The  presence  of  mineralizing  agents 
is  also  a  factor  of  importance.1  Certain  constituents  of  the 
magma  are  more  soluble  in  them  than  others  and  thus  a  magma 
rich  in  silica  and  alkali,  containing  many  rarer  metals,  may  have 
accumulated  at  the  upper  levels  of  a  magma  basin,  while  the 
basic  portion  of  the  magma  remained  below.  G.  F.  Becker 
has  indicated  the  possible  importance  of  fractional  crystalliza- 
tion, thus  regarding  the  differentiation  as  a  consequence  of  the 
general  cooling  process.  Along  the  cooler  walls  the  difficultly 
fusible  minerals  will  separate  first,  and  the  process  is  aided  by 
convection  currents.  The  last  portion  of  the  fused  mass  to 
solidify  will  be  the  portion  with  lowest  temperature  of  fusion 
and  will  therefore  approximate  a  eutectic  mixture.  Along  the 
walls  of  a  dike  basic  minerals  and  iron  ores  may  thus  solidify, 
while  the  center  will  have  a  different  composition.  In  the  case 
of  titanic  iron  ores  the  ilmenite  probably  crystallized  first,  and 
settled  to  the  bottom. 

If  the  component  parts  of  a  slowly  cooling  magma  are  not 
miscible,  a  liquation  will  take  place  and  the  heavier  parts,  such  as 
the  molten  sulphides,  will  settle  to  the  bottom. 

As  pointed  out  by  L.  V.  Pirsson,1  the  phenomena  accompany- 
ing the  eruption  or  intrusion  of  a  magma  are  extremely  complex, 
and  no  fully  satisfactory  explanation  can  be  given  of  the  process 
of  differentiation.  Liquation,  influence  of  mineralizers,  as- 
similation of  wall-rocks,  and  pressure  during  consolidation  are 

1  C.  H.  Smyth,  Jr.,  The  chemical  composition  of  the  alkaline  rocks,  etc., 
Am,  Jour.  Sci.,  4th  ser.,  vol.  36,  1913,  pp.  33-46. 

2  Bull.  237,  U.  S.  Geol.  Survey,  1905£p.  196. 


786  MINERAL  DEPOSITS 

undoubtedly  all  of  importance,  but  the  most  general  cause  of 
differentiation  is  probably  fractional  crystallization.  The  more 
closely  the  composition  of  a  magma  approaches  eutectic  ratios 
the  less  capable  of  fractionation  it  becomes.  That  crystals  sink 
or  float  in  melts  and  even  in  those  of  considerable  acidity  has  been 
proved  experimentally  by  Bowen.1  He  also  concludes  that  crys- 
tallization controls  differentiation  of  the  sub-alkaline  series  of  ig- 
neous rocks.  Perhaps  this  is  going  too  far  for  it  seems  probable 
that  in  deep  magma  basins  there  must  have  been  some  differen- 
tiation before  the  crystals  began  to  separate  out.  The  fact  that 
magnetite  and  ilmenite  sometimes  form  dikes  suggests  differ- 
entiation before  consolidation. 

PRINCIPAL  TYPES  OF  DEPOSITS 

Among  the  valuable  minerals  formed  during  the  consolidation 
of  magmas  are  diamond,  platinum,  chromite,  ilmenite,  magnetite, 
corundum,  cassiterite,  pyrrhotite,  pentlandite,  pyrite,  chal- 
copyrite,  molybdenite  sperrylite,  and  apatite.  A  much  more 
complex  series  of  minerals  is  contained  in  the  pegmatite  dikes, 
which  are  described  separately.  For  each  kind  of  rock  certain 
minerals  are  characteristic  and  most  of  the  rocks  are  of  the 
deep-seated  type,  crystallizing  with  granular  structure. 

Diamonds,  chromite,  platinum,  and  sometimes  corundum  are 
associated  with  peridotites,  corundum  also  with  certain  nephe- 
line  syenites.  Chalcopyrite,'  pyrite,  pentlandite,  and  pyrrhotite 
follow  the  basic  rocks,  especially  gabbros.  Apatite  and  magnet- 
ite are  connected  with  alkali-rich  syenites;  ilmenite  and  titan- 
iferous  magnetites  with  anorthosites  (labradorite  rocks)  and 
gabbros;  cassiterite  with  granite. 

DIAMONDS2 

Diamond  is  pure  carbon,  crystallizing  in  the  isometric  system. 
It  is  the  hardest  of  all  minerals  and  has  a  specific  gravity  of 
3.52.  Usually  it  is  white  or  yellowish,  but  other  pale  colors  are 

»N.  L.  Bowen,  Am.  Jour.  Sci.,  4th  ser.,  vol.  39,  1915,  pp.  175-191; 
Idem.,  vol.  40,  1915,  pp.  161-185. 

N.  L.  Bowen,  The  later  stages  of  the  evolution  of  igneous  rocks,  Jour. 
Geology,  vol.  23,  1915,  supplement,  p.  91. 

2  Gardner  F.  Williams,  The  genesis  of  the  diamond,  Trans.,  Am.  Inst. 
Min.  Eng.,  vol.  35,  1905,  p.  440. 

The  diamond  mines  of  South  Africa,  2  vols.  New  York,  1905. 


CONCENTRATION  'IN  MOLTEN  MAGMAS       787 

not  uncommon.  Rounded  forms  with  aggregate  structure  are 
called  bort,  while  the  dark  gray  or  black  carbonado  is  granular 
and  without  visible  cleavage.  The  last  two  varieties,  mainly 
found  in  placers  in  Brazil,  are  used  for  drilling,  set  in  steel. 
Diamond  powder  is  used  as  an  abrasive. 

Until  1871,  diamonds  were  obtained  only  from  placers.  They 
occurred  thus  in  the  Deccan  mines  in  India,  where  the  parent 
rocks  may  be  pegmatite  dikes,  or  perhaps  serpentines.  In  Brazil, 
in  the  province  of  Minas  Geraes,  they  occur  in  sands  or  gravels 
derived  from  conglomerates.  Orville  A.  Derby  says  that  they 
have  no  known  relation  to  peridotites. 

In  the  gold  belt  of  California  many  small  diamonds  have  been 
found — for  instance,  at  Placerville  and  Cherokee — very  probably 
derived  from  the  extensive  masses  of  serpentine  occurring  in  the 
Sierra  Nevada.  Scattered  diamonds  have  been  found  in  the 
northern  drift  area  in  Indiana  and  Ohio.  Along  the  Vaal  River 
in  South  Africa  fine  stones  are  found  which,  according  to  some 
authors,  cannot  have  been  derived  from  the  peridotites  and 
serpentines.  Some  have  thought  that  their  original  home  was 
in  the  diabases  or  pegmatites  of  that  region. 

The  only  American  occurrence  of  note  is  near  Murfreesboro, 
Pike  County,  Arkansas,1  where  stones  of  good  quality  are  found 
in  decomposed  peridotite.  Up  to  1911  a  total  of  1,260  stones, 
aggregating  574  carats,  had  been  found.  No  later  information 
is  available. 

In  1871  the  Kimberley  diamond  field  in  South  Africa  was  dis- 
covered, the  first  known  occurrence  of  diamonds  in  primary  rock. 
The  district  lies  in  the  northern  part  of  Cape  Colony  and  the 
adjacent  part  of  the  Orange  Colony.  Another  district  centers 
at  Jagerfontein,  in  the  Orange  Colony;  still  another  at  the  Pre- 
mier mine,  near  Pretoria  in  the  Transvaal. 

The  diamonds  in  the  Kimberley  field  are  disseminated  in 

R.  A.  F.  Penrose,  The  Premier  diamond  mine,  Transvaal,  S.  A.,  Econ. 
Geol,  vol.  2,  1907,  pp.  275-284. 

D.  B.  Sterrett  and  W.  Schaller,  Gems  and  Precious  Stones,  Mineral 
Resources,  U.  S.  Geol.  Survey  annual  publication. 

See  also  Stelzner  and  Bergeat,  Die  Erzlagerstatten,  and  F.  W.  Clarke, 
Geochemistry,  Bull.  616,  U.  S.  Geol  Survey,  1916,  pp.  322-326. 

1  G.  F.  Kunz  and  H.  S.  Washington,  Diamonds  in  Arkansas,  Trans.,  Am. 
Inst.  Min.  Eng.,  vol.  39,  1908,  pp.  169-176. 

D.  B.  Sterrett,  "Gems,"  Mineral  Resources,  U.  S.  Geol.  Survey,  pt.  2, 
1915,  pp.  843-858. 


788  MINERAL  DEPOSITS 

volcanic  necks,  commonly  called  "  pipes,"  of  "kimberlite," 
a  serpentine  derived  from  peridotite  and  containing  a  little 
garnet  and  biotite.  This  rock  breaks  through  the  horizontal 
quartzitic  sandstones,  volcanic  flows,  and  shales  of  the  Karroo 
formation  (Carboniferous  to  Triassic) .  The  Kimberley  pipe  has 
been  worked  down  to  a  depth  of  about  2,000  feet,  its  diameter 
being  about  500  feet.  Near  the  surface  the  serpentine  was 
decomposed,  forming  the  so-called  "yellow  ground,"  but  at 
greater  depth  the  not  yet  oxidized  "blue  ground"  was  met. 
The  latter  is  now  mined  exclusively  and  the  tough  rock  is  allowed 
to  slack  on  the  surface  for  many  months  before  it  can  be  washed. 
The  washing  is  effected  on  greased  tables,  the  grease  having 
the  property  of  holding  the  diamonds  while  the  other  constituents 
of  the  rock  are  washed  over  it.  The  blue  ground  is  a  rock  of  dull, 
greasy  appearance  consisting  chiefly  of  serpentine  with  abundant 
secondary  carbonates  and  partly  altered  remnants  of  olivine 
enstatite,  biotite,  vaalite  (a  brown  mica),  garnet,  diopside,  chrom- 
ite,  ilmenite,  diamonds,  zircon,  sapphire,  kyanite,  rutile,  pero- 
fskite,  apatite,  and  tourmaline.  Apophyllite,  chlorite,  and 
calcite  are  secondary  minerals.  The  blue  ground  is  distinctly 
breccia  ted  and  many  of  the  fragments  of  harder  undecomposed 
rocks  still  remaining  are  rounded.  The  pipes  are  probably  to  be 
regarded  as  explosion  vents  and  the  rock  filling  them  is  undoubt- 
edly of  igneous  origin.  As  to  the  diamonds,  they  are  often  crys- 
tallized as  octahedrons,  with  convex  or  concave  faces,  but  most 
of  the  crystals  as  recovered  are  broken.  The  color  is  white, 
yellowish,  greenish,  lilac,  and  even  deep  yellow.  Small  rounded 
masses  with  concentric  growth,  have  also  been  noted  and  many 
dark-gray  semi-transparent  pieces  are  found. 

The  blue  ground  contains  fragments  of  carbonaceous  shale 
and  Carvil  Lewis  thought  that  this  rock  might  have  furnished 
the  material  for  the  diamonds.  According  to  later  investigations 
this  mode  of  origin  is  improbable.  Stelzner,  Bonney,  and  others 
have  shown  that  the  gem  crystallized  as  an  integral  part  of 
the  magma.  Stelzner  mentions  intergrowths  of  pyrope  garnet 
and  diamonds,  and  in  the  last  few  years  Gardner  F.  Williams 
has  collected  specimens  from  Kimberley  which  show  crystallized 
diamonds  still  partly  enclosed  by  garnet  and  ferromagnesian 
minerals.  Triibenbach  and  Bonney  have  also  found  that  dia- 
monds actually  occur  in  fresh  eclogite,  a  garnet-pyroxene  rock 
closely  allied  to  peridotite,  from  the  Newlands  mine,  about  40 


CONCENTRATION  IN  MOLTEN  MAGMAS       789 

miles  west  of  Kiinberley.  T.  W.  E.  David  recently  described  a 
K-carat  diamond  from  New  South  Wales,  embedded  in  a  solid 
matrix  of  hornblende  diabase. 

The  production  at  Kimberley  by  the  De  Beers  Company  is 
from  1,500,000  to  3,000,000  carats  per  annum;  the  average 
price  for  the  rough  stone  ranges  from  $7  to  $12  per  carat.  The 
best  ground  is  said  to  average  about  1  metric  carat  (200  milli- 
grams) per  ton. 

The  deposit  at  the  Premier  mine  is  a  pipe  half  a  mile  by  a 
quarter  of  a  mile  in  horizontal  section  penetrating  the  older 
Pretoria  series  of  sediments.  About  10,000,000  tons  of  material 
is  treated  annually,  yielding  about  2,000,000  carats.  In  this 
mine  the  largest  diamond  ever  found  was  obtained;  it  is  known 
as  the  Cullinan  diamond  and  formed  a  broken  octahedron  4 
inches  long  and  2  inches  wide;  its  weight  was  3,024%  carats, 
measuring  2  by  4  inches. 

Much  of  the  product  from  the  South  African  mines  is  sold  in 
the  United  States,  the  imports  having  a  value  of  thirty  to  forty 
million  dollars  a  year.  Good  cut  stones  sell  at  from  $150  to  $200 
per  carat. 

Carbon  is  soluble  in  molten  magmas  and  can  crystallize  from 
them.  Small  diamonds  have  been  artificially  produced  in  several 
ways,  well  summarized  by  F.  W.  Clarke.  They  have  been 
obtained  by  dissolving  carbon  in  molten  iron,  fused  olivine, 
and  other  lime-magnesia  magmas.  The  discovery  of  small 
diamonds  in  meteorites  of  iron  or  peridotitic  rock  is  another 
fact  clearly  pointing  to  a  magmatic  origin  of  this  mineral.  Lately 
it  has  been  shown  by  R.  A.  A.  Johnson1  that  chromite,  a  mineral 
of  the  peridotite  rocks,  contains  microscopic  diamonds. 

OTHER  PRECIOUS  STONES 

Other 'precious  stones  contained  in  igneous  rocks  are  sapphire 
(p.  807),  garnet,  and  peridot. 

Pyrope  (magnesium-aluminum  garnet)  of  the  beautiful  deep- 
red  color  which  is  necessary  for  gem  quality  is  usually  found 
in  basic  rocks  of  igneous  origin.  The  garnets  of  Bohemia, 
obtained  in  washing  a  Cretaceous  conglomerate,  are  probably 
derived  from  a  serpentine.  The  diamond-bearing  serpentine 
of  South  Africa  contains  pyrope  of  gem  quality,  called  Cape 

1  Mem.  22,  Geol.  Survey  Canada,  1913,  p.  83. 


790  MINERAL  DEPOSITS 

ruby.  Almandite  (iron-aluminum  garnet)  is  not  so  extensively 
used.  It  occurs  in  granite  and  aplite,  and  also,  as  a  product  of 
metamorphic  action,  in  crystalline  schists.  In  the  Navajo 
Reservation,  Arizona,  pyrope  and  peridot  (yellowish-green 
olivine)  are  obtained  as  disintegration  products  of  a  basaltic  rock. 

PLATINUM  AND  PALLADIUM1 

Nearly  all  the  platinum  of  the  world  is  derived  from  placers, 
mainly  in  the  Ural  Mountains  in  Russia,  though  smaller  quan- 
tities come  from  Colombia,  California,  and  New  South  Wales 
(p.  242). 

Native  platinum  occurs  as  an  alloy  with  other?  of  the 
platinum  group — osmium,  iridium,  palladium,  ruthenijm  and 
rhodium.  Native  iridium,  iridosmine,  and  other  alloys  are  found 
with  it. 

An  analysis  of  the  crude  platinum  sand  of  California  by  Deville 
and  Debray  showed  the  following  percentages:  Platinum, 
85.50;  iridium,  1.05;  palladium,  0.60;  rhodium,  1.00;  gold,  0.80; 
copper,  1.40;  iron,  6.75;  iridosmine,  1.10;  sand,  2.95. 

The  platinum  in  the  placers  forms  small  rounded,  also  concre- 
tionary and  knobby  dark-gray  pieces.  Bright  silvery  scales  of 
iridosmine  occurs  with  it.  In  the  Ural  large  pieces  of  platinum 
have  been  found,  the  largest  weighing  about  26  pounds. 

Platinum  has  been  found  in  primary  deposits,  but  few  of  them 
are  of  economic  importance.  The  modes  of  occurrence  of  plat- 
inum are  as  follows:  1.  In  placers;  2.  Disseminated  in  peridotite 
and  olivine  gabbro,  associated  with  chromite;  3.  In  magmatic 
deposits  in  basic  rocks,  associated  with  chalcopyrite  and  pyrrho- 
tite  (with  palladium);  4.  In  small  quantities  in  quartz  veins; 
5.  In  contact  metamorphic  deposits;  6.  In  traces  in  copper  de- 
posits of  many  kinds  (with  palladium);  7.  Concentrated  by 

1  J.  F.  Kemp,  Bull.  193,  U.  S.  Geol.  Survey,  1902. 

C.  W.  Purington,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  29,  1899,  p.  3. 
R.  Beck,  Lehre  von  den  Erzlagerstatten,  1909,  pp.  22-25. 

D.  T.  Day,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  30,  1900,  p.  702. 

F.  W.  Clarke,  Geochemistry,  Bull.  616,  TJ.  S.  Geol.  Survey,  1916,  pp. 
700-704. 

C.  W.  Dickson,  Jour.,  Canadian  Min.  Inst.,  vol.  8,  1905,  p.  192. 

D.  T.  Day,  W.  Lindgren  and  B.  M.  Hill,  Platinum,  Mineral  Resources, 
U.  S.  Geol.  Survey,  1900-1917. 

G.  F.  Kunz,  Platinum,  Bull.  Pan  American  Union,  Nov.,  1917. 


CONCENTRATION  IN  MOLTEN  MAGMAS       791 

processes  of  oxidation  in  replacement  ores  of  copper  and  gold  in 
limestone  (with  palladium). 

Daubree  showed,  in  1875,  that  the  Russian  platinum  is  inter- 
grown  with  olivine,  pyroxene,  and  serpentine.  Beck,  on  author- 
ity of  S.  Conradi,  reports  it  in  dunite  rocks  in  Solowioff  Mountain, 
in  the  Ural  Mountains,  and  states  that  the  metal  forms  zonar 
crystals  of  unquestionable  magmatic  origin  lying  between  grains 
of  chromite.  Kemp  has  found  platinum  in  dunite  from  the 
Tulameen  River,  British  Columbia.  The  serpentines  of  this 
region  also  yield  traces  of  platinum.  The  platinum  of  California 
is  found  only  in  placers,  but  the  metal  is  believed  to  be  derived 
from  the  serpentine  areas  so  common  in  the  Sierra  Nevada. 

In  the  nickel  deposits  at  Sudbury,  Ontario,  which  are  con- 
sidered of  magmatic  origin,  platinum  arsenide,  sperrylite,  prob- 
ably accompanied  by  palladium?arsenide,  is  formed  as  small 
silvery- white  cubes  intergrown  with  pyrrhotite  and  chalcopyrite. 
The  same  mineral  was  discovered  at  the  Ramblerjnine,1. Wyoming, 
in  copper  ores,  mainly  chalcopyrite  and  covellite,  probably 
of  igneous  origin  and  forming  a  lens  in  a  dioritic  rock.  Much 
palladium  is  also  present. 

Another  rare  mode  of  occurrence  of  platinum  is  in  quartz 
veins,  described  by  Bell,2  from  the  southern  island  of  New 
Zealand,  from  northern  Finland,  and  from  Canada. 

A  peculiar  occurrence  of  platinum  with  wollastonite  and 
grossularite  in  a  contact-metamorphic  rock  has  been  reported 
from  Sumatra.3 

Palladium  occasionally  occurs  alloyed  with  gold.  E.  Hussak4 
found  in  Brazil  such  palladium  gold  in  a  limestone  close  to  the 
contact  of  an  igneous  rock.  Platinum  and  palladium  are  also 
recovered  in  the  electrolytic  refining  of  copper  bullion. 

A  gold-platinum-palladium  deposit  concentrated  by  processes 
of  oxidation  has  recently  been  described  by  Adolph  Knopf.5 

The  ore  occurs  in  a  lead-copper-gold  replacement  deposit 
in  limestone  in  the  Yellow  Pine  district,  southern  Nevada. 
The  material,  which  contains  the  precious  metals,  is  plumbo- 
jarosite,  a  sulphate  of  iron,  lead  and  bismuth,  and  occurs  in  im- 

1  S.  F.  Emmons,  Butt.  213,  U.  S.  Geol.  Survey,  1903,  pp.  94-97. 

2  J.  M.  Bell,  Econ.  Geol.,  vol.  1,  1906,  p.  749. 

3  L.  Hundeshagen,  Trans.,  Inst.  Min.  and  Met.  vol.  13,  1903-4. 

«  Sitz.-Ber.  Akad.  Wiss.  Wien.  vol.  113,  No.  1,  July,  1904,  pp.  1-88. 
5  Bull  620,  U.  S.  Geol.  Survey,  1915,  pp.  1-44. 


792  MINERAL  DEPOSITS 

portant  amounts.  It  may  average  in  ounces  to  the  ton:  Gold, 
3.46;  silver,  6.4;  platinum,  0.70;  palladium,  3.38.  All  metals  are 
in  minute  division.  The  gold  is  rough,  black  and  spongy,  the 
palladium-platinum  appears  as  microscopic  black  grains.  The 
platinum  metals  were  probably  contained  in  the  primary  sul- 
phides and  most  likely  have  been  concentrated  by  solutions  in 
which  they  were  present  in  colloid  suspension. 

Production  and  Use. — At  best  the  world's  annual  output  of 
platinum,  mainly  from  the  Ural  Mountains,  is  400,000  troy 
ounces.  War  conditions  have  decreased  the  Ural  production, 
while  Colombia  has  attained  20,000  ounces.  The  output  of 
platinum  metals  from  all  domestic  sources  is  now  (1917)  about 
7,400  ounces  of  which  less  than  1,000  ounces  is  from  Pacific 
Coast  placers;  the  rest  comes  from  electrolytic  copper  refineries 
and  oxidized  ores;  about  one-half  of  this  consists  of  palladium. 
The  price  of  platinum  per  troy  ounce  has  risen  from  $20  per 
ounce  or  less  to  the  present  fixed  price  of  $105;  palladium  to 
$135,  and  iridium  obtained  in  refining  crude  platinum  or  by 
picking  out  iridium  grains  from  the  concentrate  to  $175. 

Platinum  is  used  for  jewelry,  in  dentistry,  for  chemical  utensils, 
for  spark  devices  and  in  the  contact  process  for  making  sul- 
phuric acid.  Attempts  are  now  made  to  save  platinum  by  using 
alloys  of  tungsten,  molybdenum,  chrome-nickel  and  palladium. 

Palladium,  a  silvery  white  ductile  metal,  but  soluble  in  HNOs, 
is  used  for  silvering  circles  on  surveying  instruments,  and  other 
electroplating;  also  in  dentistry  and  in  alloy  with  gold  as  sub- 
stitute for  platinum. 

Iridium,  exceedingly  hard  and  resistant,  is  used  for  hardening 
platinum;  5  to  20  per  cent,  are  added.  It  is  also  employed  for 
chemical  and  physical  instruments,  contact  devices,  etc. 

Rhodium  finds  use  with  platinum  for  thermo  couples.  There 
is  little  demand  for  it. 

Osmium  is  available  in  considerable  quantities  from  the 
refining  of  platinum  sand.  Formerly  it  was  used  in  incandescent 
lamps. 

IRON  AND  NICKEL 

Native  iron  occurs  sparingly  in  some  basalts.  Large  masses 
have  been  found  in  a  basalt  at  Ovifak,  west  Greenland,  where 
the  natives  used  the  metal  for  their  tools  and. weapons.  The 
iron  contains  from  2  to  3  per  cent,  of  nickel  and  3  per  cent,  of 


CONCENTRATION  IN  MOLTEN  MAGMAS       793 

carbon  and  was  long  thought  of  meteoric  origin.  The  basalt 
breaks  through  Tertiary  beds  containing  coal,  and  it  is  believed 
by  some  that  the  metal  was  reduced  from  the  rock  by  means  of 
the  coal.  The  nickel  it  contains  militates  against  this  view; 
more  likely  it  was  carried  up  from  some  deep-seated  source  by 
the  basalt. 

Awaruite  (FeNi2)  is  disseminated  in  gravels  and  also  as  small 
grains  in  the  serpentine  and  peridotite  of  Red  Mountain,  on  the 
south  island  of  New  Zealand,  and  is  found  also  in  sluice  boxes 
for  gold  washing  at  Hoole  Canyon,  Yukon  Territory.1  A  simi- 
lar mineral,  josephinite  (FeNi6),  has  been  found  in  detritus  in 
areas  of  serpentine  in  southwestern  Oregon  and  a  few  other 
localities. 

CHROMITE' 

Chromite  (FeO.Cr2O3),3  a  mineral  of  the  spinel  group  and 
usually  admixed  with  other  spinel  molecules,  is  an  almost 
constant  accessory  of  peridotites  and  of  the  serpentines  derived 
from  them  and  is  often  found  in  them,  as  accumulations  large 
enough  to  be  mined.  The  ore,  more  or  less  mixed  with  the  rock, 
forms  irregular  bunches  and  masses  along  the  contacts  or  in  the 
interior  of  the  intrusive  masses;  frequently  also  it  forms  ill- 
defined  streaks  or  "schlieren."  In  part  the  ore  may  have  a 
secondary  origin,  being  developed  together'  with  magnetite 
during  the  process  of  serpentinization  from  primary  chromite, 
picotite,  chromium-diopside,  etc.  Late  investigations,  particu- 
larly those  of  Vogt,  have  shown  that  chromite  in  large  masses 

1  J.  Keele,  Summ.  Rept.,  Geol.  Survey  Canada,  1910,  p.  257. 

2  J.  H.  L.  Vogt,  Zeitschr.  prakt.  Geol,  1894,  pp.  384-393. 

E.  Glasser,  Les  richesses  minerales  de  la  Nouvelle  Cale"donie,  Ann.  des 
Mines  (10),  vol.  4,  1903,  pp.  299-536. 

J.  H.  Pratt,  The  occurrence,  etc.,  of  chromite,  Trans.,  Am.  Inst.  Min. 
Eng.,  vol.  29,  1899,  pp.  17-39. 

W.  Glenn,  The  chromites  of  Maryland,  Trans.,  Am.  Inst.  Min.  Eng.,  vol. 
25,  1896,  p.  481. 

E.  C.  Harder,  Some  chromite  deposits  in  western  and  central  California, 
Bull.  430,  U.  S.  Geol.  Survey,  1910,  pp.  167-183. 

J.  S.  Diller,  "Chromite,"  Mineral  Resources,  U.  S.  Geol.  Survey,  Annual 
publication. 

3  Theoretically,  chromite  should  contain  68  per  cent.  Cr2O3  and  32  per  cent. 
FeO,  but  A12O3  and  MgO  are  always  present  and  the  actual  content  of 
Cr2Q3  is  rarely  more  than  60  per  cent.     It  is  one  of  the  most  difficultly  fusible 
of  minerals,  melting  at  1,850°  C.  (A.  Brun). 


794  MINERAL  DEPOSITS 

mainly  represents  purely  magmatic  separations  in  peridotite 
magmas.  Vogt  showed  that  the  succession  in  the  Norwegian 
deposits  was  chromite,  olivine,  and  soda-lime  feldspar,  and  in 
all  cases  the  chromite  appears  to  be  the  earliest  consolidated  con- 
stituent. Deposits  in  serpentine  are  often  admixed  with  magne- 
tite. It  is  noteworthy  that  during  the  weathering  of  peridotite 
few  chromium  silicates  are  formed,  while  nickel  silicates  often 
develop.  A  little  chromiferous  muscovite  (mariposite),  also 
ouvarovite  or  chrome  garnet,  as  well  as  chloritic  chromium  miner- 
als, in  places  accompany  the  chromite. 

Copper  minerals,  especially  chalcopyrite,  are  occasionally 
found  with  the  chromite.  The  reddish  niccolite  (NiAs)  has 
been  found  in  serpentines  and  peridotites  at  Malaga^  Spain.1 
The  mineral  is  later  than  the  chromite  and  according  to  R.  Beck2 
cements  crystals  of  augite. 

Chromite  ores  should  contain  45  to  50  per  cent.  Cr20s.  De- 
posits occur  in  many  countries.  An  important  occurrence  has 
been  found  lately  at  Selukwe,  Rhodesia,  from  which  in  1910 
44,000  tons  of  ore  were  produced.  Until  lately  the  largest 
supplies  were  received  from  Asia  Minor,  near  Antiochia,  Smyrna, 
and  Brussa.  These  deposits  are  of  unusual  size.  The  Dag- 
hardy  mine,3  near  Brussa,  yielded  annually  12,000  to  15,000 
metric  tons  of  ore.  One  of  the  masses  from  this  mine  was  70 
meters  long,  25  meters  wide,  and  20  meters  deep.  Much  chro- 
mite is  also  exported  from  New  Caledonia,  where  the  ore  occurs 
in  part  as  residual  masses  which  are  concentrated,  in  part  as 
"vein-like  segregations"  in  serpentine.  Smaller  masses  have 
been  mined  near  Baltimore,  Maryland,  in  North  Carolina,  in 
California,  and  in  Oregon.  Deposits  of  chromite  in  the  serpen- 
tine areas  of  Quebec  are  now  worked  by  concentration.  Chro- 
mium is  extensively  used  as  a  steel-hardening  metal  and  also 
for  the  preparation  of  various  salts,  among  which  the  bichro- 
mate of  potassium  is  most  important.  Under  war  conditions 
prices  for  ore  containing  50  per  cent.  Cr203  have  risen  to  $80 
per  ton  but  even  at  that  domestic  sources  have  only  contributed 

1  F.  Oilman,  Notes  on  the  ore  deposits  of  the  Malaga  serpentines,  Trans., 
Inst.  Min.  and  Met.  (London),  1896,  pp.  159-165. 

2  Erzlagerstatten,  1,  1909,  p.  89. 

3  R.  E.  Weiss,  Zeitschr.  prakt.  Geol,  1901,  pp.  249-262. 

W.  F.  A.  Thomae,  Emery,  chrome  ore,  etc.,  in  Asia  Minor,  Trans., 
Am.  Inst.  Min.  Eng.,  vol.  28,  1899,  pp.  208-225. 


CONCENTRATION  IN  MOLTEN  MAGMAS       795 

40,000  tons  (1917)  whereas  in  1916,  116,000  long  tons,  besides 
much  chromate,  was  imported. 


ILMENITE  OR  TITANIC  IRON  ORE 

General  Features. — At  many  places  in  the  world  large  masses 
of  ilmenite  (FeTi03,  containing  oxygen  31.6,  titanium  31.6, 
iron  36.8),  are  found  associated  with  more  or  less  magnetite, 
olivine,  pyroxene,  and  soda-lime  feldspars.  Petrographic  re- 
search has  long  ago  shown  that  ilmenite,  with  magnetite,  is  one 
of  the  earlier  products  of  consolidation  in  magmas  and  is  con- 
tained in  almost  all  diabases,  basalts,  and  gabbros;  it  occurs 


.A  B 

FIG.  262A. — Photomicrograph  of  polished  section  showing  intergrowth  of 

hematite    (light);    ilmenite    (dark);  St.   Urbain,   Quebec.     Magnified  100 

diameters. 
FIG.  2625. — Intergrowth  of  magnetite  (dark) ;  ilmenite  (light) ;  a  grain  of 

olivine  in  upper  right  corner,  Cumberland,  Rhode  Island.     Magnified  180 

diameters.    Both  after  C.  H.  Warren. 

also  in  other  less  basic  rocks,  but  the  real  home  of  ilmenite  is  in 
the  rocks  poor  in  silica.  The  larger  masses  of  ilmenite  are  simply 
facies  of  the  rock  itself  produced  by  concentration  from  the  same 
magma.  Near  such  masses  the  dark  constituents  first  increase; 
finally  the  feldspar  disappears  and  the  ore-body  consists  of  a 
mixture  of  ilmenite  with  ferromagnesian  silicates.  Vogt1  first 

1  Die  Bildung  der  Erzlagerstatten  durch  Differentiation  in  basischen 
Eruptivmagmata,  Zeitschr.  prakt.  GeoL,  1893,  pp.  4-11,  125-143,  257-284; 
also  in  the  same  journal,  1894,  pp.  381-399;  1900,  pp.  233-242,  370-382; 
1901,  pp.  9-19,  180-186,  289-296,  327-340. 


796  MINERAL  DEPOSITS 

called  attention  to  this  well-defined  group  of  ore  deposits  and  ex- 
plained its  origin. 

The  ilmenite  deposits,  though  large,  have  thus  far  been  little 
utilized  on  account  of  difficulties  in  the  metallurgical  treat- 
ment; but  these  do  not  seem  to  be  insuperable,  and  as  it  has 
recently  been  discovered  that  titanium  confers  valuable  quali- 
ties of  hardening  on  steel  it  may  not  be  long  before  the  ores  will 
become  important  in  metallurgy.  During  the  last  few  years 
experiments  in  their  utilization  have  been  in  progress  in  the 
United  States. 

Microstructure  of  Ilmenite. — The  complex  intergrowths  of 
ilmenite  with  magnetite,  rutile  and  hematite  have  been  described 
lately  by  J.  T.  Singewald1  and  by  C.  H.  Warren.2 

Warren  summarizes  his  results  as  follows:  One  type  repre- 
sented by  occurrences  at  Miask,  Arendal,  and  Snarum  are  homo- 
geneous, though  ilmenites  from  the  latter  two  localities  contain 
a  great  excess  of  Fe20s  compelling  the  belief  that  there  is  a  wide 
range  of  miscibility  between  the  molecules  RTiO3  and  Fe203 
if  these  are  really  present.  A  second  type  presents  an  inter- 
growth  of  grains  of  homogeneous  ilmenite  and  magnetites.  The 
occurrences  at  Lake  Sanford,  Iron  Mountain,  Wyoming  belong 
here.  Many  so-called  titanic  iron  ores  are  magnetic. 

A  third  type  represented  by  St.  Urbain  (Fig.  2624)  and  Eger- 
sund,  Norway,  consists  of  a  well-defined  crystallographic  inter- 
growth  of  ilmenite  an.d  hematite,  possibly  caused  by  the  unmixing 
of  an  originally  homogeneous  solid  solution. 

A  fourth  type  is  illustrated  by  specimens  from  Cumberland, 
Rhode  Island  and  Iron  Mountain,  Wyoming,  showing  an  extraor- 
dinary minute  regularly  oriented  intergrowth  of  magnetite 
and  ilmenite  (Fig.  2625).  Warren  believes  that  there  exists  a 
limited  solid  solution  of  the  ilmenite  and  magnetite  molecules, 
with  a  eutectic;  and  that  ilmenite  and  hematite  form  a  complete 
solid  solution  at  higher  temperatures,  with  an  inversion  inter- 
val and  limited  miscibility  at  lower  temperatures. 

Irregular  Bodies. — The  titanic  iron  ores  form  irregular  masses 
or  rather  sharply  outlined  streaks  in  the  central  parts  of  gabbro 

1  J.  T.  Singewald,  The  microstructure  of  titaniferous  magnetite,  Econ. 
Geol.,  vol.  8,  1913,  pp.  207-214;  also  Bull.  13,  U.  S.  Bureau  of  Mines,  1913. 

2  C.  H.  Warren,  Am.  Jour.  Sci.,  4th  ser.,  vol.  25,  1908,  pp.  12-38.    Am. 
Jour.  .Sd.,  4th  ser.,  vol.  33,   1912,  pp.  263-277.     The  microstructure  of 
certain  titanic  iron  ores,  Econ.  Geol.,  vol.   13,  1918,  pp.  419-446. 


CONCENTRATION  IN  MOLTEN  MAGMAS       797 

or  norite  intrusives.  The  transitions  to  the  country  rock  indi- 
cate that  these  masses  have  been  formed  by  differentiation  in 
the  magma  after  the  irruption  in  its  present  place.  In  these 
differentiated  magmas  ilmenite  and  magnetite  have,  as  a  rule, 
crystallized  after  the  silicates.  Where  pyrite  and  spinel  are 
present  the  order  of  crystallization  is  ferromagnesian  silicates, 
pyrite,  spinel,  ilmenite  (specularite),  and  magnetite.  Probably 
little  water  was  present  and  the  temperature  of  consolidation 
was  high,  perhaps  near  1,450°  C.,  the  fusion  point  of  ilmenite, 
according  to  Brun.  Vogt  has  shown  that  during  the  differentia- 
tion in  a  gabbro  or  norite  magma  a  concentration  of  ferric  oxide 
takes  place,  as  well  as  of  titanium,  chromium,  and  vanadium; 
the  lime,  magnesia,  and  particularly  silica  diminish  greatly,  the 
silica  to  such  an  extent  that  the  alumina  and  magnesia  are 
forced  to  crystallize  as  corundum  and  spinel,  both  of  which  occur 
frequently  in  these  deposits.  Little  sulphur  or  phosphorus  is 
present. 

Dikes. — Separated  by  a  deeper-seated  differentiation,  veri- 
table dikes  of  almost  pure  ilmenite  may  be  injected  into  the  pre- 
vailing rock,  which  then  is  usually  an  anorthosite. 

Occurrences. — Vogt  and  Kolderup  have  described  the  Nor- 
wegian occurrences  in  norite  and  anorthosite  in  the  great  intru- 
sive region  in  Ekersund;  the  largest  body  is  3  kiK  meters  long  and 
from  30  to  70  meters  thick.  Its  composition  is  about  21  per 
cent,  plagioclase,  41  per  cent,  hypersthene,  and  38  per  cent,  il- 
menite. At  Routivare,  in  northern  Sweden,  a  gigantic  mass  of 
titanic  iron  ore  is  included  in  a  mass  of  highly  altered  gabbro, 
intruded  in  Cambro-Silurian  strata.  Some  pyrrhotite  is  asso- 
ciated with  the  ore. 

A  large  deposit  at  St.  Urbain,  in  Quebec,  is  described  by  C. 
H.  Warren.1  Elongated,  sometimes  dike-like  masses  of  ilmenite 
are  inclosed  in  anorthosite.  Specularite  is  intimately  intergrown 
with  ilmenite.  Much  rutile  and  blue  grains  of  sapphirine 
(Mg5Ali2Si2027),  also  andesine,  biotite,  and  spinel,  are  contained  in 
some  of  the  ore.  Other  varieties  in  which  no  rutile  is  present  con- 
tain only  5  to  6  per  cent,  of  other  minerals. 

J.  F.  Kemp2  has  described  the  large  deposits  in  the  Adiron- 
dacks  of  New  York,  near  Elizabethtown  and  Lake  Sanford. 

1  Am.  Jour.  Sri.,  4th  ser.,  vol.  33,  1912,  pp.  263-277. 

2  Titaniferous  iron  ores  of  the  Adirondacks,  Nineteenth  Ann.  Rept.,  U.  S. 
Geol.  Survey,  pt.  3,  1898,  pp.  383-422. 


798  MINERAL  DEPOSITS 

These  ores  are  contained  in  a  gabbro  which  is  intrusive  in  a  large 
' '  massif  "  of  anorthosite.  They  are  granular  mixtures  of  magnetite 
and  ilmenite  with  a  maximum  of  15  per  cent.  Ti02,  and  form 
irregular  or  tabular  masses  presenting  transitions  to  the  country 
rock  or  appearing  as  distinct  dikes.  They  contain  plagioclase, 
pyroxene,  olivine,  hornblende,  garnet,  pyrite,  apatite,  spinel,  and 
quartz,  but  are  low  in  sulphur  and  phosphorus.  At  the  principal 
locality  several  million  tons  of  ore  are  probably  exposed  above  the 
level  of  the  lake. 

Other  deposits  are  known  to  occur  in  Minnesota1  and  Ontario. 

In  eastern  Wyoming,  at  Iron  Mountain,2  a  dike  of  almost  solid 
ilmenite,  in  places  300  feet  wide,  breaks  through  anorthosite  con- 
taining but  little  pyroxene  and  scarcely  any  ilmenite.  This  is  a 
most  remarkable  instance  of  complete  deep-seated  differentiation 
of  magmas.  An  analysis  of  the  ore  is  as  follows: 


SiO2  

.  .     0.76 

MnO  

1.38 

TiO2  

.  .    23.49 

MgO  

1  .  56 

A1203  

.  .     3.98 

CaO  

1.16 

Cr203  

.  .      2.45 

PA  

trace 

Fe,O 

45.03 

s 

1  44 

A  ^2^3  

FeO  

.  .    17.96 

ZnO  

0.47 

99.68 

Total  Fe. 

.  .   4.5  .  49 

Some  olivine,  spinel,  and  magnetite  are  present  as  inclusions 
in  the  ore. 

Concentrations  of  ilmenite  with  prevailing  magnetite  are  not 
uncommon  in  gabbros  and  diabases,  though  rarely  of  economic 
importance. 

Taberg,  in  southern  Sweden,  is  of  interest  as  one  of  the  first 
places  in  which  the  existence  of  magmatic  ore  deposits  was 
demonstrated,  by  A.  Sjogren,  Tornebohm,  and  Igelstrom. 
Taberg  is  a  prominent  hill  400  feet  high,  composed  of  norite. 
Toward  the  center  the  ilmenite  and  magnetite  are  greatly  con- 

1  F.  J.  Pope,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  29,  1900,  pp.  372-405. 

N.  H.  and  H.  V.  Winchell,  The  iron  ores  of  Minnesota,  Geol.  Survey  of 
Minn.,  1891. 

T.  M.  Broderick,  Magnetic  surveys  of  the  magnetite  deposits  of  the 
Duluth  gabbro,  Econ.  Geol,  vol.  13,  1918,  pp.  35-49. 

2  W.  Lindgren,  Science,  new  ser.,  vol.  16,  1902,  pp.  984-985. 
S.  H.  Ball,  Bull.  315,  U.  S.  Geol.  Survey,  1907,  pp.  206-212. 
J.  F.  Kemp,  Zeitschr.  prakt.  Geol,  1905,  p.  71. 


CONCENTRATION  IN  MOLTEN  MAGMAS       799 

centrated  and  form  a  mass  of  ore  with  some  olivinc,  biotite 
and  plagioclase.  This  body  is  1  kilometer  long  and  450  meters 
wide  and  the  material,  although  of  low  grade,  has  at  times  been 
smelted  as  iron  ore;  it  contains  about  6  per  cent,  titanic  dioxide. 
A  similar  but  smaller  deposit  contained  in  gabbro  at  Cumber- 
land, Rhode  Island,  has  been  described  by  B.  L.  Johnson  and 
C.  H.  Warren.1  This  ore  contains  45  per  cent.  FesC^  and  10 
per  cent.  TiO2. 

Influence  of  Pressure. — Kolderup  has  shown  that,  under 
dynamo-metamorphic  influences,  the  ilmenite  in  deposits  of 
this  class  changes  to  rutile,  pyroxene  is  replaced  by  amphibole, 
garnet  is  developed,  and  new  biotite  and  feldspar  aggregates 
are  formed.2 

MAGNETITE 

As  iron  is  contained  in  some  basic  igneous  rocks  in  great 
quantities  and  as  even  the  average  of  this  metal  in  all  igneous 
rocks,  according  to  F.  W.  Clarke,  is  4.46  per  cent.,  it  might  be 
expected  that  magmatic  concentrations  of  magnetite  would  be 
abundant.  This  does  not  seem  to  be  the  case,  however.  In 
the  early  stages  of  consolidation  of  igneous  rocks  some  magnetite 
is  crystallized,  together  with  other  accessories,  but  the  tendency 
to  form  silicates  is  strong  and  sufficient  silica  is  usually  available 
to  take  care  of  all  the  iron  in  the  form  of  ferromagnesian  silicates. 

Commercially  valuable  deposits  of  magnetite  as  differentiation 
products  of  magmas  have  been  found  only  in  connection  with 
syenites,  syenite  porphyries,  and  keratophyres,  and  here  the 
magnetite  is  usually  later  than  the  ferromagnesian  silicates 
and  the  feldspars.  Certain  magnetite  deposits  contained  in 
ortho-gneiss  at  Sterling,  New  Jersey,  are  also  believed  to  be 
caused  originally  by  magmatic  differentiation.  Apatite  fre- 
quently accompanies  the  magnetite,  but  sulphides  are  rare. 
Some  basalts  contain  as  much  as  8  per  cent.  FeO  and  4  per  cent. 
Fe2O3;  some  gabbros  and  norites  as  much  as  15  per  cent.  FeO 
and  1  or  2  per  cent.  Fe20s.  Magnetite  requires  69  per  cent, 
ferric  oxide  and  31  per  cent,  ferrous  oxide.  Any  process  of 
differentiation  favoring  the  separation  of  magnetite  thus  requires 
a  transfer  of  part  of  the  iron  to  a  ferric  state. 

Magnetite  deposits  are  rarely  found  in  surface  lavas,  doubtless 

1  Am.  Jour.  Sci.,  4th  ser.,  vol.  25,  1908,  pp.  1-38. 

2  R.  Beck,  Erzlagerstatten,  3d  ed.,  1,  1909,  p.  57. 


800  MINERAL  DEPOSITS 

because  time  before  consolidation  has  not  been  long  enough  to 
permit  of  differentiation.  Such  concentrations  in  basalt  have 
been  described  from  Colorado1  and  a  similar  occurrence  in  ande- 
site  is  on  record  from  Nevada,2  though  in  the  latter  case  the 
author  considers  the  magnetite  and  apatite  to  have  been  formed 
by  replacement  after  consolidation. 

The  Iron  Ores  of  Northern  Sweden.3 — The  great  magnetite 
deposits  in  the  extreme  northern  part  of  Sweden,  the  largest  in 
the  world,  have  been  studied  lately  by  many  geologists.  For 
many  years  they  remained  unworked  on  account  of  their  high 
percentage  of  phosphorus,  which,  since  the  invention  of  the 
Thomas  process,  is  no  longer  objectionable. 

The  deposit  at  Kiruna  forms  a  north-south  ridge  which  is,  as 
exposed  at  the  surface,  about  2.8  kilometers  in  length  and  rises 
248  meters  above  Lake  Luossajarvi,  or  748  meters  above  the  sea. 
The  magnetite  forms  the  highest  part  of  the  ridge  and  is  from 
32  to  152  meters  wide.  The  total  amount  of  ore  proved  above 
the  level  of  the  lake  is  said  to  be  265,000,000  metric  tons,  and 
the  total,  to  a  depth  of  300  meters,  proved  by  borings,  about 
740,000,000  tons.  The  present  annual  production  from  big 
open-cut  workings  is  about  3,000,000  metric  tons;  the  ore  is 
exported  to  England,  and  to  Germany. 

The  ore-body  and  surrounding  formations  are  of  pre-Cambrian 
age  and  dip  steeply  to  the  east  (Fig.  263).  In  the  footwall  lies 
a  syenite  porphyry  with  fluidal  structure  and  rich  in  soda  (61 
SiO;>,  8  iron  oxides,  and  6  to  7  Na20),  the  pyroxene  of  which  is 
largely  altered  to  amphibole,  chlorite,  and  epidote.  Magnetite 
is  present  in  two  generations,  the  later  of  which  may  surround  the 

1  H.  S.  Washington  and  E.  S.  Larsen,  Jour.,  Washington  Acad.  Sci.,  vol. 
3,  1913,  pp.  449-452. 

2  J.  C.  Jones,  Econ.  Geol,  vol.  8,  1913,  pp.  247-263. 

3  Hj.  Lundbohm,  Kiirunavaara  och  Luossavaara  jarnmalmsfalt,  Sveriges 
Geol.,  Undersokn.,  Ser.  C,  No.  175,  1898.     Refs.  Neues  Jahrb.,  1900,  1,  pp. 
79-80;  Zeitschr.  prakt.  Geol,  1898,  pp.  423-426. 

O.  Stutzer,  Die  Eisenerzlagerstatten  bei  Kiruna,  Zeitschr.  prakt.  Geol., 
vol.  14,  1906,  pp.  65-71,  137-142.  Ref.  Econ.  Geol.,  vol.  2,  1907,  pp. 
88-91. 

P.  Geijer,  Igneous  rocks  and  iron  ores  of  Kiirunavaara,  etc.,  Stockholm, 
1910,  pp.  278.  Author's  abstract,  Econ.  Geol.,  vol.  5,  1910,  pp.  699-718 
(English). 

P.  Geijer,  Studies  on  the  geology  of  the  iron  ores  of  Lappland,  Geol. 
For.,  Forh.,  vol.  34,  1912,  pp.  727-789  (English) 

R.  A.  Daly,  Origin  of  the  iron  ores  at  Kiruna,  Stockholm,  1915,  pp.  31. 


CONCENTRATION  IN  MOLTEN  MAGMAS       801 

feldspars  and  enter  them  along  cleavage  planes.  The  contact 
between  the  ore-body  and  footwall  porphyry  is  apparently  sharp, 
but  shows  in  detail  a  narrow  zone  of  transition  due  to  small, 
sharply  denned  dikelets  of  magnetite  in  the  porphyry.  An 
ultimate  product  of  this  zone  is  a  mixture  of  magnetite  with  green 
hornblende;  it  contains  vugs  filled  with  apatite,  titanite,  and 
magnetite.  The  deposit  itself  is  free  from  vugs.  The  reddish 
quartz  porphyry  of  the  hanging  wall  is  essentially  similar  in 
microscopic  character,  but  contains  about  71  silica,  5  iron'oxides 
and  5  to  6  Na2O,  and  has  been  classified  as  a  quartz  keratophyre. 
It  contains  fragments  of  magnetite  ore  and  occasionally  of  the 
footwall  porphyry.  Above  this  hanging  wall  porphyry  lie  quartz- 
ites,  clay  slates,  and  conglomerates,  with  water-worn  fragments 
of  iron  ores  and  keratophyre. 

The  ore  is  compact  and   fine-grained,    consisting  chiefly  of 
magnetite    and    apatite    in    intimate    intergrowth,    apparently 


II 


FIG.  263. — Schematic  cross-section  of  iron  deposit  at  Kiruna.  1,  Soda 
greenstone;  2,  Kurravara  conglomerate;  3,  syenite  porphyry;  4,  magnetite 
deposit;  5,  quartz  porphyry;  5  and  6,  Hauki  complex;  7,  amphibolite;  8, 
quartz  porphyry. 

having  crystallized  together.  In  places  it  contains  pyroxene. 
The  ore  is  said  to  average  68  per  cent.  iron.  The  phosphorus  is, 
as  a  rule,  above  2  per  cent.,  and  some  parts  of  the  ore  yield 
from  3  to  4  per  cent,  or  even  more  of  this  substance.  The 
sulphur  is  not  above  0.05  per  cent. ;  manganese  not  above  0.70 
per  cent.;  a  similar  amount  of  magnesia  is  recorded,  about  1.5 
per  cent,  silica,  0.75  per  cent,  alumina,  about  3  per  cent,  lime 
and  0.3  per  cent.  TiO2.  In  places  a  fluidal  structure  of  the 
magnetite  and  branching  veinlets  of  apatite  are  observed  in  the 
ore. 

Early  views  on  the  genesis  of  the  Kiruna  deposit  suggested 
pneumatolytic  agencies.  In  1898,  Hogbom  prowd  its  magmatic 
origin  though  he  believed  that  the  differentiation  had  proceeded 
in  place.  The  investigations  of  Stutzer  and  the  later  mono- 
graphic work  by  Geijer  have  shown  plainly  that  the  ore  was 


802  MINERAL  DEPOSITS 

differentiated  from  magmas  in  depth  and  that  it  has  been  brought 
to  its  present  position  in  molten  condition.  The  differences 
largely  depend  upon  whether  the  porphyries  are  to  be  considered 
as  effusive  or  intrusive.  Stutzer  held  the  latter  and  more  prob- 
able view,  regarding  the  syenite  porphyry  as  the  earlier  rock 
followed  by  the  intrusion  of  a  dike  of  magnetite.  On  the  east 
side  the  magnetite  was  later  intruded  by  quartz  porphyry  which 
includes  fragments  of  ore.  Geijer,  holding  that  the  porphyries 
were  extrusive,  thought  that  the  ore  was  erupted  at  the  surface 
as  a  sheet  of  molten  material  while  the  flows  were  still  hori- 
zontal. Lately  R.  A.  Daly  has  again  suggested  a  differentiation 
in  place  by  gravitative  settling  of  magnetite  from  the  quartz 
porphyry  which  he  considers  to  be  an  intrusive  sheet. 

The  great  iron  mines  of  Gellivare,  a  short  distance  south 
of  Kiruna,  produce  about  1,500,000  metric  tons  of  ore  per 
annum. 

The  ore  is  principally  mined  in  open  workings  and  contains 
the  same  minerals  as  that  of  Kiruna — that  is,  magnetite  and 
apatite — but  it  has  a  coarser  grain.  Locally  it  contains  pyrite, 
chalcopyrite,  fluorite,  calcite,  and  zeolites.  The  ore  is  rudely 
schistose,  conforming  with  the  steep  dip  of  the  country  rock, 
and  forms  large  lenses,  partly  imbricating  on  parallel  and  curving 
strike  lines. 

The  rocks  are  chiefly  gneisses.  The  red  gneiss  is  most  common 
near  the  deposit  and  is  traversed  by  many  irregular  veins  of 
magnetite.  It  consists  of  albite  with  some  quartz,  chlorite, 
apatite,  and  biotite. 

The  reddish-gray  gneiss  is  similar  in  composition  but  contains 
also  soda-lime  feldspar,  microperthite,  augite,  and  hornblende. 
Both  rocks  are  rich  in  soda. 

Dikes  of  acidic  rocks,  locally  called  granite  but  really  quartz 
diorite  or  quartz  keratophyre,  cut  across  the  ore-body. 

The  deposit  at  Gellivare  has  probably  a  similar  origin  to 
that  at  Kiruna.  Epigenetic  hypotheses  are  advanced  by  Lund- 
bohm,  von  Post,  and  Lofstrand,  the  last  two  considering  the 
deposit  as  a  magmatic  dike.  This  view  is  supported  by  the 
tectonic  relationship;  the  ore-body  is  by  no  means  confined  to  a 
single  horizon  in  the  gneiss.  On  the  whole  the  analogy  with 
Kiruna  is  very  striking,  though  at  Gellivare  the  rocks  are  clearly 
of  intrusive  origin.  At  both  places  the  same  genetic  relations 
seem  to  exist;  the  earliest  rock  is  rich  in  soda  and  of  low  to 


CONCENTRATION  IN  MOLTEN  MAGMAS       803 

medium  acidity,  then  follows  an  intrusion  of  magnetite-apatite 
rock,  and  lastly  a  quartzose  soda-rich  igneous  rock  was  intruded. 

Gellivare  is,  then,  simply  a  dynamo-metamorphosed  Kiruna. 

The  hematite  deposits  at  Iron  Mountain,1  Missouri,  are  be- 
lieved by  Geijer2  to  be  closely  allied  to  the  magnetite  of  Kiruna. 
He  thinks  the  hematite  may  have  been  derived  by  alteration 
from  magnetite  and  points  to  the  association  with  apatite  and  the 
occurrence  as  dike-shaped  masses  in  the  pre-Cambrian  porphyry. 

The  Magnetites  of  the  Ural  Mountains. — According  to  the 
recent  investigations  of  Loewinson-Lessing  and  Jakowleff  the 
magnetite  deposits  of  Wyssokaia  Gora  and  Goroblagodat,  in 
the  Urals,  are  products  of  differentiation  in  magmas,  although  at 
the  former  locality  contact-metamorphic  deposits  also  appear 
to  be  present.  A  summary  of  the  Russian  literature  has  been 
given  by  Beck.3  In  both  places  the  igneous  rocks  are  augite 
syenites;  at  Goroblagodat  the  ore  forms  streaks  or  "schlieren" 
in  this  rock;  it  has  a  brecciated  structure,  the  magnetite  cement- 
ing the  augites  and  feldspar.  The  deposits  show  marked  dif- 
ferences from  the  Swedish  deposits  just  described  in  that  they 
contain  very  little  apatite  and  that  the  ores  are  not  injected 
dikes,  but  perhaps  rather  products  of  differentiation  in  place. 

The  Magnetites  of  the  Adirondacks.4— The  eastern  part  of 
the  Adirondack  Mountains,  in  northern  New  York,  contains  a 
number  of  deposits  of  magnetite  which  have  been  worked  since 
the  early  part  of  the  last  century  and  which  still  possess  consid- 
erable economic  importance.  The  total  output  is  estimated  at 
40,000,000  long  tons.  The  annual  mine  production  in  the  last 
25  years  has  varied  from  1,000,000  to  2,000,000  long  tons.  The 
latter  figure  was  reached  in  1917.  The  more  important  opera- 
tions are  carried  on  in  the  Mineville  district,  but  the  deposits 
are  spread  over  a  large  area.  As  some  of  the  ores  contain 
much  apatite,  magnetic  concentration  is  used.  The  concentrates 

1  C.  W.  Crane,  The  iron  ores  of  Missouri,  Missouri  Bur.  Geol.  and  Mines, 
2d  ser.,  vol.  10,  1912,  pp.  107-144. 

2  Econ.  Geol,  vol.  10,  1915,  p.  324. 

3  Erzlagerstatten,  3d  ed.,  1,  1909,  pp.  29-34. 

4  J.  F.  Kemp,  Geology  of  the  magnetites  near  Port  Henry,  Trans.,  Am. 
Inst.  Min.  Eng.,  vol.  27,  1898,  pp.  146-203. 

David  H.  Newland  and  J.  F.  Kemp,  Geology  of  the  Adirondack  magnetic 
iron  ores,  Bull.  119,  N.  Y.  State  Museum,  1908,  p.  182. 

D.  H.  Newland,  Magnetites  in  the  Adirondacks,  Econ.  Geol.,  vol.  2,  1907, 
pp.  763-773. 


804  MINERAL  DEPOSITS 

contain  60  to  65  per  cent,  of  iron,  and  a  by-product  of  impure 
apatite  is  obtained  which  is  used  as  a  fertilizer.  The  tailings 
consist  mainly  of  ferromagnesian  minerals.  The  ores  are  ex- 
tracted through  shafts,  the  deepest  of  which,  at  Lyon  Mountain, 
is  1,500  feet  deep  on  the  incline. 

Until  recently  all  the  deposits  in  this  region  were  considered 
as  of  sedimentary  origin,  for  they  are  contained  in  crystalline 
gneissoid  rocks,  some  of  which  are  certainly  metamorphosed 
sediments.  In  recent  years,  however,  Kemp  and  Newland  have 
shown  that  the  ores  stand  in  most  intimate  relationship  to 
augite  syenites. 

The  associated  rocks  include  syenitic,  granitic,  and  dioritic 
gneisses,  garnetiferous  schists,  amphibolites,  and  crystalline  lime- 
stones. The  deposits  considered  of  magmatic  type  occur  in 
a  belt  of  syenitic  gneisses,  in  part  also  massive  syenites  and 
their  pegmatites,  whose  igneous  origin  is  well  established. 
These  rocks  contain  from  1.5  to  6.5  per  cent,  magnetite. 

In  the  Archean  sedimentary  rocks,  known  as  the  Grenville 
series,  are  a  number  of  smaller  deposits,  many,  of  which  contain 
pyrite  as  well  as  magnetite;  in  the  rocks  graphite,  sillimanite, 
garnet,  and  scapolite  have  been  noted.  The  genesis  of  these 
deposits  is  in  doubt;  they  may  be  of  sedimentary  origin  and 
subsequently  metamorphosed. 

The  magnetites  associated  with  undoubtedly  igneous  rocks 
appear  as  long  lenses  or  pod-like  bodies,  often  bent,  curved,  or 
folded,  and  have  clearly  participated  in  the  general  metamor- 
phism  of  the  country;  at  first  they  were  probably  tabular  bodies. 
The  ore  lenses  range  in  thickness  from  a  few  feet  up  to  25  feet 
or  more,  especially  where  curved  or  folded.  In  part  the  magnet- 
ite ore  is  pure,  but  more  commonly  it  is  mixed  with  the  minerals 
that  make  up  the  wall-rocks,  into  which  the  ores  often  grade; 
these  minerals  are  feldspar,  quartz,  pyroxene,  and  hornblende. 

According  to  the  percentage  of  phosphorus  present  the  mag- 
netites may  be  divided  into  low-phosphorus,  Bessemer,  and 
non-Bessemer  grades.  Apatite  is  usually  present,  and  the 
non-Bessemer  grade  may  contain  as  much  as  10  per  cent,  of 
this  mineral.  While  much  of  the  ore  yields  60  per  cent,  iron, 
there  are  large  masses  of  ore  with  50  per  cent,  iron  or  less  that 
are  suitable  for  concentration.  According  to  Newland  the  lowest 
grade  of  workable  milling  ore  carries  about  35  per  cent.  iron. 
An  average  analysis  of  65  carloads  from  pit  21  of  the  Mineville 


CONCENTRATION  IN  MOLTEN  MAGMAS       805 

group  of  mines  gave,  in  per  cent.,  iron,  60.03;  silica,  4.48;  phos- 
phorus, 1.635;  sulphur,  0.021 ;  and  titanium,  0.12.  The  result  of 
concentration  from  the  "Old  Bed"  ore  at  Mineville  in  1903  is 
shown  in  the  subjoined  table  in  percentages. 


Crude  ore 

Concen- 
trates 

First-grade 
apatite 

Second- 
grade 
apatite 

Iron  

59.59 

67.34 

3.55 

12.14 

Phosphorus  
Phosphorus  as  "bone 
phosphate." 

1.74                  0.675 

12.71 
63.55 

8.06 
40.30 

The  intimate  association  and  intergrowth  of  the  magnetite 
with  the  feldspar,  augite,  hypersthene,  and  hornblende  of  the 
augite  syenite  are  considered  by  Kemp  to  prove  its  origin  by 
magmatic  differentiation.  Syenitic  pegmatites  are  also  present 
and  the  processes  of  pegmatization  are  considered  to  have  con- 
tributed to  the  formation  of  the  ore;  fluorite  and  titanite  are 
often  found  in  the  ores. 

CORUNDUM1 

General  Mode  of  Occurrence. — Corundum  (A1203)  has  long 
been  known  as  a  product  of  regional  and  contact  metamor- 
phism;  that  it  may  also  result  from  direct  crystallization  from  a 
molten  magma  has  been  established  by  late  investigations. 
Alumina  is  contained  in  all  magmas,  usually  forming  about 
14  to  17  per  cent.  Certain  syenites,  nepheline  syenites,  and 

1  The  blue  transparent  corundum  is  called  sapphire;  the  red  trans- 
parent variety  forms  ruby;  both  varieties  are  valuable  gems.  Colorless, 
yellow,  and.  green  varieties  also  occur.  The  ordinary  bluish  or  gray  cor- 
undum is  an  inconspicuous  mineral  with  good  basal  cleavage  and  great 
hardness,  whence  its  principal  use  as  an  abrasive.  Mixed  with  magnetite, 
mainly  in  metamorphic  rocks,  it  is  termed  emery,  the  name  being  derived 
from  Cape  Emeri,  on  the  island  of  Naxos. 

J.  H.  Pratt,  Corundum,  Bull.  269,  U.  S.  Geol.  Survey,  1906. 

A.  E.  Barlow,  Corundum,  Mem.  57,  Geol.  Survey  Canada,  1915,  p. 
377. 

G.  P.  Merrill,  The  non-metallic  minerals,  1904,  pp.  69-85;  2d  ed.,  1910, 
pp.  73-89. 

J.  Morozewicz,  Tsch.  Min.  pet.  Mitt.,  vol.  18,  1898,  pp.  22-83. 


806  MINERAL  DEPOSITS 

anorthosites  may  contain  as  much  as  30  per  cent.  Theperido- 
tites,  on  the  other  hand,  contain  only  from  a  fraction  of  1  per 
cent,  up  to  10  per  cent,  of  alumina.  The  corundum  of  magmatic 
origin  is  chiefly  associated  with  those  rocks  of  exceptionally 
high  or  low  content  of  alumina,  in  which  at  the  same  time  the 
silica  is  low. 

By  some  observers  the  corundum  of  igneous  rocks  is  regarded 
as  due  to  recrystallization  of  included  shale  fragments.  This 
view  has  been  advanced  by  L.  V.  Pirsson  in  regard  to  the  sap- 
phire-bearing dike  of  Yogo  Gulch,  Montana.  On  the  whole 
the  differentiation  theory  fits  the  facts  better. 

Corundum  is  fusible  only  at  2,250°  C.  (Moissan),  but  it  by  no 
means  follows  that  it  crystallized  from  the  magma  at  this  tem- 
perature; Hautefeuille  and  Perrey  showed,  for  instance,  that 
alumina  is  soluble  in  a  nepheline  or  leucite  melt  and  that  upon 
cooling  the  greater  part  of  it  crystallizes  as  corundum.  Very 
likely  it  will  be  found  that  similar  conditions  would  exist  in 
melted  olivine.  Morozewicz  has  shown  that  when  in  a  silicate 
melt  the  alumina  is  in  excess  of  the  ratio  RO  :A1203 :  :1: 1, 
corundum,  spinel,  sillimanite,  or  cordierite  will  form. 

Corundum  in  Igneous  Magnesian  Rocks. — A  long  belt  of 
magnesian  rocks  of  probable  pre-Cambrian  age,  mainly  perido- 
tites,  gabbros,  and  norites,  extends  along  the  Appalachian  region 
from  Alabama  to  Massachusetts,  and  in  these  rocks  corundum 
has  been  found  in  commercial  quantities  at  a  number  of  places. 

In  North  Carolina  and  Georgia  the  mineral  occurs  as  vein-like 
bodies  from  a  few  inches  up  to  15  feet  in  width  at  the  contact  of 
peridotite  with  gneisses  and  schists,  in  part  also  in  the  peridotite 
itself.  Chlorite,  enstatite,  and  spinel  are  associated  with  the 
corundum.  Among  the  principal  localities  are  the  Laurel 
Creek  mine,  in  Rabun  County,  Georgia;  Corundum  Hill,  Macon 
County,  and  Webster,  Jackson  County,  North  Carolina.  None 
of  the  southern  localities  have  been  productive  in  late  years. 

Deposits  of  emery,  an  impure  corundum  mixed  with  magnet- 
ite, are  worked  by  open  cuts  near  Peekskill,  Westchester  County, 
New  York.  The  emery  here  occurs  in  the  intrusive  Cortlandt 
series  of  rocks  described  by  G.  H.  Williams1  and  consisting  of 
peridotites,  norites,  and  diorites.  The  corundum  and  mag- 
netite are,  according  to  Williams,  simply  segregations  in  the 
norite,  the  constituent  minerals  of  which  occur  even  in  the  present 

1  Am.  Jour.  Sci.,  3d  ser.,  vol.  33,  1887,  p.  135. 


CONCENTRATION  IN  MOLTEN  MAGMAS       807 

emery.  Hercynite — an  iron  spinel — and  garnet  accompany  the 
magnetite  and  the  corundum. 

Regional  metamorphism  easily  changes  many  of  these  magne- 
sian  rocks  to  amphibolites  and  chloritic  schists.  During  this 
process  the  corundum  is  apparently  little  affected.  At  Chester, 
Massachusetts,  according  to  B.  K.  Emerson,1  emery  occurs  for  a 
long  distance  along  a  belt  of  amphibolite  contained  in  sericite 
schists.  The  emery-bearing  part  of  the  schist  is  in  places  12  feet 
wide  and  has  been  mined  to  a  depth  of  300  feet  below  the  surface. 

Corundum  of  gem  quality  is  occasionally  found  in  these  depos- 
its or  in  the  gravels  derived  from  them,  but  most  of  the  sapphires 
obtained  in  the  United  States  are  derived  from  a  different  source. 
At  Yogo  Gulch,  Fergus  County,  Montana,  long  dikes  belonging 
to  the  monchiquite-camptonite  series  of  lamprophyric  rocks 
contain  sharply  crystallized  rhombohedral  sapphires  of  excellent 
quality.  The  decomposed  rock  is  allowed  to  slack  and  is  washed 
in  sluice  boxes.  The  deposits  have  been  described  by  Weed,2 
Pirsson,3  and  Sterrett.4 

Pale  blue  or  greenish  sapphires  have  also  been  mined  on  a  com- 
mercial scale  from  the  alluvial  deposits  extending  for  20  miles  a- 
long  the  Missouri  River  near  Helena,  Montana.  According  to 
G.  F.  Kunz  the  gems  are  derived  from  dikes  of  a  vesicular  mica- 
augite  andesite,  but  the  primary  deposits  have  not  been  worked. 
The  sapphires  of  Queensland  are  found  in  placers,  associated  with 
a  basaltic  rock.  Those  of  India,  Burma  and  Siam  also  occur  in 
placers,  and  are  derived  from  gneissoid  or  syenitic  rocks. 

Corundum  in  Syenite. — The  most  important  deposits  of  corun- 
dum in  Canada  were  discovered  by  W.  F.  Ferrier  about  1896. 
The  mineral  occurs  as  an  essential  constituent  of  syenites  and 
nepheline  syenites  and  their  pegmatites  in  Haliburton,  Peterboro, 
Hastings,  and  Renfrew  counties,  Ontario.5  The  rocks  form 
dikes  or  intrusive  masses  in  gneisses  and  contain  as  much  as 
12  or  15  per  cent,  of  bluish  or  gray,  often  roughly  crystallized 
corundum,  many  of  the  crystals  being  2  or  3  inches  in  diameter. 
The  rock  is  quarried,  crushed,  and  concentrated  on  tables.6 

1  Mon.  29,  U.  S.  Geol.  Survey,  1908,  pp.  117-147. 

2  Twentieth  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  3,  1899,  pp.  454-460. 

3  Am.  Jour.  Sci.,  4th  ser.,  vol.  4,  1897,  p.  421. 

4  Mineral  Resources,  U.  S.  Geol.  Survey,  1907,  pt.  2,  p.  816. 

5  A.  P.  Coleman,  Eighth  Rept.,  Ontario  Bur.  Mines,  1899,  pp.  250-253. 
•  A.  E.  Barlow,  Op.  cit. 


808  MINERAL  DEPOSITS 

The  production,  formerly  great,  has  now  diminished.  The 
value  of  corundum  is  $100  to  $160  per  ton. 

Minor  deposits  of  similar  character  occur  in  Gallatin  County, 
Montana,  in  a  syenite  composed  of  orthoclase,  biotite,  and 
corundum. 

Other  Occurrences. — Corundum  has  also  been  found  in  anor- 
thosites — rocks  consisting  principally  of  labradorite,  or  anorthite 
feldspar.  According  to  T.  H.  Holland1  corundum  is  abundant 
in  India  and  representatives  of  the  various  classes  already 
described  are  present. 

In  a  number  of  occurrences  of  corundum  in  "gneiss"  we  have 
probably  to  deal  with  igneous  rocks  like  syenite  made  schistose 
by  pressure.  Corundum  is,  however,  unquestionably  also 
developed  by  the  contact  metamorphism  of  limestone,  probably 
by  transfer  of  alumina  from  the  magma.  The  largest  known 
deposits  of  emery  occur  on  the  island  of  Naxos,  in  the  Greek  Arch- 
ipelago, and  near  Smyrna  in  Asia  Minor;  they  are  contained  in 
metamorphosed  limestone.  Corundum  may  also  be  developed 
by  the  regional  metamorphism  of  clay  shale  and  shaly  quartzite. 
Many  minor  occurrences  of  this  kind  in  the  Southern  States 
have  been  described  by  Pratt. 

Production  in  the  United  States. — At  present  little  or  no 
corundum  is  produced  in  the  United  States.  In  1916,  15,000 
tons  of  emery  at  $8  per  ton  represented  the  domestic  output. 
Imports  of  corundum  and  emery  from  Canada  and  other  countries, 
in  1916,  had  a  total  value  of  about  $241,000.  In  response  to  a 
great  demand,  artificial  corundum  (alundum)  is  now  manufac- 
tured at  Niagara  Falls  by  fusing  bauxite  in  the  electric  furnace. 

Uses. — Corundum  finds  its  principal  use  as  an  abrasive,  wheels 
and  abrading  tools  of  all  kinds  being  manufactured  from  the 
crushed  material. 

SULPHIDE  ORES  OF  IGNEOUS  ORIGIN 

General  Principles. — That  sulphide  minerals  may  crystallize 
from  a  magma  has  been  ascertained  beyond  doubt,  but  the 
number  of  minerals  which  have  this  origin  is  limited  to  a  few 
species,  mainly  pyrrhotite,  pyrite,  chalcopyrite,  molybdenite, 
sphalerite,  and  pentlandite ;  arsenides  like  niccolite  and  sperrylite 
are  also  known.  This  view  of  the  igneous  origin  of  certain  ores 

1  A  manual  of  the  geology  of  India,  Economic  geology,  part  1,  Corundum, 
Calcutta,  1898. 


CONCENTRATION  IN  MOLTEN  MAGMAS        809 

lias  been  gained  comparatively  lately,  and  largely  by  the  labors 
of  J.  H.  L.  Vogt.1 

While  it  is  clear  that  sulphides  are  not  freely  miscible  with 
silicate  magmas,2  Vogt  has  shown  that  the  monosulphides  are 
soluble  to  some  extent  in  dry  melts  of  basic  character — that  is, 
with  much  iron,  calcium,  and  manganese;  under  favorable  cir- 
cumstances, particularly  at  temperatures  of  about  1,500°  C., 
as  much  as  6  or  7  per  cent,  of  these  sulphides  may  become  dis- 
solved. But  it  is  also  to  be  noted  that  Vogt  found  but  little 
solubility  for  the  sulphides  of  copper,  nickel,  lead,  and  silver. 
Upon  crystallization  the  sulphides  always  separate  out  first,  as 
oldhamite  (CaS),  alabandite  (MnS),  troilite  (FeS),  and  zinc 
blende  (ZnS).  These  experimental  results  do  not  exactly  cor- 
respond with  those  found  in  nature,  for  of  the  sulphides  men- 
tioned zinc  blende  is  the  only  one  encountered  at  all  in  igneous 
rocks,  and  the  sulphides  of  slight  solubility,  like  those  of  copper 
and  nickel,  are  the  most  abundant.  Later  investigations  by  W. 
Wanjukoff3  have  shown  that  the  sulphides  of  copper,  nickel,  iron, 
zinc  and  cadmium  are  soluble  in  basic  slags  to  a  notable  degree 
and  in  the  order  of  abundance  indicated.  Very  likely  the  pres- 
ence of  mineralizers  other  than  sulphur  would  increase  this 
solubility  as,  in  fact,  already  suggested  by  Vogt. 

In  the  surface  lavas  which  correspond  most  closely  to  dry 
melts  primary  sulphides  are  extremely  rare,  although  grains  of 
chalcopyrite  are  occasionally  found.  The  sulphides  of  economic 
importance  are  almost  wholly  confined  to  the  peridotites,  norites, 
and  gabbros,  all  rocks  of  deep-seated  crystallization;  the  charac- 
teristic metallic  association  is  iron,  copper,  nickel,  platinum, 
and  occasionally  a  little  arsenic. 

Many  observers  have  stated4  that  the  pyrrhotite  and  chalcopyr- 
ite which  often  occurs  in  basic  rocks,  more  or  less  intergrown 
with  magnetite,  are  of  primary,  magmatic  origin.  None  has 
treated  the  subject  better  and  more  convincingly  than  E.  Howe5 
who  described  the  gabbro-norite  and  pyroxenite  of  the  Cortlandt 

1  J.  H.  L.  Vogt,  Zeitschr.  prakt.  Geol.,  1893. 

2  J.  H.  L.  Vogt,  Die  Silikatschmelzlosungen,  pt.  1,  Videnskabs-Selskabets. 
Skrifter,  Math.-Naturv.  Klasse,  Kristiania,  No.  8,  1903. 

3  Metallurgie,  vol.  9,  1912,  pp.  1-48. 

4  See  W.  Lindgren,  Gold  quartz  veins  of  Nevada  City  and  Grass  Valley, 
Seventeenth  Ann.  Rept.,  U.  S.  Geol.  Survey,  1892,  pp.  67-70.      . 

6  Sulphide  bearing  rocks  from  Litchfield,  Ct.,  Econ.  Geol.,  vol.  10,  1915, 
pp.  330-347. 


810  MINERAL  DEPOSITS 

series.  His  conclusions  are  as  follows:  The  extremely  fresh  rocks 
contain  small  amounts  of  pyrrhotite,  pentlandite  and  chalcopyr- 
ite;  there  is  no  post  magmatic  alteration.  The  sulphides  are  as 
essentially  magmatic  as  the  silicates.  Although  most  of  the 
sulphides  separated  from  solutions  at  an  early  stage  of  the  cooling 
of  the  magma,  small  quantities  continued  to  separate  or  to  re- 
dissolve  until  the  magma  had  nearly  crystallized.  The  form  and 
the  interstitial  relations  of  the  sulphides  seem  to  show  that 
although  they  may  have  separated  early  from  the  silicates,  they 
remained  liquid  until  the  silicates  had  crystallized.  The  sul- 
phide bearing  rocks  are  regarded  as  products  of  differentiation 
from  magmas  poorer  in  these  substances. 

The  magmatic  sulphide  ores  contain  magnetite,  pyrrhotite, 
pentlandite,  chalcopyrite  and  bornite,  but  no  gangue  minerals 
other  than  the  primary  rock  forming  minerals.  They  contain 
no  sericite,  chlorite,  garnet  or  epidote.  The  sulphides  often  cor- 
rode and  embay  the  older  silicates  but  without  any  indication  of 
secondary  substances.  One  often  wonders  what  became  of  these 
dissolved  portions.  There  is  a  certain  succession  among  the 
sulphides.  Where  pyrite1  is  present  it  is  often  octahedral  and 
etched  depressions  on  its  faces  are  sometimes  filled  with  chal- 
copyrite. Pyrrhotite  is  the  most  abundant  mineral.  The  usual 
succession  of  minerals  as  established  by  Tolman  and  Rogers,2 
is  as  follows:  Silicates,  magnetite,  hematite,  pyrrhotite,  pent- 
landite, chalcopyrite  and  bornite.  "Any  change  in  this  order, 
they  state,  is  due  to  rearrangement  subsequent  to  the  magmatic 
period. "  Nevertheless  it  seems  certain  that,  in  these  rocks,  there 
is  also  magnetite,  earlier  than  the  silicates.  It  will  be  remem- 
bered that  Vogt  in  his  studies  on  silicate  melts  frequently  ob- 
tained two  generations  of  this  mineral. 

At  times  it  becomes  extremely  difficult  to  hold  closely  to  the 
above  mentioned  definitions  and  rules.  Most  deposits  of  mag- 
matic sulphides  are  not  uniform  disseminations  but  rather 
marginal  concentrations,  quite  plainly  injected  after  the  surround- 
ing rock  had  crystallized,  or  else  they  are  accompanied  by 
gangue  materials  like  chlorite,  quartz,  garnet,  epidote,  etc.  Such 
deposits  were  probably  formed  by  highly  concentrated  sulphide 

1  Pyrite  as  a  magmatic  mineral  is  not  accepted  by  all  authors.     See 
Tolman  and.  Rogers,  A  study  of  the  magmatic  sulfid  ores,  Stanford  Univ. 
Pub.  Univ.  Ser.,  1916,  p.  69. 

2  Idem. 


CONCENTRATION  IN  MOLTEN  MAGMAS       811 

melts  with  water  and  other  mineralizers.  When  the  dominating 
influence  is  no  longer  the  straight  melt  but  is  instead  the  con- 
centrated and  controlling  gases  we  may  perhaps  employ  the 
much  abused  term  pneumatolytic  action,  which  again  grades 
into  hydrothermal  action. 

For  the  plainly  magmatic  ores,  be  they  sulphide  or  oxides, 
Graton  and  McLaughlin1  have  suggested  the  term  orthotectic 
while  for  the  slightly  later  processes  in  which  the  strictly  mag- 
matic influences  were  modified  by  mineralizers  they  would  use 
the  expression  pneumotectic.  Naturally,  the  boundaries  are 
sometimes  indistinct.  The  Sudbury  ores  would  be  considered 
pneumotectic. 

In  the  orthotectic  deposits  the  temperature  may  have  been 
very  high  but  in  the  subsequent  though  still  magmatic  phases 
temperatures  as  low  as  400°  to  500°  C.  may  have  been  reached. 
We  are  not  well  informed  on  this  subject. 

The  magmatic  sulphide  ores  have  lately  been  discussed  by 
W.  H.  Goodchild  from  a  physico-chemical  standpoint.2 

Types  of  Deposits. — Some  of  the  magmatic  sulphide  deposits 
are  simply  basic  rocks  abnormal  in  containing  much  pyrrhotite, 
chalcopyrite,  and  pentlandite.  Other  occurrences  are  clearly 
related  to  contacts  and  bear  evidence  of  later  magmatic  injection. 
There  is  still  another  class  in  which  the  magmatic  origin  is  only 
dimly  perceived  on  account  of  the  metamorphic  changes  which 
the  rocks  have  undergone.  The  basic  igneous  rocks  are  easily 
modified  by  pressure  and  more  or  less  schistose  amphibolites 
are  developed,  which  besides  amphibole  contain  garnet,  quartz, 
epidote,  and  chlorite.  Any  primary  Sulphide  segregations 
contained  in  such  rocks  will  be  correspondingly  affected  and  a 
new  type  of  deposit  of  metamorphic  appearance  will  result; 
the  sulphides  themselves  apparently  undergo  little  change. 

Sulphides  in  Peridotites  and  Gabbros. — E.  S.  Bastin3  has  de- 
scribed a  rock  from  Knox  County,  Maine  (Fig.  264),  which  shows 
convincingly  the  magmatic  origin  of  sulphide  ores.  This  rock, 
which  occupies  a  small  area  surrounded  by  drift,  consists  of 
60  per  cent,  olivine,  21.53  per  cent,  pyrrhotite,  some  andesine- 
labradorite  feldspar,  hornblende,  and  magnetite,  1.03  per  cent, 
chalcopyrite,  and  pyrite,  biotite,  and  spinel.  The  analysis 

1  Econ.  Geol,  vol.  13,  1918,  p.  85. 

2  Mining  Mag.  (London),  Jan.-June,  1918. 

3  Jour.  Geology,  vol.  16,  1908,  pp.  124-138. 


812 


MINERAL  DEPOSITS 


shows  0.94  per  cent,  nickel  oxide,  and  the  material  is  thus 
practically  a  very  low  grade  ore.  The  constituents  are  inter- 
grown,  showing  simultaneous  crystallization  except  that  the  mag- 
netite, enclosed  in  olivine,  is  the  earliest  mineral  separated; 
the  chalcopyrite  is  associated  irregularly  with  the  pyrrhotite. 
There  has  been  some  serpentinization  but  not  enough  to  obscure 
the  relations.  The  complete  analysis  is  as  follows: 


FIG.  264. — Thin  section  of  olivine  corroded  by  pyrrhotite  and  chalcopyrite 
East  Union,  Maine.     Magnified  15  diameters.     After  E.  S.  Baslin. 


SiO2 

A1.G,.... 

FeO 

Fe203 

MgO.. 

CaO.. 

•no,.. 

P205-. 
K2O.. 
Na,0. 


28.04 
3.51 

MnO  

Fe7S8  
NiS 

0.24 
21.53 
0  94 

14.95 

CoS 

0  03 

21  97 

CuFeS2 

1.03 

1.78 
0.20 
0.04 
n  ns 

H20+  
H2O-  
C02  

2.54 

1.48 
1.01 

0.28 


99.65 


In  the  deposit  at  Mittel-Sohland,  in  Saxony,  described  by  Beck,1 
the  sulphides  form  a  rather  rich  nickel  ore.  They  occur  in  an 
olivine  diabase  of  gabbroic  habit,  containing,  in  order  of  deposi- 
tion, magnetite,  ilmenite,  silicates,  pyrrhotite,  and  chalcopyrite. 
The  ores  lie  along  the  contact  between  the  diabase  and  a  granite 

1  Zeitschr.  Deutsch.  geol.  Gesell.,  1903,  pp.  296-331. 


CONCENTRATION  IN  MOLTEN  MAGMAS       813 

and  extend  in  a  belt  about  2  meters  wide  for  a  distance  of  150 
meters,  gradually  merging  into  normal  diabase;  the  granite  con- 
tains disseminated  sulphides  close  to  the  contact.  Beck  believes 
that  the  ores  were  introduced  after  the  consolidation  of  the 
rock. 

Vogt  has  described  the  numerous  Norwegian  occurrences  in 
great  detail.  The  ore-bearing  intrusives  are  norites  or  allied 
rocks,  often  with  biotite  and  brown  hornblende,  and  are  in- 
truded in  pre-Cambrian  gneiss.  In  part  the  gabbros  are  pressed 
to  amphibolites.  The  nickeliferous  pyrrhotites  occur  largely 
along  the  contacts.  They  contain  little  copper  and  only  1  to 
1.5  per  cent,  nickel.  In  the  amphibolitized  deposits  considerable 
migration  has  taken  place.  Garnet  is  formed  along  the  streaks 
of  pyrrhotite.  The  hornblende  is  in  part  transformed  to  bluish- 
green  amphibole. 

A  similar  occurrence  is  that  of  Lancaster  Gap,  Pennsylvania, 
described  by  Kemp,  where  the  nickeliferous  pyrrhotite  lies  along 
the  contacts  of  a  mass  of  amphibolite,  contained  in  mica  schist. 
Much  nickel  ore  was  mined  here  up  to  1893. 

Many  copper  deposits  in  amphibolite  are  really  dynamo- 
metamorphosed  forms  of  such  magmatic  deposits  as  have  been 
described  above.  The  examination  of  several  such  small  de- 
posits in  Colorado1  led  to  this  conclusion.  Deposits  at  Sedalia 
and  Turret,  in  Chaffee  County,  are  basic  dikes  in  a  pre-Cambrian 
sedimentary  series  contact-metamorphosed  by  later  granitic 
intrusion  and  still  later  altered  to  amphibolite.  Chalcopyrite, 
zinc  blende,  and  magnetite,  are  intergrown  with  bluish-green 
amphibole,  garnet,  spinel,  and  labradorite.  The  diabase  is 
probably  a  differentiated  offshoot  from  a  neighboring  large  mass 
of  coarse  diabase. 

Sudbury,  Ontario.2 — The  nickel  deposits  of  Sudbury  now  yield 

l.W.  Lindgren,   Notes  on  copper  deposits  in  Colorado,  Bull.  340,  U.  S. 
Geol.  Survey,  1908,  pp.  157-174. 
2  The  literature  is  extensive ;  only  a  few  articles  are  cited  here. 

C.  W.  Dickson,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  34,  1904,  pp.  3-67. 

A.  E.  Barlow,  Report  on  the  nickel  and  copper  deposits  of  the  Sudbury 
mining  district,  Fourteenth  Ann.  Rept.,  Canada  Geol.  Survey,  pt.  H,  1904. 

W.  Campbell  and  C.  W.  Knight,  Econ.  Geol.,  vol.  2,  1907,  pp.  350-366. 

A.  P.  Coleman,  The  nickel  industry,  Canada  Dept.  Mines,  1913. 

E.  Howe,  Econ.  Geol,  vol.  9,  1914,  pp.  505-522. 

C.  F.  Tolman  and  A.  F.  Rogers,  Op.  cit.,  pp.  21-37. 

A.  P.  Coleman,  Econ.  Geol,  vol.  12,  1917,  p.  427. 


814  MINERAL  DEPOSITS 

the  larger  part  of  the  world's  production,  and  the  once  pre- 
eminent ores  of  New  Caledonia  are  now  of  decreasing  importance. 
Minor  amounts  of  nickel  are  derived  from  deposits  in  Norway 
and  elsewhere.  No  important  nickel  deposits  have  been  found 
in  the  United  States.  The  production  from  Sudbury  in  1910 
was  18,600  long  tons  of  nickel  while  in  1917  the  output  exceeded 
40,000  tons  and  was  derived  from  12  mines,  which  yielded  about 
1,500,000  tons  of  ore.  Besides  nickel  the  ores  contain  an  impor- 
tant percentage  of  copper,  with  a  little  gold,  silver,  palladium 
and  platinum.  It  is  not  necessary  to  specify  the  uses  of  nickel; 
they  depend  on  its  properties  of  toughening,  whitening,  hardening, 
increasing  the  elasticity  and  preventing  the  oxidation  of  certain 
alloys.  Nickel  steels  are  the  most  important  of  all  alloy  steels. 

The  geology  of  the  region  is  complicated.  On  a  basement  of 
Keewatin  greenstone  and  the  Sudbury  quartzite  (lower  Huronian) 
rests  a  syncline  of  upper  Huronian  or  Animikie  rocks  including 
conglomerate,  tuff  and  slates.  This  syncline  is  36  miles  long 
and  16  miles  wide  (Fig.  265).  Between  the  basement  and  the 
Animikie  there  is  intruded  a  thick  sheet  of  igneous  massive 
rocks  which  may  be  of  Keweenawan  age  and  is  also  referred 
to  as  the  "nickel  eruptive"  on  account  of  its  unquestionable 
connection  with  the  nickel  deposits.  This  sheet  is  strongly 
differentiated,  supposedly  by  gravitative  settling  of  crystals  in 
the  magma.  It  grades  from  a  norite  or  hypersthene  gabbro 
in  the  lower  parts  to  a  micropegmatitic  granite  in  the  upper 
parts.  Even  in  the  lower  section  there  is  a  certain  amount  of 
" acid  extract"  of  micropegmatite  between  the  other  constituents. 

Granitic  rocks  are  intruded  in  the  basement  and  there  are 
even  some  granite  dikes  in  the  norite  at  the  Murray  mine  and 
possibly  at  the  Creighton  mine. 

The  deposits  are  found  (1)  as  rudely  tabular  masses  at  the 
contact  of  the  norite  with  the  basement  rocks  (marginal  de- 
posits); they  dip  30°  to  60°  toward  the  center  of  the  syncline 
(Creighton,  Gertrude,  Murray  mines).  (2)  As  mineralized 
dikes  or  "offset  deposits"  in  the  basement  rocks.  These  are 

A.  P.  Coleman,  The  Sudbury  laccolitic  sheet,  Jour.  Geology,  vol.  15, 
1917,  p.  252. 

W.  G.  Miller  and  C.  W.  Knight,  Nickel  deposits  of  the  world,  Royal 
Ontario  Nickel  Comm.,  Toronto,  1917. 

A.  M.  Bateman,  Econ.  Geol,  vol.  12,  1917,  pp.  391-426. 

M.  A.  Dresser,  Econ.  Geol,  vol.  12,  1917,  pp.  563-580. 


CONCENTRATION  IN  MOLTEN  MAGMAS       815 

steep,  irregular  or  columnar  and  have  been  followed  to  depths  of 
1,200  or  1,400  feet  (Copper  Cliff  and  Victoria  mines). 

The  ore  minerals  are  pyrrhotite,  pentlandite  and  chalcopyrite 
with  occasional  magnetite,  pyrite,  sphalerite,  sperrylite  (PtAs2), 
polydymite  (NuSs).  The  order  of  succession  as  established  by 
Tolman  and  Rogers  is  silicates,  magnetite,  pyrrhotite,  pentlandite 
and  chalcopyrite.  Pentlandite  of  ten  forms  veinlets  in  pyrrhotite 
and  can  easily  be  distinguished  from  the  latter  in  polished 
section  by  etching  with  hydrochloric  acid.  The  precious  metals 
seem  to  follow  the  chalcopyrite  and  occur  most  abundantly  in 
some  of  the  "offset  deposits"  like  Victoria  and  Vermilion.  The 
ores  of  the  Creighton  mine  are  the  richest,  containing  in  per 
cent,  about  4  nickel  and  1.4  copper.  Other  deposits  yield  poorer 
ore  with  2  nickel  and  0.8  copper.  The  values  of  the  precious  met- 
als aggregates  $1  to  $2  per  ton. 


Scale  of  Miles 


FIG.  265.— Section  across  Sudbury  syncline  showing  intrusive  norite  sheet. 
After  A.  P.  Coleman. 

The  best  developed  marginal  deposit  is  at  the  Creighton  mine 
where  it  has  been  followed  to  a  depth  of  2,000  feet  on  the  incline 
(Fig.  266).  It  lies  between  granite  and  norite  but  as  in  other 
marginal  deposits  the  ore  is  a  breccia  or  mass  of  subangular 
boulders  of  almost  barren  norite,  cemented  by  the  ore  minerals, 
which  often  form  a  hard  crust  on  the  rock.  The  sulphides  also 
enter  norite  and  granite  as  veinlets.  The  massive  ore  contains 
abundant  remnants  of  partly  replaced  rock  minerals  and  in  the 
poorer  ores  the  sulphides  have  corroded  and  replaced  the  rock 
minerals.  However,  the  interpretation  of  the  facts  observed 
under  the  microscope  varies  considerably.  No  gangue  minerals 
are  formed.  The  ore  shoots  are  from  a  few  feet  to  150  feet 
thick.  Note  similarity  of  Fig.  266  to  sections  of  Norwegian 
deposits  given  by  Vogt 

In  the  "offset  deposits"  the  relations  are  similar  although 


816 


MINERAL  DEPOSITS 


we  here  find  various  gangue  minerals  such  as  chlorite  (Copper 
Cliff),  epidote  and  quartz  indicating  somewhat  different  solutions 
and  probably  lower  temperatures. 

The  earlier  view  of  a  gravitative  settling  of  the  sulphides  in  the 
norite  sheet  has  given  way  to  the  theory  of  an  injection  of  sul- 
phide magma  more  or  less  charged  with  mineralizers  along  cer- 
tain brecciated  or  fractured  zones.  Graton's  term  of  "pneu- 
motectic  deposits"  (p.  811)  is  applicable  to  Sudbury  and  in 
places  the  deposits  even  show  transitions  to  high  temperature 
veins.  In  some  respects  they  present  strong  similarity  to  the 


FIG.  266. — Generalized  section  through  Creighton  ore-body,  Sudbury, 
Ontario,  extending  to  eighteenth  level.  The  norite  contains  blebs  of  ore 
about  size  of  peas,  for  2000  feet  beyond  the  ore-body.  After  C.  W.  Knight. 


" injected  pyritic  deposits"  (p.  818).  In  minor  part  they  may 
have  been  formed  by  direct  magmatic  segregation  from  the 
nickel  eruptive,  but  in  greater  part  they  have  been  formed  at 
the  end  of  the  magmatic  period  by  replacement  of  the  silicates 
by  a  very  liquid  melt  charged  with  sulphides  and  developed 
by  differentiation  in  a  magma  reservoir  in  depth.  With  these 
views  agree,  wholly  or  partly,  Tolman  and  Rogers,  Howe, 
Bateman,  and  even  Coleman  and  Knight.  That  the  nickel 
ores  are  genetically  connected  with  the  norite  admits  of  no 
doubt. 


CONCENTRATION  IN  MOLTEN  MAGMAS       817 

Cape  Colony. — A  deposit  at  Insizwa,  Cape  Colony,  similar  to 
that  of  Sudbury,  has  been  described  by  A.  E.  Dutoit  and  W.  H. 
Goodchild.1  Pyrrhotite,  chalcopyrite,  and  pentlandite,  with  a 
little  bornite  and  niccolite,  occur  in  a  thick  sheet  of  gabbro  or 
norite,  at  its  contact  with  underlying  sediments.  The  ores 
contain  also  a  little  platinum,  osmiridium,  gold,  and  silver. 
The  sulphides  separated  from  the  magma  at  the  end  of  the 
period  of  consolidation  and  corrode  idiomorphic  olivine  (Fig. 
267),  pyroxene,  and  feldspar. 

Bornite  Deposits. — Bornite  is  occasionally  recorded  as  a  minor 
constituent  of  pegmatite  dikes  and  sometimes  occurs  in  the 
deep  vein  zone.  A  small  but  remarkable  bornite  deposit  in 
an  igneous  rock,  described  by  E.  Hitter2  and  lately  by  E.  S.  Bas- 
tin  and  J.  M.  Hill,3  occurs  at  the  Evergreen  mine,  Gilpin  County, 
Colorado.  Dikes  of  a  monzonitic  rock,  in  part  brecciated,  con- 
tain, intergrown  with  the  primary  minerals,  bornite  and  chalcopyr- 
ite, also  garnet,  calcite,  and  wollastonite.  All  these  minerals 
are  contemporaneous  with  the  ordinary  rock  minerals.  This 
seems  to  be  a  case  of  digestion  of  material  from  calcareous  rocks; 
possibly  the  sulphides  are  also  of  foreign  origin,  perhaps  derived 
by  absorption  from  an  older  deposit;  an  origin  by  direct  differ- 
entiation is,  however,  not  unlikely.  The  ore  extracted  con- 
tained 3  per  cent,  copper  and  $5  in  gold  and  silver  per  ton. 

The  remarkable  and  rich  bornite  deposits  of  Ookiep  in  Nama- 
qualand,  Cape  Colony,  have  been  regarded  as  magmatic  by 
Stutzer,  -Kuntz  and  other  authors.  However,  here  too,  the 
idea  of  fractured  zones  directing  or  enriching  the  ores  has  been 
brought  forward.4 

Tolman  and  Rogers5  have  lately  examined  these  ores  and  con- 
clude that  they  are  of  typical  magmatic  origin.  Magnetite,  hema- 
tite, chalcopyrite  and  bornite  replace  the  silicates  in  hypersthenite 
and  diorite. 

The  Engels  Mine,  Plumas  County,  California,  contains  dis- 
seminated bornite  and  chalcocite  in  a  mass  of  norite.  Some 

1  Fifteenth  Ann.  Rept.,  Geol.  Comm.,  Cape  of  Good  Hope  (1910),  1911, 
pp.  110-142. 

W.  H.  Goodchild,  The  economic  geology  of  the  Insizwa  range,  Trans. 
Inst.  Min.  and  Met.  (London),  1916. 

2  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  38,  1907,  pp.  751-765. 
»  Econ.  Geol.,  vol.  6,  1911,  pp.  465-472. 

4  A.  Schenk,  Vortr.  Zeilschr.  d.  d.  Geol.  Ges.,  53,  vol.  4  H,  1902,  p.  64. 
'  Op.  tit. 


818  MINERAL  DEPOSITS 

authors1  have  attributed  the  bornite  to  strictly  magmatic  proc- 
esses while  the  chalcocite  was  believed  deposited  by  concentrat- 
ing thermal  waters.  Later,  Tolman  and  Rogers2  regarded  the 
bornite  as  deposited  by  the  aid  of  mineralizers  at  a  later  mag- 
matic stage. 

Still  later  Graton  and  McLaughlin3  classified  the  deposit  as 
of  pneumatolytic  origin  with  amphibole,  albite,  tourmaline, 
magnetite  as  gangue  minerals  and  bornite  and  chalcopyrite 
as  ore  minerals.  This  was  followed  by  hydrothermal  action 
producing  chlorite,  sericite,  epidote  and  bornite,  and  finally, 
zeolites.  The  chalcocite  is  regarded  as  a  product  of  descending 
meteoric  waters. 

INJECTED  PYRITIC  DEPOSITS 

General  Features. — Some  deposits  in  which  the  ore  consists 
mainly  of  solid  pyritic  minerals  present  features  which  can  hardly 


FIG.  267. — Thin  section  of  olivine  norite,  Insizwa  mine,  Cape  Colony. 
Magnified  18  diameters.  Black  areas,  pyrrhotite;  ol,  olivine;  py,  pyroxene; 
bi,  biotite;  la,  labradorite.  After  A.  L.  Du  Toil. 

be  explained  otherwise  than  by  actual  injection  of  molten  sul- 
phides, perhaps  to  be  considered  as  residual  solutions  from 
adjoining  intrusive  bodies.  A.  Bergeat4  first  summarized  these 
peculiar  occurrences,  among  which,  it  must  be  confessed,  are 

1  H.  W.  Turner  and  A.  F.  Rogers,  Econ.  Geol,  vol.  9,  1914,  pp.  359-391. 

2  A  study  of  the  magmatic  sulfid  ores,  Stanford  Univ.  Pub.,  Univ.  Ser., 
1916. 

3  L.  C.  Graton  and  D.  H.  McLaughlin,  Econ.  Geol.,  vol.  12,  1917,  pp.  1-38. 

4  A.  W.  Stelzner  and  A.  Bergeat,  Die  Erzlagerstatten,  vol.  2,  pp.  964-987. 


CONCENTRATION  IN  MOLTEN  MAGMAS       819 

some  of  the  most  enigmatic  of  ore  deposits.  The  examina- 
tions of  the  deposit  at  Bodenmais,  in  Bavaria,  by  J.  Lehmann 
and  E.  Weinschenk  appear  to  have  led  up  to  the  definite  view 
suggested  above,  and  since  then  a  number  of  other  occurrences 
have  been  added  to  this  class,  which  presents  strong  points  of 
similarity  to  the  Sudbury  deposits  as  interpreted  above. 

These  ores  are  usually  enclosed  in  schist  or  gneiss  and  that 
they  originated  by  metasomatic  replacement  of  limestone  appears 
to  be  out  of  the  question,  though  it  must  not  be  overlooked 
that  the  same  was  thought  once  of  the  ores  at  Ducktown,  Ten- 
nessee, which  now  have  been  shown  by  W.  H.  Emmons  to  be 
replacements  of  small  limestone  lenses  in  a  prevailing  schist 
formation  (p.  751). 

That  fluid  sulphides  may  penetrate  silicate  rocks  in  veinlets 
and  corrode  the  various  primary  minerals,  like  augite,  has  been 
shown  in  interesting  experiments  by  O.  Stutzer,1  and  by  previous 
observations  by  von  Gotta  on  the  brickwork  of  old  lead  furnaces. 
In  Stutzer's  experiments  the  sulphide  veinlets  of  pyrrhotite, 
zinc  blende,  and  galena  penetrated  the  rocks  along  minute 
cracks  and  along  the  cleavage  planes  of  the  minerals.  In  gabbros 
the  pyroxene  grains  were  corroded,  in  a  manner  similar  to  that 
noted  in  the  ores  of  Sudbury.  The  sulphide  melt  would  probably 
be  under  high  pressure  and  would  force  its  way  into  the  adjoin- 
ing rocks.  Deposits  of  this  kind  are  decidedly  rare.  The  igneous 
rocks  near  whose  contact  injected  deposits  lie  are  of  many  kinds, 
not  always  of  basic  character. 

The  minerals  of  the  ores  include  magnetite,  pyrrhotite,  pyrite, 
zinc  blende,  chalcopyrite,  and  rarely  galena.  The  associated 
gangue  minerals  surely  indicate  high  temperature  and  are  present 
in  scant  quantity;  they  are  quartz,  orthoclase,  plagioclase,  am- 
phibole,  hypersthene,  biotite,  cordierite,  spinel,  especially  zinc 
spinel,  and  garnet.  The  various  minerals  are  practically  con- 
temporaneous. The  feldspars  have  a  characteristic  greenish 
color. 

Bavaria.— At  Bodenmais2  granite  intersects  gneisses.  The  ore 
deposits  lie  in  cordierite  gneiss.  The  ores  contain  pyrrhotite 
and  pyrite,  with  some  zinc  blende  rich  in  cadmium  and  galena 
rich  in  silver;  the  bodies  lie  in  general  parallel  to  the  dip  of  the 
gneiss,  but,  according  to  Weinschenk,  the  contact  between  ore 

1  Zeitschr.  prakt.  Geol,  vol.  16,  1908,  pp.  119-122. 

2  E.  Weinschenk,  Zeitschr.  prakt.  Geol.,  1900,  pp.  65-71. 


820  MINERAL  DEPOSITS 

and  gneiss  is  sharp,  though  there  are  some  disseminated  sulphides 
in  the  surrounding  rock.  Many  of  the  gangue  minerals  in  the 
ore  are  rounded  or  corroded. 

Sweden. — The  renowned  copper  deposit  at  Falun,1  in  Sweden, 
forms  a  huge  inverted  cone  enclosed  in  gray  quartzose  and 
gneissoid  rocks  and  extending  to  a  depth  of  1,200  feet.  The  ore- 
body  is  really  composed  of  the  same  rock,  impregnated  to  greater 
or  less  extent  with  pyrite,  pyrrhotite,  and  chalcopyrite.  The 
gangue  minerals  accompanying  the  ore  are  cordierite,  magnetite, 
andalusite,  spinel,  and  garnet.  It  is  difficult  to  arrive  at  a 
definite  conclusion  regarding  the  origin  of  this  deposit;  at  any 
rate  it  was  formed  at  high  temperature.  According  to  Vogt 
the  total  production  of  copper  from  Falun  from  1300  to  the  pres- 
ent time  is  about  480,000  metric  tons. 

The  copper  deposit  at  Bersbo,2  in  Sweden,  is  also  considered 
by  Bergeat  to  belong  to  this  class.  The  ores  are  quartzose  and 
are  embedded  in  gray  fine-grained  "granulite"  or  "leptite"  (p. 
757),  which  is  now  by  many  considered  an  igneous  and  intrusive 
rock.  In  thin  section  the  ores  show  a  texture  resembling  that  of 
contact-metamorphosed  schist  and  contain  as  gangue  minerals 
quartz,  cordierite,  spinel,  biotite,  hornblende,  and  garnet.  On 
the  whole,  the  succession  is  magnetite  (oldest),  pyrite,  pyrrhotite, 
zinc  blende,  and  chalcopyrite. 

Norway. — A  number  of  remarkable  pyritic  deposits  are  found 
in  Norway;  among  them  are  the  well-known  localities  of  Roras. 
Vigsnas,  and  Sulitjelma,  all  of  which  have  been  the  subject  of 
extended  discussion.  They  occur  in  metamorphic  schists  in- 
cluding clay  slate,  chloritic  schist,  amphibolite,  and  in  part 
certainly  in  dynamo-metamorphosed  gabbro  intrusions.  The 
ores  consist  of  early  pyrite,  chalcopyrite,  sphalerite  and  pyrrhotite. 
A  little  biotite  and  magnetite  is  present.3  Flat  ore  lenses  prevail, 
in  some  places  strictly  parallel  to  the  schistosity,  in  other  places, 
as  at  Roras,  distinctly  cutting  across  it.  In  large  part  they  are 
massive  pyritic  bodies,  but  the  neighboring  rock  is  usually  im- 

1  Hj.  Sjogren,  The  Falun  copper  mine,  Guide  exc.,  XI"  Cong.  geol.  inter- 
nat.,  Stockholm,  No.  31,  1910. 

A.  E.  Tornebohm,  Geol.  For.,  Fork.,  Stockholm,  vol.  15,  1893,  pp.  609- 
690. 

2  A.  E.  Tornebohm,  Geol.  For.  Fork.,  vol.  7,  1885,  pp.  562-597. 
A.  Bergeat,  op.  tit.,  p.  978. 

3H.  Hies  and  R.  E.  Somers,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  58,  1918, 
pp.  244-264. 


CONCENTRATION  IN  MOLTEN  MAGMAS        821 

pregnated  with  pyritic  ore.  One  of  the  flat  ore-bodies  at  Roras 
extended  along  its  dip  for  1,900  meters  and  was  100  meters  wide, 
averaging  8  meters  in  thickness.1  At  Sulitjelma  the  contact 
phenomena  have  been  interpreted  as  injections.  The  ore  brec- 
ciates  the  schist  and  enters  into  it  on  veins  and  seams.  Feld- 
spar, chlorite  and  garnet  of  the  schist  are  embayed  but  not 
sericitized  or  otherwise  altered. 

Quartz,  actinolite,  garnet,  epidote,  and  biotite  accompany  the 
ore  minerals  at  some  places.  Th.  Kjerulf  and  J.  H.  L.  Vogt2 
among  others  consider  these  deposits  as  igneous  injections,  the 
latter  author  placing  them  in  genetic  association  with  the  gabbro 
intrusions. 

1  Stelzner  and  Bergeat,  vol.  1,  1904,  p.  298. 

2  Vogt  has  asserted  a  relationship  between  these  ores  and  those  of  Ram- 
melsberg,  in  the  Harz,  Germany.     As  shown  on  pp.  644-648,  the  latter 
belong  in  an  entirely  different  class,  with  barite  gangue,  and  show  an 
absence  of  high  temperature  minerals. 


CHAPTER  XXX 

METAMORPHOSED  DEPOSITS 

PROCESSES  INVOLVED 

Mineral  deposits  are  usually  formed  during  comparatively 
brief  epochs,  in  which  uniform  conditions  prevail,  rendering  a 
given  set  of  minerals  stable.  In  the  development  of  the  epi- 
genetic  deposits  this  is  not  invariably  true,  for  we  sometimes 
find  evidence  of  successive  changes  in  the  mineral-bearing 
solutions;  early  minerals  are  dissolved  and  a  new  set  formed. 
The  replacement  of  calcite  veins  by  silica  offers  an  instance 
of  this  process,  as  do  also  the  successive  generations  of  minerals 
in  zeolitic  copper  deposits  and  in  pegmatite  dikes. 

After  the  epoch  of  mineralization  has  passed  the  deposit  will, 
as  a  rule,  be  subjected  to  different  temperatures  and  different 
degrees  of  pressure,  and  solutions  of  various  kinds  will  percolate 
through  it.  Consequently,  in  many  deposits  the  minerals  of 
their  ore  are  now  unstable  and  only  the  slowness  of  the  changes 
may  prevent  them  from  being  wholly  altered.  "Persistent" 
minerals  remain  unaffected  except  by  mechanical  deformation, 
but  very  few  minerals  are  persistent  in  all  zones. 

In  general,  when  by  erosion,  intrusion,  or  dynamo-metamor- 
phism  a  mineral  deposit  is  transferred  to  a  new  zone,  the  char- 
acteristic minerals  of  this  zone  will  develop  in  it  and  become 
superimposed  upon  the  original  minerals.  Some  deposits  have 
a  complicated  history,  having  been  subjected  to  several  changes, 
each  of  which  has  left  its  imprint  on  the  ores. 

It  often  happens  that  a  deposit  becomes  involved  in  folding 
or  dynamic  metamorphism  of  general  or  local  kind;  there  will 
then  be  mechanical  deformation;  veins  and  irregular  masses 
will  be  squeezed  out  into  lenses  which  may  in  places  overlap 
or  imbricate.  The  minerals  of  dynamic  metamorphism,  such 
*as  garnet,  amphibole,  and  biotite  of  the  deeper  zones,  or  chlorite, 
epidote,  zoisite,  muscovite,  albite,  and  talc  of  the  upper  zones, 
will  be  formed  from  the  old  constituents.  Hydrates  may  lose 
their  water  and  carbonates  their  carbon  dioxide.  The  quartz- 

822 


METAMORPHOSED  DEPOSITS  823 

sulphide  veins  are  least  affected,  their  minerals  being  compara- 
tively persistent. 

Most  deposits  have  been  exposed  to  static  metamorphism 
at  moderate  temperature,  during  which  chlorite,  carbonates, 
and  epidote  have  developed.  Increased  temperature  may 
leave  some  deposits  unaltered,  while  others  in  the  vicinity  of 
igneous  masses  may  be  profoundly  modified.  Examples  are 
known  of  sedimentary  deposits  of  limonite  or  siderite  which, 
close  to  intrusive  rocks,  change  to  magnetite  and  specularite 
and  in  which  garnets  and  other  silicates  develop.  Such  deposits 
may  simulate  those  of  contact-metamorphic  origin,  but  in  the 
latter  the  ores  did  not  exist  in  the  sedimentary  rocks  but  were 
introduced  by  solutions.  Some  of  the  deposits  in  the  pre- 
Cambrian  terranes  owe  their  complex  nature  to  successive 
changes,  and  their  history  may  be  most  difficult  to  unravel. 

DEFORMED  PYRITIC  DEPOSITS 

The  copper  deposit  at  Rammelsberg,1  in  the  Harz  Mountains 
(p.  644),  illustrates  well  the  effects  of  local  dynamo-metamor- 
phism  at  no  great  depth.  Under  strong  pressure  the  softer 
minerals  like  galena,  chalcopyrite,  and  zinc  blende  are  easily  de- 
formed and  pressed  out  to  plastic  streaky  masses.  Pyrite,2  being 
harder,  is  crushed  without  plastic  deformation  and  subsequently 
cemented. 

W.  H.  Emmons3  has  examined  some  copper  deposits  in  New 
Hampshire,  the  origin  of  which  antedated  the  metamorphism 
of  the  surrounding  rocks.  At  the  Milan  mine  there  are  two 
overlapping  lenses  of  cupriferous  pyrite  that  are  clearly  portions 
of  a  single  ore-body  which  was  separated  during  the  process 
of  regional  metamorphism.  The  surrounding  quartz-chlorite 
schist  was  in  its  zone  of  flow  while  the  pyrite  deposit  was  in 
its  zone  of  fracture  (Fig.  268-).  The  massive  pyrite  shows 
little  deformation,  but  near  the  walls,  in  the  lower  grade  ore, 
quartz  and  pyrite  have  been  pressed  out  into  schistose  form. 
Thin  sections  show  crushing  and  re-cementing  of  the  pyrite, 
which  seems  massive  in  the  hand  specimen.  The  gangue  at  the 
Milan  mine  consists  of  quartz,  muscovite,  biotite,  and  chlorite; 

1  Econ.  Geol,  vol.  6,  1911,  pp.  303-313. 

2  F.  D.  Adams,  An  experimental  investigation  into  the  action  of  differen- 
tial pressure,  etc.,  Jour.  Geology,  vol.  18,  1910,  pp.  480-535. 

3  Bull  432,  U.  S.  Geol.  Survey,  1910,  p.  62. 


824 


MINERAL  DEPOSITS 


probably  the  three  last  named  minerals  are  of  dynamometamor- 
phic  origin. 

In  the  interpretation  of  these  deposits  it  is  necessary  to  search 
for  relics  of  older  gangue  minerals,  more  or  less  affected  and 
changed  by  pressure.  In  many  deposits  such  mineralsjjrnay 
have  been  entirely  obliterated. 


FIG.  268. — Diagram  showing  deformation  of  pyritic  vein  at  Milan,  New 
Hampshire.     After  W.  H.  Emmons,  U.  S.  Geol.  Survey. 

REGIONALLY  METAMORPHOSED  IRON  ORES 

General  Features. — In  regionally  metamorphosed  sediments 
or  in  crystalline  schists,  the  origin  of  which  may  be  in  doubt, 
bedded  deposits  of  magnetite  or  specularite,  or  both,  are  often 
encountered.  The  well-known  fact  that  iron  ores  such  as 
limonite,  siderite,  hematite,  or  iron  silicates  (chamosite  and 
thuringite)  form  integral  parts  of  sedimentary  series  of  all  ages 
suggests  strongly  that  the  beds,  of  these  ores  in  metamorphosed 
rocks  also  had  a  sedimentary  origin.  As  a  rule  this  is  no  doubt 
true,  but  the  metamorphism  may  have  gone  so  far  that  the 
original  sedimentary  nature  of  the  surrounding  rocks  may  be 
open  to  doubt,  and  many  observers  maintain  an  igneous  origin 
for  some  such  deposits.  Indirectly,  igneous  rocks  have  often 
brought  about  the  accumulation  of  bedded  iron  ores,  either 
by  the  weathering  and  denudation  of  intrusive  rocks  or  lavas 
rich  in  iron,  or  possibly  by  direct  emanations  from  volcanic 
rocks. 


METAMORPHOSED  DEPOSITS  825 

Bedded  metamorphic  iron  ores  are  accompanied  by  silicate 
minerals,  like  feldspar,  actinolite,  and  garnet,  usually  also  by 
quartz,  and  they  have  assumed  a  thoroughly  crystalline  texture 
similar  to  that  of  other  crystalline  schists,  the  constituents 
being  generally  interpenetrating,  indicating  almost  simultaneous 
development.  Relic  structure  showing  the  sedimentary  origin 
is  rarely  observed. 

Swedish  "Dry  Ores."1 — Sweden  and  Norway  are  rich  in  these 
bedded  ores,  which  often  appear  in  the  vicinity  of  other  iron 
deposits  of  different  kind.  Some  are  found  near  the  great 
magmatic  deposit  of  Kiruna,  interbedded  in  tuff  and  shales  of 
late  pre-Cambrian  age.  Others,  which  are  worked  more  ex- 
tensively, appear  near  the  metasomatic  magnetites  of  central 
Sweden  (p.  755)  and  form  part  of  the  complicated  leptite 
series  (p.  757).  They  are  designated  "dry  ores"  (torr-sten) 
and  are  usually  siliceous,  the  accompanying  beds  averaging  84 
per  cent,  silica.  The  ores  average  50  per  cent,  iron,  contained 
in  micaceous  fine-grained  specularite  with  a  little  magnetite. 
The  accompanying  beds  in  places  contain  garnet,  amphibole,  or 
epidote,  each  mineral  often  forming  a  separate  streak.  They  are 
markedly  banded.  Many  of  the  beds  are  10  or  15  feet  thick, 
though  some  considerably  exceed  15  feet,  and  have  been  followed 
with  regular  steep  dip  to  a  depth  of  several  hundred  feet.  These 
ores  contain  little  phosphorus.  An  analysis  of  such  ore  from 
Striberg  is  as  follows: 

Fe 52.20  CaO 1.05 

Fe2O3 60.21  A12O3 0.89 

FeO 13.93  SiO2 23.61 

MnO 0.09  P2OB 0.043 

Mg 0.31  S .     0.021 

Until  recently  little  doubt  has  been  expressed  about  the  sedi- 
mentary origin  of  these  ores.  Lately,  however,  H.  Johannson 
has  announced  his  opinion  that  the  fine-grained  leptites  are 
simply  a  product  of  extreme  magmatic  differentiation  and  that 
the  accompanying  bedded  iron  ores  are  also  of  magmatic  origin. 
He  even  believes  that  the  metasomatic  limestone  and  "skarn 
ores"  (p.  757)  have  this  origin.  Hj.  Sjogren  does  not  share  this 

1  H.  E.  Johannson,  Geol.  For.,  Fork.,  vol.  32,  1910. 
H.  Sjogren,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  38,  1908,  pp.  766-835. 
See  also  references  on  p.  755. 


826  MINERAL  DEPOSITS 

opinion  but  holds  that  the  bedded  ores  and  limestone  ores  are 
caused  by  injection  or  replacement  by  "granitic  extracts"  while 
the  differentiated  granulites  were  in  the  anamorphic  zone. 
Holmquist  thinks  that  by  deep  burial  these  originally  sedimen- 
tary ores  have  been  subjected  to  igneous  metamorphism  followed 
by  slight  regional  metamorphism  (p.  759). 

It  does  not  seem  that  the  opponents  to  the  sedimentary  genesis 
of  the  ores  have  proved  their  case. 

Norwegian  Ores.1 —  Northern  Norway  is  rich  in  deposits  of  the 
type  here  discussed.  Banded  magnetites,  variously  interpreted, 
occur  on  a  large  scale  in  thick  beds  that  are  traceable  for  several 


FIG.  269. — Thin  section  of  typical  Syd  Varanger  ore.  Black,  magnetite ; 
white,  quartz ;  striated,  hornblende.  Magnified  25  diameters.  After  J.  H. 
L.  Vogt. 

miles  in  South  Varanger,  near  the  Russian  frontier.  The  ores 
are  mined  on  a  large  scale  and  concentrated.  Some  of  the  larger 
bodies  are  1,000  feet  long  and  25  feet  thick  and  contain  about  35 
per  cent.  iron.  One  hundred  million  metric  tons  are  available 

1  J.  H.  L.  Vogt,  Norway,  in  Iron-ore  resources  of  the  world,  Stockholm, 
1910. 

J.  H.  L.  Vogt,  Norges  jernmalmforekomster,  Norges  Geol.  Undersok., 
No.  51,  Kristiania,  1910. 

Hj.  Sjogren,  Om  jernmalmerna  i  granit  p&  Lofoten,  Geol.  For.,  Forh., 
vol.  30,  1908. 

Per  Geijer,  Contributions  to  the  geology  of  the  Sydvaranger  iron-ore 
deposits,  Geol.  For.,  Forh.,  vol.  33,  1911,  pp:  312-343. 


METAMORPHOSED  DEPOSITS  827 

for  open-cut  mining.  An  analysis  given  by  Vogt  shows  3G.71 
per  cent.  Fe2O3,  15.40  per  cent.  FeO,  43.92  per  cent.  SiO2,  0.07 
per  cent.  P2O5,  and  0.04  per  cent.  S.  There  is  little  alumina, 
lime,  or  magnesia.  The  ores  are  beautifully  banded  and 
according  to  P.  Geijer  are  associated  with  fine-grained  "leptites" 
(granulites)  rich  in  quartz  with  some  orthoclase  and  oligoclase; 
hornblende,  garnet,  and  diopside  accompany  the  ore  (Fig.  269). 
While  Vogt  considers  the  ores  to  be  due  to  igneous  differentiation 
and  Sjogren  believes  similar  ores  from  the  Lofoten  Islands  to  be 
intrusive  into  an  igneous  rock,  Geijer  gives  good  reasons  why  they 
should  be  held  to  be  of  sedimentary  origin  and  deposited  as 
chemical  sediments.  It  seems  that  the  advocates  of  intrusive 
origin  for  these  occurrences  have  few  cogent  arguments. 

Ores  of  distinctly  sedimentary  origin  are  found  at  Dunder- 
land  and  Naeverhaugen,  also  in  northern  Norway.  They  form 
beds  traceable  for  many  miles,  with  a  thickness  of  3  to  10  meters, 
or  in  places  even  50  meters.  They  are  intercalated  jn  a  thick 
series  of  mica  schist  and  crystalline  marbles  believed  to  be  of 
Paleozoic  age.  The  closely  banded  ores  carry  mainly  specularite 
and  magnetite,  but  are  of  low  grade.  The  concentration,  at- 
tempted on  a  large  scale  and  at  great  expense,  has  failed  because 
of  the  scaly  character  of  the  specularite.  The  average  content  in 
iron  is  said  to  be  40  per  cent.  Hornblende,  garnet,  epidote, 
and  feldspar  are  accessory  minerals.  There  is  little  sulphur,  but 
phosphorus  is  present  in  quantities  as  great  as  0.3  per  cent- 
United  States. — The  Lake  Superior  ores  do  not  strictly  belong 
to  the  type  here  discussed;  they  are  rather  rich  concentrations 
in  the  zone  of  oxidation  from  low-grade  sedimentary  beds. 
In  places  where  the  iron  formations  have  been  exposed  to  contact- 
metamorphism  ores  with  magnetite  and  grunerite  result. 

Typical  quartz-magnetite  ores,  described  by  Sidney  Paige,1 
occur  in  the  Llano  region  in  Texas,  but  have  not  yet  been  util- 
ized. The  pre-Cambrian  series  of  crystalline  schists  in  this 
region  consists  of  gneiss,  mica  schist,  and  quartzite  with  some 
limestone  lenses.  The  ores  are  thinly  bedded  and  occur  in  granu- 
lar schists  or  gneisses.  A  specimen  of  lean  ore  consisted  of  mag- 
netite 22  per  cent.,  quartz  50  per  cent.,  albite  and  albite-oligoclase 
26  per  cent.  There  is  much  more  soda  than  potash.  The 
iron  was  probably,  according  to  Paige,  deposited  as  glauconite, 
and  contact-metamorphism  by  later  pre-Cambrian  granite  has 
1  Bull.  450,  U.  S.  Geol.  Survey,  1911. 


828  MINERAL  DEPOSITS 

effected  the  removal  of  potash  and  introduction  of  soda.  Ex- 
amples of  adinole  and  other  contact-metamorphic  rocks  are 
cited  to  support  this  view.  Low-grade  ore  representing  a  bed 
17  feet  thick  contained  Fe  35.87  per  cent.,  Si02  34.57  per  cent., 
Mn  1.05  per  cent.,  P  0.07  per  cent.,  S  0.04  per  cent.,  and  Ti02 
0.15  per  cent. 

The  Grenville  series  of  pre-Cambrian  metamorphosed  sedi- 
ments in  northern  New  York  contain,  according  to  D.  H.  New- 
land,1  similar  deposits  of  magnetite.  The  ores  are  mostly  en- 
closed in  quartzose  gneisses  with  hornblende  and  biotite  and  are 
believed  to  be  of  sedimentary  origin. 

THE  ZINC  ORES  OF  AMMEBERG,  SWEDEN 

One  of  the  most  mysterious  of  ore  deposits  is  that  of  Amme- 
berg,2  in  Sweden,  where  the  zinc  ore  is  disseminated  in  banded 
and  contorted  gray  gneissoid  "leptite."  Some  layers  of  gabbro, 
crystalline,  limestone,  and  lime-silicate  rocks  are  intercalated 
in  the  steeply  dipping  leptite.  Zinc  blende,  with  very  little 
galena,  is  widely  disseminated  in  the  leptite,  seemingly  taking 
the  place  of  magnetite,  and  along  certain  zones  has  accumulated 
as  long,  lenticular  folded  bands,  some  of  which  are  30  to  50  feet 
in  width  and  have  been  followed  to  depths  of  1,000  feet.  The 
ores  are  rich  in  zinc  blende  but  contain  few  other  minerals.  A 
"fahlband"  of  disseminated  pyrrhotite  and  arsenopyrite  lies  in 
the  leptite  of  the  footwall. 

The  deposit  certainly  seems  to  be  of  syngenetic  origin  and 
the  mineral  association  indicates  that  it  has  been  subjected  to 
high  temperatures. 

1  Geology  of  the  Adirondack  magnetic  iron  ores,  Bull.  119,  N.  Y.  State 
Mus.,  1908,  pp.  27,  40-41.  o 

2  H.  E.  Johansson,  The  Ammeberg  zinc  ore  field,  Geol.  For.,  Fork.,  vol. 
32,  Stockholm,  April,  1910.     Guide  exc.,  XIe  Cong.  geol.  internat.,  Stock- 
holm, No.  35,  1910. 


CHAPTER  XXXI 

OXIDATION  OF  METALLIC  ORES1 
3ENERAL  CONDITIONS 

The  upper  part  of  a  mineral  deposit,  within  the  zone  of  weather- 
ing, is  usually  more  or  less  altered  by  surface  waters  containing 
free  oxygen.  The  direct  effects  of  this  weathering  cease  in  many 
deposits  at  the  permanent  water  level,  but  in  deposits  of  sulphides 
the  indirect  effects,  due  to  the  action  of  sulphates  generated  by 
the  oxidation  of  primary  sulphides,  may  persist  to  a  considerable 
depth  below  the  water  level.  Generally  speaking,  the  zone  above 
the  water  level  is  that  of  the  oxy-salts,  haloid  salts,  and  native 
metals;  underneath  this  in  many  deposits  lies  a  zone  of  varying 
depth  in  which  secondary  sulphides  appear  and  strong  enrichment 
has  taken  place.  Finally,  beneath  these  zones  of  extensive  changes 
and  molecular  rearrangements  is  found  the  original  or  "  primary  " 
ore. 

The  oxidation  of  mineral  deposits  is  naturally  a  process  analo- 
gous to  rock-weathering.  In  deposits  free  from  sulphides  the 
changes  are  relatively  simple,  consisting  of  disintegration,  solu- 
tion, oxidation,  and  hydration.  Siderite  alters  to  limonite,  car- 
bonates of  manganese  to  pyrolusite;  calcite  is  dissolved;  the  rock 
minerals  change  to  kaolin.  The  final  products  are  likely  to  be 
residual  quartz,  limonite,  pyrolusite,  and  kaolin.  Where  native 
copper  is  present  malachite  and  cuprite  are  also  found  if  the 
leaching  has  not  been  carried  too  far. 

In  deposits  which  contain  sulphides,  but  no  pyrite,  the  changes 
are  rather  slow  and  inconspicuous.  Galena  changes  slowly  to 
anglesite  and  cerussite,  zinc  blende  to  calamine  and  smithsonite; 
galena  and  enargite  often  remain  unoxidized  close  to  the  surface. 
The  presence  of  pyrite,  which  easily  gives  off  one  atom  of  sulphur, 
changes  and  complicates  the  whole  trend  of  the  oxidizing  processes. 
During  oxidation  the  various  metals  behave  very  differently  and 
thus  many  separations  are  effected. 

1  For  a  fuller  treatment  of  this  subject  see  W.  H.  Emmons,  The  enrich- 
ment of  ore  deposits,  Bull.  625,  U.  S.  Geol.  Survey,  1917. 

829 


830  MINERAL  DEPOSITS 

DEPTH  OF  OXIDATION 

Oxidation  is  a  relatively  slow  process.  Some  of  the  more 
conspicuous  cases  of  deep  oxidation  have  required  long  geolog- 
ical time.  The  copper  deposits  at  Bisbee,  Arizona,  for  example, 
where  large  bodies  of  oxidized  ores  are  present,  were  probably 
attacked  by  oxidation  in  Cretaceous  time.  In  glaciated  areas, 
such  as  Canada  and  northern  Europe,  oxidation  has  made  little 
progress  since  the  ice  swept  away  the  older  accumulations  of 
weathered  products,  and  sulphide  ores  are  usually  found  close  to 
the  surface.  Comparatively  little  effect  has  been  produced  by 
an  exposure  of  several  thousand  years. 

In  non-glaciated  regions  provided  with  a  liberal  rainfall  the 
ground-water  level  is  usually  less  than  100  feet  below  the  surface 
and  the  oxidation  is  correspondingly  shallow.  On  the  other 
hand,  in  regions  with  deficient  rainfall  the  ground  water  may 
stand  several  hundred  feet  below  the  surface  and  the  oxygen  has 
had  an  opportunity  to  decompose  the  ores  to  a  similar  depth. 
At  the  copper  mines  of  Butte,  Montana,  the  sulphides  are  oxi- 
dized to  a  depth  of  at  most  400  feet;  in  the  silver-gold  veins  of 
Tonopah,  Nevada,  700  feet;  at  Tintic,  Utah,  in  limestone,  as 
much  as  1,600  or  2,200  feet.  At  Bisbee,  Arizona,  also  in  lime- 
stone, oxidized  copper  ores  are  found  at  a  depth  of  1,400  feet. 
At  the  Durango  lead  mine,  Mapimi,  Mexico,  the  ground-water 
level  is  said  to  stand  2,300  feet  below  the  surface. 

In  a  general  way  the  depth  of  the  water  table  corresponds  to 
the  depth  of  the  oxidized  sulphides,  but  this  is  a  rule  with  ex- 
ceptions and  qualifications.  In  most  districts  sulphide  ores  may 
be  found  in  local  masses  almost  or  quite  at  the  surface,  and,  on 
the  other  hand,  oxidation  may  penetrate  to  a  depth  of  several 
hundred  feet  below  the  water  level.  In  the  Cripple  Creek  dis- 
trict, Colorado,  for  example,  at  the  Golden  Cycle  mine  oxidized 
ores  were  found  200  feet  below  the  water  level.  It  is  simply  a 
question  of  the  presence  or  absence  of  decided  local  movement 
of  the  oxygen-charged  water  downward  along  the  fissures.  The 
changes  are,  of  course,  greatest  along  the  fissures,  where  oxida- 
tion is  usually  far  in  advance  of  the  weathering  of  the  general 
country  rock.  Changes  in  climate  or  elevation  with  correspond- 
ing changes  in  the  water  level  must  not  be  overlooked.  In 
Arizona  we  find  at  many  places— Clifton,  Globe,  and  Ray,  for 
instance — zones  of  secondary  chalcocite  which  assuredly  were 


OXIDATION  OF  METALLIC  ORES  831 

formed  below  the  water  level,  but  which  now  lie  high  above  the 
permanent  water.  At  Butte,  Montana,  on  the  other  hand,  there 
is  evidence  of  a  depression  of  the  block  containing  the  deposits 
which  has  had  the  effect  of  raising  the  water  level  high  above 
the  position  it  occupied  when  the  chalcocite  enrichment  took 
place.  The  facts  observed  in  some  districts  can  be  explained 
only  on  the  supposition  of  repeated  and  relatively  rapid  changes 
of  water  level. 

The  temperature  also  plays  a  part.  We  may  expect  a  deeper 
oxidation  in  warm  climates  than  in  cold  climates  simply  because 
heat  accelerates  reactions.  Frequent  alternation  of  moisture 
and  dryness  promotes  oxidation.  Porosity  and  fissuring  of  the 
rocks  and  ores  are  factors  extremely  favorable  to  oxidation.1 

In  a  region  of  dry  climate  where  mountain  ranges  are  separated 
by  valleys  filled  with  saline  deposits,  the  winds  carry  the  salt 
to  the  oxidizing  outcrops  and  the  development  of  chloride  of 
silver,  for  instance,  is  facilitated.2 

The  essential  factors  entering  into  the  problem  of  oxidation 
of  ore  deposits  are,  then,  ore;  metal;  country  rock;  fissuring; 
permeability;  climate,  water  level,  and  rainfall;  topography; 
geological  age  and  history  of  deposit. 

OUTCROPS 

The  outcrops  of  deposits  in  glaciated  areas  are  likely  to  be 
inconspicuous,  except  where  the  principal  gangue  mineral  is 
unusually  hard,  like  quartz.  In  non-glaciated  regions  the 
outcrop  form  is  determined  by  the  difference  in  the  rate  of 
erosion  of  the  deposit  and  the  country  rock.3  A  thick  and  hard 
quartz  vein  or  a  mass  of  solid  silicified  rock  or  garnet  rocks  in 
contact-metamorphic  deposits  will  remain  as  little  ridges  or 
series  of  knobs  above  the  general  level  of  a  softer  country  rock. 
The  quartz  veins  of  California,  for  instance,  are  ordinarily 
easily  traceable  on  the  surface.  Where  the  quartz  contains 
much  pyrite,  a  honeycombed  or  cellular  mass  of  limonite  and 
quartz  remains  more  or  less  conspicuously  above  the  surrounding 

1  A.  M.  Finlayson,  Economics  of  secondary  enrichment,  Min.  and  Sci. 
Press,  July  16  and  23,  1910. 

2  C.  R.  Keyes,  Origin  of  certain  bonanza  silver  ores  of  the  arid  region, 
Trans.  Am.  Inst.  Min.  Eng.,  vol.  42,  1911,  pp.  500-517. 

3  W.  H.  Emmons,  Outcrop  of  ore-bodies,  Min.  and  Sci.  Press,  Dec.  4  and 
11,  1909. 


832  MINERAL  DEPOSITS 

country  rock.  Such  weathered  croppings  the  German  miner 
calls  "eiserner  Hut,"  the  Cornishman  a  "gossan,"  the  Australian 
"  ironstone."  The  Spanish- American  gives  these  oxidized  lim- 
onite  ores  the  names  "colorados,"  "pacos,"  "podridos,"  or 
"quemados,"  according  to  their  reddish  color,  their  soft  or  rotten 
character,  or  their  burnt  appearance. 

Where  the  minerals  of  the  deposit  are  softer  than  the  country 
rock  a  depression,  or  little  gap,  or  saddle  may  mark  its  outcrop. 
At  Butte  the  outcrops  of  the  rich  copper  veins,  which  contain 
little  gangue,  are  inconspicuous,  while  the  silver  veins  can  be 
easily  followed  along  the  surface.  Along  a  single  vein  there 

Gossan 


Leached  Zone 

Barren ) 
xidized  Secondary 

Sulphides 
ich  Secondary 
Sulphides 


FIG.  270. — Diagram  showing  normal  course  of  oxidation  in  pyritic  veins 
and  influence  of  rapid  erosion  on  exposed  secondary  sulphide  zone.  In  the 
deposit  to  the  right  the  gossan  has  been  eroded  and  the  upper  part  of  the 
secondary  sulphide  zone  leached,  causing  a  thinner  but  richer  secondary 
zone. 

may  be  great  variation  in  the  croppings.  Barren  parts  tend 
to  stand  up  prominently,  while  the  ore  shoots,  containing  softer 
metallic  minerals,  may  easily  become  effaced  at  the  surface. 
The  typical  gossan  of  pyritic  bodies,  under  uniform  conditions 
of  high  water  level  and  slow  erosion,  probably  remains  without 
much  change  for  long  periods.  When  a  gradual  lowering  of  the 
water  level  and  a  quickening  of  the  erosion  expose  new  parts  of 
the  deposit  to  the  decomposing  influence  of  oxygenated  waters 
the  transfer  of  material  downward  becomes  more  active,  and 
in  a  copper  deposit,  it  may  happen  that  the  surface  portion  be- 
comes entirely  leached  of  metallic  minerals  and  consists  simply 
of  cellular  quartz  and  of  the  more  resistant  parts  of  the  country 
rock.  Some  such  croppings  of  pyritic  copper  ores  contain 
scarcely  a  trace  of  iron  or  copper  (Fig.  270)  (p.  858). 


OXIDATION  OF  METALLIC  ORES  833 


NOMENCLATURE 

The  terms  primary  and  secondary  as  applied  to  ores  are  in- 
convenient and  often  misleading.  They  will,  therefore,  be 
avoided  as  far  as  possible. 

A  great  number  of  ore  deposits  are  formed  by  ascending  waters. 
Such  waters  and  such  deposits  are  termed  hypogene.1  Most 
changes  during  direct  and  indirect  oxidation  are  caused  by  de- 
scending surface  waters.  Such  waters  and  the  ores  formed  by 
them  by  the  rearrangements  of  the  hypogene  ores  are  called 
super  gene.1- 

In  many  cases,  valueless  but  mineralized  material  has  been 
worked  over  by  descending  surface  waters  and  valuable  ore- 
bodies  have  been  formed  from  it.  Ransome  has  proposed  the 
term  protore2  to  designate  any  primary  material  of  too  low  tenor 
to  constitute  ore  but  which  may  be  concentrated  into  ore.  Thus, 
the  low  grade  pyritic  dissemination  underneath  the  chalcocite 
blanket  at  Ely,  Nevada,  is  protore. 

PRINCIPLES  OF  OXIDATION 

The  powerful  reagents  of  oxidation  are  oxygen,  carbon  dioxide, 
and  sulphuric  acid.  The  last  combines  with  iron  to  form  ferric 
and  ferrous  sulphate,  the  former  being  an  especially  active  agent 
of  oxidation,  while  the  latter  is  an  important  reducing  agent. 
Sodium  chloride  and  sulphuric  acid  yield  hydrochloric  acid, 
which  easily  combines  with  iron  to  make  the  strongly  oxidizing 
ferric  chloride.  Under  the  influence  of  sulphuric  acid  the  waters 
change  from  the  calcium  carbonate  type  characteristic  of  the 
normal  surface  conditions  to  the  calcium  sulphate  type.  The 
aluminous  silicates  are  attacked  by  sulphuric  acid  and  by  car- 
bon dioxide;  sulphates,  carbonates  and  hydrous  silicates  result. 

Insoluble  minerals,  like  cassiterite,  wolframite,  and  often  also 
gold,  remain  without  change  in  the  outcrops,  enrich  them  upon 
contraction  of  volume,  or  on  their  disintegration  are  concen- 
trated into  placers.  Soluble  salts,  especially  sulphates,  are 
carried  away.  Newly  formed  compounds  are  precipitated, 
chiefly  by  reactions  between  carbonates  and  sulphates  or  be- 
tween sulphates.  Below  a  certain  point,  usually  the  water 

1  F.  L.  Ransorae,  Bull.  540,  U.  S.  Geol.  Survey,  1912,  p.  152. 

2  W.  H.  Emmons,  Bull.  625,  U.  S.  Geol.  Survey,  1917,  p.  68. 


834  MINERAL  DEPOSITS 

level,  the  free  oxygen  rapidly  diminishes  and  sulphides  are  pre- 
cipitated by  reactions  between  sulphates  and  sulphides  or  by 
other  processes. 

Much  of  the  dissolved  material  is  naturally  removed  by  the 
running  water  of  the  vicinity,  but  the  greater  part  of  it  sinks 
in  the  deposit  itself  and  there  becomes  re-deposited,  thus  con- 
tributing to  the  general  process  of  enrichment  by  the  descend- 
ing waters.  Some  enriched  deposits  are  the  product  of  long- 
continued  descending  concentration  from  a  great  thickness  of 
rocks  now  removed  by  erosion. 

In  ore  deposits  free  from  great  amounts  of  resistant  quartz 
gangue  oxidation  obliterates  structure.  Heavy  pyritic  deposits 
appear  at  the  surface  as  cellular  masses  of  quartz  and  limonite; 
the  sheeted  gold-telluride  veins  of  Cripple  Creek,  Colorado,  which 
do  not  carry  much  quartz,  appear  as  brown  clayey  bands  with- 
out visible  structure.  There  is  thorough  rearrangement  of 
metal  association,  and  often  also  segregation  of  new  minerals 
in  large  masses.  Limestone  country  rock  especially  favors 
such  changes.  Lead  and  zinc  closely  associated  in  galena  and 
zinc  blende  part  company;  the  oxidized  zinc  ores  wander  farther 
away  from  the  original  deposit  than  does  the  cerussite.  Copper 
and  iron,  so  intimate  in  primary  ores,  separate  more  or  less  in 
the  zone  of  oxidation,  the  former  exhibiting  a  centripetal,  the 
latter  a  centrifugal  tendency,1  and  arrange  themselves  concen- 
trically, just  as  happens  in  fragments  of  sulphide  ore  subjected 
to  "kernel  roasting." 

Masses  of  nearly  pure  kaolin  and  alunite  often  form  in  this 
zone. 

Some  sulphates,  like  anglesite  or  basic  ferric  sulphate,  are 
insoluble:  others,  like  goslarite  (Zn),  mallardite  (Mn),  epsom- 
ite  (Mg),  ferrous  sulphate,  and  aluminous  sulphate,  are  most 
easily  carried  away.  Gypsum,  common  as  a  product  of  exchange 
in  reactions  leading  to  the  formation  of  insoluble  carbonates  in 
limestone,  is  also  rather  easily  removed  in  solution.  The  native 
carbonates  of  zinc  and  copper  are  relatively  insoluble  and  may 
remain  for  a  long  time  in  the  gossan.  Other  minerals  character- 
istic of  the  oxidized  zone  are  native  metals  (copper,  gold,  silver, 
and  mercury),  chloride,  bromide,  and  iodide  of  silver,  phosphates, 
arsenates,  antimoniates,  molybdates,  vanadates,  rarely  chro- 

1  W.  Lindgren,  L.  C.  Graton,  and  C.  II.  Gordon,  The  ore  deposits  of  New 
Mexico,  Prof.  Paper  68,  U.  S.  Geol.  Survey,  1910,  p.  55;  see  also  PI.  IV,  B. 


OXIDATION  OF  METALLIC  ORES  835 

mates;  also  hydroxides  and  oxychlorides;  and  a  few  hydrous 
silicates,  like  calamine  and  chrysocolla. 

There  is  then,  during  oxidation,  both  dissipation  and  con- 
centration of  metals.  The  concentration  may  take  place  either 
in  the  deposit  itself  or  may  be  effected  by  the  precipitating 
influence  of  the  country  rock  on  the  solutions;  bodies  of  oxidized 
zinc  ores  often  form  in  the  limestone  surrounding  a  deposit. 

In  the  zone  of  supergene  sulphides  below  the  direct  oxidation 
we  meet  the  copper  sulphides — mainly  chalcocite  and  covellite, 
rarely  chalcopyrite  and  bornite;  also,  argentite,  and  complex 
sulphantimonides  and  sulpharsenides,  associated  with  native 
silver.  Pyrite  and  zinc  blende  are  seldom  found  as  products  of 
this  zone. 

Generally  speaking,  solution  prevails  near  the  surface  and  pre- 
cipitation and  cementation1  in  the  supergene  sulphide  zone.  In 
the  zone  of  direct  oxidation  enrichment  may  or  may  not  take 
place.  If  there  is  a  supergene  sulphide  zone  this  always  involves 
enrichment. 

The  character  of  the  solutions  changes  gradually  in  depth. 
Oxygen  is  removed;  the  free  acid  decreases;  reduction  replaces 
oxidation;  gases  like  H2S  and  CO2  may  be  generated.  The 
general  character  of  gangue  and  wall  rock  is  of  great  importance. 
If  carbonates  prevail,  the  minerals  that  form  may  differ  from 
those  that  are  developed  -in  a  quartzose  gangue.  The  results 
show  an  infinity  of  variations  and  complexity. 

TEXTURES  AND  CRITERIA  OF  THE  OXIDIZED  ZONE 

The  action  of  meteoric  waters,  aided  by  sulphuric  acid  in 
pyritic  deposits,  opens  spaces  in  the  zones  of  oxidation  resulting 
in  vuggy,  cellular,  honeycombed  structures  with  clayey  masses  if 
silicates  are  present.  Deposition  proceeds  in  part  by  replacement 
without  constancy  of  volume,  in  part  by  crustification  in  open 
cavities;  mammillary,  concretionary,  and  stalactitic  forms  are 
common,  alternating  with  crusts  of  crystallized  minerals  and 
pseudomorphs  stable  at  the  particular  moment.  Nodular  tex- 
tures are  common  coupled  with  rearrangement  of  the  oxy-salts 
in  shells,  so  that  copper  ores  may  surround  limonite  or  zinc 
ores  have  a  core  of  cerussite.  Reticulating  fractures  are  filled 
with  oxidized  products.  Concentric  rings  of  the  same  products 

1  P.  Krusch,  Die  Eintheilung  der  Erze,  etc.,  Zeitschr.  prakt.  Geol.,  1907, 
pp.  129-139. 


836  MINERAL  DEPOSITS 

surround  sulphides  (Fig.  271).  Colloidal  solutions  and  suspen- 
sions are  equally  common  as  electrolytes.  Solutions  change 
rapidly  in  composition  so  that  calcite  and  even  quartz  crusts 
may  alternate  with  limonite,  kaolin  and  oxidized  salts  of  copper, 
lead  and  zinc.  The  volume  is  diminished  and  enrichment  of 
relatively  insoluble  constituents  follows. 

The  presence  of  limonite  with  other  hydrous  oxy-salts  is  one  of 
the  safest^'criteria  of  oxidation,  but  the  absence  of  sulphides  is 
not  necessary  for  oxidation  proceeds  extremely  capriciously  and 
residual  sulphides  may  be  found  at  all  levels  in  the  zone  of 
oxidation.  To  a  limited  extent  supergene  sulphides  like  chalco- 
cite,  covellite  and  argentite  may  be  formed  in  the  oxidized  zone 
wherever  there  was  a  temporary  shortage  of  oxygen.1 


•A  B 

FIG.  271. — A.  Photomicrographs  of  polished  sections  showing  oxidation 

of  enargite,  Tintic,  Utah.   Light,  enargite ;  dark,  branching  veinlets  of  copper 

arsenates.    Light  streaks  in  vein,  chalcocite.     Magnified  25  diameters. 
/?.  Concentric  texture  in  stromeyerite,  developed  by  oxidation,  Broken 

Hill,  N.  S.  W.    Light,  tetrahedrite ;  dark,  oxidized  material.    Magnified  25 

diameters. 

TEXTURES  OF  THE  SUPERGENE  SULPHIDE  ZONE 

In  the  zone  of  supergene  sulphides,  important  in  copper  and 
silver  deposits,  precipitation  and  deposition  prevails  and  the  tex- 
tures become  more  compact,  though  in  places  loose,  earthy  ores 
are  present.  The  so-called  "sooty"  chalcocite  at  Butte  and 

1  W.  Lindgren,  Econ.  Geol,  vol.  10,  1915,  p.  236. 

W.  Lindgren  and  G.  F.  Loughlin,  Prof.  Paper  107,  U.  S.  Geol.  Survey, 
1919. 


OXIDATION  OF  METALLIC  ORES  837 

many  other  places  exemplifies  this  latter  condition.  The  super- 
gene  sulphide  ores  are  permeable  and  more  or  less  porous.  With 
increasing  compactness  replacement  becomes  more  evident  and 
proceeds  volume  for  volume.  The  secondary  sulphides  re- 
place the  primary  ore  minerals  in  manifold  succession  and  form. 
Reticulating  veinlets  are  mostly  formed  by  replacement;  grains 
and  crystals  of  pyrite  are  concentrically  replaced  by  chalcocite 
(Figs.  275,  276).  The  supergene  sulphides  also  fill  veinlets  or 
vugs  or  form  thin  coatings.  Regular  "graphic  intergrowths " 
often  similar  to  eutectic  textures  in  metals  develop  at  many 
places  (Fig.  272).  This  was  formerly  thought  a  criterion  of  pri- 


FIG.  272.— Intergrowth  of  bornite  (6)  and  chalcocite  (cc).  Wall  mine, 
Virgilina,  North  Carolina,  cc  (sec),  Secondary  chalcocite.  Magnified  50 
diameters.  After  L.  C.  Graton  and  J.  Murdoch. 

mary  contemporaneous  origin  but  it  is  now  known  to  be  a  feature 
of  replacement  most  commonly  of  supergene  origin.1  Thus  chal- 
cocite replaces  bornite  and  covellite  replaces  galena;2  stromeyer- 

1  F.  B.  Laney,  Econ.  Geol,  vol.  6,  1911,  p.  399. 

C.  C.  Gilbert  and  G.  E.  Pogue,  Proc.  Nat.  Mus.,  vol.  45,  1913,  pp.  609- 
625. 

L.  C.  Graton  and  J.  Murdoch,  Trans.,  Am.  Inst.  Min.  Eng.,  vol  45,  1913, 
p.  768. 

J.  Segall,  Econ.  Geol,  vol.  10,  1915,  p.  469. 
W.  L.  Whitehead,  idem,  vol.  11,  1916,  pp.  1-13. 
F.  N.  Guild,  idem,  vol.  12,  1917,  pp.  297-353. 
*  W.  L.  Whitehead,  op.  tit. 


838  MINERAL  DEPOSITS 

ite  replaces  chalcopyrite  and  galena;  proustite  replaces  galena. 
Generally  speaking,  the  supergene  sulphides  replace  primary 
sulphides  but  very  rarely  primary  gangue  minerals. 

SOLUTION 

In  the  presence  of  water  oxygen  attacks  the  sulphides  and  car- 
bon dioxide  the  silicates.  Alkaline  solutions  would  attack  the 
quartz  and  the  silicates  but  under  the  influence  of  free  sulphuric 
acid  generated  by  oxidation  of  pyrite  they  are  generally  absent. 
Distinction  must  be  drawn  between  solution  and  decomposition; 
most  of  the  changes  in  the  oxidized  zone  involve  decomposition. 
In  general  oxidation  tends  to  transform  sulphides,  sulphosalts, 
arsenides,  tellurides,  etc.,  to  oxygen  salts  and  native  metals  both 
of  which  may  be  further  changed  or  carried  away  by  surface 
waters.  The  silicates  in  the  deposit  are  changed  to  a  few  stable 
minerals:  kaolin,  limonite,  manganese  dioxide  and  quartz. 
Carbonates  of  earthy  metals  and  alkalies  are  carried  away; 
original  quartz  is  rarely  attacked  but  new  silica  from  the 
decomposition  of  the  silicates  may  be  deposited  as  opal  and 
chalcedony. 

Oxidation  tends  to  thorough  change  of  composition  and  to 
obliteration  of  original  texture  and  structure. 

The  simple  sulphides  are  very  slightly  soluble  in  water,  the 
solubility  decreasing  as  follows :  Mn,  Fe,  Ni,  Cd,  Zn,  Cu,  Pb,  As, 
Ag,  Bu,  Hg.1 

In  dilute  sulphuric  acid  (Ko  normal),  pyrrhotite,  chalcopyrite, 
galena,  zinc  blende  and  cadmium  sulphide  are  dissolved  or  readily 
attacked.  Argentite,  galena,  bornite,  arsenopyrite,  stibnite,  pyrar- 
gyrite  and  polybasite  are  slightly  attacked,2  while  many  others 
like  cinnabar,  molybdenite,  realgar,  orpiment,  bismuthinite, 
covellite,  and  chalcocite  are  not  attacked. 

The  decomposition  or  solution  is  often  hastened  if  the  dilute 
sulphuric  acid  contains  ferric  sulphate.  Few  sulphides  resist  this 
reagent.3 

Some  sulphides  react  with  alkaline  solutions  at  ordinary  tem- 

1  Oscar  Weigel,  Zeitschr.  physikal  Chemie,  vol.  58,  1907,  pp.  293-300. 

2  H.  C.  Cooke,  Jour.  Geology,  vol.  21,  1913,  pp.  1-28. 

3  For  experimental  data  see  G.  S.  Nishihara,  The  rate  of  reduction  of 
acidity  of  descending  waters  by  certain  ore  and  gangue  minerals,  Econ. 
Gaol,  vol.  9,  1914,  pp.  743-757. 


OXIDATION  OF  METALLIC  ORES  839 

perature.1  Orpiment,  realgar,  stibnite  and  pyrrhotite  are 
strongly  attacked  by  a  1  per  cent,  solution  of  NaHCO3;  many 
others  are  slightly  attacked;  arsenopyrite,  cinnabar,  enargite, 
chalcocite,  bornite,  light  colored  zinc  blende  and  niccolite  are 
practically  resistant.  ' 

Experiments  with  pyrite  have  not  given  consistent  results. 
A.  N.  Winchell2  exposed  powdered  pyrite  for  10  months  to  the 
action  of  distilled  aerated  water  and  obtained  a  very  slow  rate 
of  oxidation,  the  solution  containing  Fe2(S04)3  and  H2SO4.  His 
results  were  in  general  confirmed  by  F.  F.  Grout.3  H.  A. 
Buehler  and  V.  H.  Gottschalk4  obtained  a  much  more  rapid 
attack  and  in  3  months  the  filtrate  yielded  2.5  to  3.7  per  cent,  of 
the  original  weight  of  the  iron  in  the  sample.  Sphalerite  in  the 
same  time  yielded  only  0.2  per  cent,  of  its  zinc,  galena  0.005 
per  cent,  of  its  lead,  covellite  2.7  per  cent,  of  its  copper,  and  chal- 
copyrite  1  per  cent,  of  the  same  metal.  On  the  other  hand,  enar- 
gite showed  no  solubility.  When  the  various  sulphides  were 
mixed  with  pyrite  the  action  was  much  more  energetic.  In  the 
time  specified  sphalerite  with  pyrite  yielded  4.2  per  cent,  of  its 
zinc,  galena  with  pyrite  0.7  per  cent,  of  its  lead,  covellite  with 
pyrite  2.7  per  cent,  of  its  copper,  covellite  with  marcasite  27.6 
per  cent.,  and  enargite  with  pyrite  10  per  cent,  of  its  copper. 
After  an  exposure  of  only  7  weeks,  pyrite  had  oxidized  to  the 
amount  of  0.1  to  0.28  per  cent,  of  its  original  weight. 

In  a  second  paper  Gottschalk  and  Buehler5  show  that  while 
in  a  mixture  of  two  sulphides  there  is  a  large  increase  in  the 
solution  of  one,  there  is  also  a  protective  action  exerted  on  the 
other;  and  further  that  there  exists  a  difference  of  potential 
between  the  sulphides,  which  can  be  arranged  in  a  series  similar 
to  the  electrolytic  series  of  metals.  Acceleration  of  reaction 
is  due  to  electric  currents  generated  by  contact  of  minerals 
of  different  potential;  the  currents  flow  from  the  mineral  of  the 
higher  potential,  and  the  mineral  of  lower  potential  will  dissolve 
more  rapidly.  In  mixtures  with  pyrite  the  iron  transferred  is 

1  F.  F.  Grout,  On  the  behavior  of  acid  sulphate  solutions  of  copper,  silver 
and  gold  with  alkaline  extracts  of  metallic  sulphides,  Econ.  Geol,  vol.  8, 
1913,  p.  427. 

2  A.  N.  Winchell,  Econ.  Geol,  vol.  2,  1907,  pp.  290-294. 

3  Idem,  vol.  3,  1908,  p.  532. 

« Idem,  vol.  5,  1910,  pp.  28-35. 

5  Econ.  Geol,  vol.  7,  1912,  pp.  15-34. 


840 


MINERAL  DEPOSITS 


but  a  small  portion  of  that  obtained  when  iron  disulphide  is 
treated  alone. 

From  this  it  follows  that  the  order  of  oxidation  in  a  mixture 
of  minerals  varies  with  conditions  of  mass,  aggregate  and  charac- 
ter of  solution.  No  general  rule  can  be  formulated.  It  is  known, 
however,  that,  for  instance,  zinc  blende  oxidizes  before  chalcopyr- 
ite  and  the  latter  before  pyrite.  In  a  pyrite-chalcocite  ore  the 
chalcocite  is  attacked  first. 

The  relative  solubility  of  the  various  carbonates  and  sul- 
phates are  important  for  the  distribution  of  metals  in  the  oxidized 
zone.  The  following  data  have  been  determined  by  Kohlrausch. 
The  low  solubilities  of  the  carbonates  are  considerably  increased 
in  the  presence  of  CC>2. 

SOLUBILITY    OF    SULPHATES  AND  CARBONATES    AT  18°  C.  IN  GRAMS  OF 
ANHYDROUS  SALT  PER  100  GRAMS  OF  H2O. 


Salt 

Grams 

Salt 

Grams 

BaSO,  
PbSO 

0.00023 
0  0041 

PbCO3  
CaCO 

0.0001 
0  0013 

CaSO4 

0  20 

Ae,CO 

0  0017 

Ag,SO 

0  55 

BaCO 

0  0023 

K2S04  
Na2SO4  
CuS04  
FeSO 

11.11 

16.83 
19.30 
23  00 

ZnCO3  
MgCO3  
FeC03  
MnCO 

0.0047 
0.100 
0.073* 

A12(SO4)3 

31  301 

CuCO3 

NiSO4  

34  202 

Na,CO, 

19  38 

MgS04  
ZnSO4  
MnS04  

35.43 
53.18 
65.  OO3 

K2C03  

108.00 

1  With  18  mol.  H2O  at  0°;  89.1  with  18  mol.  H2O  at  100°  C. 

'At  15°. 

*  At  30°. 

4  Sat.  with  CO2  at  7  at. 

The 'descending  solutions  contain  many  salts  and  in  places 
colloids,  which  even  in^the  presence  of  electrolytes  may  fail 
to  be  coagulated,  if  "protected"  by  the  presence  of  certain 
other  colloids.  Near  the  surface  in  pyritic  deposits  ferric  sul- 
phates and  even  aluminum  sulphate  and  free  sulphuric  acid  may 
be  abundant  but  with  increasing  depth  the  ferrous  sulphate 


OXIDATION  OF  METALLIC  ORES  841 

predominates  and  the  solutions  tend  to  become  neutral.  Gases 
like  CO2  and  H2S  may  be  generated  locally.  In  general,  during 
oxidation  there  is  a  great  dissipation  of  the  sulphates  of  iron, 
zinc  and  calcium. 

PRECIPITATION 

Precipitation  is  effected  by  reactions  between  solutions,  by 
hydrolysis,  by  coagulation,  by  gases  and  by  reactions  between  the 
solutions  and  solids.  The  latter  phase  is  very  important.  Many 
reactions  take  place  by  the  action  of  solutions  on  sulphides 
or  on  gangue  minerals  or  on  country  rock  of  sedimentary  or 
igneous  origin.  The  investigations  of  E.  C.  Sullivan1  have  shown 


FIG.  273. — Photomicrograph  of  'thin  section  showing  azurite  crystals 
replacing  kaolin,  Clifton,  Arizona.     Magnified  15  diameters. 

that  silicates  may  precipitate  oxygen  salts  by  chemical  reactions. 
In  a  general  way  this  was  known  long  ago,  and  E.  Kohler2  showed 
that  cupric  sulphate  lost  its  copper  when  filtered  through  kaolin. 
This  was  attributed  to  adsorption — that  is,  an  accumulation  of 
dissolved  substance  on  the  contact  between  liquid  and  solid- — but 
Sullivan  shows  that  a  chemical  change  takes  place.  The  natural 
silicates  such  as  kaolin,  albite,  orthoclase,  amphibole,  pyroxene, 
and  mica  precipitate  the  metals  from  salt  solutions,  while  at  the 
same  time  the  bases  of  the  silicates  are  dissolved  in  quantities 

1  The  interactions  between  minerals  and  water  solutions,  Butt.  312,  U.  S. 
Geol.  Survey,  1907;  Econ.  GeoL,  vol.  1,  1905,  p.  67. 

2  Zeitschr.  prakt.  Geol,  vol.  11,  1903,  p.  49. 


842  MINERAL  DEPOSITS 

nearly  equivalent  to  the  precipitated  metals.  The  latter  precipi- 
tates take  the  form  of  hydroxides  or  basic  salts,  though  silicates 
may  also  be  formed  to  some  extent.  Thus  by  a  simple  chem- 
ical exchange  the  metal  may  be  removed  from  a  solution  and  fixed 
in  the  solid  state  and  thus  concentrated  by  contact  with  even  the 
most  insoluble  of  silicates. 

These  experiments  elucidate  the  deposition  of  brochantite 
and  chrysocolla  in  granitic  and  porphyritic  rocks,  as  well  as  the 
deposition  of  cuprite  and  azurite  in  shale.1  (Fig.  273.)  A 
solution  of  silver  sulphate  yielded  its  metal  completely  to  a  pow- 
dered clay  gouge,  metallic  silver  being  probably  formed.  With 
kaolin  the  reaction  is  rapid  and  the  copper  solution  soon  becomes 
colorless.  The  iron  in  ferric  and  ferrous  sulphate  is  easily  re- 
tained by  kaolin  as  limonite. 

The  direct  oxidation  of  galena,  for  instance,  yields  carbonate 
and  sulphate  of  lead.  By  further  reactions  with  ferric  sulphate 
basic  sulphates  of  iron  and  lead  are  formed  like  plumbojarosite, 
and  in  this  way  the  lead  is  further  distributed.  Many  other 
difficultly  soluble  basic  sulphates  form  during  oxidation;  alunite 
is  one  of  the  most  common  of  these. 

SUPERGENE  SULPHIDES 

The  development  of  secondary  sulphides  may  take  place 
by  direct  precipitation  from  solutions  by  means  of  hydrogen 
sulphide  or  other  reducing  solutions  or  ga?es;  or  it  may  result 
from  a  metasomatic  interchange  between  a  solution  and  a  solid, 
usually  another  sulphide.  Dilute  sulphuric  acid  generated  by 
the  decomposition  of  pyrite,  for  instance,  attacks  a  few  sulphides, 
with  the  evolution  of  hydrogen  sulphide.  This  gas  is  produced 
in  abundance  by  the  attack  on  pyrrhotite  and  to  a  less  extent, 
according  to  R.  C.  Wells,  when  zinc  blende  is  exposed  to  the  acid. 
If  copper  is  present  in  the  solutions,  a  precipitate  of  cupric  sul- 
phide (CuS)  will  be  formed,  besides  some  cuprous  sulphide(Cu2S). 
Sulphides  are  formed  mainly  where  the  supply  of  oxygen  from 
the  surface  becomes  nearly  exhausted. 

Previous  to  the  year  1900  the  presence  of  secondary  sulphides 
as  indirect  products  of  oxidation  had  been  noted  by  some  ob- 
servers and  had  been  definitely  stated  by  L.  de  Launay.2  In  the 

1  W.  Lindgren,  Prof.  Paper  43,  U.  S.  Geol.  Survey,  1905,  p.  191. 

2  Les  variations  des  filons  me'talliferes  en  profondeur,  Revue  generate  des 
Sciences,  etc.,  No.  8,  April  30,  1900. 


OXIDATION  OF  METALLIC  ORES  843 

year  referred  to  S.  F.  Emmons,  C.  R.  Van  Hise,  and  W.  H.  Weed 
in  three  notable  papers1  formulated  the  important  law  of  the 
accumulation  of  sulphides  as  a  concentration  from  the  overlying 
oxidized  zone,,  at  or  below  the  water  level.  It  was  shown  that 
in  copper  deposits  chalcocite  and  covellite  were  precipitated  by 
pyrite  from  sulphate  solutions  and  that  under  similar  conditions 
in  silver  deposits  argentite,  stephanite,  polybasite,  and  pyrargyrite 
or  proustite  might  form;  it  was  also  shown  that  zinc  blende 
and  galena  were  probably  precipitated  in  a  similar  manner. 
The  chemical  reasons  for  these  reactions  were  found  in  the 
so-called  Schiirmann's  law,2  by  which  it  was  established  that 
in  the  presence  of  the  sulphides  of  certain  of  the  metals  the 
salts  of  other  metals  would  be  decomposed  and  the  metals  pre- 
cipitated as  sulphides.  This  was  thought  to  indicate  that  the 
metals  which  were  precipitated  possessed  a  greater  affinity  for 
sulphur  than  the  other  metals. 

Schiirmann's  series  was  as  follows:  Mercury,  silver,  copper, 
bismuth,  cadmium,  lead,  zinc,  nickel,  cobalt,  iron,  and  manga- 
nese. The  solution  of  a  salt  of  any  of  these  metals  will  be  de- 
composed by  the  sulphide  of  any  succeeding  metal  and  the  first 
metal  will  be  precipitated  as  a  sulphide.  Thus  from  a  solution  of 
silver  or  copper  salts  the  metal  would  be  precipitated  by  a  sul- 
phide of  lead,  zinc,  or  iron.  If  secondary  deposition  of  sulphides 
by  reaction  of  pyritic  ores  on  descending  sulphate  waters  had 
taken  place  in  an  ore  deposit  containing  silver,  copper,  lead,  and 
zinc,  these  sulphides  would  theoretically  be  arranged  in  the  fol- 
lowing order:  Argentite,  chalcocite,  galena,  and  zinc  blende,  the 
last'at  the  lowest  level.  It  was  shown  later  by  R.  C.  Wells3  that 
the  influencing  factor  was  the  relative  solubility  of  the  sulphides 
(p.  838)  rather  than  the  "affinity  for  sulphur." 

The  farther  apart  any  two  sulphides  are  in  Schiirmann's 
series  the  more  nearly  complete  is  the  replacement.  The  full 
series  is  not  represented  by  natural  sulphides  and  in  ore  deposits 
the  reactions  of  copper  and  silver  solutions  are  the  most  im- 
portant. Supergene  sulphides  of  bismuth,  nickel  and  cobalt 

1  S.  F.  Emmons,  The  secondary  enrichment  of  ore  deposits. 

C.  R.  Van  Hise,  Some  principles  controlling  the  deposition  of  ores. 

W.  H.  Weed,  Enrichment  of  gold  and  silver  veins. 

All  in  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  30,  1901. 

2E.  Schurmann,  Leibig's  Ann.  der  Chemie,  vol.  249,  1888,  pp.  326-350. 
3Econ.  Geol.,  vol.  5,  1910,  p.  14. 


844  MINERAL  DEPOSITS 

are  not  known  and  supergene  cinnabar  is  rare.  On  the  other 
hand  chalcocite  and  covellite  replace  galena  and  zinc  blende  as 
well  as  pyrite  and  they  may  also  replace  sulphosalts,  such  as 
enargite  and  tetrahedrite  and  iron-copper  sulphides  like  bornite 
and  chalcopyrite;  argentite  replaces  the  sulphides  of  lead,  zinc  and 
iron;  galena  replaces  zinc  blende  though  the  reaction  is  of  slight 
economic  importance.  The  supergene  sulphosalts  of  silver  such 
as  polybasite  replace  galena  and  other  sulphides.  Complex 
sulphosalts  of  lead,  e.g.,  jamesonite  may  replace  simple  sulphides 
as  well  as  galena. 

Supergene  sulphide  deposition  is  accompanied  by  few,  if  any, 
characteristic  gangue  minerals;  opal,  chalcedony  and  kaolin 
are  occasionally  present;  quartz  is  rare. 

The  role  of  colloidal  solutions  may  also  involve  the  trans- 
portation of  sulphides.  It  has  long  been  known  that  sulphides 
may  be  transferred  into  colloid  solutions  by  certain  dispersing 
agents.  Experiments  by  John  D.  Clark  and  others1  have  shown 
the  extent  of  this  possibility. 

Nearly  all  sulphides,  arsenides  and  sulphosalts  may  become 
highly  dispersed  as  colloids  under  the  influence  of  hydrogen 
sulphide  in  solutions  of  mild  alkalinity.  These  minerals  are  then 
in  condition  to  migrate  with  the  solutions.  With  the  escape  of 
H2S  or  by  contact  with  calcareous  and  argillaceous  material 
precipitation  and  replacement  may  occur.  Minerals  may  crys- 
tallize directly  from  colloid  solutions.  Secondary  upward 
migrations  may  thus  occur  in  connection  with  repeated  invasions 
of  hypogene  solutions.  Where  H2S  is  locally  developed  this 
process  may  be  of  some  importance  in  the  supergene  sulphide 
zone. 

A.  F.  Rogers2  has  suggested  that  enrichment  by  secondary 
chalcocite  has  taken  place  by  such  ascending  solutions,  specially 
citing  the  case  of  the  Butte  deposits.  This  can  not  be  regarded 
as  proved. 

The  temperature  during  direct  oxidation  of  pyritic  ores  may 
in  places  rise  considerably  above  50°  C.  It  is  probable  that  even 
in  the  supergene  sulphide  zone  fairly  high  temperatures  of  30° 
or  40°  C.  may  obtain  at  times.3 

»  C.  F.  Tolman  and  John  D.  Clark,  Econ.  Geol.,  vol.  9,  1914,  pp.  559-592. 
John  D.  Clark  and  P.  L.  Menaul,  Econ.  Geol.,  vol.  11,  1916,  pp.  37-41. 

2  Econ.  Geol,  vol.  8,  1913,  pp.  781-794. 

3  W.  H.  Emmons,  Econ.  Geol,  vol.  10,  1915,  pp.  151-160. 


OXIDATION  OF  METALLIC  ORES  845 

It  has  already  been  stated  that  in  deep  oxidized  zones  super- 
gene  sulphides  may  well  form  together  with  oxy-salts.  Generally, 
however,  the  sulphides  begin  at  the  water  level  and  extend  for  a 
varying  distance  below  it  thus  forming  a  zone  of  secondary  sul- 
phides, which  may  be  many  hundreds  of  feet  deep  or  which  may 
only  occupy  a  thickness  of  a  few  feet.  Permeability  of  the  pri- 
mary ore  has  much  to  do  with  this  but  time  and  climatic  condi- 
tions are  also  potent  factors.  The  water  level  may  have  changed 
its  position  during  geologic  times  and  so  we  may  find  chalcocite 
zones  "marooned"  high  above  the  water  level  and  now  in  active 
process  of  oxidation.  In  the  same  way  the  products  of  direct 
oxidation  of  a  former  low  water  level  may  now  be  buried  in  the 
underground  water. 

In  such  cases  physiography  may  come  to  the  rescue  and  elu- 
cidate the  changes  which  have  taken  place  in  the  configuration 
of  the  ground.1 

Such  well-defined  zones  of  supergene  sulphides  are  common 
only  in  the  case  of  copper  and  silver.  In  the  case  of  silver 
deposits  the  products  of  direct  oxidation  and  sulphide  deposition 
are  greatly  mingled.  No  great  zones  of  supergene  lead  or  zinc 
sulphides  are  known. 

Certain  elements  like  iron,  zinc  and  arsenic,  which  may  be  com- 
mon in  the  primary  ore,  may  be  completely  eliminated  in  the 
oxidized  ore  and  in  the  supergene  sulphides. 

CRITERIA  OF  SUPERGENE  SULPHIDE  ENRICHMENT 

The  question  whether  or  not  secondary  sulphides  have  been 
deposited  in  an  ore-body  by  descending  waters  is  most  important. 
If  the  ore  minerals  are  only  a  part  of  a  shallow  enriched  layer 
and  poorer  ore  is  to  be  expected  at  lower  levels,  knowledge  of 
this  fact  is  greatly  to  be  desired  from  the  mine  owner's  standpoint. 

The  best  geological  evidence  of  enrichment  consists  in  the 
progressive,  uniform  impoverishment  of  all  similar  sulphide  de- 
posits in  a  given  district,  coupled  with  the  condition  that  the 
change  in  ore  should  be  dependent  upon  post-mineral  topographic 
development.2  If  the  enriched  zone  is  shallow  such  evidence 
may  be  conclusive.  If  it  is  deep  there  may  be  difficulties  in  ar- 
riving at  a  correct  conclusion. 

1  W.  W.  Atwood,  The  physiographic  condition  at  Butte,  Mont.,  Econ. 
Geol.,  vol.  11,  1916,  pp.  697-740. 

2  F.  L.  Ransome,  Econ.  Geol,  vol.  5,  1910,  pp.  205-220. 


846  MINERAL  DEPOSITS 

The  occurrence  of  exceptionally  rich  ores  just  below  the  zone  of 
oxidation  is  generally  suggestive  of  enrichment.  Graton  and 
Murdoch1  find  the  important  criterion  in  the  microscopic  struc- 
ture of  the  ore  but  this  is  not  always  reliable  because  of  the  simi- 
larity of  the  latest  hypogene  replacements  to  those  of  the  de- 
scending surface  waters.  If  microscopic  structure  were  always  a 
safe  guide,  we  should  know  more  about  the  genesis  of  the  great 
copper  deposits  at  Butte  than  we  now  do. 

Generally  speaking  it  is  believed  that  the  presence  of  chalcocite 
and  covellite  in  large  amounts  is  a  safe  indication  of  supergene 
sulphide  enrichment,  while  in  silver  deposits,  the  occurrence  of 
rich  silver  sulphantimonides  and  argentite  just  below  the  zone  of 
oxidation  is  also,  in  most  cases,  a  reliable  criterion. 

IRON 

In  iron  deposits  with  siderite  and  iron  silicates  oxidizing  con- 
ditions result  in  abundant  limonite.  Hematite  and  magnetite 
deposits  are  very  slowly  oxidized  but  ultimately  form  some 
limonite.  The  small  amount  of  magnetite  in  certain  deposits  of 
hematite  and  limonite  may  be  residual  or  it  may  have  been  gener- 
ated during  mild  regional  or  contact  metamorphism.  Oxida- 
tion under  tropical  conditions  generates  hematite  from  ferrous 
silicates  in  rocks. 

It  is  known  that  magnetite  may  alter  to  hematite;  pseudo- 
morphs  (martite)  of  hematite  after  magnetite  are  not  uncommon. 
The  nature  of  this  alteration  is  as  yet  in  some  doubt.  Some 
authors2  noting  the  abundance  of  hematite  near  the  surface  in 
some  magnetite  deposits  and  the  anastomosing  veinlets  of 
hematite  in  the  magnetite  itself  have  believed  that  this  slight 
oxidation  of  a  resistant  mineral  is  caused  by  descending  surface 
waters.  Undoubtedly  th*e  two  minerals  may  form  together  in- 
dependently of  oxidation,  or  hematite  may  be  introduced  later 
than  the  magnetite  during  high  temperature  processes. 

It  has  been  stated  lately3  that  the  iron  oxides  are  in  part  solid 

1  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  45,  1913,  pp.  26-31. 

2  W.  Lindgren  and  C.  P.  Ross,  The  iron  deposits  of  Daiquiri,  Cuba,  Trans., 
Am.  Inst.  Min.  Eng.,  vol.  53,  1916,  pp.  40-66. 

See  also  on  same  subject,  M.  Roesler,  Trans.,  Am.  Inst.  Min.  Eng., 
vol.  56,  1917,  pp.  77-127. 

3  R.  B.  Sosman  and  J.  C.  Hostetter.  Trans.,  Am.  Inst..  Miu.  Ens.,  vol.  58, 
1918,  pp.  409-444. 


OXIDATION  OF  METALLIC  ORES  847 


solutions  of  FesC^  in  FeaOa  while  others  are  mixtures  of  the  two 
compounds.  Metallographic  methods  do  not  appear  to  have 
been  used  in  this  examination. 

In  sulphide  deposits  pyrite,  pyrrhotite  and  marcasite  are  the 
principal  iron  minerals.1 

Pyrite  is  a  persistent  mineral  forming  in  all  deposits  and  at  all 
temperatures  even  locally  at  the  surface  under  reducing  condi- 
tions. It  may  be  reproduced  in  alkaline  solutions,  or,  with  mar- 
casite, in  slightly  acid  solutions. 

The  oxidation  of  pyrite  is  started  by  oxygen  and  hastened  by 
the  ferric  sulphate  developed. 

FeS2+70+H20  =  FeS04+H2S04. 

This  reaction  involves  several  intermediate  stages  during  which 
sulphur  dioxide,  sulphur,  or  hydrogen  sulphide  may  form.  The 
well-known  smell  from  old  dumps  containing  pyrite  indicates 
the  development  of  sulphur  dioxide,  according  to  the  equation 
FeS2+6O+H20  =  FeS04+H2S03,  and  this  sulphurous  acid  is 
further  oxidized  to  sulphuric  acid.  The  presence  of  sulphur  is 
often  observed  near  the  surface  in  the  casts  of  dissolved  pyrite 
crystals. 

Ferrous  sulphate  easily  changes  to  the  ferric  salt  and  to  the 
hydroxide: 

2FeS04+H2S04+  0  =  Fe2(SO4)  3+  H2O. 

6FeS044-30+3H20  =  2Fe2(S04)3+Fe2(OH)6. 
Ferric  sulphate  hydrolyzes  to  hydroxide  and  free  acid  : 
Fe2(SO4)  3+6H20  =  2Fe(OH)  3+3H2SO4. 

The  ferric  sulphate  is  a  strong  oxidizing  agent,  which,  according 
to  Stokes,  attacks  pyrite  rapidly  at  100°  C.  and  more  slowly  in 
the  cold: 

Fe2(S04)3+FeS2  =  3FeS04+2S. 

The  sulphur  may  be  oxidized  to  sulphuric  acid.  The  colloid 
ferric  sulphate  changes  easily  to  various  basic  sulphates,  like 
coquimbite,  copiapite,  or  jarosite,  often  found  in  the  lower  part 
of  the  oxidized  zone.  Limonite  is  usually  the  final  product. 
Melanterite  (FeSO4.7H2O)  .  often  forms  as  crusts  and  stalactites 
by  dripping  mine  waters. 

1  E.  T.  Allen,  J.  L.  Crenshaw  and  John  Johnson,  The  mineral  sulphides  of 
iron,  Am.  Jour.  Sci.,  4th  ser.,  vol.  33,  1912,  pp.  169-236. 


848  MINERAL  DEPOSITS 

Ferrous  sulphate  and  calcite  yield  limonite  and  soluble  gyp- 
sum. In  deposits  in  limestone  this  is  a  most  important  reaction. 

The  ferric  sulphate  and  the  sulphuric  acid  may  attack  other 
sulphides  present,  such  as  chalcopyrite  or  zinc  blende.  In  the 
deeper  levels  of  the  oxidizing  zone  much  or  all  of  the  sulphate  is 
likely  to  remain  in  the  ferrous  form.  The  sulphate  solutions 
sink  to  the  ground-water  level  and  may  here  produce  manifold 
changes  by  reaction  with  primary  sulphides. 

Marcasite  forms  only  in  acid  solutions,  but  may  then  crystal- 
lize together  with  pyrite.  Above  450°  C.  it  passes  into  pyrite. 
Low  temperature  and  free  acid  favor  its  development.  Marcasite 
is  thus  a  relatively  unstable  mineral  formed  mainly  near  the 
surface.  A  few  ore  deposits  of  igneous  affiliations  like  that  at 
Goldfield,  Nevada,  contain  marcasite,  but  it  is  usually  a  very 
late  product  forming  in  fissures  and  vugs.  It  is  most  prominent 
in  the  lead-zinc  ores  of  the  Mississippi  valley  type.  Nevertheless 
it  is  not  found  among  the  supergene  sulphides  probably  because 
descending  sulphates  are  likely  to  attack  it  vigorously. 

It  oxidizes  much  more  easily  than  pyrite  but  the  reactions 
are  the  same. 

Pyrrhotite,  regarded  as  a  solid  solution  of  sulphur  in  FeS,  is 
in  nature  a  high  temperature  mineral  not  known  to  occur  in  ores 
of  shallow  or  intermediate  depths.  It  is  readily  attacked  by 
dilute  H2S04.with  evolution  of  H2S,  which  in  copper  deposits 
may  precipitate  copper  sulphide  and  prevent  the  development 
of  deep  chalcocite  zones.  It  is  also  easily  attacked  by  oxida- 
tion, the  H2SO4  formed  accelerating  its  destruction. 

Fe7S8+310+H2O  =  7FeS04+H2SO4. 


COPPER 

Minerals. — The  most  common  primary  sulphides  of  copper 
and  iron  include  chalcopyrite  (CuFeS2)  and  bornite  (CusFeS4). 
The  pale  yellow  mineral  chalmersite  (CuFe2S3)  is  probably  more 
common  than  has  been  suspected.1  There  are  further  tetrahe- 
drite  (Cu8Sb2S7)  with  its  arsenical  analogon  tennantite,  and 
enargite  (Cu3AsS4),  all  three  important  copper  ores  in  some 
places.  Bournonite  (CuPbSbSs)  is  sometimes  an  ore  mineral  and 
there  are  a  number  of  rare  copper  and  lead-copper  sulph-bis- 

1  B.  L.  Johnson,  Econ.  Geol,  vol.  12,  1917,  pp.  519-525. 


OXIDATION  OF  METALLIC  ORES  849 

muthides.     Native  copper  is  an  important  primary  ore  mineral 
in  some  districts  while  the  arsenides  are  rare  minerals. 

Chalcocite  (Cu2S)  is  rarely,  if  ever,  of  hypogene  origin. 

The  supergene  copper  minerals  are  very  numerous.  Those 
of  some  economic  importance  include  native  copper,  cuprite 
(Cu2O),  several  indefinite  minerals  of  colloid  origin  containing 
CuO,  M2O,  ZnO,  Si02  and  H2O  (copper  pitch  ores),  also  the 
oxychloride  atacamite  (Cu2Cl(OH)3)  which  is  found  where 
windblown  sodium  chloride  is  available. 

There  are  further  chalchantite  (CuSO4.5H2O)  with  its  related 
minerals  krohnkite  (CuSO4.Na2SO4+2H2O)  and  natrochalcite 
(Na2SO4.Cu4(OH)2(SO4)2-f-2H2O),  all  important  minerals  in  arid 
countries  like  Chile.  Phosphates  are  rare. 

Malachite  (Cu2(OH)2CO3),  azurite  (Cu3(OH)2(CO3)2)  and 
chrysocolla  (CuSiO3.2H20)  are  the  most  abundant  of  all  the 
oxidized  copper  minerals.  Basic  sulphates  like  brochantite 
(Cu4SO4(OH)6)  are  locally  abundant  as  are  various  arsenates 
most  common  among  which  is  olivenite  (Cu3As2O8.Cu(OH)2). 

The  secondary  sulphides  of  prime  importance  are  chalco- 
cite  (Cu2S)  and  covellite  (CuS)  while  bornite  and  chalcopyrite 
may  also  be  of  supergene  origin. 

Solutions  and  Precipitation. — Copper  is  one  of  the  most  easily 
transported  metals  and  as  it  is  also  easily  precipitated  its  super- 
gene  deposits  have  great  importance.  Copper  migrates  down- 
ward by  stages  through  the  oxidized  zone  and  through  the 
supergene  sulphide  zone  so  that  a  considerable  concentration 
may  eventually  be  reached. 

Chalcopyrite  is  readily  attacked  by  oxygen  and  by  ferric 
sulphate.  It  is  slightly  attacked  by  dilute  sulphuric  acid. 

CuFeS2+80  =  CuS04+FeS04. 

As  the  ferrous  salt  is  easily  transformed  into  limonite,  pseudo- 
morphs  of  that  mineral  after  chalcopyrite  are  extremely  common. 
Bornite  is  more  strongly  attacked  by  dilute  sulphuric  acid 
than  chalcopyrite,1  but  like  all  copper  sulphides  easily  decom- 
posed by  ferric  sulphate. 

Cu5FeS4+2H2SO4+180  =  5CuSO4+FeSO4+2H2O. 
Chalcocite  like  covellite  is   very  slightly  attacked  by  dilute 

1  E.  G.  Zies,  E.  T.  Allen  and  H.  E.  Merwin,  Some  reactions  involved  in 
secondary  copper  sulphide  enrichment,  Econ.  Geol,  vol.  11,  1916,  p.  476. 


850  MINERAL  DEPOSITS 

sulphuric  acid  but  is  decomposed  by  ferric  solutions  which  trans- 
form it  to  sulphates,  probably  also  to  covellite: 


Enargite  and  tetrahedrite  are  likewise  slowly  decomposed 
by  dilute  solutions  of  ferric  sulphate.  Normally  the  antimony 
remains  as  insoluble  oxide  while  the  arsenic  is  carried  away  unless 
fixed  as  arsenates  of  copper  by  carbonate  solutions  or  limestone. 

The  universally  resulting  cupric  sulphate1  is  more  or  less 
completely  fixed  as  malachite,  azurite  and  brochantite,  chrys- 
colla  and  similar  products  by  solutions  containing  carbonates  or 
silica.  Ultimately  even  these  minerals  will  be  leached  for  they 
are  slightly  soluble  in  water  containing  carbon  dioxide  and  easily 
soluble  in  dilute  sulphuric  acid.  They  may,  however,  be  re- 
duced to  cuprite  and  the  cuprite  to  native  copper  which  again 
may  go  into  solution  with  H2S04. 

The  oxidation  products  of  the  chalcocite  zone  are  described 
on  p.  858. 

Supergene  Copper  Sulphides.  —  So  far  we  have  considered  the 
products  of  direct  oxidation,  in  the  uppermost  part  of  pyrite 
deposits;  they  form  under  the  influence  of  acid  solutions  contain- 
ing free  oxygen  and  ferric  salts;  limonite  and  oxy-salts  of  copper 
result,  mixed  with  residual  quartz.  At  a  certain  depth,  usually 
at  the  water  level,  if  that  remains  in  its  original  horizon,  the 
material  changes  from  a  brown  to  grey  or  bluish  color  and  copper 
sulphides  begin  to  appear.  At  first  they  form  pulverulent  or 
sooty  masses  with  little  residual  pyrite.  Their  quantity  gradu- 
ally decreases;  we  find  grains  of  pyrite,  chalcopyrite,  bornite, 
or  zinc  blende  covered  by  coatings  of  covellite  or  chalcocite  and 
the  microscope  gives  evidence  that  the  process  is  a  replacement 
of  primary  sulphides  by  the  two  minerals  mentioned  (Figs. 
274,  275,  276)  .  Occasionally  secondary  chal  copyrite  or  bornite 
will  appear  but  no  secondary  pyrite.  The  alteration  proceeds 
from  reticulating  veinlets  or  in  concentric  shells.  Below  the 
upper  part  of  the  zone  the  chalcocite  may  be  compact  with  dark 

1  Cuprous  sulphate  may  form  as  intermediate  product  in  some  reactions 
but  it  is  unstable  and  according  to  W.  H.  Emmons  has  not  been  discovered 
in  any  analysis  of  mine  waters. 


OXIDATION  OF  METALLIC  ORES 


851 


FIG.  274. — Pyrite  (Py}  intergrown  with  zinc  blende  (Zn),  which  is  alter- 
ing to  covellite  (Cv~),  Kyschtim,  Russia.  Magnified  45  diameters.  After 
L.  C.  Graton  and  J.  Murdoch. 


FIG.  275. — Bornite  (6)  altering  along  fissures  to  chalcopyrite  (cp),  Ajo, 
Arizona,  g,  Gangue;  cc  (sec),  secondary  chalcocite.  Magnified  50  diame- 
ters. After  L.  C.  Graton  and  J.  Murdoch. 


852 


MINERAL  DEPOSITS 


grey  metallic  luster.  Veins  of  pyrite  may  be  converted  to  sooty 
or  massive  chalcocite  with  only  few  residual  grains  of  the  original 
mineral.  In  some  deposits  large  and  rich  masses  of  chalcocite, 
more  rarely  covellite,  may  be  formed  in  this  manner. 

Kaolin  may  form  along  with  the  secondary  sulphides  partly 
at  the  expense  of  the  sericite;  other  minerals  are  chalcedonic  and 
opaline  silica,  also  alunite.  Pyrite  is  unstable  in  this  zone. 
Deposition  of  quartz  is  very  unusual. 


FIG.  276. — Pyrite  altering  to  chalcocite  (dark);  black  is  open  field;  Clifton, 
Arizona.     Magnified  30  diameters. 


The  upper  limit  of  the  chalcocite  zone  is  usually  sharp.  In 
depth,  the  secondary  sulphides  may  cease  equally  suddenly 
(Fig.  277),  but  it  is  more  common  to  find  a  gradual  decrease. 
Chalcocite  zones  of  wide  extent  are  usually  explored  by  churn 
drilling  and  the  plotting  by  graphic  methods  of  the  assays  obtained 
gives  a  clear  idea  of  the  sharp  changes  or  gradual  transitions  due 
to  the  enrichment.1  The  depth  of  chalcocite  zone  often  reaches 
1,000  feet  and  in  exceptional  cases  considerably  more.  On  the 

1  E.  H.  Perry  and  A.  Locke,  Interpretation  of  assay  curves  for  drill  holes, 
Trans.,  Am.  Inst.  Min.  Eng.,  vol.  54,  1917,  pp.  93-99. 


OXIDATION  OF  METALLIC  ORES 


853 


other  hand,  the  enrichment  may  be  confined  to  a  thin  layer  of  a 
thickness  of  a  few  feet  only  (Fig.  278). 

The  development  of  the  chalcocite  zones  is  dependent  on 


sw 


Barren  and  Leached  Zone 


Chalcocite  Zone 


NE 


4700- 
4600- 


Zone  of  Primary  Cupriferous  Pyrite 


and  Zinc  Blende 


Scale 
0        100     200       300     400      500      COO  Feet 

FIG.  277. — Longit  udinal  section  of  chalcocite  zone  in  a  vein  at  Morenci, 
Arizona. 


climate  and  water  level  as  well  as  on  composition  and  texture 
of  the  primary  rocks  and  ores.  Permeable  rocks  like  serici- 
tized  granite,  porphyry  or  schists  are  [particularly  favorable 


Gossan  iron  ore      Horizon  of    Low-grade  iron  and 
chalcocite       copper  sulphides 

FIG.  278. — Chalcocite  zone  at  Ducktown,  Tennessee.     After  W.  H.  Emmons, 
U.  S.  Geol.  Survey. 


environments.  In  compact  contact-metamorphosed  shales  and 
limestones,  the  secondary  sulphides  do  not  readily  develop. 
In  limestone  the  zone  is  usually  irregular  and  shallow  because 


854  MINERAL  DEPOSITS 

the  basic  copper  carbonates  here  form  so  easily.1  Wherever 
the  chalcocite  zone  is  present  a  marked  enrichment  has  taken 
place. 

Chalcocite  zones  may  develop  in  primary  ores  of  economic 
value.  They  may  also  form  by  enrichment  of  low-grade  material 
(protore,  p.  833)  whether  this  be  contained  as  heavy  pyrite  in 
veins  of  low  tenor  in  copper,  as  at  Clifton,  Arizona;  or  as  dis- 
seminations of  pyrite  in  larger  mineralized  areas  as  at  Ely, 
Nevada,  Miami,  Arizona,  and  many  other  places.  Such  enrich- 
ments, usually  of  not  greater  thickness  than  100  to  300  feet, 
but  considerable  horizontal  extent,  are  often  referred  to  as 
"chalcocite  blankets"  and  rarely  contain  more  than  2  or  3  per 
cent,  of  copper,  except  perhaps  in  their  uppermost  levels  where 
progressive  enrichment  has  been  proceeding. 

The  secondary  copper  sulphides  are  not  necessarily  con- 
fined below  the  water  level.  They  may  be  deposited  at  any 
place  in  the  oxidized  zone  where  there  is  a  deficiency  in  oxygen 
and  ferric  sulphate,  as  well  shown  at  Tintic,  Utah,  and  other 
places.2  Such  supergene  sulphides  are,  however,  likely  to 
be  spotted  and  irregular  in  occurrence. 

A  lowering  of  the  water  level  and  resulting  oxidation  of  the 
chalcocite  causes  a  progressive  enrichment  and  an  enlargement 
of  the  zone  for  wherever  the  cupric  sulphate  reaches  the  primary 
ore,  fresh  cupric  or  cuprous  sulphide  will  form.  Zinc  blende, 
galena  and  bornite  are  easily  attacked,  galena  more  easily  than 
any  other  sulphide;  next  follows  chalcopyrite,  while  pyrite  is 
not  readily  replaced  by  secondary  sulphides  as  long  as  the  other 
minerals  are  present.  The  whole  process  involves  removal  of  iron 
on  a  large  scale.  Zinc  and  arsenic  are  also  carried  away. 

Theory  of  Supergene  Copper  Sulphides.3 — The  recent  litera- 
ture on  the  subject  of  sulphide  enrichment  in  copper  deposits  is 

1  A.  C.  Spencer  (Prof.  Paper  96,  U.  S.  Geol.  Survey,  1917,  p.  82)  states, 
however,  that  calcite  does  not  precipitate  copper  carbonates  from  a  solution 
of  cupric  and  ferrous  sulphate;  also  that  secondary  copper  sulphides  may 
form  on  pyrite  and  chalcopyrite  in  the  presence  of  large  amounts  of  calcite. 

2  W.  Lindgren,  Econ.  Geol,  vol.  10,  1915,  p.  236. 

3  H.  N.  Stokes,  On  the  solution,  transportation  and  deposition  of  copper, 
silver  and  gold,  Econ.  Geol.,  vol.  1,  1906,  pp.  644-650.     Also  Idem,  vol.  2, 
1907,  pp.  12-23. 

E.  Posnjak,  E.  T.  Allen  and  H.  E.  Merwin,  The  sulphide  ores  of  copper, 
Econ.  Geol,  vol.  10, 1915,  pp.  491-535. 

E.  G.  Zies,  E.  T.  Allen  and  H.  E.  Merwin,  Some  reactions  involved  in 


OXIDATION  OF  METALLIC  ORES  855 

voluminous  and  it  is  not  possible  to  follow  it  in  detail  in  this  place. 
The  deposition  is  governed  by  Schurmann's  reactions  (p.  843) 
so  that  in  general  the  simple  sulphides  of  copper  replace  the  simple 
sulphides  of  iron,  lead  and  zinc.  They  also  replace  the  copper- 
iron  sulphides  like  chalcopyrite  and  bornite  and  the  sulphanti- 
monides  like  tetrahedrite  and  the  sulpharsenides  like  tennantite 
and  enargite.  There  is  not  much  evidence  of  deposition  of  copper 
sulphides  by  precipitation  by  hydrogen  sulphide  or  by  alkaline 
sulphides  though  no  doubt  such  reactions  may  also  take  place, 
especially  when  pyrrhotite  is  one  of  the  primary  minerals. 
Spencer  has  pointed  out  that  the  action  of  cupric  sulphate  on 
sulphides  is  really  a  process  of  oxidation  of  iron,  and  he  as  well 
as  Graton  and  others  have  suggested  that  the  formation  of 
secondary  chalcocite  probably  involves  a  series  of  transitions 
with  gradually  increasing  richness  of  copper,  such  as  pyrite, 
chalcopyrite,  bornite,  covellite  and  chalcocite.  All  the  stages  are 
rarely  observed  together  and  the  last  two  minerals  are  surely 
the  most  important.  The  extent  of  bornite  as  a  secondary 
mineral  is  as  yet  in  doubt. 

Near  the  surface  the  mine  waters  are  solutions  of  sulphuric 
acid  and  ferric  sulphate;  in  depth  their  acidity  decreases  and 
ferrous  sulphate  increases;  at  greater  depth  the  waters  become 
neutral  and  finally  alkaline.1  Cupric  sulphate  is  present  all 
along  but  the  secondary  sulphides  are  evidenlty  not  readily 
precipitated  in  the  presence  of  much  ferric  sulphate  though  they 
are  normally  found  in  the  presence  of  ferrous  sulphate  of  slight 

secondary  copper  sulphide  enrichment.  Contribution  No.  7.  Secondary 
enrichment  investigation,  Econ.  Geol.,  vol.  11,  1916,  pp.  407-503. 

C.  F.  Tolman,  Jr.,  Secondary  sulphide  enrichment,  Min.  &  Sci.  Press, 
vol.  106,  1913,  pp.  38-43,  141-145,  178-181. 

C.  F.  Tolman,  Jr.,  Observations  on  certain  types  of  chalcocite,  etc., 
Trans.,  Am.  Inst.  Min.  Eng.,  vol.  54,  1917,  pp.  402-442. 

L.  C.  Graton  and  J.  Murdoch,  The  sulphide  ores  of  copper,  Trans.,  Am. 
Inst.  Min.  Eng.,  vol.  45,  1914,  pp.  126-181. 

A.  C.  Spencer,  Geology  and  ore  deposits  of  Ely,  Nevada,  Prof.  Paper  96, 
U.  S.  Geol.  Survey,  1917,  pp.  76-91.  Excellent  review  of  subject. 

W.  H.  Emmons,  The  enrichment  of  ore  deposits,  Bull.  625,  U.  S.  Geol. 
Survey,  1917,  pp.  154-249.  Excellent  and  complete  review.  Bibliography 
on  pp.  20-33. 

1  G.  S.  Nishihara,  The  rate  of  reduction  of  acidity  by  descending  waters, 
etc.,  Econ.  Geol.,  vol.  9,  1914,  p.  743-757. 

F.  F.  Grout,  On  the  behavior  of  cold,  acid  solutions,  etc.,  Econ.  Geol., 
vol.  8,  1913,  p.  429. 


856  MINERAL  DEPOSITS 

acidity.  The  early  work  by  H.  V.  Winchell,  C.  F.  Tolman,  Jr., 
H.  N.  Stokes,  E.  C.  Sullivan,  T.  T.  Read,  T.  W.  B.  Welsh, 
C.  A.  Stewart  and  A.  C.  Spencer  showed  that  secondary  sul- 
phides of  copper  may  replace  pyrite  and  other  sulphides  in  cupric 
sulphate  solution.  In  1906  Stokes  had  established  quantita- 
tively the  reaction  with  pyrite.  In  the  notable  paper  by  E.  G. 
Zies,  E.  T.  Allen  and  H.  E.  Merwin  of  the  Geophysical  Labora- 
tory all  the  various  reactions  were  quantitatively  determined 
at  temperatures  of  40°  C.  and  200°  C. 

The  reaction  with  sphalerite  is  as  follows: 

ZnS+CuS04=CuS+ZnS04. 

The  presence  of  sulphuric  acid  accelerates  the  reaction.  When 
cupric  sulphate  and  galena  react  at  35°  C.  cupric  sulphide  is 
first  formed  which  is  further  attacked  by  cupric  sulphate  yield- 
ing cuprous  sulphide.  The  attack  on  chalcopyrite  at  40°  and  at 
200°  C.  is  expressed  by  the  equation 

•:        CuFeS2+CuSO4  =  2CuS+FeSO4. 

In  this  reaction  the  cupric  sulphide  again  alters  to  cuprous 
sulphide  on  further  attack  by  CuSO4.  The  presence  of  sulphuric 
acid  does  not  retard  the  reaction. 

The  action  between  bornite  and  cupric  sulphate  at  the  same 
temperature  is  expressed  by  the  equations 

5Cu5FeS4+llCuS04+8H20  =  18Cu2S+-5FeSO4-r-8H2SO4. 
Cu5FeS4-fCuS04  =  2Cu2S+2CuS+FeSO4. 

Bornite  is  attacked  by  H2S04  resulting  in  CuS  and  Cu2S,  and 
FeS04,  hydrogen  sulphide  developing  at  the  same  time.  These 
products  will  react  and  form  secondary  chalcopyrite. 

Pyrite  alters  to  chalcocite  and  covellite  according  to  Stokes' 
formula:1 

5FeS2+ 14CuSO4+  12H2O  =  7Cu2S+5FeS04+  12H2SO4 

1  Stokes  verified  this  reaction  at  80°  and  100°  C.  with  neutral  solution. 
Some  CuS  was  also  formed,  less  at  100°  than  at  180°.  Cuprous  sulphate  also 
forms  as  an  intermediate  product.  Cupric  and  ferrous  sulphate  mix  in  all 
proportions  without  change,  except  at  high  temperatures  (200°  C.),  when, 
according  to  Stokes,  cuprous  sulphate  and  ferric  sulphate  form;  the  latter 
hydrolyzes  to  ferric  hydrate  and  £[2804,  while  cuprous  sulphate  deposits 
copper  upon  cooling.  This  reaction  is  not  likely  to  take  place  at  tempera- 
tures ordinarily  existing  in  the  oxidized  zone. 


OXIDATION  OF  METALLIC  ORES  857 

The  formation  of  covellite  is  expressed  by  the  following 
formula: 

4FeS2+7CuSO4+4H20  =  7CuS+4FeSO4+4H2S04. 

Sulphuric  acid  exerts  a  markedly  retarding  influence  on  these 
reactions. 

According  to  Zies,  Allen  and  Merwin  pyrrhotite  alters  to 
"chalcopyrite  and  probably  later  to  bornite  when  attacked  by 
cupric  sulphate  but  the  reaction  was  not  followed  quantitatively. 
Most  observers  have  assumed  that  covellite  is  earlier  than  chal- 
cocite  and  Zies,  Allen  and  Merwin  confirm  this  experimentally: 

5CuS+3CuSO4+4H20  =  4Cu2S+4H2S04. 

It  is  probable  that  this  reaction  is  reversible  for  in  many  cases 
covellite  is  an  alteration  product  of  chalcocite. 

According  to  Posnjak,  Allen  and  Merwin1  cupric  sulphide  is 
formed  when  cuprous  sulphide  is  treated  with  dilute  acid  solution 
in  the  presence  of  oxygen.  This  probably  explains  the  develop- 
ment of  covellite  in  oxidizing  chalcocite  and  its  occasional  crys- 
tallization together  with  products  of  oxidation  like  anglesite. 

The  Relation  of  Chalcocite,  Covellite  and  Bornite.— According 
to  the  authors  just  cited  all  chalcocite  crystals  so  far  found — 
they  are  not  common — crystallize  in  the  rhombic  system,  while 
all  synthetic  chalcocite  formed  above  91°  C.  is  isometric.  They 
also  found  that  chalcocite  may  hold  covellite  in  solid  solution 
and  that  above  8  per  cent,  covellite  there  is  no  inversion  point. 
Chalcocite  when  pure  is  white  in  reflected  light  but  many  varie- 
ties are  more  or  less  bluish,  and  this  is  held  to  be  caused  by  dis- 
solved CuS.  While  this  is  undoubtedly  correct,  it  is  true  that 
some  of^the  "blue  chalcocites"  under  high  objectives  are  re- 
solved into  a  mixture  of  chalcocite  with  covellite.  Some  even 
show  pinkish  mottling  and  are  found  to  contain  particles  of 
bornite  and  chalcopyrite.  The  etch  figures  of  normal  low  tem- 
perature chalcocite  often  show  a  network  of  three  partings, 
suggesting  isometric  symmetry.  This  has  been  investigated  by 
L.  C.  Graton  and  J.  Murdoch,  and  C.  F.  Tolman,  Jr.,  who  suggest 
that  this  parting  may  have  been  inherited  from  the  bornite  from 
which  the  chalcocite  was  derived.  The  question  of  "primary 
chalcocite"  has  occupied  much  space  in  the  literature  of  late. 
In  so  far  as  the  view  of  chalcocite  as  a  hypogene  mineral  in  ore 

1  Op.  cit.,  p.  528. 


858  MINERAL  DEPOSITS 

deposits  is  based  on  the  regular  "  intergrowth  "  of  chalcocite  and 
bornite  it  is  decidedly  untenable  for  these  "intergrowths" 
are  surely  replacements  by  chalcocite  and  could  well  have  been 
formed  by  supergene  solutions.  The  problem  is  not  solved  but 
it  may  be  said  that  chalcocite  has  not  yet  been  proved  a  primary 
or  hypogene  mineral  in  sulphide  deposits  of  hypogene  origin.1 

L.  C.  Graton,2  J.  C.  Ray3  and  others  have  also  held  that 
covellite  may  be  of  hypogene  origin  because  it  replaces,  in  crystal 
form,  other  sulphides  like  pyrite,  enargite  and  sphalerite.  How- 
ever, covellite  formed  by  undoubtedly  supergene  solutions  always 
shows  an  exceedingly  strong  tendency  to  develop  blades  and 
crystals.  Covellite  has  not  yet  been  proved  a  primary  or  hypo- 
gene  mineral  in  sulphide  deposits  of  hypogene  origin. 

Oxidation  of  Chalcocite  Zones. — Where  erosion  and  water  level 
have  been  stationary  for  a  long  time  no  changes  take  place  in 
the  chalcocite  zone  except  by  the  gradual  increase  due  to  con- 
tinued slow  leaching  of  the  oxidized  zone.  But  when  erosion 
is  quickened  or  the  water  level  subsides  the  reactions  of  oxidation 
may  invade  the  chalcocite  zone.  The  gossan  may  be  entirely 
removed  and  then  oxidation  will  be  working  on  enriched  sulphide 
ore  in  which  there  will  be  comparatively  little  iron  as  pyrite. 
Other  elements  like  arsenic  may  also  have  been  removed  during 
chalcocitization.  The  oxidation  of  such  materials  may  result 
in  almost  complete  leaching  of  copper  and  a  residual  outcrop 
consisting  of  quartz  with  some  sericite  in  which  even  copper 
stains  may  be  lacking.  Such  conditions  exist  at  Clifton,  Miami, 
and  many  other  places  in  Arizona.  The  outcrops  at  Butte 
represent  also  a  leached  chalcocite  zone  and  contain  little  copper. 

The  course  of  the  oxidation  depends  largely  on  the  amount  of 
residual  pyrite.  Where  much  of  this  is  present  the  chalcocite 
is  decomposed  to  cupric  sulphate  while  pyrite  protected  by  the 
chalcocite  remains  a  little  longer  until  finally  decomposed  into 

1  For  discussion  regarding  this  subject  see  B.  F.  Laney,  Econ.  Geol.,  vol. 
6,  1911,  pp.  399-411  (Virgilina  district). 

A.  F.  Rogers,  Econ.  Geol,  vol.  8,  1913,  pp.  781-795  (Butte). 

H.  W.  Turner  and  A.  F.  Rogers,  Econ.  Geol,  vol.  9,  1914,  pp.  359-391 
(Engel's  mine,  Cal.). 

L.  C.  Graton  and  D.  H.  McLaughlin,  Econ.  Geol,  vol.  12,  1917,  pp.  1-38 
(Engel's  mine). 

J.  F.  Tolman,  Jr.,  Econ.  Geol,  vol.  12,  1917,  pp.  379-386. 

2  Trans.,  Am  Inst.  Min.  Eng.,  vol.  45,  1914,  p.  51. 
»  Econ.  Geol,  vol.  9,  1914,  p.  473. 


OXIDATION  OF  METALLIC  ORES 


859 


iron  sulphate.  When  there  is  little  or  no  pyrite  the  chalcocite 
normally  changes  by  oxidation  to  covellite  and  cuprite,  the 
small  amount  of  sulphuric  acid  and  ferric  sulphate  available 
seems  to  be  sufficient  to  dissolve  these  (Fig.  279).  Again,  the 
chalcocite  may  change  to  brochantite  which  seems  to  be  particu- 
larly common  in  oxidizing  supergene  sulphide  zones,  or  more 
rarely  to  malachite  and  chrysocolla. 

2Cu2S+0  =  2CuS+Cu20. 


2Cu2S+10O4-4H20  =  H6Cu4SOio  (brochantite)  +H2SO4. 
The  cuprite  is  often  reduced  by  ferrous  sulphate  to  native  copper 
NW  SE 

Probable  Croppings 

COOOFeet 

above  Sea  Level 


Pyritic 
Zone 


Porphyry 


Scale 


FIG.  279. — Secondary  zones  in  copper  veins  in  contact-metamorphic  rocks 
Clifton,  Arizona. 

and  this  may  again  be  dissolved  by  sulphuric  acid.  At  times 
chalcocite  is  directly  reduced  to  native  metal  which  may  preserve 
the  structure  of  the  black  sulphide.1  This  might  have  been 
effected  by  ferric  sulphate  according  to  the  following  reaction, 
and  probably  in  many  other  ways. 

Cu2S+3Fe2(SO4)3+4H20  =  2Cu+6FeS04+4H2S04. 
In  the  chalcocite  blankets  it  is  not  common  to  find  much  native 
1  W.  Lindgren,  Prof.  Paper  43,  U.  S.  Geol.  Survey,  1905,  p.  101. 


860  MINERAL  DEPOSITS 

metal,  but  at  Chino,  New  Mexico,  most  of  the  secondary  sul- 
phide appears  to  have  been  converted  to  copper.1 

EXAMPLES  OF  OXIDATION  OF  COPPER  DEPOSITS 

General  Features. — The  study  of  the  various  modes  of  enrich- 
ment in  copper  deposits  is  a  subject  full  of  difficulties.  We  find 
the  most  diverse  development  even  in  a  region  of  uniform  general 
climate.  Take,  for  instance,  the  Sonora- Arizona  province,  where 
the  rainfall  is  small  and  the  climate  warm.  At  Los  Pilares, 
Sonora,  near  Nacozari,  a  gossan  of  barren  hematite  100  feet  deep 
is  underlain  by  an  ill-defined  zone  with  bornite  and  chalcocite, 
changing  below  the  500-foot  level  to  primary  chalcopyrite-pyrite 
ore.  In  other  parts  of  Sonora,  according  to  Finlayson,2  are 
gossan  and  chrysocolla  ores  extending  to  a  depth  of  200  to  400 
feet;  below  this  is  a  shallow  zone  of  secondary  sulphides.  Again, 
at  Clifton,  Arizona,  there  are  in  the  contact-metamorphic  depos- 
its in  limestone  strong  gossans,  sometimes  rich  in  copper,  under- 
neath which  no  secondary  sulphides  are  found.  Pyritic  veins 
in  porphyry  at  the  same  place  have  a  barren  siliceous  outcrop 
without  gossan  and  perhaps  150  feet  thick,  below  which  lies  a 
rich  chalcocite  zone  that  in  a  few  hundred  feet  or  less  changes  to 
lean  primary  sulphides.  Other  veins  near  by  show  chrysocolla 
from  the  surface  down  to  a  shallow  chalcocite  zone  at  100  feet. 
At  Miami,  Arizona,  where  enrichment  has  taken  place  through 
concentration  in  large  masses  of  pyritized  and  sericitized  rocks, 
there  is  a  thick,  almost  barren  zone  of  oxidation  below  which, 
at  depth  of  200  to  1,100  feet,  lies  a  blanket  of  chalcocitized  rock 
from  50  feet  or  less  to  300  feet  in  thickness.  Almost  everywhere 
in  Arizona  the  chalcocite  zone  is  far  above  the  water  level, 
though  according  to  the  accepted  theory  the  chalcocite  zone 
should  form  at  and  below  the  water  level.  The  water-table  may 
formerly  have  been  higher  and  coincided  with  the  top  of  the 
chalcocite  zone,  but  this  cannot  always  be  proved. 

In  the  normal  course  of  oxidation  a  gossan  must  form  and  the 
three  zones  should  be  distinct;  if  the  gossan  is  not  present  it  has 
been  eroded  and  the  barren  upper  zone  has  then  been  formed  by 
leaching  of  the  zone  of  sulphide  enrichment,  the  copper  solution 

1  L.  C.  Graton,  Prof.  Paper  68,  U.  S.  Geol.  Survey,  1910,  p.  316. 
Sidney  Paige,  Econ.  Geol,  vol.  7,  1912,  pp.  547-559. 

2  A.  M.  Finlayson,  Economics  of  secondary  enrichment,  Min.  and  Sci. 
Press,  July  16  and  23,  1910. 


OXIDATION  OF  METALLIC  ORES  861 

descending  to  further  enrich  the  deposit  in  depth.  In  regions 
of  deep  erosion  it  is  exceedingly  rare  to  find  a  strong  chalcocite 
enrichment  in  deposits  exposed  in  the  lower  parts  of  the  canyons. 
In  glaciated  or  rapidly  eroded  regions  almost  all  enrichments 
may  be  lacking. 

Rio  Tinto. — The  pyritic  deposits  of  Rio  Tinto,  southern  Spain,1 
are  situated  in  a  country  of  sub-tropical  climate,  with  an  annual 
rainfall  of  about  25  inches  and  mature  topography,  where  erosion 
makes  slow  headway.  The  primary  deposits  are  thick  lenses  of 
pyrite  containing  less  than  1  per  cent,  of  copper.  There  is  a 
heavy  gossan  of  massive  hematite,  45  to  90  feet  thick,  containing 
no  copper,  over  50  per  cent,  iron,  and  10  to  15  per  cent,  of  sili- 
ceous and  argillaceous  matter.  The  depth  of  oxidation  has 
everywhere  been  determined  by  the  ground-water  level.  The 
lower  limit  of  the  gossan  is  sharp  and  the  line  is  often  marked  by 
a  thin  earthy  zone  with  notable  quantities  of  gold  and  silver,2 
believed  to  represent  an  enrichment  caused  by  leaching  of  the 
gossan  by  solutions  containing  chlorine  and  ferric  sulphate.  The 
top  of  the  sulphide  zone  for  a  thickness  of  a  few  feet  is  composed  of 
leached  pyrite  with  a  trace  of  copper,  resembling  the  upper  part 
of  the  chalcocite  zone  of  Morenci,  Arizona.  Below  this  begins 
the  zone  of  enriched  sulphides,  containing  in  the  upper  part  3  to 
12  per  cent,  copper,  gradually  becoming  poorer  downward  and 
passing  into  lean  pyritic  ore  assaying  1  per  cent,  or  less  of  copper. 
The  depth  at  which  the  unaltered  ore  is  reached  ranges  from  200 
to  1,500  feet  below  the  outcrop.  The  enriched  pyrite  contains 
mainly  chalcocite,  but  also  some  secondary  chalcopyrite.  The 
bulk  of  the  Rio  Tinto  copper  production  to-day  is  derived 
from  enriched  ore. 

Mount  Morgan.3 — The  great  gold  and  copper  deposit  of  Mount 
Morgan,  in  Queensland,  which  since  1886  has  yielded  about 
$65,000,000  in  gold  and  now  bids  fair  to  become  a  great  copper- 
producing  property,  is  most  interesting  and  shows  the  peculiar 
feature  of  great  gold  enrichment  with  almost  entire  absence  of  a 

1  A.  M.  Finlayson,  The  pyritic  deposits  of  Huelva,  Spain,  Econ.  Geol., 
vol.  5,  1910,  pp.  357-372;  403-437. 

2  J.  H.  L.  Vogt,  Zeitschr.  prakt.  Geol,  1899,  p.  250. 

'  J.  M.  Maclaren,  Gold,  London,  1908,  pp.  333-337. 

W.  F.  Gaby,  Petrography  of  the  Mt.  Morgan  mine,  Trans.,  Am.  Inst. 
Min.  Eng.,  vol.  55,  1917,  pp.  263-283- 

J.  F.  Newman  and  J.  F.  C.  Brown,  Trans.,  Austral.  Inst.  Min.  Eng., 
vol.  15,  pt.  2,  1910. 


862  MINERAL  DEPOSITS 

zone  of  secondary  copper  ores.  The  region  has  a  tropical  climate 
and  moderate  rainfall;  the  topography  is  of  the  moderately 
mature  type.  The  water  level  is  probably  deep.  The  irregular 
deposit  is  apparently  a  replacement  in  Carboniferous  rocks, 
surrounded  on  both  sides  by  intrusive  granite,  thus  recalling 
the  deposits  worked  by  the  Reforma  mine,  in  Guerrero,  Mexico, 
and  the  Mount  Lyell  mine,  in  Tasmania. 

At  the  outcrop  there  was  an  extremely  rich  zone  with  free 
gold  in  kaolin,  limonite,  and  black  manganese.  Below  this 
was  found  a  zone  of  a  cellular,  almost  pumiceous  siliceous  mass, 
evidently  a  quartz  skeleton  resulting  from  the  removal  of  pyrite; 
this  was  poorer  in  gold,  but  the  kaolin  that  was  in  places  associated 
with  it  was  rich  in  silver.  The  sharply  defined  lower  limit  of  the 
oxidized  ore  was  met  at  180  to  300  feet  below  the  surface,  and 
the  primary  ore  consisted  at  first  of  pyrite,  then  of  pyrite  with 
chalcopyrite,  carrying  2  to  3  per  cent,  copper  and  $1  to  $8  in  gold 
to  the  ton.  It  is  difficult  to  account  for  the  lack  of  a  chalcocite 
zone.  Unquestionably  there  has  been  concentration  of  gold  on 
a  large  scale  at  the  surface,  probably  caused  by  the  presence  of 
unusual  amounts  of  chlorine.  It  is  noteworthy  that  the  gold 
has  been  precipitated  mainly  at  the  surface  and  could  not  be 
carried  down  into  lower  levels. 

Butte. — The  copper  deposits  of  Butte,  Montana, 1  form  a  system 
of  east-west,  steeply  dipping  veins  cutting  quartz  monzonite  as 
well  as  dikes  of  aplite  and  granite  porphyry  (Modoc  porphyry) 
intrusive  into,  this  granular  rock.  They  are  mainly  dissemi- 
nated pyritic  replacements  along  fissures  and  contain  mainly 
pyrite  with  enargite,  bornite,  chalcocite,  zinc  blende  and  a  little 
chalcopyrite  and  covellite.  There  is  a  scant  gangue  of  quartz. 

1  S.  F.  Emmons,  W.  H.  Weed,  and  G.  W.  Tower,  Jr.,  Butte  folio  (No.  38), 
Geol.  Atlas  U.  S.  Geol.  Survey.  W.  H.  Weed,  Geology  and  ore  deposits 
of  the  Butte  district,  Mont,,  Prof.  Paper  74,  U.  S.  Geol.  Survey,  1912.  J.  F. 
Simpson,  Econ.  Geol,  vol.  3,  1909,  pp.  628-636.  See  also  Reno  H.  Sales, 
Superficial  alteration  of  Butte  veins,  Econ.  Geol.,  vol.  5,  1910,  pp.  15-21; 
vol.  3,  1908,  pp.  326-331.  Ore  deposits  of  Butte,  Montana,  Trans.,  Am. 
Inst.  Min.  Eng.,  vol.  40,  1914,  pp.  3-106.  C.  T.  Kirk,  Conditions  of  min- 
eralization in  the  copper  veins  at  Butte,  Econ.  Geol,  vol.  7,  1911,  pp.  35-82. 
J.  C.  Ray,  Paragenesis  of  the  ore  minerals  in  the  Butte  district,  Econ.  Geol. , 
vol.  9,  1914,  pp.  463-481.  D.  C.  Bard  and  H.  M.  Gidel,  Mineral  associa- 
tions in  the  Butte  district,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  46,  1914,  pp. 
123-127.  A.  P.  Thompson,  The  occurrence  of  covellite  at  Butte,  idem, 
vol.  52,  1916,  pp.  563-596.  W.  W.  Atwood,  The  physiographic  conditions 
at  Butte,  etc.,  Econ.  Geol,  vol.  11,  1916,  pp.  697-740. 


OXIDATION  OF  METALLIC  ORES  863 

Extensive  sericitization  along  the  veins  is  characteristic.  These 
veins  are  cut  by  a  system  of  veins  trending  northwest  and  char- 
acterized by  much  enargite  besides  the  other  minerals  mentioned. 
Finally  there  are  northeastward-trending  veins  that  have  caused 
dislocations  of  the  older  veins  amounting  in  places  to  several 
hundred  feet.  The  supergene  ores  are  found  mainly  in  the  first 
two  classes  of  fractures  and  consist  of  chalcocite  with  smaller 
amounts  of  covellite.  Many  of  the  fault  veins  carry  chalcocite 
as  drag  but  owing  to  their  clayey  character  the  circulation  along 
them  has  been  sluggish  and  they  do  not  contain  large  masses  of 
ore  (Figs.  46  and  47). 

The  outcrops  are  not  prominent  and  the  copper  is  leached 
from  them;  in  some  places  they  contain  chrysocolla.  The  depth 
of  this  oxidized  barren  zone  of  honeycombed  quartz  veins  ranges 
from  10  to  400  feet.  In  the  central  copper  area,  where  the  granite 
is  greatly  altered,  the  upper  limit  of  the  sulphides  is  practically  a 
plane  in  spite  of  surface  inequalities  of  nearly  300  feet,  evidently 
depending  more  upon  the  alteration  of  the  country  rock  than  on 
the  topography.  In  the  leached  zone  there  is  a  slight  enrichment 
of  silver,  the  material  containing  as  much  as  30  ounces  per  ton, 
in  contrast  to  2  5  ounces  per  ton  in  the  sulphide  ore.  No  sec- 
ondary silver  sulphides  have  been  noted.  The  gold  tenor  is 
uniform  throughout  the  copper  area,  indicating  that  practically 
no  secondary  concentration  has  taken  place.  The  ores  average 
30  to  50  cents  per  ton  in  gold,  with  little  difference  between  oxi- 
dized ore  and  that  of  the  sulphide  zone.  A  sharp  line  of  demar- 
cation separates  the  two  zones,  the  change  in  many  places  occurring 
within  2  or  3  feet  vertically.  At  this  level  the  ores  contained  much 
chalcocite  and  averaged  8  per  cent,  or  more  of  copper.  Solid 
masses  of  glance  ore,  15  feet  or  more  in  thickness,  were  found. 
Covellite  is  less  abundant  and  secondary  chalcopyrite  is  rare. 
Corroded  crystals  of  chalcocite  and  others  coated  by  quartz 
are  mentioned  by  H.  V.  Winchell.  In  depth  the  enriched  ore 
gradually  decreases  in  value,  but  low-grade  ore  of  about  2.5 
per  cent,  persists  to  depths  of  3,000  feet  or  more,  particularly 
along  planes  where  the  circulation  of  water  was  energetic.  In 
general,  chalcocitization  in  the  upper  levels  was  accompanied 
by  kaolinization. 

It  is  generally  conceded  that  the  upper  exceptionally  rich 
parts  of  the  veins  contained  an  abundance  of  supergene  chalco- 
cite (Fig.  47).  Sales  designates  these  as  the  "sooty"  chalcocite 


864  MINERAL  DEPOSITS 

zone  implying  its  soft  and  pulverulent  character.  However, 
much  of  that  ore  mined  in  the  early  days  was  compact  and 
showed  bornite. 

Below  this,  enargite  began  to  come  in  but  the  chalcocite 
still  persisted  and  is  even  now  found  in  the  deepest  levels  (3,500 
feet  below  the  surface)  though  there  is  much  pyrite  and  enargite 
and  some  tetrahedrite.  Covellite  occurred  in  considerable 
amounts  in  the  upper  levels  and  as  deep  as  2,000  feet  or  more. 
Many  observers,  among  them  Reno  Sales,  J.  C.  Ray  and  A.  P. 
Thompson  believe  that  this  deeper  chalcocite  is  mainly  of  hy- 
pogene  origin  though  later  than  the  first  succession,  which  is 
represented  by  quartz,  pyrite  and  enargite.  The  bornite  is 
generally  older  than  the  chalcocite  which  even  in  depth  seems 
to  be  the  later  mineral.  Almost  any  specimen  of  the  older, 
rich  chalcocite  ore  shows  ill-defined  remnants  of  bornite.  The 
observers  mentioned  above  also  believe  that  the  covellite  may 
be  of  hypogene  origin. 

There  is  no  doubt  that  the  Butte  ores  show  repeated  re- 
placements of  a  complicated  order,  but  I  am  inclined  to  believe 
that  the  hypogene  origin  of  chalcocite,  covellite,  and  perhaps 
bornite  has  not  been  proved.  As  far  as  can  be  judged  the 
chalcocite  is  of  the  rhombic  modification  which  is  only  stable 
below  91°  C.,  a  low  temperature  which  can  hardly  have  been 
reached  during  hypogene  condition  in  the  Butte  veins.  It  must 
be  remembered  that  wherever  cupric  sulphate  is  carried  super- 
gene  sulphides  may  form  and  there  is  evidence  of  a  distinct 
downward  movement  of  the  groundwater  far  below  the  water 
level. 

I  am  inclined  to  doubt  the  hypogene  origin  of  the  bornite 
remnants  in  the  chalcocite.  Even  those  who,  like  Ray,  believe 
in  the  hypogene  origin  admit  that  it  is  a  secondary  mineral. 
I  have  seen  exactly  the  same  association  in  the  small  veins  of 
Chuquicamata  (for  instance  at  the  Emilia  mine)  which  in  their 
lower  levels  carry  only  pyrite-enargite  ore. 

According  to  this  view  the  Butte  veins  contain  a  hypogene 
pyrite-enargite  ore  and  in  them  has  been  developed  an  ex- 
ceedingly strong  chalcocite  zone  with  elimination  of  arsenic 
in  the  upper  levels.  The  oxidized  zone  as  well  contains  no 
arsenic  and  has  been  formed  by  oxidation  of  the  chalcocite 
zone  which,  below,  became  progressively  enriched. 

In  explanation  of  the  deep  chalcocite  zone  Emmons  and  Weed 


OXIDATION  OF  METALLIC  ORES  865 

state  that  the  block  in  which  the  veins  are  contained  has  been 
faulted  down  probably  several  thousand  feet  and  that  thus  the 
water  level  might  formerly  have  stood  lower  than  at  the  present 
time.  This  explanation  is  not  satisfactory,  for  it  is  not  probable 
that  the  water  level  in  this  region  has  ever  been  at  a  considerable 
depth  below  the  surface,  and  in  the  adjacent  and  higher  block 
on  the  east  the  water  level  still  remains  high.  It  is  conceded 
that  the  present  barren  zone  was  created  by  leaching  of  the  former 
upper  part  of  the  chalcocite  zone  and  that  during  this  process 
the  copper  was  sharply  concentrated  in  the  upper  part  of  the 
present  enriched  zone.  Therefore  the  latter,  like  many  other 
bodies  of  secondary  chalcocite,  must  be  of  considerable  geological 
age  and  long  antedate  the  faulting.  When  it  was  accumulated 
the  water  level  was  assuredly  much  higher  than  at  present. 
This  view  is  supported  in  a  notable  paper  by  Atwood  in  which 
physiographic  principles  are  applied  to  the  study  of  supergene 
enrichment. 

Ely. — An  important  deposit  of  secondary  chalcocite  is  now 
being  worked  on  a  large  scale  at  Ely,  Nevada,1  by  the  Nevada 
Consolidated  Copper  Company. 

The  reserves  of  the  company  are  estimated  at  50,000,000  tons, 
containing  from  1.5  to  2  per  cent,  of  copper.  The  ore  carries  also, 
in  ounces  per  ton,  0.018  in  gold  and  0.088  in  silver. 

The  geological  relations  are  similar  to  those  of  the  Arizona 
deposits.  Intrusions  of  monzonite  porphyry  in  Paleozoic  lime- 
stone caused  contact  metamorphism  of  the  limestone,  silicifica- 
tion  of  both  rocks,  and  some  development  of  copper  deposits,  few 
of  which  are  of  economic  importance.  After  intrusion  the  por- 
phyry became  impregnated  with  disseminated  pyrite  with  a  little 
chalcopyrite,  the  silicates  being  altered  to  sericite  and  pyrite. 
When  the  intrusive  masses  became  exposed  by  erosion  to  the 
action  of  oxidizing  waters  a  downward  migration  of  copper  sul- 
phate, either  from  the  porphyry  itself  or  from  the  overlying 
contact  deposits,  effected  a  chalcocitization  over  wide  areas. 

The  leached  zone  is  from  50  to  200  feet  in  depth  and  forms  an 
iron-stained  soft  mass,  in  places  containing  oxidized  copper 
ores;  below  this  lies  the  chalcocite  zone,  consisting  of  white 
earthy  porphyry  with  disseminated  grains  and  flakes  of  chalco- 
cite and  a  little  pyrite.  This  zone  has  a  maximum  depth  of 
about  500  feet,  the  copper  minerals  gradually  diminishing  down- 

1  A.  C.  Spencer,  Prof.  Paper  96,  U.  S.  Geol.  Survey,  1917. 


866 


MINERAL  DEPOSITS 


ward  to  the  pyritic  valueless  pro  tore;  the  upper  limit  of  the  chal- 
cocite  is  rather  sharply  denned.  Water  is  beginning  to  come 
in  a  depth  of  100  feet,  though  the  general  water  level  in  the 
porphyry  is  said  to  be  385  feet  below  the  surface  (Fig.  280). 


306821  31    9^4  35 


1    72 


'A/A////^X^~        Longitudinal  Section,    -  -^ 

t-A/J-^^^thTough  Ltberty,JHecla'and  Eureka  "• 
u      *    *  »       „  Nevada  Consolidated,  Copper  Co.  <    "  < 


FIG.  280. — Plan  and  section  of  chalcocite  deposit  at  Ely,  Nevada, 
annual  report  of  Nevada  Consolidated  Copper  Co. 


From 


Bingham. — Relations  similar  to  those  at  Ely  exist  at  Bingham, 
Utah,1  in  a  region  of  much  sharper  relief  and  medium  aridity. 
A  small  mass  of  monzonite  is  here  intruded  into  Carboniferous 
(Pennsylvanian)  quartzite  and  limestone,  and  has  apparently 
caused  the  rich  mineralization  of  the  Bingham  deposits.  A 
large  part  of  the  monzonite,  one  mile  in  length  and  half  a  mile 

1  J.  M.  Boutwell,  Prof.  Paper  38,  U.  S.  Geol.  Survey,  1905. 

J.  J.  Beeson,  The  disseminated  copper  ores,  Bingham  Canyon,  Utah, 
Trans.,  Am.  Inst.  Min.  Eng.,  vol.  54,  1917,  pp.  356-401. 


OXIDATION  OF  METALLIC  ORES 


867 


in  width  has  been  subjected  to  hydrothermal  metamorphism 
resulting  in  the  development  of  disseminated  pyrite,  chalcopyrite 
and  bornite.  This  "protore"  which  probably  contains  1  per 
cent,  copper  or  less  has  been  enriched  by  supergene  solution,  de- 
positing chalcocite  and,  in  its  upper  part,  also  covellite,  resulting 
in  a  low-grade  ore  containing  1.5  per  cent,  copper  and,  in  ounces 
per  ton,  0.018  (37  cents)  in  gold  and  0.25  (25  cents)  in  silver. 
The  output  of  ore  per  day  is  almost  30,000  tons;  steam  shovels 
are  now  used  almost  exclusively.  Below  a  leached  surface  zone 
(with  some  oxidized  ore)  about  70  feet  in  depth,  lies  the  chal- 


FIG.  281. — Longitudinal  section  through  central  portion  of  ore-body  of 
the  Utah  Copper  Co.,  Bingham  Canyon,  Utah. 

cocite  blanket.  The  total  thickness  of  the  enriched  zone  is  not 
fully  determined.  The  developed  ore  amounts  to  about  360,- 
000,000  tons  (Fig.  281). 

Beeson  has  shown  that  some  supergene  chalcopyrite  may  be 
formed  as  an  intermediate  product  between  pyrite  and  chal- 
cocite, and  bornite  as  an  intermediate  mineral  between  chalco- 
pyrite and  chalcocite.  Bornite  and  chalcopyrite  may  also  be 
developed  in  reverse  order  by  replacement  of  chalcocite  and 
covellite. 

The  Southwestern  Chalcocite  Deposits.— In  the  arid  country  of 
southern  Arizona  and  New  Mexico  we  find  an  interesting  group 
of  secondary  sulphide  deposits  similar  to  the  last  two  examples 
given.  They  are  sometimes  called  chalcocite  blankets  or  dissemi- 


868  MINERAL  DEPOSITS 

nated  chalcocite  deposits,  and  excellent  representatives  of  them 
are  found  at  Clifton,  Globe,  Ray,  and  Santa  Rita,  and  in  the 
Burro  Mountains.  In  brief,  the  concentration  has  been  proceed- 
ing in  porphyry,  granite,  or  schist  containing  disseminated  pyrite 
with  a  little  chalcopyrite.  Enrichment  through  replacement  of 
pyrite  by  chalcocite  has  in  places  occurred  along  fissures  or 
fissured  zones,  or  still  more  commonly  in  irregular  areas  of  frac- 
tured and  brecciated  rocks.  The  result  is  a  chalcocite  ore  con- 
taining 2  to  4  per  cent,  copper  and  also  some  residual  pyrite; 
this  zone  is  from  100  feet  or  less  up  to  several  hundred  feet  in 
thickness.  Above  it  lies  a  barren  oxidized  and  leached  zone 
reaching  to  the  surface  and  from  50  to  1,000  feet  in  thickness; 
in  places  this  zone  contains  some  oxidized  ore.  Below  the  chal- 
cocite, the  primary  pyritic  dissemination  extends  to  an  unknown 
depth,  the  rock  containing  but  a  fraction  of  a  per  cent,  of  copper. 
The  upper  limit  of  the  chalcocite  zone  is  sharply  defined;  the 
richest  ore  is  found  here,  gradually  decreasing  in  tenor  as  depth 
increases.  The  water  level  usually  lies  at  or  below  the  lower 
limit  of  the  chalcocite  zone,  and  the  zone  itself,  or  at  any  rate 
the  top  of  it,  is  for  the  most  part  high  above  the  present  drainage 
level. 

Evidently  the  secondary  sulphides  could  not  have  been  formed 
in  their  present  places  under  present  conditions,  for  their  upper 
parts  are  now  being  actively  oxidized.  They  give  evidence  of 
having  been  accumulated  during  a  long  period,  probably  begin- 
ning in  the  late  Tertiary,  when  the  climate  was  damp  and  the 
water  level  high,  before  erosion  had  cut  to  its  present  depth. 
The  overlying  lean  porphyry  was  leached  of  its  scant  copper 
content,  the  copper  descending  as  sulphate  to  become  precipi- 
tated as  chalcocite  on  the  primary  pyrite  in  depth.  At  some 
places  the  copper  solutions  may  have  been  derived  partly  from 
once  overlying,  now  eroded  contact-metamorphic  deposits. 

These  deposits  are  then  old — marooned,  as  it  were,  high  above 
their  normal  position  and  in  an  unstable  condition.  Probably 
they  were  once  thicker  and  poorer  than  now  and  covered  by  a 
gossan.  Erosion  has  carried  away  the  surface  gossan,  and  the 
scant  rain  waters  have  leached  the  upper  part  of  the  underlying 
chalcocite  zone — now  the  barren  zone — and  driven  the  copper 
downward  to  replace  the  remaining  pyrite  at  the  level  in  the  zone 
where  the  oxygen  of  the  descending  water  became  exhausted. 
Thus  is  explained  the  richness  near  the  top,  and  it  follows jis  a 


OXIDATION  OF  METALLIC  ORES  869 

corollary  that  chalcocite  may  be  deposited  above  the  permanent 
water  level,  provided  not  much  oxygen  is  present. 

Globe. — The  copper  deposits  at  Globe,  Arizona,  and  the  geol- 
logy  of  the  surrounding  region  have  been  described  by  Ransome.1 

A  few  miles  from  Globe,  in  a  region  of  moderate  relief,  there 
is  an  area  of  granite  (Schultze  granite)  intrusive  into  the  pre- 
Cambrian  Final  schist;  in  the  latter  near  the  contact  several 
disseminated  chalcocite  deposits  have  lately  been  discovered. 

At  the  Miami  mine2  the  leached  zone  is  about  200  feet  deep 
and  contains  in  places  oxidized  ores;  a  sharp  line  of  demarca- 
tion separates  it  from  the  underlying  chalcocite.  The  deposit 
forms  a  flattened  mass  which  in  depth  gradually  increases  in 
extent.  On  the  270-foot  level  the  chalcocite  area  occupies 
1  acre;  on  the  370-foot  level,  3  acres;  on  the  470-foot  level,  16 
acres.  The  average  tenor  of  the  ore  is  over  3  per  cent,  of  copper 
near  the  top  of  the  chalcocite  zone,  but  falls  to  2.65  per  cent, 
on  the  570-foot  level.  At  greater  depth  the  percentage  of  copper 
in  the  ore  changes  abruptly  from  2  per  cent,  to  1  per  cent,  or  less. 
The  mine  produces  a  little  water  on  the  450-foot  level. 

At  the  neighboring  Inspiration  mine  the  leached  surface  zone 
is  from  50  to  575  feet  deep,  averaging  367  feet,  while  the  under- 
lying enriched  zone  averages  155  feet  in  thickness.3  It  is  stated 
that  21,000,000  tons  of  ore  containing  2  per  cent,  copper  has 
been  developed  over  an  area  of  40  acres.  To  develop  this  ore 
81  holes  were  drilled  and  underground  development  work  aggre- 
gating 27,500  feet  was  done.  In  the  whole  ore  zone,  it  is  said, 
80,000,000  tons  of  2  to  2.5  per  cent,  ore  has  been  developed. 

These  deposits  are  thought  to  have  been  formed  during  the 
last  part  of  the  Tertiary  period.  Their  oxidation  is  now  in 
progress,  with  enrichment  and  concentration  of  the  underlying 
chalcocite. 

Ray. — At  Ray  Arizona,4  about  25  miles  southwest  of  Globe, 
a  similar  but  more  extensive  chalcocite  blanket  has  been  dis- 

1  F.  L.  Ransome,  Prof.  Paper  12,  U.  S.  Geol.  Survey,  1903. 

2  C.  F.  Tolman,  Min.  and  Sci.  Press,  Nov.  13,  1909;  E.  Higgins,  Eng.  and 
Min.  Jour.,  April  9,  1910;  R.  L.  Herrick,  Mines  and  Minerals,  July,  1910; 
F.  L.  Ramsome,  Prof.  Paper  (in  press),  U.  S.  Geol.  Survey. 

3  Recent  developments  have  shown  that  the  chalcocite  zone  at  one  place 
lies  at  a  depth  of  1,200  feet  below  the  surface. 

4  W.  Lindgren,  personal  observations;  C.  F.  Tolman,  Min.  and  Sci.  Press, 
Nov.  6,  1909;  W.  H.  Truesdell,  Min.  and  Sci.  Press,  June  5,  1909;  W.  H. 
Weed,  Mining  World,  Jan.  14,  1911. 


870  MINERAL  DEPOSITS 

covered  and  developed  by  churn  drills.  The  Ray  mines  are 
situated  in  a  basin  on  the  upper  part  of  a  creek,  at  an  elevation 
of  about  2,200  feet.  The  deposits  are  in  an  area  of  crushed  and 
altered  pre-Cambrian  schist,  cut  by  dikes  of  granite  porphyry 
and  diabase.  The  upper  leached  zone,  containing  some  oxidized 
copper  ore,  is  from  50  to  150  feet  thick.  The  chalcocite,  dis- 
closed by  drilling  and  underground  operations,  extends  over  a 
large  area,  probably  more  than  100  acres;  its  thickness  is  from 
20  to  300  feet  and  in  a  considerable  part  of  the  area  averages 
60  feet.  The  chalcocite  zone  is  richest  at  the  top  and  gradually 
becomes  poorer  in  depth.  An  ore-body  of  about  83,000,000 
tons,  said  to  average  2.2  per  cent,  copper,  has  been  shown  to  exist, 
and  the  exploitation  of  this  great  deposit  began,  in  1910.  The 
region  had  long  been  known  as  copper-bearing  and  futile  opera- 
tions on  small  masses  of  oxidized  ore  along  diabase  dikes  had 
been  undertaken.  Until  about  ten  years  ago,  however,  it  was 
not  thought  that  ore  of  so  low  a  grade  could  be  mined  profitably. 
Water  begins  to  come  in  at  the  lower  limits  of  the  ore-body, 
which  lies  below  the  level  of  the  creek. 

According  to  Weed  the  granite  porphyry  is  later  than  the 
diabase,  and  it  is  probable  that  both  at  Miami  and  at  Ray  the 
intrusion  of  the  granite  porphyry  was  the  cause  of  the  primary 
mineralization  of  lean  cupriferous  pyrite  in  the  schists.  Accord- 
ing to  the  same  author  there  are  some  exceptions  to  the  rule  of 
sharp  definition  between  the  leached  zone  and  the  ore,  for  at 
the  Ray  Central  mine  the  change  is  gradual  and  for  some 
distance  above  the  200-foot  level  the  ore  consists  of  one-half 
chalcocite  and  one-half  cuprite.  The  passage  into  primary 
sulphides  is  usually  effected  within  50  feet.  In  this  deepest  zone 
are  found  disseminated  pyKte,  a  little  chalcopyrite  and  molybde- 
nite, and  quartz  veinlets.  This  material  contains  less  than  1  per 
cent,  copper. 

Chuquicamata. — The  great  copper  lode  at'Chuquicamata  in 
which  the  developed  ore  is  estimated  to  be  over  300,000,000  tons 
carrying  an  average  of  a  little  more  than  2  per  cent,  copper,  is 
situated  in  Northern  Chile.  The  climate  is  exceedingly  arid.  The 
mass  of  the  ore  so  far  developed  is  oxidized  and  carries  mainly 
brochantite,  near  the  surface  also  atacamite,  the  latter  formed 
by  aid  of  windblown  sodium  chloride.  The  primary  ore  as  shown 
by  borings  and  by  adjacent  smaller  deposits  contains  mainly 
quartz,  pyrite  and  enargite.  The  present  great  oxidized  body 


OXIDATION  OF  METALLIC  ORES  871 

is  formed  by  the  oxidation  of  a  deep  chalcocite-covellite  zone 
of  which  the  lower  part  only  now  remains.  This  is  in  part  mixed 
with  oxidized  ore  and  these  ores  are  somewhat  richer  than  those 
of  the  oxidized  zone.  During  the  chalcocitization  the  arsenic 
was  removed  and  the  oxidized  zone  now  contains  only  traces  of 
that  element.  During  oxidation  in  the  nearly  rainless  climate 
very  little  copper  has  been  carried  downward.  The  brochantite 
which  contains  nearly  as  much  copper  (70  per  cent.)  as  chalcocite 
(80  per  cent.)  has  evidently  simply  replaced  the  supergene  sul- 
phides. Further  alteration  of  brochantite  results  in  the  poorer 
sodium-copper  sulphates,  krohnkite  and  natrochalcite.  Close  to 
the  surface  is  an  irregular  and  shallow  zone  of  leaching,  in  which 
the  copper  is  partly  removed  and  basic  iron  sulphates,  hematite 
and  gypsum  have  formed. 

ZINC 

Minerals. — The  primary  ore  minerals  of  zinc  are  few  in  number. 
In  fact  sphalerite  or  zinc  blende  (ZnS)  may  be  said  to  be  the  only 
one  but  that  is  almost  universally  present  in  sulphide  ores  and  is 
a  persistent  mineral  ranging  from  deposits  of  magmatic  origin 
to  deposits  formed  practically  at  the  surface  where  reducing 
conditions  obtain.  Wurtzite,  the  hexagonal  modification  of 
zinc  sulphide  is  questionable  as  to  its  hypogene  origin.  The 
absence  of  zinc  sulpharsenides  and  sulphantimonides  is  remark- 
able and  the  small  quantities  of  zinc  reported  in  analyses  of  such 
minerals  may  well  be  caused  by  mechanically  admixed  zinc 
sulphide.  Zinc  spinel  or  gahnite  ZnO.Al203)  occasionally  occur- 
ring in  high  temperature  deposits  is  of  no  economic  importance. 
The  minerals  zincite  (ZnO),  franklinite  (ZnO.Fe2O3),  willemite 
(Zn2SiO4),  troostite  (Zn(Mn)2Si04),  and  several  rare  silicates  are 
almost  exclusively^  confined  to  the  unique  deposits  at  Franklin 
Furnace,  New  Jersey. 

The  oxidized  ores  of  supergene  origin  comprise  the  m 
common  smithsonite  (ZnCO3),  the  calamine  (ZnH2Si05)  and  the 
hydrozincite  ZnCO3.2ZnO2H2).  Willemite  may  apparently  also 
be  formed  during  oxidation.  There  is  also  the  rarer  auri- 
chalcite,  a  basic  carbonate  of  copper  and  zinc,  and  several 
still  more  infrequent  arsenates  and  vanadates.  Goslarite  (ZnS04.- 
7H2O)  forms  efflorescences  but  is  of  principal  interest  as 
the  form  in  which  zinc  is  usually  transported  in  solution. 


872 


MINERAL  DEPOSITS 


Monheimite  (ZnFe)  CO3,  the  iron  rich  variety  of  sinithsonite, 
is  not  uncommon. 

Solubility  and  Mineral  Development.  —  The  sulphate  and 
chloride  of  zinc  are  very  easily  soluble,  whereas  the  carbonate  and 
the  silicates  are  difficultly  soluble.  Zinc  blende  is  attacked 
by  oxygenated  water  and  the  carbonate  forms  slowly.  In 
sulphuric  acid  zinc  blende  is  fairly  easily  soluble  with  develop- 
ment of  H2S  and  hence  the  oxidation  of  the  mineral  proceeds 
most  rapidly  in  pyritic  deposits.  Unless  limestone  or  some  other 
precipitant  is  available  the  zinc  of  the  oxidized  zone  is  rapidly 
dispersed  as  sulphate  and  many  examples  are  known  of  zinc- 
bearing  sulphide  deposits  from  the  oxidized  part  of  which  the 
metal  has  wholly  disappeared.  Zinc  is,  in  fact,  the  most  mobile 
of  the  common  metals  in  ore  deposits.  Like  galena,  zinc  blende 
is  also  readily  attacked  by  ferric  sulphate: 


and  the  resulting  free  acid  starts 
the  decomposition  again. 

Smithsonite  is  supposed  to  form 
according  to  the  reaction  ZnSO4+ 
CaC03  =  ZnC03+CaS04  but  there 
is  reason  to  believe  that  where  the 
replacement  is  effected  by  equal 
volumes  the  dilute  solution  of  zinc 
carbonate  is  rather  the  reagent 
than  the  sulphate.  It  is  not  un- 
common to  find  limestone  re- 
placed by  smithsonite  with  perfect 
preservation  of  structure.  The 
oxidized  zinc  ores  are  often  incon- 
spicuous earthy  or  admixed  with 
clay  and  ferric  hydroxide,  and, 
therefore,  easily  escape  attention.  Zinc  sulphate  is  often  found 
in  mine  waters. 

Supergene  Shoots  of  Zinc  Ore.  —  In  calcareous  rocks  the  de- 
scending zinc  solutions  are  easily  arrested  and  there  it  is  common 
to  find  secondary  zinc  shoots  below  the  primary  ore  (Fig.  282). 
In  case  of  the  extremely  common  combination  of  zinc  and  lead 
the  latter  metal  remains  in  its  original  place  as  residual  galena 


FIG.  282.— Diagram  illustrat- 
ing development  of  oxidized 
zinc  ore  in  limestone  below  pri- 
mary bodies  of  lead-zinc  ore, 
May  Day  mine,  Tintic,  Utah. 
After  G.  F.  Loughlin. 


OXIDATION  OF  METALLIC  ORES  873 

or  cerussite  while  the  smithsonite  is  found  lower  down  or  along 
convenient  paths  used  by  the  downward  moving  waters.  G. 
F.  Loughlin1  has  described  many  cases  of  this  kind  from  Tintic, 
Utah,  and  other  places.  Usually  smithsonite  forms  first  and 
hydrozincite,  calamine  and  aurichalcite  are  distinctly  secondary 
after  smithsonite. 

Supergene  Zinc  Sulphide.— Zinc  is  not  as  a  rule  deposited  as  a 
secondary  (supergene)  sulphide  and  no  case  has  been  recorded 
where  it  replaces  pyrite,  as  chalcocite  so  often  does.  Many  cases 
have,  however,  been  described  showing  that  zinc  sulphide  may 
form  below  the  oxidized  zone.  W.  H.  Weed  describes  such  an 
occurrence  at  Neihart,  Montana,  and  F.  Bain  regards  a  certain 
red  variety  of  zinc  blende  at  Joplin,  Missouri,  as  of  secondary 
origin  (p.  451).  White  amorphous  zinc  sulphate  has  been  found 
precipitated  by  hydrogen  sulphide  from  mine  waters.  Crystals 
of  zinc  blende  have  been  observed  in  old  workings  opened  after 
having  been  flooded  for  many  years.2 

Wurtzite,  the  hexagonal  form  of  zinc  sulphide,  which,  except 
by  its  optical  qualities,  is  almost  indistinguishable  from  sphalerite 
has  been  lately  discovered  at  several  places  in  the  United  States, 
particularly  at  Joplin,  at  Butte,  at  the  Hornsilver  Mine,  Utah, 
at  Goldfield,  Nevada,  and  at  the  Era  district,  Idaho.3 

Allen  and  Crenshaw4  have  shown  a  remarkable  analogy  between 
pyrite  and  marcasite  on  one  hand  and  sphalerite  and  wurtzite 
on  the  other.  While  pyrite  and  sphalerite  may  crystallize  from 
alkaline  or  from  acid  solutions  the  presence  of  free  acid  is  essential 
for  the  formation  of  marcasite  and  wurtzite.  It  is  believed  that 
either  wurtzite  develops  from  sphalerite  under  the  influence  of 
acidic  solutions  or  that  both  crystallize  together  from  such 
solutions.  Butler  believed  that  the  wurtzite  which  has  formed 
abundantly  in  the  lower  levels  of  the  oxidized  zone  at  the  Horn- 
silver  mine5  is  of  secondary  origin.  The  oxidation  of  sulphides 
yielded  sulphates  and  free  acid.  The  difficultly  soluble  copper 
sulphide  was  precipitated  as  covellite  on  the  more  easily  soluble 

1  Econ.  Geol,  vol.  9,  1914,  p.  1. 

2  C.  R.  Keyes,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  31,  1901,  p.  611. 
W.  P.  Jenney,  idem,  vol.  33,  1903,  p.  470. 

3  For  a  review  of  these  occurrences  see  J.  B.  Umpleby,  Prof.  Paper  97, 
U.  S.  Geol.  Survey,  1917,  pp.  87-89. 

4  Am.  Jour.  Sci.,  4th  ser.,  vol.  34,  1912,  pp.  341-396;  idem,  vol.  38,  1914, 
pp.  393-431. 

6  B.  S.  Butler,  Prof.  Paper  80,  U.  S.  Geol.  Survey,  1913,  p.  154. 


874  MINERAL  DEPOSITS 

sphalerite,  which,  however,  could  not  precipitate  wurtzite,  this 
being  the  more  soluble  of  the  two  zinc  sulphides.  After  the  acid 
had  become  partly  neutralized,  the  wurtzite  would  be  precipitated 
by  the  hydrogen  sulphide  generated  by  the  attack  of  H2SO4  on 
the  sphalerite.  Occurrences  of  "Schalen  blende"  or  concentric 
intergrowths  of  sphalerite  and  wurtzite  are  known  from  Vieille 
Montagne,  in  Belgium,  and  other  places.  There  is,  as  yet,  much 
uncertainty  as  to  the  conditions  under  which  wurtzite  develops  in 
nature. 

LEAD 

Minerals. — Among  the  primary  lead  minerals  galena  (PbS)  is 
by  far  the  most  common;  bournonite  (PbCuSbS3),  jamesonite 
(Pb2Sb2S5)  and  a  host  of  other  lead  sulphantimonides  such  as 
boulangerite,  geocronite,  etc.,  are  of  little  economic  importance. 

Far  more  numerous  are  the  oxidized  lead  ores.  They  com- 
prise the  red  oxide,  minium  (Pb304),  the  yellow  massicot  (PbO) 
and  the  dark  plattnerite  (Pb02).  All  these  are  comparatively 
rare  as  are  the  chloride  and  the  oxy-chloride.  The  really  abund- 
ant oxidized  lead  minerals  are  anglesite  (PbSO4)  and  especially 
cerussite  (PbC03).  Of  considerable  importance  are  also  a  series 
of  hydrous  basic  sulphates  of  lead  with  copper  and  iron,  including 
the  blue  linarite  (Pb,Cu)SO4(Pb,Cu)(OH)2,  the  yellow  plumbo- 
jarosite  (PbO.3Fe203.4S03.6H20),  the  microscopic  foils  of  which 
cling  to  the  finger  like  graphite  and  several  other  yellow  earthy 
lead  copper  sulphates  of  varying  composition. 

Lead  chloro-phosphate  called  pyromorphite  (PbaPaO^Cl) 
and  the  corresponding  arsenate  (mimetite)  and  vanadate  (vana- 
dinite)  are  not  uncommon  and  may  be  considered  ore  minerals. 
The  same  applies  to  wulfenite  (PbMoO4),  and  crocoite  (PbCrO4) 
and  stolzite  (PbWO4). 

Silicates  of  lead  with  several  other  oxy-salts  like  antimonio- 
silicate  of  manganese  are  found  sparingly  at  the  two  abnormal 
deposits  of  contact -metamorphic  type,  Franklin  Furnace,  New 
Jersey  and  Langban,  Sweden. 

Reactions  in  the  Oxidized  Zone. — Lead  in  contrast  to  zinc 
shows  slight  mobility  in  the  oxidized  zone.  All  the  salts  are 
difficultly  soluble,  particularly  the  carbonate.  The  sulphate  is 
very  slightly  soluble.  Most  soluble  is  the  chloride1  by  means  of 
which  some  transportation  may  be  effected.  Galena  is  slightly 

1  At  15°  C.,  0.909  grams  in  100  grams  H2O;  at  100°  C.,  3.340  grams. 


OXIDATION  OF  METALLIC  ORES  875 

attacked  by  dilute  H2SO4  and  especially  by  the  same  solvent 
together  with  ferric  sulphate.1  The  first  change  in  galena  is 
usually  to  anglesite  (PbS+40  =  PbS04).  Residual  nodules  of 
galena  are  surrounded  by  dark  concentric  rings  of  anglesite,  the 
color  being  caused  by  remaining,  finely  disseminated  lead  sul- 
phide (Fig.  283).  Anglesite  is  also  seen  well  crystallized. 
Cerussite  appears  to  form  easily  from  anglesite  and  usually  pre- 
dominates (PbSO4+H2C03  =  PbC03+H2SO4);  it  appears  as 
beautiful  crystal  groups  but  is  more  commonly  earthy,  white  or 
yellowish  and  of  sandy  texture  (sand  carbonate). 


FIG.  283. — Photomicrograph  of  polished  section  showing  residual  galena 
(light)  in  anglesite  (dark).     Magnified  420  diameters. 

Once  formed  these  two  minerals  are  exceedingly  stable.  How- 
ever, if  there  is  free  sulphuric  acid  or  ferric  sulphate  or  chlorides 
present  the  lead  may  be  rendered  more  mobile.2  Soluble  chloride 
and  oxy-chlorides  form  and  a  whole  series  of  basic  yellow  lead 
iron  sulphates  may  be  developed.  Plumbojarosite,  .one  of  this 

»•  H.  C.  Cooke,  Econ.  Geol,  vol.  21,  1913,  p.  11. 

1  One  liter  of  water  dissolves  only  4.4  milligrams  PbSO4  while  the  same 
amount  of  saturated  NaCl  solution  dissolves  660  milligrams,  slowly  decom- 
posing it  to  chloride. 


876  MINERAL  DEPOSITS 

series,  has  been  used  as  an  ore.1  Considerable  migration  of  lead 
in  oxidized  ore  has  been  carried  on  by  the  aid  of  the  two  reagents 
just  referred  to. 

"Steel  galena"  owes  its  fine-grained  texture  either  to  me- 
chanical deformation  of  larger  crystals  or  to  a  beginning  trans- 
formation to  anglesite.  It  is  supposed  to  be  rich  in  silver  but 
this  is  by  no  means  always  true.  Oxidized  lead  ores  are  usually 
poor  in  silver,  but  here  again  there  may  be  many  exceptions 
noted. 

Wulfenite  is  formed  from  the  oxidation  of  galena  and  molyb- 
denite; the  latter  is  often  present  only  in  microscopic  particles. 
The  enrichment  of  lead  in  the  oxidized  zone  is  generally  a  con- 
sequence of  the  solution  of  associated  minerals  and  the  resulting 
reduction  of  volume  of  the  ore. 

Supergene  Sulphides. — Well-defined  zones  of  supergene  lead 
sulphide  have  never  been  observed.  According  to  its  position 
in  Schurmann's  series  lead  should  be  deposited  as  sulphide  ore 
on  sphalerite  and  pyrite. 

Examples  are  known  of  galena  formed  as  well-defined  crystals 
on  iron  spikes  from  old  workings  of  a  lead  mine  in  Missouri. 
Thin  films  of  galena  are  sometimes  deposited  on  zinc  blende; 
this  has  been  observed  by  Irving  and  Bancroft  at  Lake  City, 
Colorado;  by  Boutwell  at  Bingham,  Utah;  and  by  Ransome  at 
Breckenridge,  Colorado. 

Oxidation  in  the  Coeur  d'Alene  District. — In  the  Coeur  d' 
Alene  district,2  northern  Idaho,  the  precipitation  is  heavy, 
the  topography  is  accentuated,  and  the  water  level  stands  near 
the  surface  of  the  ground.  The  veins,  which  are  enclosed  in 
quartzite  country  rock,  contain  galena,  zinc  blende,  and  siderite. 
Little  pyrite  is  present.  The  lower  limit  of  oxidation  is  very 
irregular.  Complete  oxidation  is  confined  to  the  vicinity  of 
the  surface,  but  cerussite  occurs  in  vugs  and  fractures  several 
hundred  feet  below  the  surface,  while  the  galena  may  in  places 
occur  in  the  outcrops.  The  minerals  first  attacked  are  pyrite 
and  sphalerite,  while  the  solid  galena  is  very  resistant.  The 
chief  products  of  oxidation  are  limonite,  occurring  as  great  masses 
at  the  outcrops,  and  cerussite,  to  which  as  the  latest  products 
pyromorphite,  also  plattnerite  (PbO2)  are  added.  Owing  to  the 

1  B.  S.  Butler,  Prof.  Paper  80,  U.  S.  Gteol.  Survey,  1913,  p.  109. 

2  F.  L.  Ransome  and  F.  C.  Calkins,  Prof.  Paper  62,  U.  S.  Geol.  Survey, 
1908,  p.  132. 


OXIDATION  OF  METALLIC  ORES  877 

prevalence  of  siderite,  cerussite  is  the  predominating  oxidized 
lead  mineral;  anglesite  is  absent.  The  quantity  of  silver  con- 
tained in  the  galena  is  small  and  there  is  no  evidence  of  enrich- 
ment either  in  the  oxidized  zone  or  below  it. 

Oxidation  in  the  Mississippi  Valley  District. — The  effects  of 
oxidation  on  the  lead  and  zinc  deposits  of  the  Ozark  region  have 
been  described  by  Bain,1  Siebenthal  and  Smith,2  and  Buckley 
and  Buehler.3  The  oxidation  is  mostly  confined  to  the  zone 
between  the  surface  and  the  water  level,  which  is  rarely  as  much 
as  100  feet  below  the  surface.  The  oxidized  ore  is,  to  a  consider- 
able degree,  a  product  of  the  residual  weathering  of  limestone 
and  chert  and  thus  consists  of  a  confused  mass  of  red  residual 
clay,  with  layers  and  fragments  of  white  chert,  in  which  are 
found  galena  and  the  oxidized  ores  of  lead  and  zinc.  Galena 
is  the  only  sulphide  found  in  quantity  above  the  water  level. 

The  zinc  blende  alters  either  to  calamine  or  to  smithsonite  and 
nodular  masses  of  each  sometimes  hold  a  kernel  of  the  sulphide. 
A  soft  "  tallow  clay,"  mainly  an  impure  kaolin  with  admixed 
calamine,  occurs  in  many  deposits.  Bain  asserts  that  secondary 
sulphides  of  zinc  and  lead  with  pyrite  and  chalcocite  have  been 
deposited  at  Joplin,  below  the  zone  of  oxidation.  Possibly 
this  has  taken  place  on  a  small  scale,  but  most  of  the  ore  immedi- 
ately below  the  oxidized  zone  appears  to  be  of  primary  origin. 
Zinc  blende  and  galena  may  easily  have  been  deposited  by  the 
reduction  of  sulphate  solutions  by  means  of  metallic  iron  (spikes), 
as  illustrated  in  Bain's  report,  or  by  organic  matter.  There  is 
scarcely  enough  pyrite  in  the  Missouri  deposits  to  cause  extensive 
replacement  (according  to  Schiirmann's  reactions)  of  pyrite  by 
galena  and  zinc  blende.  Moreover,  should  the  deposition  have 
taken  place  by  precipitation  on  sulphides,  galena  could  replace 
zinc  blende,  but  the  reverse  reaction  could  not  take  place. 
There  are  undoubtedly  two  generations  of  blende,  one  earlier 
in  the  chert  and  another  and  less  important  deposition  of  small 
red  crystals  on  dolomite,  but  this  scarcely  proves  that  enrichment 
has  taken  place.  During  oxidation  the  solutions  are  probably 

1  H.  Foster  Bain,  Lead  and  zinc  deposits  of  the  Ozark  region,  Twenty- 
second  Ann.  tiept.,  U.  S.  Geol.  Survey,  pt.  2,  1901,  pp.  155-162. 

2  C.  E.  Siebenthal  and  W.  S.  Tangier  Smith,  Joplin  folio,  Geol.  Atlas  148, 
U.  S.  Geol.  Survey,  1907. 

3  E.  R.  Buckley  and  H.  A.  Buehler,  Geology  of  the  Granby  area,  Missouri 
Bur.  of  Geology  and  Mines,  vol.  4,  2d  ser.,  1906. 


878  MINERAL  DEPOSITS 

acid  only  where  considerable  amounts  of  iron  sulphide  are  present. 
A  frequently  occurring  association  is  that  of  calamine  surmounted 
by  crystals  of  dolomite,  which  could  not  have  been  deposited 
from  solutions  containing  free  sulphuric  acid. 

In  the  upper  Mississippi  Valley  (p.  457),  according  to  C.  R. 
Van  Hise,1  the  main  valuable  minerals  are  smithsonite  and  galena; 
above  the  level  of  ground  water,  which  lies  close  to  the  surface.  En- 
crusting the  galena  are  some  cerussite  and  less  anglesite;  with 
the  smithsonite  is  some  zinc  blende.  The  smithsonite  may  ex- 
tend 15  to  30  feet  below  water  level,  but  at  greater  depth  the  oxi- 
dized products  almost  wholly  disappear.  Below  this  level  zinc 
blende  with  much  marcasite  forms  the  principal  ore-bodies. 
Above  the  water  level,  then,  the  iron  sulphide  has  been  dissolved, 
as  well  as  much  of  the  zinc  blende,  leaving  a  richer  concentrate 
of  galena.  The  galena  at  or  below  the  water  level  may  possibly 
in  part  be  secondary,  precipitated  by  a  reaction  between  lead 
sulphate  and  iron  sulphide.  At  a  still  lower  level  secondary 
zinc  blende  may  have  been  deposited,  but  no  cogent  proof  of 
this  has  been  furnished. 

GOLD 

Gold  shows  slight  mobility  in  ore  deposits;  it  is  less  easily 
transported  than  silver,  and,  compared  to  copper  and  zinc, 
it  is  almost  stationary.  The  solubility  of  gold  has  been  discussed 
briefly  in  the  chapter  on  placers.  Gold  usually  occurs  native, 
in  coarse  or  in  fine  distribution. 

The  tellurides  form  the  only  definitely  known  combinations  of 
gold  with  other  elements.  Among  them  are  calaverite  and  the 
less  common  sylvanite  and  krennerite,  all  with  the  general 
formula  of  (Au,  Ag)  Te2,  and  petzite  (Au,  Ag)2  Te.  The  tellu- 
rides are  apparently  able  to  form  under  widely  differing  conditions, 
though  they  are  generally  absent  from  the  deposits  formed  under 
conditions  of  very  high  pressure  and  temperature.  They  de- 
compose easily  above  the  water  level;  the  tellurium  is  in  part 
carried  away  as  soluble  compounds,  in  part  fixed  as  tellurite 
(TeC>2)  or  tellurates  of  iron  like  emmonsite  and  durdenite.  The 
gold  remains  in  minute  brownish  grains  (mustard  gold).  In 
most  cases  there  is  little  evidence  of  solution  and  transportation 
of  this  gold. 

Certain  deposits  formed  by  hot  ascending  waters  near  the 

1  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  30,  1900,  pp.  102-109. 


OXIDATION  OF  METALLIC  ORES  879 

surface  contain  selenium,  either  alone  or  together  with  tellurium 
(Republic,  Washington;  Tonopah,  Nevada;  Radjang  Lebong, 
Sumatra),  and  probably  they  carry  a  selenide  of  gold,  though  its 
existence  has  not  been  definitely  proved.  Little  is  known  about 
the  oxidation  products  of  selenium. 

Much  more  commonly  the  gold  occurs  in  native  form,  in 
gold-quartz  veins  with  small  amounts  of  sulphides.  In  such  de- 
posits there  is  little  evidence  of  solution  and  transportation  of 
the  gold.  Enrichment  often  takes  place  in  them,  but  rather 
by  reduction  of  volume  of  the  ore  than  by  solution  of  gold. 
The  sulphides  in  these  veins  are  usually  rich  in  finely  distributed 
gold  which  remains  behind  upon  oxidation.  The  oxidation 
of  a  crystal  of  pyrite  will  generally  result  in  a  pseudomorph 
of  limonite  which  contains  flakes  of  native  gold,  indicating 
that  within  the  crystal  a  certain  amount  of  transportation  of 
gold  has  taken  place. 

Very  common  also  are  the  deposits  in  which  the  sulphides 
abound  and  which  contain  no  visible  free  gold.  The  Gilpin 
County  veins,  Colorado;  Mount  Morgan,  Queensland;  and 
the  Haile  Deposit,  South  Carolina,  may  serve  as  examples.  It 
is  in  these  deposits  that  most  evidence  is  found  of  the  solution  and 
transportation  of  gold. 

Gold  has  undoubtedly  been  transported  in  the  form  of  chloride, 
though  its  migration  in  colloidal  suspension  derived  by  solution 
processes  from  native  gold  is  probably  much  more  common 
than  has  been  suspected.  Now,  while  chloride  of  gold  is  easily 
soluble  in  water,  it  is  also  most  easily  precipitated  by  reducing 
reagents,  such  as  organic  matter,  ferrous  sulphate,  metals  or  sul- 
phides, like  pyrite.  In  the  older  literature  ferric  chloride  and 
ferric  sulphate  were  frequently  given  as  solvents  of  gold,  and  the 
question  had  previously  been  discussed  by  Pearce,  Don,  and  T. 
A.  Rickard.  H.  N.  Stokes1  and  J.  R.  Don2  showed  that  ferric 
sulphate  is  ineffective,  and  the  recent  work  of  W.  H.  Emmons3 
and  A.  D.  Brokaw4  has  shown  that  gold  is  insoluble  in  ferric 
chloride  also  and  goes  into  solution  only  when  nascent  chlorine 

1  Econ.  Geol,  vol.  1,  1906,  p.  650. 

*  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  27,  1898,  p.  599. 

3  The  agency  of  manganese  in  the  superficial  alteration  and  secondary 
enrichment  of  gold  deposits  in  the  United  States,  Trans.,  Am.  Inst.  Min. 
Eng.,  vol.  40,  1910,  pp.  767-837. 

4  Jour.  Geology,  vol.  18,  1910,  p.  322;  vol.  21,  1913,  pp.  251-267. 


880  MINERAL  DEPOSITS 

is  present.     The  metal  is  easily  precipitated  from  chloride  solution, 
most  easily  and  completely  by  ferrous  sulphate. 

In  deposits  containing  much  pyrite  oxidation  results  in  the 
liberation  of  sulphuric  acid.  Sodium  chloride  is  present  in  some 
degree  in  all  mine  waters  and  is  abundant  in  some.  Reaction 
between  sulphuric  acid  and  sodium  chloride  results  in  hydro- 
chloric acid,  and  it  should  be  recalled  that  Don  actually  found 
free  HC1  in  a  number  of  superficial  mine  waters  from  New 
Zealand.1  If  dioxide  of  manganese  is  present  in  the  deposit 
nascent  chlorine  will  be  generated  according  to  the  formula: 

Mn02+4HCl  =  2H20+MnCl2+2Cl. 

Ferric  and  cupric  salts  have  similar  power,  but  chlorine  devel- 
ops very  slowly,  if  at  all,  in  the  cold.  It  should  be  expected, 
according  to  Emmons,  that  auriferous  deposits  which  contain 
manganese  would  show  the  effect  of  solution  and  migration  of 
gold  more  clearly  than  non-manganiferous  ores.  According  to 
experiments  by  A.  D.  Brokaw,  quoted  by  Emmons,  gold  is  not 
dissolved  in  hydrochloric  acid,  ferric  sulphate,  or  ferric  chloride.2 
It  is  dissolved  at  38°  C.  in  concentrated  solution  containing  both 
ferric  sulphate  and  hydrochloric  acid;  also  at  the  same  tempera- 
ture in  concentrated  solution  of  cupric  chloride  and  hydrochloric 
acid;  the  dilute  solutions  are  not  effective.  Brokaw's  experi- 
ments verified  the  solubility  of  gold  by  nascent  chlorine  in 
presence  of  manganese  as  outlined  above.  It  was  shown  that 
a  small  piece  of  rolled  gold  weighing  about  0.15  gram  in  a  solu- 
tion of  50  c.c.  of  one-tenth  normal  HC1  with  1  gram  of  powdered 
MnO2  in  14  days  lost  0.01369  gram.  To  approximate  natural 
waters  a  solution  one-tenth  normal  was  made  as  to  ferric  sul- 
phate and  sulphuric  acid  and  one  twenty-fifth  normal  as  to 
sodium  chloride.  In  a  second  experiment  1  gram  of  Mn02  was 
added;  the  time  allowed  was  14  days: 

Fe2(S04)3+H2S04+NaCl+Au. 

No  weighable  loss. 

Fe2(SO4)3+H2S04+NaCl+Au+Mn02. 
Loss  of  gold,  0.00505  gram. 

1  J.  R.  Don,  op.  cit. 

2  Don  in  1897  had  already  stated  that,  in  the  absence  of  free  chlorine, 
gold  is  insoluble  both  in  FeCl3  and 


OXIDATION  OF  METALLIC  ORES  881 

Where  much  MnOg  is  present,  ferrous  sulphate  is  almost  im- 
mediately transformed  to  ferric  sulphate  and  the  precipitation 
of  the  gold  is  delayed.1 

The  gold  dissolved  in  the  presence  of  Mn02  and  held  in  solu- 
tion by  the  absence  of  FeSO4  moves  downward  until  the  excess 
of  acid  is  reduced,  and  simultaneously  the  iron  and  manganese 
compounds  tend  to  hydrolyze  and  deposit  oxides.  At  this 
stage  FeSO4  becomes  increasingly  prominent  and  effective  as  a 
precipitant.  The  transportation  of  gold  is  thus  dependent  upon 
the  oxidation  of  ferrous  sulphate  by  manganese  dioxide.  In 
the  presence  of  MnC>2  gold  may  even  be  carried  down  and  depos- 
ited below  the  water  level. 

The  greater  enrichment  in  gold  will  be  found  in  the  lower  part 
of  the  oxidized  zone.  A  slight  enrichment  is  sometimes  found 
in  the  upper  part  of  chalcocite  zones.  In  zones  of  deep  oxidation 
as  at  Tintic,2  Utah,  supergene  concentrations  of  gold  may  be 
found  at  all  levels.  Manganese  is  apparently  always  present 
here.  The  supergene  gold  is  usually  very  pure. 

Many  examples  of  actual  redeposition  of  gold  are  mentioned  in 
the  literature,  such  as  films  of  gold  in  fissures  and  on  other 
minerals — for  instance,  on  zinc  blende  at  mines  in  Lake  City, 
Colorado.3 

Crystals  of  gold  are  sometimes  deposited  on  the  surface  of 
pyrite  crystals;  fine  examples  of  this  have  been  described  from  the 
Crystal  mine,  Port  Snettisham,  Alaska.4  Associated  with  the 
gold  were  small  crystals  of  galena.  This  is,  however,  probably 
a  primary  deposition  from  alkaline  ascending  solutions. 

Organic  matter,  hydrogen  sulphide,  carbon,  sulphides,  tellu- 
rides  and  carbonates5  precipitate  gold  from  chloride  solution. 
Palmer  and  Bastin6  have  shown  that  most  sulphides  easily  pre- 
cipitate gold;  pyrite  and  galena,  which  do  not  precipitate  silver 
readily  bring  down  the  gold.  With  dilute  solutions  a  protective 
colloid  may  inhibit  precipitation.7 

A  peculiar  feature  in  certain  gold  deposits  where  extensive 

1  W.  H.  Emmons,  op.  cit.     (Experiment  20.) 

2  W.  Lindgren,  Econ.  Geol,  vol.  10,  1915,  p.  237. 
a  J.  D.  Irving,  oral  communication. 

4  J.  E.  Pogue,  Zeitschr.  Kryst.  u.  Min.,  vol.  49,  1911,  p.  455. 
*  V.  Lenher,  Econ.  Geol,  vol.  13,  1918,  pp.  161-184.    . 
« Econ.  Geol,  vol.  8,  1913,  pp.  140-170. 
7  E.  S.  Bastin,  Jour.  Washington  Acad.  Sci.,  1916,  p.  64. 


882  MINERAL  DEPOSITS 

kaolinization  has  taken  place  near  the  surface  is  the  occurrence 
of  white  kaolin  extraordinarily  rich  in  gold  so  fine  that  it  is 
scarcely  visible  when  the  material  is  washed  in  the  pan.1  This 
is  undoubtedly  an  effect  of  oxidation,  but  the  mode  of  this  en- 
richment is  not  fully  explained.  Possibly  the  gold  has  been  pre- 
cipitated in  this  extremely  finely  divided  form  from  colloidal 
solutions. 

Examples  of  Oxidation  of  Gold  Deposits. — In  the  Blue  Moun- 
tains of  Oregon,2  a  region  of  heavy  precipitation  but  dry  summers 
and  rather  high  topographic  relief,  gold-quartz  veins  are  con- 
tained in  Paleozoic  argillites  and  in  intrusive  diorite.  The  ores, 
which  in  places  carry  much  free  gold,  are  oxidized  down  to  a 
depth  of  100  to  300  feet.  At  the  Sanger  mine,  on  Eagle  Creek, 
the  uppermost  100  feet  showed  a  narrow  vein,  caused  by  collapse 
of  outcrops,  yielding  $25  per  ton,  while  farther  down  the  vein 
widened  and  its  gold  was  reduced  to  $12  per  ton. 

In  the  highly  productive  gold  veins  of  the  Cracker  Creek  and 
Granite  districts  the  sulphides  and  arsenopyrite  are  in  fine  dis- 
tribution and  much  of  the  ore  is  rather  hard.  The  water  level  is 
high,  but  on  the  steep  hillsides  the  oxidized  zone  is  in  places  250 
feet  deep.  The  oxidation  to  this  depth  is  only  partial,  but 
there  is  a  surprisingly  slight  difference  in  tenor  between  the  sur- 
face ore  and  the  primary  ore.  In  the  latter  the  gold  is  contained 
mainly  in  the  sulphides;  free  gold  is  present  in  the  oxidized  ore, 
but  there  is  not  enough  to  convert  the  material  into  free-milling 
ore.  No  great  reduction  of  volume  has  taken  place,  and  weather- 
ing has  only  slightly  increased  the  tenor  of  gold,  while  the  small 
silver  content  has  been  slightly  leached. 

The  gold-telluride  lodes  of  Cripple  Creek,  Colorado,3  are  mainly 
sheeted  zones  in  which  the  seams  are  filled  with  quartz,  fmorite,. 
and  calaverite  (Au(Ag)Te2).  These  deposits  oxidize  to  brownish 
clayey  material  in  which  the  original  vein  structure  is  no  longer 
apparent.  As  quartz  is  not  abundant,  the  main  product  of  the 
oxidation  is  kaolin,  with  some  limonite.  The  fluorite  is  carried 
away,  while  the  tellurides  are  very  easily  reduced  to  dark-brown 

1  W.  Lindgren,  Twentieth  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  3, 1900,  p.  171. 
F.  Guitermann,  Proc.,  Colorado  Sci.  Soc.,  vol.  3,  1891,  pp.  264-268. 

2  W.  Lindgr-en,  The  gold  belt  of  the  Blue  Mountains  of  Oregon,  Twenty- 
second  Ann,  Rept.,  U.  S.  Geol.  Survey,  pt.  2,  1901. 

8  W.  Lindgren  and  F.  L.  Ransome,  Prof.  Paper  54,  U.  S.  Geol.  Survey, 
1906. 


OXIDATION  OF  METALLIC  ORES  883 

powdery  gold.  The  tellurium  is  partly  carried  away  in  solution 
but  to  some  extent  remains  as  colorless  tellurite  (TeO2)  and  green 
ferric  tellurites  like  durdenite  and  emmonsite. 

The  oxidation  extends  to  the  water  level,  which  is  from  300  to 
900  feet  below  the  surface,  and  in  places  the  ore  is  oxidized  for 
some  distance  below  the  water  level.  There  has  been  little  or 
no  enrichment  of  gold  in  the  oxidized  zone,  but  a  decided  leaching 
of  the  small  amount  of  primary  silver  originally  contained  as 
telluride  or  tetrahedrite.  No  secondary  silver  sulphides  were  de- 
tected, nor  is  there  evidence  of  secondary  deposition  of  tellurides. 

At  Creede,1  Colorado,  and  in  the  Tomboy  and  the  Camp 
Bird  mines,1  Colorado,  as  well  as  in  the  Granite-Bimetallic1  mine, 
Montana,  in  the  Tintic  mines,2  Utah,  and  in  the  Mount  Morgan 
mine,  Queensland  (p.  861)  examples  are  found  of  the  deposition 
of  supergene  gold  in  the  lower  part  of  the  oxidized  zone. 

SILVER 

Minerals. — Of  the  primary  silver  minerals  argentite  (Ag2S) 
is  easily  the  most  important.  Hessite  (Ag2Te)  is  not  uncommon 
in  gold-quartz  veins  but  plays  no  great  part  as  an  ore  mineral, 
and  the  selenide  is  a  rarity.  Native  silver  may  under  some 
circumstances — as  in  the  Lake  Superior  copper |mines — appear  as 
a  primary  mineral.  Of  more  importance  are 'tetrahedrite  (Cu8- 
Sb2S7)  and  tennantite  (Cu8As2S7)  for  they  usually  contain  silver 
sulphide  in  chemical  combination.  Finally  we  have  a  long  series 
of  sulphantimonides,  sulpharsenides  and  sulphobismuthides  of 
silver  or  silver-lead.  All  of  these  are  invariable  of  late  though 
not  necessarily  of  supergene  origin.  At  least  two  of  them — prous- 
tite  and  pyrargyrite — are  probably  often  of  hypogene  derivation. 

Galena  and  silver  are  particularly  frequently  associated  and 
it  is  now  known  that  this  silver  is  contained  in  galena  as  minute 
inclusions  of  argentite3  easily  made  visible  by  etching  a  polished 
surface  with  HC1.  Occasionally  tetrahedrite  is  also  included 
in  galena.  It  is  probable  that  the  argentite  is  also  contained 
in  other  of  the  common  sulphides  and  sulpho  salts.  Galena 
and  argentite  form  a  eutectic  at  77  per  cent.  Ag2S  at  630°  C. 
but  this  is  not  developed  where  there  is  less  than  2.70  per  cent. 

1  W.  H.  Emmons,  Bull.  625,  U.  S.  Geol.  Survey,  1917,  pp.  324-349. 

2  W.  Lindgren,  Econ.  Geol,  vol.  10,  1915,  p.  237. 

3  A.  E.  Nissen  and  S.  L.  Hoyt,  Econ.  Geol.,  vol.  10.  1915,  pp.  172-179. 


884  MINERAL  DEPOSITS 

Ag2S  and  it  has  not  been  observed  in  nature.  Silver  sulphide 
may  exist  in  solid  solution  in  PbS  but  the  limit  of  this  lies  below 
0.2  per  cent.  Ag2S. 

The  distinctly  supergene  silver  minerals  comprise  native  sil- 
ver, argentite,  cerargyrite  (AgCl),  embolite  Ag  (ClBr),  bromy- 
rite  (AgBr)  and  iodyrite  (Agl).  The  first  four  are  of  common 
occurrence. 

Stromeyerite  (CuAgS,)  closely  allied  to  chalcocite,  is  probably 
always  of  supergene  origin  and  occurs  in  chalcocite  zones,  for 
instance,  at  Broken  Hill,  New  South  Wales. 

Of  the  group  of  sulpho  salts,  referred  to  above,  the  following 
are  surely  supergene  though  they  may  well  also  develop  as  a 
last  effect  of  hypogene  action,  perhaps  by  an  "upward  secondary 
enrichment:" 

Pyrargyrite  Ag3SbS3  Proustite     Ag3AsS3 

Miargyrite  Ag  SbSz 

Stephanite  Ag6SbS4 

Polybasite  Ag9SbS6  Pearceite     Ag9AsS6 

Dyscrasite  is  really  an  alloy  of  silver  and  antimony  with 
varying  composition;  it  has  formed  important  ore  at  Broken 
Hill  and  Chanarcillo  and  is  also  known  from  Cobalt. 

Solubility  and  Mineral  Development. — Argentite  is  oxidized 
to  cerargyrite  (horn  silver),  probably  by  way  of  the  fairly  soluble 
silver  sulphate;  as  follows:  Ag2S+4O  =  Ag2S04;  Ag2S04+2NaCl 
=  2AgCl+Na2S04.  Pseudomorphs  of  cerargyrite  after  argentite 
are  well  known. 

Argentite  is  very  slightly  attacked  by  dilute  sulphuric  acid, 
but  is  much  more  rapidly  decomposed  by  ferric  sulphate  or  by  a 
mixture  of  the  two  solvents.1  Pyrargyrite  and  polybasite  are 
also  decomposed  by  sulphuric  acid,  silver  and  a  little  antimony 
going  in  solution.  The  reaction  is  increased  by  ferric  sulphate. 

It  is  thus  evident  that  the  presence  of  oxidizing  pyrite  will 
greatly  facilitate  the  movement  of  silver  in  ore  deposits  and  while 
the  metal  is  not  as  mobile  as  copper  or  zinc  it  is  transported  much 
easier  than  gold.  The  universal  presence  of  chlorine  in  waters, 
particularly  in  those  of  arid  climates,  and  the  insolubility  of  the 
silver  chloride  account  for  the  common  occurrence  of  the  latter 
mineral  in  the  oxidized  zones  of  ore  deposits. 

Though  both  cerargyrite  and  argentite  are  easily  reduced  to 

1  H.  C.  Cooke,  Jour.  Geology,  vol.  21,  1913,  p.  13. 


OXIDATION  OF  METALLIC  ORES  885 

native  silver  they  are  very  slightly  soluble  in  solutions  usually 
occurring  in  nature. 

The  solubility  of  the  various  silver  salts  is  as  follows,  in  grams 
of  anhydrous  salt  per  100  grams  of  water:  Ag2S04  at  25°  C.,  0.83; 
at  100°  C.,  1.46.  Ag2CO3  at  25°  C.,  0.003;  in  water  saturated 
with  CO2,  at  15°  C.,  0.08.  AgCl  at  13°  C.,  0.00014;  at  43°  C., 
0.0004.  AgBr  at  25°  C.,  0.00001.  Agl  is  still  less  soluble  than 
the  bromide.  It  will  be  noted  that  the  solubility  of  AgCl  in- 
creases rapidly  with  the  temperature.1 

Another  way  to  facilitate  transportation  of  silver,  which  seems 
to  have  escaped  general  attention,  is  by  colloid  solutions  or  sus- 
pensions. Such  solutions  of  AgCl,  AgBr,  Agl  and  Ag2S  have 
been  prepared  by  methods  which  might  easily  be  used  by  nature 
and  by  adding  protective  colloids  like  silica  they  may  be  made 
very  stable.  The  possibilities  of  such  transportation  are 
evident.2 

Precipitation. — Native  silver  is  easily  attacked  by  ferric  sul- 
phate; the  silver  salts  are  easily  reduced  by  ferrous  sulphate.3 

The  reaction  with  ferric  sulphate  is  reversible  silver  being 
dissolved  on  heating  and  reprecipitated  on  cooling.  It  may  be 
written  as  follows:  2Ag+Fe2(S04)3±=?Ag2SO4+2FeSO4. 

Silver  is  further  precipitated  from  its  solutions  by  organic  mat- 
ter as  well  as  by  copper,  cuprite  and  various  sulphides.4  The 
precipitation  by  sulphides  and  arsenides  has  been  investigated 
by  C.  Palmer  and  E.  S.  Bastin6  and  also  by  F.  F.  Grout6  who 
found  that  alabandite,  chalcocite,  covellite  and  enargite  precipi- 
tate silver  rapidly  while  pyrite,  chalcopyrite,  galena  and  sphal- 
erite are  comparatively  inactive.  All  simple  arsenides  are  also 
very  effective  precipitants  while  the  sulpharsenides  do  not 
react. 

Silver  is  further  precipitated  by  many  silicates  such  as  kaolin 

1  A.  G.  Melcher,  The  solubility  of  silver  chloride,  etc.,  Jour.  Amer.  Chem. 
Soc.,  vol.  32,  1910,  pp.  50-56. 

2  See  E.  S.   Bastin,  Experiments  with  colloidal  gold  and  silver,  Jour. 
Washington  Acad.  Sci.,  1916,  p.  64. 

3  H.  N.  Stokes,  Econ.  Geol.,  vol.  1,  1906,  p.  649. 

H.  C.  Cooke,  Jour.  Geology,  vol.  21,  1913,  pp.  1-28. 

4  J.  H.  L.  Vogt,  Ueber  die  Bildung  des  gediegen  Silbers,  Zeitschr.  prakt. 
Geol,  1899,  p.  113. 

5  Econ.  Geol,  vol.  8,  1913,  pp.  140-170;  C.  Palmer,  Idem,  vol.  12,  1917, 
pp.  207-218. 

6  Econ.  Geol,  vol.  8,  1913,  pp.  407-433. 


886  MINERAL  DEPOSITS 

and  orthoclase.1  Calcite,  siderite  and  rhodochrosite  do  not  pre- 
cipitate silver  from  weak  solutions.2 

Silver  sulphide  is  precipitated  by  hydrogen  sulphide  which 
may  result  from  the  attack  of  H2SO4  on  pyrrhotite,  zinc  blende  or 
galena. 

Supergene  Sulphide  Enrichment. — According  to  Schurmann's 
table  (p.  843)  silver  sulphide  should  replace  many  other  sul- 
phides such  as  those  of  copper,  lead,  zinc  and  iron,  following 
the  reaction  Ag2S04+ZnS  =  Ag2S+ZnS04.  Such  replacements 
may  be  found  but  they  are  rare  and  the  sulphides  generally 
precipitate  native  silver  (in  places  with  some  AgS)  and  in  the  case 
of  galena,  sphalerite  and  pyrite  the  reaction  is  weak.  On  the 
other  hand  almost  all  of  the  common  hypogene  sulphides  as  well 
as  tetrahedrite,  tennantite  and  enargite  are  easily  replaced  by 
the  silver  sulphantimonides,3  such  as  pyrargyrite  and  to  this 
action  much  of  the  supergene  sulphide  enrichment  is  due.  These 
rich  silver  minerals  as  well  as  secondary  argentite  are  also  found 
in  druses,  veinlets  and  crusts  in  which  form  they  do  not  represent 
replacements  but  are  perhaps  precipitated  by  hydrogen  sulphide 
or  by  complex  reactions  between  solutions. 

According  to  the  investigations  of  F.  F.  Grout4  and  L.  G. 
Ravicz5  the  silver  sulphantimonides  and  the  less  common  sul- 
pharsenides  are  formed  where  the  descending  solutions  have 
become  alkaline  through  the  neutralizing  reactions  going  on  all 
the  time  during  their  downward  travel.6  Grout  obtained  a 
product  of  the  approximate  composition  of  stephanite  by  adding 
silver  sulphate  to  a  solution  obtained  by  digesting  stibnite  in  a 
1  per  cent,  solution  of  sodium  carbonate.  At  higher  tempera- 
tures of  250°  C.  and  300°  C.  de  Senarmont  and  Doelter  obtained 
proustite,  pyrargyrite  and  stephanite  by  treating  alkaline  sul- 
pharsenite  or  sulphantimonite  with  alkaline  carbonates. 

It  is  a  difficult  task  to  separate  sharply  the  effects  of  super- 
gene  alkaline  waters  from  those  of  the  ascending  hypogene 
solutions.  Both  may  yield  argentite  and  sulpho  salts.  In  all 
silver  deposits  there  is  a  progressive  hypogene  series  beginning 

1  E.  C.  Sullivan,  Bull.  312,  U.  S.  Geol.  Survey,  1907,  pp.  37-64. 

2  L.  G.  Ravicz,  Econ.  Geol,  vol.  10,  1915,  pp.  368-389. 
3F.  N.  Guild,  Econ.  Geol.,  vol.  12,  1917,  pp.  297-352. 

4  Econ.  Geol.,  vol.  8,  1913,  pp.  407-433. 

5  Econ.  Geol,  vol.  10,  1915,  pp.  378-484. 

6  G.  S.  Nishihara,  The  rate  of  reduction  of  acidity  of  descending  waters, 
etc.,  Econ.  Geol,  vol.  9,  1914,  pp.  743-757. 


OXIDATION  OF  METALLIC  ORES  887 

with  arsenopyrite,  pyrite,  sphalerite,  tetrahedrite  and  galena,  the 
•later  replacing  the  earlier;  then  follows  a  series  of  richer  silver 
minerals  such  as  pyrargyrite  and  proustite,  which  may  replace 
any  and  all  of  the  earlier  sulphides1  besides  filling  druses  and 
veinlets.  These  may  be  either  of  supergene  or  hypogene  origin. 

Zones  of  Supergene  Deposition.- — Zones  of  the  regularity  of 
those  of  copper  deposits  are  rarely  found  in  case  of  silver  ores. 
Still  there  is  a  more  or  less  marked  succession,  dependent  on 
the  association  of  minerals.  Silver  enrichment  apparently 
necessitates  ferric  sulphate  as  an  active  solvent;  this  may  be 
furnished  by  pyrite  or  by  arsenopyrite.  In  the  gossan  and  down 
to  the  water  level  we  find  mainly  cerargyrite  sometimes  as  large 
masses,  though  even  here  there  may  be  local  accumulations  of 
native  silver  and  argentite.  At  Lake  Valley,  New  Mexico,2  100 
tons  of  silver  was  obtained  from  a  comparatively  small  stope, 
called  the  Bridal  Chamber;  it  was  contained  in  Paleozoic  lime- 
stone, and  within  100  feet  of  the  surface.  Below  the  chloride 
zone  we  may  find  the  chlorobromides  and  below  this  a  horizon 
where  the  iodyrite  is  precipitated.3  It  would  be  expected  to  find 
the  most  difficultly  soluble  salt  at  the  top  instead  of  at  the 
bottom  but  according  to  W.  H.  Emmons4  the  explanation  is  that 
if  in  the  solution  containing  the  three  halogens  chlorine  is  in  vast 
excess  silver  chloride  though  most  soluble  will  be  precipitated 
first.  The  three  halogens  are  probably  of  wind  blown  origin, 
from  the  salt  pans  of  the  desert  or  from  the  sea. 

Below  the  chloride  zone  we  find  usually  more  native  silver  and 
dyscrasite  and  this  may  extend  to  considerable  depths,  in  fact 
many  hundred  feet  below  the  water  level.  The  deposition  of 
this  native  silver  may,  like  that  of  argentite  and  sulpho  salts,  be 
accompanied  by  calcite,  barite  and  other  gangue  minerals  usually 
deposited  by  the  ascending  waters.  Theoretically  the  zone  of  the 
argentite  and  the  sulpho  salts  should  begin  at  the  water  level 
but  in  silver  deposits  this  transition  line  is  far  less  sharply  marked 
than  in  copper  deposits.  The  bulk  of  the  supergene  argentite 
usually  lies  above  the  sulpho  salts. 

1  F.  N.  Guild,  Econ.  Geol,  vol.  12,  1917,  pp.  297-352. 

2  Ellis  Clark,  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  24,  p.  148,  1895. 

3  F.    A.   Moesta,    Ueber   das   Vorkommen  der  Chlor-Brom-und   Jodver 
bindungen  in  der  natur,  Marburg,  1870,  pp.  57. 

J.  A.  Burgess,  Econ.  Geol,  vol.  6,  1911,  p.  13;  Idem.,  vol.  12,  1917,  p.  590. 

4  Bull.  625,  U.  S.  Geol.  Survey,  1917,  p.  257. 


888 


MINERAL  DEPOSITS 


The  secondary  silver  sulphides  may  descend  to  considerable 
depths,  in  places  500  to  1,000  feet  below  the  surface.  Poorer. 
ore  of  hypogene  origin  will  be  found  below  the  enriched  zone 
(Fig.  284).  In  many  cases  the  oxidized  ore  and  the  supergene 
sulphides  appear  inextricably  mixed. 

Enrichment  at  Granite-Bimetallic  Mine.—  The  Granite  Bi- 
metallic vein,  in  Montana,  described  by  W.  H.  Emmons,1  is  con- 
tained in  granite  and  has  been  opened  to  a  depth  of  2,600  feet. 
The  early  production  amounted  to  $32,000,000  in  gold  and  silver. 
The  strong,  steep  fissure  has  a  filling  from  1  to  20  feet  wide,  of 


FIG.  284. — Longitudinal  vertical  projection  of  Granite-Bimetallic  vein, 
Philipsburg,  Montana,  showing  zone  of  enrichment.  After  W.  H.  Emmons 
and  F.  C.  Calkins. 

simple  structure  and  remarkable  horizontal  and  vertical  persist- 
ence. The  primary  ore  consists  of  quartz  and  rhodochrosite  with 
much  pyrite,  arsenopyrite,  galena,  and  tetrahedrite,  some  galena 
and  zinc  blende,  and  specks  of  pyrargyrite.  The  ore  is  of  com- 
paratively low  grade,  containing  20  to  30  ounces  of  silver  and 
about  $2  in  gold  per  ton  (Fig.  284). 

The  uppermost,  oxidized  zone,  from  50  to  300  feet  deep, 
contains  a  poor  iron-stained  quartz,  with  little  silver  and  a  trace 
of  gold.  The  material,  which  is  so  poor  that  many  of  the  claims 

1  Prof.  Paper  78,  U.  .8  Geol.  Survey,  1913,  p.  202. 


OXIDATION  OF  METALLIC  ORES  889 

along  the  vein  were  abandoned  during  its  early  history,  also 
carries  some  cerussite,  malachite,  etc.,  as  well  as  remnants  of 
unoxidized  sulphides.  Emmons  considers  that  this  leached  zone 
is  derived  by  incomplete  oxidation  of  the  lower  zone  of  enriched 
sulphides. 

Below  this  leached  zone  lies  the  zone  of  enriched  oxidized  ore; 
it  is  for  the  most  part  between  the  100-foot  and  400-foot  levels. 
This  ore  is  composed  of  quartz,  stained  by  iron  and  manganese, 
but  contains  much  cerargyrite  and  native  silver  as  thin  seams. 
There  is  also  some  argentite,  galena,  .etc.  This  ore  contains  much 
silver  and  from  $5  to  $16  in  gold  per  ton.  Its  value  is  generally 
less  than  that  of  the  ore  in  the  next  lower  zone. 

The  zone  of  enriched  sulphides  extends  in  the  main  from 
300  to  800  feet  below  the  surface.  It  is  extremely  rich  in  silver 
and  yields  from  $4  to  $8  in  gold  per  ton.  Quartz  and  rhodo- 
chrosite  are  the  gangue  minerals;  there  is  much  argentite,  pyrar- 
gyrite,  and  proustite,  as  well  as  abundant  remains  of  primary  ore 
minerals.  The  ruby  silver,  which  is  the  most  valuable  mineral, 
is  contained  in  veinlets  or  seams. 

There  is  then  here  a  distinct  leached  zone,  a  zone  of  moderate 
gold  enrichment,  and  a  zone  of  strong  silver  enrichment. 

While  it  cannot  be  asserted  that  all  of  the  primary  ore  was 
poor  it  is  very  unlikely  that  its  value  reached  the  high  figures 
shown  in  the  zone  of  secondary  sulphides,  which  for  a  vertical 
distance  of  300  to  400  feet  averaged  from  100  to  175  ounces  of 
silver  to  the  ton.  Emmons  concludes  that  the  silver  and  gold 
from  the  upper  parts  of  the  vein,  now  eroded,  have  been  carried 
down  and  that  a  successive  enrichment  has  thus  taken  place. 
The  moderate  gold  enrichment  in  the  lower  part  of  the  oxidized 
zone  is,  according  to  Emmons,  due  to  solution  of  gold  by  free 
chlorine  (p.  880)  and  precipitation  by  ferrous  sulphate. 

Enrichment  at  Georgetown. — In  the  Georgetown  district,  Colo- 
rado,1 argentiferous  veins  cut  pre-Cambrian  granites  and  schists. 
The  climate  is  rigorous,  the  relief  strong,  the  water  level  high. 
The  zone  of  complete  oxidation,  in  which  the  ores  are  rich  in 
silver,  extends  at  most  40  feet  below  the  surface.  Below  this  are 
friable  black  sulphides  and  secondary  galena  rich  in  silver  and  with 
more  gold  than  occurs  at  greater  depth;  the  primary  sulphides, 
which  contained  about  25  ounces  of  silver  per  ton,  are  here  enriched 

1  J.  E.  Spurr  and  G.  H.  Garrey,  Prof.  Paper  63,  U.  S.  Geol.  Survey,  1908, 
p.  144. 


890  MINERAL  DEPOSITS 

and  carry  more  than  200  ounces  per  ton.  Below  the  zone  where 
the  soft  secondary  sulphides  occur  and  irregularly  overlapping 
the  lower  portion  of  this  zone  are  rich  ores  containing  polybasite, 
and  ruby  silver,  better  crystallized  and  more  massive  than  the 
pulverulent  sulphides,  but  also  subsequent  in  origin  to  the  pri- 
mary galena-blende  ore.  These  richer  ores  diminish  in  quantity 
as  depth  increases,  although  gradually  and  irregularly,  so  that  the 
lower  portion  of  the  veins  contains  relatively  less  silver  and  lead. 
The  best  silver  ore  in  most  veins  has  been  found  in  the  upper- 
most 500  feet,  although  good  ore  extends  locally  down  to  700  or 
800  feet,  and  in  one  case  1,000  feet  below  the  surface. 

Enrichment  at  Tonopah. — At  Tonopah,  Nevada,1  a  series  of 
rich  silver-gold  quartz  veins  (Au:  Ag  =  9:l  by  weight),  containing 
hypogene  gold,  argentite,  polybasite,  pyrite,  etc.,  with  rhodonite, 
adularia,  and  carbonates,  cut  across  Tertiary  andesites.  The 
climate  is  exceedingly  dry,  and  the  veins  are  situated  in  a  group 
of  hills  rising  from  the  desert.  The  deposits  are  oxidized  down 
to  a  depth  of  about  700  feet;  ground  water  is  lacking,  but  from 
1,000  feet  downward  tepid  and  hot  waters,  containing  mainly 
alkaline  sulphates,  are  met.  The  oxidation  is  irregular  and  in- 
complete; the  pyrite  is  changed  to  limonite,  and  much  chloride 
of  silver,  with  some  bromide  and  iodide,  has  formed.  In  general, 
cerargyrite  is  found  in  the  upper  part  of  the  oxidized  zone; 
bromyrite  in  the  middle,  and  iodyrite  in  the  lower  part.  Other 
minerals  of  the  oxidized  zone  are  kaolin,  hyalite,  gypsum,  limonite, 
hematite,  jarosite,  and  wulfenite.  On  the  whole  the  oxidized  ore 
contains  more  silver  than  the  primary  ore. 

Below  the  oxidized  zone  the  fissures  and  cracks  in  the  hard 
quartzose  ore  contain  some  secondary  argentite,  polybasite, 
pyrargyrite,  and  also  chalcopyrite.  Even  the  oxidized  zone 
contains  some  veinlets  filled  with  pyrargyrite.  There  is  no  well- 
defined  zone  of  sulphide  enrichment;  secondary  pyrite,  blende, 
and  galena  are  absent.  Spurr  believes  that  "the  secondary 
sulphides  in  the  oxidized  zone  originated  from  descending  sur- 
face waters,  and  probably  part  but  not  all  of  the  sulphides  in 
druses  in  the  sulphide  ore  have  a  similar  origin."  Manganese  is 
present  but  no  evidence  of  transportation  of  gold  has  been  found. 

Analyses  show  that  the  carbonates  were  removed  from  the 

1  J.  E.  Spurr,  Prof.  Paper  42,  U.  S.  Geol.  Survey,  1905. 
J.  A.  Burgess,  Econ.  Geol,  vol.  6,  1911,  pp.  13-21. 

E  S.  Bastin  and  F.  B.  Laney,  Prof.  Paper  104,  U.  S.  Geol.  Survey,  1918. 


OXIDATION  OF  METALLIC  ORES  891 

primary  ore  and  with  them  most  of  the  lime  and  magnesia;  iron, 
manganese,  copper,  lead,  and  zinc  have  been  largely  removed, 
likewise  most  of  the  selenium,  arsenic,  and  antimony.  The  argen- 
tite  has  largely  remained  unaltered,  while  polybasite  and  the 
selenides  have  been  decomposed;  a  little  As  and  Sb  remain  to 
form  ruby  silver.  The  silver  in  the  oxidized  ore  is  combined  as 
argentite,  haloid  compounds,  and  gold  alloy. 

Enrichment  at  Chanarcillo. — The  exceedingly  rich  silver  veins 
of  Chanarcillo  in  the  arid  region  of  central  Chile  have  been 
described  by  F.  A.  Moesta1  and  lately  by  W.  L.  Whitehead.2 
The  veins  intersect  Mesozoic  limestone  with  associated  volcanics. 
At  a  depth  of  from  500  to  1,000  feet  the  primary  ore  appears. 
It  consists  of  calcite  and  barite  as  the  earliest  minerals;  a  second 
stage  is  represented  by  pyrite,  zinc  blende,  chalcopyrite  and 
galena;  a  third  stage  by  arsenopyrite  and  quartz;  and  a  fourth 
stage  in  which  tetrahedrite,  pearceite,  proustite,  polybasite 
and  pyrargyrite  were  deposited.  The  sulphide  enrichment,  in 
which  processes  of  replacement  predominate,  occupies  a  vertical 
interval  of  300  to  600  feet. 

During  the  first  stage  of  enrichment  stephanite,  pearceite, 
polybasite,  stromeyerite  and  argentite  replace  earlier  sulphides. 
In  a  later  stage  silver  and  dyscrasite  develop  on  a  large  scale 
mainly  by  replacing  sulpho  salts  and  sulphides  but  also  by  re- 
placing calcite-  and  by  filling. 

The  zone  of  oxidation  is  from  150  to  600  feet  deep  and  is  marked 
by  the  rich  development  of  iron  oxides,  cerargyrite,  embolite 
and  iodyrite,  the  latter  being  mostly  found  in  depth.  The 
halides  replace  silver  and  dyscrasite,  but  also  sulphides  and 
calcite;  to  a  minor  extent  they  are  deposited  in  open  space. 
The  succession  is  closed  by  a  development  of  argentite  and 
native  silver  in  local  enrichment  due  to  a  reversal  of  oxidation 
reactions;  these  minerals  again  replace  the  halides. 

OTHER  METALS 

Platinum  and  Palladium. — The  main  source  of  platinum 
is  the  native  metal  in  alloy  with  smaller  amounts  of  others  of 
the  same  group.  Some  sulphide  deposits  of  the  high  temperature 
types  contain  sperrylite  (PtAs2)  and  usually  also  more  palladium 

1  Op.  dt. 

*  Econ.  GeoL,  vol.  14,  1919,  pp.  1-45. 


892  MINERAL  DEPOSITS 

than  platinum;  it  is  not  known  in  what  form  the  palladium 
appears.  Platinum  minerals  are  highly  resistant  to  oxidation. 

Sperrylite  is  not  easily  attacked  by  meteoric  waters  and  this 
leads  to  concentration  by  reduction  of  volume.  At  Sudbury  the 
shallow  oxidized  zone  is  richer  in  the  platinum  minerals  than 
the  primary  ore. 

The  occurrence  described  on  p.  791,  from  southern  Nevada, 
shows  that  oxidized  ores  with  much  plumbojarosite  (basic 
plumbo-ferric  sulphate)  contains  finely  divided  gold  as  well  as 
metallic  platinum-palladium.  It  is  probable  that  the  metals 
have  been  precipitated  from  colloid  solutions  or  suspensions. 

Palladium  with  some  platinum  is  found  in  the  blister  copper 
from  certain  smelters,  especially  from  some  treating  ore  from 
disseminated  chalcocite  deposits.  The  Ely  deposits,  Nevada 
(p.  865  ,  yield  more  palladium  than  others.  Selenium,  common 
in  all  blister  coppers,  is  here  also  present  in  unusual  amounts. 

A.  Eilers1  showed  that  this  copper  contained  in  per  cent. 
0.0004  Pt,  0.00016  Pd  and  0.055  Se  but  that  there  is  no  tellurium. 
This  suggests  a  concentration  of  the  platinum  metals,  possibly 
as  selenides,  during  the  supergene  chalcocite  enrichment. 

Mercury.2 — Cinnabar,  the  principal  ore  mineral  of  mercury,  is 
stable  up  to  its  sublimation  point  (680°  C.).  It  is  practically 
insoluble  in  water  (p.  838),  but  is  soluble  in  alkaline  sulphides 
forming  the  compounds  HgS.2Na2S  and  HgS.Na2S  (p.  500). 
From  such  solutions  the  mineral  is  easily  formed  as  shown  by 
Allen  and  Crenshaw.  Cinnabar  is  not  attacked  by  dilute 
solutions  of  sulphuric  acid  nor  by  a  mixture  of  ferric  and  ferrous 
sulphate,  but  it  is  dissolved  by  dilute  hydrochloric  acid  and  is 
readily  attacked  by  nascent  chlorine.  The  rare  selenides  tie- 
mannite  (HgSe)  and  onofrite  (Hg(SSe))  are  probably  primary. 

It  is  apparent  that  the  enrichment  by  oxidation  of  cinnabar 
deposits  is  not  easily  accomplished  and  the  secondary  minerals 
are  usually  confined  to  a  little  native  metal,  which  is  often  found 
in  drops  on  cinnabar  and  probably  is  reduced  by  the  hydro- 

1  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  47,  1913,  p.  217. 

2  E.  T.  Allen  and  J.  L.  Crenshaw,  The  sulphides  of  zinc,  cadmium  and 
mercury,  Am.  Jour.  Sci.,  4th  ser.,  vol,  34,  1912,  pp.  367-383. 

W.  H.  Emmons,  Enrichment  of  ore  deposits,  Bull.  625,  U.  S.  Geol. 
Survey,  1917,  pp.  392-398. 

F.  F.  Grout,  Econ.  Geol,  vol.  8,  1913,  p.  427. 

T.  M.  Broderick,  Some  experiments  bearing  on  the  secondary  enrich- 
ment of  mercury  deposits,  Econ.  Geol,  vol.  11,  1916,  pp.  645-651. 


OXIDATION  OF  METALLIC  ORES  893 

carbons  which  are  common  in  many  deposits;  and  occasionally 
to  a  little  mercurous  chloride  or  calomel.  It  is  not  impossible 
that  some  of  the  mercury  may  be  of  primary  origin  for  G.  F. 
Becker  in  some  of  his  experiments1  obtained  precipitates  of  mixed 
sulphide  and  native  metal.  The  oxidized  products  of  cinnabar 
consist  'besides  native  metal  and  calomel  (HgCl),  of  the  red  mon- 
troydite  (HgO)  and  a  number  of  oxychlorides,  such  as  terling- 
uaite  (Hg2C10)  and  eglestonite  (Hg4Cl20).  They  are  generally 
found  in  the  deposits  occurring  in  arid,  wind  swept  regions,  where 
sodium  chloride  abounds.  Large  masses  of  montroydite,  calo- 
mel and  oxychlorides  occurred  at  Terlingua,2  Texas,  but  even 
here  cinnabar  is  the  principal  mineral.  According  to  Hill,3 
pyrolusite  and  pyrite  are  quite  abundant  in  the  deposits.  All  the 
conditions  are  thus  present  for  the  development  of  nascent 
chlorine  so  that  the  cinnabar  can  be  transformed  into  HgCl2 
which  again  by  reaction  with  the  abundant  calcite  would  produce 
oxychlorides.4 

The  mercurous  sulphate  is  only  slightly  soluble  in  water  (0.058 
grams  per  liter),  and  the  mercurous  chloride  HgCl  is  still  less 
soluble  (0.002  grams  per  liter)  while  HgCU  is  easily  soluble.5 
Though  the  experiments  seem  to  indicate  the  insolubility 
of  cinnabar  in  sulphuric  acid  it  will  be  recalled  that  G.  F.  Becker 
found  cinnabar,  at  New  Idria,  in  a  crust  of  epsomite  and  other- 
soluble  sulphates.' 

It  has  been  observed  that  when  mercurial  tetrahedrite  occurs 
in  a  deposit,  cinnabar  may  be  found  in  the  oxidized  products. 
This  is  shown,  for  instance,  in  several  mines  near  Sumpter, 
Oregon.7  It  is  not  known  by  what  reactions  this  is  accomplished. 

Secondary  Sulphides  of  Quicksilver. — From  the  position  of 
quicksilver  in  the  table  of  Schurmann's  reactions  it  would  appear 
that  in  the  absence  of  oxygen  the  deposition  of  secondary  sul- 
phides should  be  easily  effected.  Broderick's  experiments 
showed,  however,  that  the  sulphide  was  not  precipitated  from 

1  Am.  Jour.  Sci.,  3d  ser.,  vol.  33,  1887,  p.  199. 

2  W.  F.  Hillebrand  and  W.  T.  Schaller,  Bvtt.  405,  U.  S.  Geol.  Survey,  1909. 

3  B.  F.  Hill,  The  Terlingua  quicksilver  deposits,  Bull.  15,  Univ.  Texas, 
1902. 

4  T.  M.  Broderick,  op.  cit. 

5  W.  H.  Emmons,  op.  tit.,  p.  393. 

6  Mon.  13,  U.  S.  Geol.  Survey,  1888,  p.  307. 

7  W.  Lindgren.  Twenty-second  Ann.  Rept.,  U.  S.  Geol.  Survey,  pt.  2,  1901, 
p.  708. 


894  MINERAL  DEPOSITS 

chloride  solutions  by  marcasite,  pyrite  or  realgar.  A  precipitate 
was  obtained  with  stibnite  and  chalcopyrite.  Cinnabar  re- 
placing sulphides  have  not,  however,  been  observed  in  nature. 
There  exists  a  black  amorphous  modification  of  HgS,  known 
as  metacinnabar,  which  has  been  found  in  the  oxidized  zone  of 
many  deposits.  Allen  and  Crenshaw  have  ascertained  that  this 
is  an  unstable  form  and  have  obtained  it  artificially  by  precipi- 
tating dilute,  acid  solutions  of  mercuric  salts  by  sodium  thiosul- 
phate,  a  reaction  which  might  well  take  place  in  nature.  Cinna- 
bar was  obtained  only  from  alkaline  solutions.  At  100°  C.  the 
black  sulphide  changes  into  cinnabar. 

Cadmium. — Cadmium  is  commonly  contained  in  zinc  blende, 
probably  as  an  isomorphous  mixture.  The  cadmium  sulphate 
is  as  easily  soluble  as  zinc  sulphate  and  it  is  present  in  some  mine 
waters  of  Joplin,  Missouri,  and  Butte,  Montana  (p.  905).  Cad- 
mium sulphide,  greenockite,1  is  often  found  as  coatings  on  zinc 
blende  in  agreement  with  Schurmann's  rule,  and  it  occurs  in 
this  way  even  in  the  zone  of  oxidation. 

Nickel  and  Cobalt. — Nickel  contained  as  silicate  in  basic 
igneous  rocks  is  concentrated  during  oxidation  to  hydrous  nickel 
silicates  like  garnierite.  Under  similar  conditions  the  cobalt 
separates  as  asbolite,  a  black  earthy  hydrous  oxide  with  manga- 
nese. Both  are  colloid  precipitates. 

The  primary  nickel  ores  consist  of  niccolite  (NiAs),  pentlan- 
dite  ((FeNi)S),  polydymite  (Ni4S5),  chloanthite  (NiAs2)  and 
minerals  allied  to  the  latter  form.  Millerite  (NiS)  is  of  no 
economic  importance. 

The  nickel  sulphides  are  easily  attacked  by  oxygen,  sulphuric 
acid  and  ferric  sulphate.  Pyrrhotite  with  which  they  usually 
occur  is  also  easily  decomposed. 

Nickel  sulphate  (NiSO^  is  easily  soluble  and  does  not  hydrolyze 
like  iron  sulphate  to  form  the  trivalent  oxide.  Therefore,  nickel 
and  iron  are .  separated  during  oxidation.  Much  of  the  iron 
remains  in  the  gossan  while  the  nickel  is  carried  away  in  solution.2 
Pyrrhotite  ores  are  likely  to  lose  nickel  if  exposed  to  oxidation 
in  stopes  and  piles. 

If  arsenic  is  present  stable  arsenates  may  be  formed  in  small 
quantities,  in  the  oxidized  zone;  like  the  apple  green  annabergite 

1  E.  T.  Allen  and  J.  L.  Crenshaw,  The  sulphides  of  zinc,  cadmium  and 
mercury,  Am.  Jour.  Sci.,  4th  ser.,  vol.  34,  1912,  p.  341. 

2  W.  H.  Emmons,  Bull.  625,  U.  S.  Geol.  Survey,  1917,  p.  460. 


OXIDATION  OF  METALLIC  ORES     •          895 

(Ni3As208+8H20).  In  a  similar  way,  erythrite  (Co3As2O8+ 
8H2O)  develops  from  cobalt  arsenides.  Basic  sulphates  of  nickel 
are  not  known.  There  is  some  evidence  that  millerite  and  poly- 
dymite  may  be  locally  precipitated  as  supergene  sulphides.  Both 
nickel  and  cobalt  are  often  found  in  acid  mine  waters. 

The  more  common  cobalt  arsenides  and  sulpharsenides  in- 
clude smaltite  (CoAs2),  cobaltite  (CoAsS)  and  linnaeite  (Co3S4). 
These  minerals  are  easily  attacked  by  reagents  and  the  sulphate 
being  very  soluble  the  metal  is  readily  dissipated  during  oxidation. 
The  hydrous  oxide,  asbolite,  seems  to  be  the  only  stable  oxidized 
mineral,  except  the  unimportant  arsenate. 

Chromium. — The  primary  chromite  of  igneous  deposits  is 
very  insoluble  and  often  remains  in  detrital  deposits.  Chromium 
mica  (mariposite  and  fuchsite)  is  known  in  sulphide  deposits 
where  they  traverse  serpentine  or  peridotite.  Chromates  such 
as  the  red  crocoite  (PbCr04)  may  be  found  in  the  oxidized  zone. 
Possibly  chromite  is  slightly  affected  by  ferric  sulphate  solution. 
Chromium  sulphates  of  green  or  purple  color  have  been  found  in 
quicksilver  veins  traversing  serpentines. 

Manganese. — The  primary  manganese  minerals  in  ore  de- 
posits are  manganosiderite  ((MnFe)C03),  more  rarely  rhodochro- 
site  (MnCOs)  or  rhodonite  (MnSi03),  still  more  rarely  alabandite 
(MnS)  or  hauerite  (MnS2).  Hausmannite  (Mn304)  occurs  in 
some  deposits  of  igneous  metamorphism. 

Manganese  is  more  soluble  than  iron  and  less  easily  precipitated. 
Calcite  which  readily  precipitates  iron  has  little  effect  on  man- 
ganese solutions.  These  facts  explain  the  separation  of  iron 
and  manganese  in  oxidized  deposits. 

The  universal  products  of  oxidation  are  pyrolusite  (Mn02)  or 
its  hydrated  derivatives  like  psilomelane  and  wad.  Manganite 
(Mn2O3.H2O)  and  braunite  (3Mn203.MnSi03)  in  places  accom- 
pany pyrolusite.  Many  of  the  hydrated  forms  of  manganese 
dioxide  contain  in  chemical  combination  the  oxides  of  lead, 
copper,  zinc,  cobalt,  barium  and  potassium  and  are  probably 
hardened  colloid  precipitates. 

Tin. — In  tin  deposits  cassiterite  is  ordinarily  the  most  abundant 
ore  mineral  and  exhibits  great  resistance  to  solution  and  trans- 
portation. Frequently  it  remains  after  other  constituents  have 
been  dissolved,  the  outcrops  appearing  enriched  in  tin.  Some  of 
the  surface  croppings  of  the  Freiberg  veins  are  reported  to  have 
contained  considerable  amounts  of  cassiterite,  probably,  ae- 


896  MINERAL  DEPOSITS 

cording  to  Stelzner,  released  from  the  zinc  blende  in  which  it  was 
disseminated  as  minute  crystals.  Above  the  water  level  the 
Cornwall  veins  contained  mainly  tin,  the  accompanying  copper 
having  been  leached.  Doelter1  has  shown  experimentally  that 
cassiterite  is  perceptibly  soluble  in  water,  but  this  is  denied  by 
J.  P.  Goldsberry.2  According  to  some  authorities3  on  Cornwall, 
cassiterite  is  occasionally  found  as  a  cement  in  gravels  and  as 
impregnations  in  long-buried  deer  horns.  Lately  Scrivenor  has 
shown  that  the  cassiterite  is  simply  carried  mechanically  into  the 
cavities  of  the  antlers.  The  occurrence  of  pebbles  of  wood 
tin — a  fibrous  form  of  cassiterite — is  reported  from  placers  of 
Saxony,  Cornwall,  several  places  in  the  western  states  (as  at 
Wood's  Creek,  Montana),  and  Bolivia.  Stelzner  regarded 
the  Bolivian  pebbles  as  derived  from  stannite  (Cu2FeSnS4)  or 
from  stanniferous  pyrite.  The  cassiterite  had  apparently  not 
been  altered. 

Tungsten.— Scheelite  (CaWO4)  and  wolframite  (Fe,Mn)W04 
are  common  in  many  deposits,  especially  in  those  formed  at  high 
temperatures.  A  sulphide  of  tungsten  (WS)  looking  like  molyb- 
denite has  recently  been  discovered  in  Utah.4  These  primary 
minerals  are  very  resistant  and  often  occur  in  placers.  Tungsten 
minerals  are  slightly  attacked  by  dilute  sulphuric  acid.5  The 
solutions  are  easily  hydrolyzed  to  insoluble  oxides,  which  also 
are  precipitated  by  ferric  salts,  acids,  etc.  Tungstite6  (WO3.H2O) 
usually  results  and  forms  a  canary  yellow  coating. 

Vanadium. — Few  primary  vanadium  minerals  are  known. 
Igneous  rocks,  according  to  F.  W.  Clarke,  contain  an  average 
of  0.018  per  cent,  vanadium  and  many  pyroxenes  and  micas 
carry  notable  amounts.  Roscoelite  is  found  in  many  gold  quartz 
veins.  It  is  also  widely  transported  in  anorganic  and  organic 
cycles  and  thus  found  in  sediments,  clays,  coal,  etc. 

Vanadates,  like  vanadinite  (PbClPb4(VO4)3)  or  descloizite 
(4  (Pb,Zn)O.V2O5H2O),  are  present  in  the  oxidized  zone  of  many 
lead  deposits  of  igneous  affiliations  and  may  occasionally  be 

1  Min.  pet.  Mitt.,  vol.  11,  1890,  p.  325. 

2  W.  H.  Emmons,  Bull.  625,  U.  S.  Geol.  Survey,  1917,  p.  399. 

3  J.  H.  Collins,  Min.  Mag.,  vol.  4,  1880,  pp.  1  and  103;  vol.  5,  1883,  p.  121. 

4  R.  C.  Wells  and  B.  S.  Butler,  Tungstenite,  a  New  Mineral,  Proc.,  Wash- 
ington Acad.  Sci.,  vol.  7,  1917,  pp.  596-599. 

6R.  W.  Gannett,  Econ.  Geol,  vol.  14,  1919,  pp.  68-78. 
•F.  L.  Hess,  Tungsten  minerals  and  deposits,  Bull.  652,   U.  S.   Geol. 
Survey,  1917,  p.  34. 


OXIDATION  OF  METALLIC  ORES  897 

of  economic  importance  for  the  vanadium  contained,  which 
probably  is  derived  from  the  country  rock. 

Regarding  the  complex  vanadium  oxysalts  in  deposits  in  sand- 
stone and  the  rare  vanadium  sulphide  patronite  see  pp.  407-412. 
The  former  include  vanadium  sulphates  and  uranium  vanadates, 
and  vanadium  mica  (roscoelite).  These  are  undoubtedly  de- 
posited by  meteoric  waters,  but  the  history  of  derivation  and 
succession  is  imperfectly  known. 

Molybdenum. — From  molybdenite  the  common  primary  ore 
mineral,  molybdite  (Mo03)  or  molybdic  ocher1  (Fe2O3.3MoO3.- 
7^H20)  is  formed  as  secondary  yellow  powder.  The  lustrous 
orange  plates  of  wulfenite  (PbMoO4)  are  common  in  the  oxidized 
zone  of  deposits  containing  galena  and  molybdenite,  and  is  in 
places  as  at  Mammoth,  Arizona,  of  economic  importance;  rarely  it 
is  accompanied  by  powellite  (CaMoO4).  W.  H.  Emmons  states 
that  molybdenite  is  not  attacked  by  H2SO4  or  HC1,  and  not  even 
by  ferric  sulphate.  The  metal  is  not  very  mobile  and  is  not 
commonly  concentrated  in  the  oxidized  zone;  neither  does  it 
appear  in  the  supergene  sulphide  zone.  Molybdenite  does 
oxidize  however.  It  appears  also  as  the  readily  soluble  blue 
ilsemannite  which  by  later  investigations2  appears  to  be  moly- 
bdenum sulphate.  It  appears  in  some  mine  waters  to  which 
it  may  give  a  deep  blue  color. 

Most  probably  this  mineral  is  of  colloidal  origin  and  has 
been  formed  from  molybdite  or  molybdic  ocher  by  colloidal 
solution  methods.  Attention  is  also  directed  to  the  presence  of 
molybdenum  in  vanadium  deposits  in  sandstone,  which  probably 
were  formed  by  meteoric  waters  and  to  the  reported  existence 
of  a  colloidal  sulphide.3 

Bismuth. — Among  the  primary  bismuth  minerals,  bismuthinite 
(Bi2S3)  the  native  metal  and  various  lead-bismuth  sulphides 
are  the  most  important.  The  principal  oxidized  minerals 
of  bismuth  are  bismite,  Bi2O3.3H2O,  bismutite,  Bi203.CO2.- 
H2O  and  several  arsenates,  for  instance,  arsenobismite  (2Bi2O3.- 
As2O5.2H2O).  In  general  the  bismuth  salts  are  difficultly  soluble 
in  water  and  the  metal  is  in  this  respect  like  lead.  They  do  not 
show  great  mobility  and  bismuth  is  not  found  abundantly  in  the 
supergene  sulphide  zone.  Nevertheless  it  seems  certain  that 

1  W.  T.  Schaller,  Am.  Jour.  Sci.,  4th  ser.,  vol.  22,  1907,  p.  297. 

2  W.  T.  Schaller,  Jour.  Washington  Acad.  Sci.,  vol.  7,  July  19,  1917. 

3  F.  Cornu,  Zeitschr.  Chem.  Indust.  Kolloide,  vol.  4,  p.  190,  1909. 


898  MINERAL  DEPOSITS 

bismuth  has  been  concentrated  in  places  in  the  zone  of  direct 
oxidation  as  shown  by  masses  of  bismutite  and  hydrous  bismuth 
arsenate  (arsenobismite)  found  at  Tintic1  and  elsewhere.  Other 
observations  point  the  same  way:  At  the  Great  Cobar2  copper 
mine  (p.  698),  in  New  South  Wales,  the  smelting  records  show 
bismuth  to  have  been  abundant  in  the  zone  of  oxidation  while 
there  is  very  little  of  it  in  the  primary  ore. 

Another  evidence  of  transportation  of  the  metal  is  its  occa- 
sional presence  with  lead  in  basic  iron  sulphates  like  jarosite. 

Arsenic.  —  The  primary  arsenical  minerals  in  ore  deposits  are 
arsenopyrite  (FeAsS),  smaltite  (CoAs2),  tennantite  (Cu8As2S7), 
enargite  (CusAsS^,  proustite  (Ag3AsS3),  and  many  rarer  sulphar- 
senides  of  copper,  silver  and  lead. 

Supergene  minerals  of  arsenic  include  native  metal,  arsenolite 
(As2O3),  mimetite  (PbCl.Pb^sjOw),  olivenite  (Cu3As208.Cu- 
(OH)2),  realgar  (AsS),  orpiment  (As2S3),scorodite(FeAs04.2H20), 
pharmacosiderite  (basic,  hydrous,  ferric  arsenate)  proustite  and 
pearceite  (Ag9AsSe). 

Arsenic  acts  as  an  acid  forming  compound  similar  to  phos- 
phorous. The  reactions  in  acid  solution  are  probably  carried 
out  by  means  of  alkaline  arsenates  and  arsenites.  The  sul- 
phides, which  are  almost  insoluble  in  dilute  H2S04,  easily  form 
soluble  double  salts  with  alkaline  sulphides,  e.g.,  Na3AsS3.  On 
the  whole  arsenic  is  fairly  mobile  in  the  oxidized  zone,  more  so 
than  antimony.  Arsenopyrite  oxidizes  to  ferrous  sulphate  and 
arsenic  trioxide: 

2FeAsS+110  =  2FeS04+As203. 

Finally  scorodite  and  pharmacosiderite  are  formed.  Arsenopyrite 
is  attacked  by  ferric  sulphate  and  sulphuric  acid.  Smaltite 
oxidizes  to  arsenates: 

3CoAs2+  140  =  Co3As208+2As203. 

Enargite  is  very  resistant  to  oxidation  but  yields  arsenate,  sul- 
phate and  sulphuric  acid: 


1  W.  Lindgren,  Prof.  Paper  107,  U.  S.  Geol.  Survey,  1919, 

A.  H.  Means,  Am.  Jour.  Sci.,  4th  ser.,  vol.  41,  1916,  p.  125. 

2  E.  C.  Andrews,  Report  on  the  Cobar  copper  and  gold  field,  Min.  Res.  17, 
Geol.  Survey  N.  S.  W.,  1913,  p.  113. 


OXIDATION  OF  METALLIC  ORES  899 

If  there  is  not  sufficient  oxygen  present,  some  chalcocite  or 
covellite  may  form. 

Native  arsenic  in  concentric  shells  is  not  uncommon.  Large 
masses  occurred  at  Washington  Camp,  Arizona.  It  is  apparently 
always  a  product  of  oxidation  and  was  deposited  'by  colloidal 
solutions.  Films  of  secondary  proustite  occurred  on  arsenic  in 
some  deep  mines  at  Freiberg,  Saxony,  which  shows  that  the 
metal  is  not  confined  to  the  uppermost  part  of  the  oxidized 
zone. 

Realgar  and  orpiment  are  probably  always  supergene  sulphides 
but  they  are  not  found  in  the  secondary  zones  of  copper  deposits. 
They  are  rather  more  characteristic  of  the  oxidized  zone,  and 
often  are  derived  from  arsenopyrite.  The  chemistry  of  their 
deposition  is  uncertain. 

Sulpharsenides  of  silver  are  characteristic  of  the  supergene 
zone  of  many  silver  deposits. 

In  limestone  the  basic  arsenates  of  copper  are  easily  fixed; 
calcium,  iron  and  zinc  also  enter  in  their  composition. 

In  non-calcareous  rocks  the  arsenates  are  scarce  and  in  fact,  in 
many  oxidized  zones  and  supergene  sulphide  zones  the  arsenic 
of  the  primary  enargite  ore  is  almost  wholly  removed.  This  is 
the  case  at  Chuquicamata,  Chile,  and  at  Butte,  Montana. 

Antimony. — The  primary  minerals  of  antimony  in  ore  deposits 
are  stibnite  (Sb2S3)  and  a  considerable  number  of  sulphanti- 
monides  in  part  of  copper  like  tetrahedrite  (4Cu2S.Sb2S3), 
and  famatinite  (3Cu2S.Sb2S5) ;  in  part  of  lead  like  jamesonite 
(Pb2Sb2S5),  and  many  others;  in  part  of  silver  like  pyrargyrite 
(Ag3SbS3). 

The  supergene  minerals  comprise  native  metal,  cervantite 
(Sb2O4),  senarmontite  (Sb203)  and  valentinite  (Sb2O3);  further, 
oxysalts  like  bindheimite  (Pb3Sb2O8.H2O)  and  stibiconite 
(2SbO2.H2O);  finally  many  silver  sulphantimonides  of  which 
stephanite  (Ag5SbS4)  and  polybasite  (Ag9SbS6)  are  the  most 
important.  Antimony  is  often  associated  with  gold,  silver  and 
lead. 

Antimony  is  considerably  less  mobile  than  arsenic  during 
oxidation  and  as  the  salts  hydrolyze  in  water  there  is  a  strong 
tendency  to  form  insoluble  white  or  yellowish  oxides.  To  these 
stibnite  usually  alters  and  once  formed  they  are  difficultly  moved. 
When,  as  often  happens,  lead  is  present  in  antimony  deposits 
insoluble  compounds  like  bindheimite  are  likely  to  develop. 


900  MINERAL  DEPOSITS 

Native  antimony  is  not  common  but  is  probably  of  supergene 
origin. 

Stibnite  is  very  slightly  attacked  by  dilute  sulphuric  acid 
and  by  ferric  sulphate,  but  reacts  readily  with  alkaline  solutions 
and  like  mercury  and  arsenic  forms  soluble  alkaline  sulpho 
salts. 

Though  according  to  Schiirmann's  series  antimony  sulphide 
would  be  expected  to  replace  various  other  sulphides  no  examples 
of  this  have  been  found.  Stibnite  is,  however,  often  found  as 
hairlike  crystals  in  vugs  where  the  mineral  certainly  is  of  very  late 
origin.  The  same  applies  even  more  to  certain  occurrences  of 
capillary  and  feathery  jamesonite  but  whether  this  is  the  effect 
of  late  hypogene  or  deep  supergene  solutions  is  not  known 
definitely. 

According  to  W.  Malcolm1  stibnite  is  being  deposited  at  the 
present  time  in  the  West  Gore  mine,  Nova -Scotia,  and  also  a 
"red  sulphide"  perhaps  kermesite  (Sb2S20)  is  also  said  to  be 
forming,  both  probably  from  alkaline  water. 

Antimony  is  thus  not  extensively  transported  in  the  supergene 
sulphide  zone.  It  becomes  of  importance  only  in  the  case  of 
silver  sulphantimonides  (p.  884). 


MINE  WATERS2 
Chloride  Waters 

In  ores  free  from  sulphides  and  other  easily  decomposed 
minerals  the  mine  waters  differ  little  from  the  ordinary  ground 
waters  of  the  region.  Examples  of  such  waters  are  found  in  the 
iron  and  copper  mines  of  Michigan.  In  the  following  table 
analyses  1,  2,  and  3  represent  average  waters  of  the  upper  cir- 
culation; 4  and  5  give  the  composition  of  the  deep  waters  of  the 
same  region,  which  differ  very  materially  from  the  shallow  waters 
and  contain  an  abundance  of  calcium  chloride.  The  deep 
waters  in  the  copper  region  contain  some  copper,  zinc,  nickel,  and 
traces  of  boron. 

1  Mem.  20-E,  Geol.  Survey  Canada,  1912,  p.  296. 

2  W.  H.  Emmons  and  G.  L.  Harrington,  A  comparison  of  the  waters  of 
mines  and  of  hot  springs,  Econ.  Geol,  vol.  8,  1913,  pp.  653-659. 

E.  T.  Hodge,  The  composition  of  waters  of  sulphide  ores,  Econ.  Geol, 
vol.  10,  1915,  pp.  123-139. 


OXIDATION  OF  METALLIC  ORES 


901 


ANALYSIS  OF  MINE  WATERS    FROM  UPPER  AND  LOWER  LEVELS  OF  IRON 

AND  COPPER  MINES  OF  MICHIGAN 

[Parts  per  million] 


1 

2 

3 

4 

5 

Cl 

3  5 

18  68 

6  00 

25  360 

176  027 

Br 

2  200 

CO3  

24.2 

163.  OO1 

41.60 

Not  det. 

SO 

18  8 

13  14 

12  10 

1  045 

110 

Ca 

12  9 

62  29 

15  20 

7902 

86  478 

Mg  
Na  '..... 
K  

2.0 
11.3 

28.20 
}    19.00 

9.60 
f      4.00 
{      1  50 

566 
}     7,290 

20 
f  15,188 
(        411 

A1203\ 
Fe,0,/" 

Mn       

4.0 

18.20 

/      0.60 
t      1.23 

}            700 

•'; 

Cu  

16 

Ni  

6 

SiO2 

14  5 

9  80 

8  43 

20 

Loss 

2  8 

Total  

94.0 

332.31 

100.26 

42,863 

280,490 

'CO,. 

1.  Mass   copper  mine,  Michigan.     Water  from  upper  levels.     Analyst, 
Dearborn  Chemical  Works,  Chicago.     A.  C.  Lane,   Mine  waters.    Thir- 
teenth Annual  Meeting,  Lake  Superior  Mining  Institute,  June,  1908,  p.  31. 

2.  Vulcan  iron  mine,  Michigan.     Analyst,  G.  Fernekes.     Idem,  p.  6. 

3.  Newport  mine,   Gogebic  district,   Michigan.     Analyst,   R.   D.   Hall. 
Free  CO2  18.0.     Residue  dried  at  100°  C.  108.30.     Van  ffise  and  Leith, 
Mon.  52,  U.  S.  Geol.  Survey,  1911,  p.  543. 

4.  Republic  iron  mine.     Seventeenth  level.     Temperature  57°  F.     Anal- 
yst, G.  Fernekes.     Calculates  to  NaCl  18,510  and  CaCl2  21,800.     A.  C. 
Lane,  op.  cit.,  p.  10. 

5.  Quincy  copper  mine.     Drippings  on  fifty-fifth  level.     Similar  waters 
from  the  Calumet  &  Hecla  mine  also  contain  some  zinc.     Analyst,  G.  Fer- 
nekes.    Chiefly  calcium  and  sodium  chlorides  and  sodium  bromide.     Op.  cit., 
p.  48.     Trace  boron  and  strontium.     No  barium  or  carbon  dioxide. 

Occasionally  mine  waters  comparatively  rich  in  silica  may  be 
encountered.  Thus,  E.  T.  Allen  analyzed  a  surface  water  from 
the  Mesabi  iron  range,  Minnesota,  which  contained  22  parts 
per  million  of  silica  and  only  about  14  parts  of  sulphates  and 
5  parts  of  carbonates  of  calcium  and  alkali  metals.1 

1  Mon.  43,  U.  S.  Geol.  Survey,  1903,  p.  264. 


902  MINERAL  DEPOSITS 

Salt  waters,  containing  mainly  sodium  chloride  to  the  amount 
of  several  per  cent.,  are  reported  from  the  Kalgoorlie  mines  in 
Western  Australia,  where  they  began  to  come  in  at  water  level, 
a  few  hundred  feet  below  the  surface.  At  the  Great  Boulder 
Proprietary  the  water  contained  9  per  cent,  of  sodium  chloride 
and  also  much  magnesium  chloride.1  Similar  waters  are  now 
coming  into  the  deep  levels  of  the  Bendigo  mines.2  Sulphides 
are  not  abundant  in  these  mines.  The  analysis  of  a  sample 
taken  4,280  feet  below  the  surface,  in  the  Victoria  Reef  quartz, 
where  the  temperature  of  the  water  is  114°  F.,  is  as  follows, 
contained  in  parts  per  million : 

NaCl,  1,308.45;  Na2SO4,  75.79;  Na2CO3,  37.18;  CaCO,,  124.41;  MgCO3, 
45.76;  SiO2,  21.45;  (Al,  Fe)2O3,  2.86;  total,  1,615.90. 

According  to  T.  A.  Rickard3  the  mine  water  at  Mammoth, 
Final  County,  Arizona,  contains  86  parts  per  million  of  sodium 
chloride,  and  that  from  Stratton's  Independence  mine,  at  Cripple 
Creek,  Colorado,  51  parts  of  the  same  salt. 

Carbonate  Waters 

The  mine  waters  from  the  Wardner  lead  mines,  in  the  Coeur 
d'Alene  district,  Idaho,  are  rich  in  ferrous  carbonate  (from 
siderite  in  the  ore)  and  Deposit  abundant  limonite.  A  sample 
from  the  Reed  level,  Bunker  Hill  &  Sullivan  mine,  showed  70 
parts  per  million  of  total  solids,  chiefly  bicarbonate  and  sulphate 
of  calcium.4 

A  number  of  analyses  of  waters  from  the  lead  mines  of  south- 
eastern Missouri  are  given  by  E.  R.  Buckley.5  The  waters  come 
from  the  La  Motte  sandstone  and  Bonneterre  dolomite,  generally 
at  depths  of  a  few  hundred  feet.  The  total  solids  are  at  most 
500  parts  per  million,  of  which  200  parts  or  more  are  calculated 
as  calcium-magnesium  carbonates.  The  sulphates,  calculated 
as  the  magnesium  salt,  are  at  most  200  parts  per  million,  while 
sodium  chloride  averages  only  50  parts.  Silica  is  low.  All 
contain  a  little  lead,  at  most  1  part  per  million,  calculated  as 

1  T.  A.  Rickard,  Formation  of  bonanzas,  Trans.,  Am.  Inst.  Min.  Eng., 
vol.  31,  1901,  pp.  198-220. 

2  W.  J.  Rickard,  Deep  mining  at  Bendigo,  Mining  Mag.,  1910,  p.  281. 

3  Trans.,  Am.  Inst.  Min.  Eng.,  vol.  31,  1901,  pp.  198-220. 

4  F.  L.  Ransome,  Prof.  Paper  62,  U.  S.  Geol.  Survey,  1908. 

5  Missouri  Bur.  Geology  and  Mines,  vol.  9,  pt.  1,  1909,  p.  249. 


OXIDATION  OF  METALLIC  ORES  903 

lead  sulphate,  and  generally  a  trace  of  zinc.  They  are  weak 
waters  mainly  on  account  of  the  small  amount  of  pyrite  in  the 
deposit. 

Sulphate  Waters 

Oxidation  of  Pyrite. — Where  pyrite  is  present  in  notable 
quantities  its  oxidation  materially  changes  the  composition  of  the 
waters.  The  sulphuric  acid  radicle  increases  rapidly  and  dis- 
places the  equilibrium  so  that  the  normal  calcium  carbonate 
waters  are  changed  into  those  containing  mainly  calcium  sulphate. 
When  the  free  sulphuric  acid  increases  still  further  the  water 
becomes  rich  in  the  sulphates  of  aluminum  (by  the  decomposi- 
tion of  sericite  and  other  silicates)  and  iron,  the  latter  present 
as  both  ferrous  and  ferric  sulphate.  Free  hydrochloric  acid  is 
sometimes  present.  In  waters  above  or  at  the  water  level  these 
sulphates  may  be  present  in  large  quantities.  Below  the  water 
level  free  acid  is  rarely  found  and  the  sulphate  of  aluminum  is 
absent.  The  iron  is  present  as  ferrous  sulphate  and  diminishes 
in  quantity  with  increasing  depth.  The  characteristic  calcium 
sulphate  waters  persist  for  wide  spaces  around  pyritic  deposits 
and  also  reach  considerable  depths.  Besides  the  sulphates 
mentioned,  the  mine  waters  of  the  oxidized  zone  contain  almost 
all  the  metals  occurring  in  the  deposit.  Zinc  sulphate  is  es- 
pecially abundant;  copper  sulphate  is  usually  present,  lead  much 
more  rarely;  arsenic  is  common  and  antimony  rare. 

The  waters  of  coal  mines  show  plainly  the  result  of  the  oxida- 
tion of  the  pyrite  and  marcasite  occurring  in  the  beds.  Such 
waters  are  often  rich  in  the  sulphates  of  ammonium,  calcium,  iron, 
and  aluminum,  and  even  in  free  sulphuric  acid.  In  the  drainage 
from  the  mines  the  iron  appears  as  ferrous  sulphate,  from  which, 
by  oxidation,  ferric  sulphate  is  formed.  Coarsely  crushed  coal 
washed  with  distilled  water  is  said  to  yield  free  sulphuric  acid 
in  the  filtrate.  Mine  waters  from  coal  mines  occasionally  con- 
tain zinc,  copper,  cobalt,  and  nickel.  A  water  from  the  coal  mine 
of  the  Dravo-Doyle  Company,  in  Pennsylvania,  showed  accord- 
ing to  analysis  by  the  Pittsburgh  testing  laboratory  of  the 
Bureau  of  Mines: 

Free  H»SO4 117  parts  per  million 

Fej(SO4)j 4,970  parts  per  million 

Alj(SO4)» 140  parts  per  million 

FeSO4 54  parts  per  million 


904 


MINERAL  DEPOSITS 


More  or  less  of  the  sulphate  of  calcium  and  magnesium  are  also 
usually  present. 

Examples. — A  series  of  analyses  of  the  Comstock  waters, 
Nevada,  by  J.  A.  Reid  well  illustrates  the  occurrence  of  sulphate 
waters.  No.  3  is  a  concentrated  sulphate  water  from  the  oxidized 
zone;  Nos.  1  and  2  are  deeper  hot  waters,  resulting  from  the 
reaction  between  an  ascending  sodium-carbonate  water  and 
sulphuric  acid  from  the  upper  zones.  The  ores  contain  mainly 
gold  and  silver  and  are  not  rich  in  pyrite. 

ANALYSES  OF  MINE  WATERS  FROM  THE  COMSTOCK  LODE 
[Parts  per  million] 


1 

2 

3 

Cl 

1  27 

19  00 

127  60 

SO4  
CO  3 

380.38 
115  03 

474.00 
20  45 

209,100.00 

K           

8  39 

53  40 

Na  
Ca  
Mg  
Al 

57.13 
148.10 
154.03 

132.00 
100.10 
5.88 
1  37 

535  .  00 
.    1,286.00 
6,590.00 
9  670  00 

Mn 

885  10 

Cu 

147  50 

SiO, 

30.50 

133  40 

616  00 

Fe              

6.33 

5,025  02 

H     

2,575.00 

Total  salinity... 

764.40 

965.60 

110,958.30 

1.  Water  from  the  600-foot  level  of  the  Savage  mine.     G.  F.  Becker, 
Mon.  3,  U.  S.  Geol.  Survey,  1882,  p.  152. 

2.  Waters  from  the  C.  &  C.  shaft  at  the  2,250-foot  level.    John  A. 
Reid,  Bull.  California  Univ.  Dept.  Geology,  vol.  4,  1905,  pp.  177-199. 

3.  Vadose  water  from  the  Central  tunnel.    Idem. 

Some  of  the  mine  waters  of  the  Joplin  zinc  region,  where  the 
deposits  contain,  besides  zinc  blende  and  galena,  some  pyrite  or 
marcasite,  are  extremely  rich  in  zinc  sulphate  and  contain  also  the 
sulphates  of  iron  and  aluminum.  (See  analysis  No.  1  in  table 
on  p.  905.) 

The  water  of  the  Rothschonberger  tunnel,  draining  the  mines 
at  Freiberg,  Saxony,  is  a  good  example  of  a  dilute  mine  water 


OXIDATION  OF  METALLIC  ORES 


905 


which  has  traversed  the  old  workings  of  veins  carrying  pyrite, 
galena,  and  zinc  blende.     (See  analysis  No.  2.) 

The  same  principle  is  illustrated  by  the  analyses  of  two  waters 
from  the  mines  at  Butte,  Montana.  No.  3  is  from  a  deep  level, 
but  rather  far  from  the  principal  vein  system;  No.  4  is  from  the 
1,200-foot  level  in  one  of  the  principal  mines;  it  has  acquired 
the  habit  of  a  water  of  the  upper  oxidized  zone  because  the  water 
level  has  been  artificially  lowered  and  the  oxidation  of  the  pyrite 
is  progressing  rapidly.  No.  5  is  a  deep  water  from  Cripple  Creek. 

ANALYSES  OF  MINE  WATERS 


1 

c\ 

3 

4 

5 

Cl  
HCOj 

2.7 

12.4 

6.8 
13  5 

13.0 

0.8 
210  0 

SO4 

6,153.2 

124  8 

406  5 

2  672  0 

1,088  0 

SiOz 

107.6 

18.0 

23  0 

47  7 

64.0 

AsO* 

Trace 

Ca  
Mg  
Na  
K 

345.3 
25.2 
49.9 
0.5 

46.4 
14.5 

151.2 
28.2 
16.2 
7  1 

132.5 
61.6 
39.6 
13.1 

564.7 
22.4 
51.2 
7.1 

Fe" 

1 

f      1  8 

Fe"' 

j    474.6 

6.6 

{..        ... 

159.8 

Mn 

1  7 

0  5 

12  0 

Zn  
Cd 

2,412.0 
9  o 

8.9 

0.3 

852.0 
41.1 



Al 

142  1 

83  5 

Cu 

3  7 

Trace 

59  1 

Co  +Ni   

0.5 

Sn 

17.0 

9,727.5 

231.6 

655.1 

4,204.5 

2,308.8 

1.  Water  from  Alabama  Coon  mine,  Joplin,  Missouri.     H.  N.  Stokes, 
analyst. 

2.  Water  from  the  Rothschonberger  Stolln,  Freiberg,  Saxony,  at  point  of 
discharge.     Analysis  by  Frenzel.     Recalculated  by  F.  W.  Clarke,  Data  of 
geochemistry,  Bull.  616,  U.  S.  Geol.  Survey,  1916,  p.  632. 

3.  Water  from  2,200-foot  level,  Green  Mountain  mine,  Butte,  Montana, 
remote  from  veins.     W.  F.  Hillebrand,  analyst. 

4.  Water  from  1,200-foot  level,  crosscut  St.  Lawrence,  Butte,  Montana. 
Tin  possibly  accidentally  introduced?     Faintly  acid.     W.  F.  Hillebrand, 
analyst.     Fe"  probably  changed  to  Fe'"  during  exposure  to  air. 

5.  Cripple  Creek,  Colorado,  water  from  El  Paso  Tunnel  draining  the 
lowest  workings.     R.  C.  Wells,  analyst.     Fe +A1,  etc.,  0.6.     Free  CO2  trace. 


906 


MINERAL  DEPOSITS 


F.  L.  Ransome1  mentions  a  mine  water  from  Goldfield, 
Nevada,  which  contained  about  4,250  parts  per  million  of 
total  solids,  mostly  sulphates  of  iron,  sodium,  magnesium,  and 
calcium.  The  silica  in  such  waters  is  generally  low. 

A.  C.  Lawson2  describes  the  mine  water  from  the  Ruth  mine 
335  feet  below  the  surface,  in  the  chalcocite  blanket  in  the 
porphyry  of  Ely,  Nevada.  The  temperature  was  16°  C.,  de- 
cidedly higher  than  the  average  annual  temperature  of  the 
region.  The  total  solids  were  1,094  parts  per  million,  of  which 
359  parts  were  calculated  as  calcium  sulphate,  130  as  magne- 
sium sulphate,  93  as  alkaline  chlorides,  160  as  ferrous  sulphate, 
and  7  as  ferric  sulphate. 

Two  analyses  tof  the  mine  waters  at  the  copper  mines  of 
Cananea,  Mexico,  have  been  received  through  the  courtesy  of 
Mr.  W.  H.  Emmons.  The  waters  come  from  an  upper  and  a 
deeper  level  and  have  percolated  through  a  sericitized  rock  with 
a  considerable  amount  of  chalcocite  and  pyrite,  though  there  are 
no  solid  masses  of  pyrite. 


ANALYSES  OF  WATERS  FROM  THE  CAPOTE  MINE,  CANANEA,  MEXICO 

F.  G.  Hawley,  Analyst 

[Parts  per  million] 


300-foot  level. 

900-foot  level. 

SO3  
Cl  
H2SO4(free)  
SiO2 

4,220 
Not  determined. 
970 
76 

3,714 

22 
nil. 
56 

FeO 

393 

674 

ALO, 

42 

CaO  
MgO  
MnO  
ZnO  

610 
102 
305 

Not  determined. 

1,053 
144 
198 
315 

CuO 

2  097 

76 

(K,Na)20  

Not  determined. 

198. 

Fe  almost  wholly  as  Fe". 

H2SO4  not  subtracted  from  total  SO3. 

1  Geology  and  ore  deposits  of  Goldfield,  Nevada,  Prof.  Paper  66,   U.  S. 
Geol.  Survey,  1909,  p.  258. 

2  Bull.  California  Univ.  Dept.  Geol.,  vol.  4,  1906,  pp.  287-357. 


OXIDATION  OF  METALLIC  ORES 


907 


The  deeper  waters  contain  much  more  calcium  sulphate  as 
well  as  ferrous  sulphate,  but  much  less  copper.  Chalcocite  proba- 
bly reduces  thfe  ferric  sulphate  to  ferrous. 

Another  instructive  series  was  collected  by  W.  H.  Emmons  and 
Laney  at  Ducktown,  Tennessee.  Here  the  water  level  is  high 
and  the  ores  consist  of  heavy  masses  of  pyrrhotite  with  some 
chalcopyrite. 

ANALYSES  OF  MINE  WATE.R  FROM  DUCKTOWN,  TENNESSEE' 

R.  C.  Wells,  Analyst 

[Parts  per  million] 


1 

2 

3 

SO4  6,664.0 

415.8 

474.8 

Cl  {               0.1 

0.7 

0.4 

SiO2  55.6 

37.0 

49.9 

H2SO4  (free)  !           129.6 

210.2 

97.5 

Al  ,.!           433.0 

14.5 

19.1 

Fe"  2,178.0 

71.4 

89.2 

Fe'"  nil. 

20.3 

55.9 

Mn  |               0.2 

0.2 

0.1 

Ca  67.6 

19.7 

30.4 

Mg  !             40.6 

5.2 

6.2 

Cu  i           312.1 

28.1 

11.0 

Zn  :....             199.8 

2.4 

2.9 

K                              1             19  8 

2.7 

2.2 

Na  1             23.4 

5.2 

5.5 

1.  Burra-Burra  mine.     Circulating  water  dripping  from  roof  of  drift 
just  below  chalcocite  zone. 

2.  Callaway  shaft,  standing  water,  at  water  level,  90  feet  below  surface. 
Both  1  and  2  were  collected  with  special  precautions  and  sealed  to  prevent 
oxidation. 

3.  Callaway  shaft,  standing  water  37  feet  below  water  level. 


All  three  waters  are  rich  in  free  acid,  the  deepest  sample  being 
the  least  acid.  The  water  of  No.  1,  collected  just  below  the 
chalcocite  zone,  is  extremely  rich  in  sulphates  but  contains  no 
ferric  sulphate,  while  the  water  standing  in  the  shaft  contains 
both  ferrous  and  ferric  sulphate.  There  is  less  copper  in  the 
lower  part  of  the  standing  body  of  water  than  at  the  surface  but 
more  calcium. 

1  Quoted  in  Butt.  529,  U.  S.  Geol.  Survey,  1913,  pp.  60-61. 


908  MINERAL  DEPOSITS 

The  Homestake  mine,  South  Dakota1  is  working  a  large  len- 
ticular body  of  gold  ore  with  some  pyrite,  pyrrhotite,  quartz, 
and  amphibole.  The  ordinary  creek  waters  are  t>f  the  normal 
calcium  carbonate  type,  and  a  salinity  of  about  300  parts  per 
million.  The  ordinary  mine  water  from  the  upper  levels  has  a 
salinity  of  510  parts  and  contains  CaS04  and  (Ca,Mg)COs. 
The  deeper  waters,  from  the  1,100  and  1,550  foot  levels,  have  a 
higher  salinity,  as  much  as  1,228  parts,  caused  mainly  by  increase 
of  CaSC>4.  The  salinity  increases  during  dry  periods. 

1  W.  J.  Sharwood,  Econ.  Geol,  vol.  6,  1911,  pp.  738-744. 


CHAPTER  XXXII 

METALLOGENETIC  EPOCHS1 

INTRODUCTION 

Wherever  universal  geological  processes  are  in  operation — 
weathering,  sedimentation,  metamorphism,  deposition  by  under- 
ground waters  and  vulcanism — there  mineral  deposits  may  be 
forming  as  they  have  done  during  the  long  ages  of  the  earth's 
history.  Over  larger  or  smaller  areas  the  conditions  may  at  a 
given  time  be  favorable  for  the  deposition  of  useful  minerals. 
Such  areas  are  called  minerogenetic  or  metallogenetic  prov- 
inces. The  Jurassic  iron  ores  of  England  and  France,  the 
bauxite  deposits  of  the  southern  Appalachian  States,  the  salt 
deposits  of  Kansas,  the  gold-quartz  veins  of  California  occupy 
minerogenetic  provinces.  The  provinces  may  widen  into  minero- 
genetic regions:  Thus  the  Cordilleras  of  the  Americas  form  such 
a  region  marked  by  gold  and  silver  deposits  of  igneous  genetic 
affiliations.  In  the  same  way  the  Paleozoic  of  the  whole  Mississ- 
ippi Valley  region  is  a  metallogenetic  region  characterized  by 
lead  and  zinc  deposits  related  in  origin  to  circulating  meteoric 
waters. 

The  time  intervals  favorable  for  the  deposition  of  certain  useful 
substances  are  called  minerogenetic  or  metallogenetic  epochs. 
Usually  speaking  they  are  short  and  transitory  but  they  likewise 
may  widen  into  periods  and  eras.  Weathering  and  sedimenta- 
tion may  go  on  for  periods  but  the  mineral  deposits  due  to  these 
processes  commonly  represent  a  comparatively  brief  time.  The 
Clinton  hematites,  for  instance,  though  widely  spread,  were 

1  This  conception  was  first  developed  by  L.  De  Launay  who  has  contri- 
buted much  to  the  study  of  this  subject. 

L.  De  Launay,  Gites  metalliferes,  vol.  1,  1913,  pp.  241-288. 
W.  Lindgren,  Metallogenetic  epochs,  Econ.  Geol.,  vol.  4,  1909,  pp.  409- 
420. 

W.  Lindgren,  Gold  and  silver  deposits  in  North  and  South  America, 
Trans.,  Am.  Inst.  Min.  Eng.,  vol.  55,  1917,  pp.  883-909. 
909 


910  MINERAL  DEPOSITS 

formed  during  a  limited  period  of  littoral  sedimentation  and  are 
covered  and  underlain  by  heavy.  Carboniferous  and  Ordovician 
limestones. 

This  applies  even  more  to  deposits  of  igneous  affiliations  which 
often-  were  formed  during  a  very  brief  interval  when  the  emana- 
tions of  each  igneous  phase  found  opportunity  for  escape  and 
transportation.  Each  igneous  province  usually  includes  many 
smaller  elements  in  which  successive  intrusions  and  effusions  were 
followed  by  short  epochs  of  various  metallizations. 

Each  mineral  deposit  has  usually  a  well  marked  paragenetic 
history  due  to  the  cooling  or  changing  of  solution.  In  any  older 
deposit  subsequent  epochs  of  mineralization  may  have  left  their 
imprints,  or  geologic  processes  like  metamorphism  and  weather- 
ing may  have  modified  them. 

The  minerogenetic  development  of  the  several  continents  is 
outlined  in  briefest  form  in  the  following  paragraphs.  It  is  a 
fascinating  subject  for  it  connects  in  logical  order  the  science 
of  general  geology  with  that  of  mineral  deposits. 

Main  Epochs. — In  the  nature  of  the  case  metallogenetic  epochs 
of  weathering,  sedimentation  and  erosion  are  not  confined  to  any 
particular  geological  period.  On  the  other  hand  the  deposits 
genetically  connected  with  igneous  rocks  and  metamorphism 
are  most  abundantly  formed  during  the  great  periods  of  such 
disturbances  which  are  more  or  less  closely  affiliated  with 
folding  and  mountain  building.  In  his  studies  on  European 
deposits,  De  Launay  divides  them  into  those  of :  1.  Pre-Cambrian 
age;  2.  Hercynian  age;  and  3.  Tertiary  age. 

It  is  necessary  to  realize,  however,  that  the  pre-Cambrian  de- 
posits, though  generally  formed  at  high  temperature  and  great 
depth,  include  many  ages  far  apart.  The  Hercynian  deposits 
were  formed  during  the  great  orogenetic  disturbance,  which  falls 
between  the  end  of  the  Paleozoic  and  the  beginning  of  the  Trias 
and  which  was  accompanied  by  many  intrusions.  To  this 
period  belong  such  deposits  as  the  tin  veins  of  Cornwall  and  the 
silver-lead  veins  of  Freiberg.  "Hercynian"  is  a  term  that  has 
been  applied  rather  loosely  and  even  metallogenetic  epochs  of 
distinctly  Paleozoic  age  or  of  Triassic  have  been  included  in  it. 
The  Tertiary  deposits  are  those  which  were  formed  during  or 
following  the  great  Alpine  foldings  and  subsequent  igneous 
activity.  Folding  is,  however,  a  process  not  necessarily  con- 
nected with  igneous  activity,  and,  therefore,  we  shall  place  less 


METALLOGENETIC  EPOCHS  911 

emphasis  than  De  Launay  has  done  on  the  connection  between 
such  tangential  thrusts  and  mineral  deposition. 

EUROPE 

Pre -Cambrian  Epochs. — We  find  the  pre-Cambrian  deposits 
mainly  in  the  great  shield  of  Fenno-Scandia,  in  Sweden,  Norway 
and  Finland.  They  carry  mainly  iron  and  copper,  more  rarely 
lead  and  zinc  or  tin.  Gold  and  silver  are  very  scarce.  Deposits 
of  contact-metamorphic  or  magmatic  type  prevail  with  struc- 
tural and  mineralogical  features  indicating  depth  and  high  tem- 
perature. Dynamic  metamorphism  frequently  has  been  super- 
imposed. Iron  deposits  of  sedimentary  origin  in  part  a  little 
similar  to  those  of  North  America  are  found  in  places. 

Paleozoic  Epochs. — Sedimentary  ores  of  hematite  and  iron 
silicates  are  rather  widespread  in  Europe.  We  find  them  in  the 
metamorphosed  Cambro-Silurian  of  northern  Norway,  in  Great 
Britain,  France,  Germany,  Bohemia  and  Spain.  Among  the 
deposits  of  igneous  origin  we  note  the  copper  and  nickel  ores  of 
northern  Norway,  connected  with  gabbros  intrusive  in  the 
Cambro-Silurian  complex. 

Hercynian  Epochs. — The  Hercynian  movements  fell  between 
the  Carboniferous  and  the  Trias  and  resulted  in  great  mountain 
chains  extending  East  and  West  across  Europe.  Intrusions  of 
granitic  rocks  of  that  age  are  laid  bare  by  erosion  which  has  also 
truncated  the  deposits  so  that  the  veins  which  form  the  dominant 
type  present  the  high  temperature  or  intermediate  types.  Tin, 
lead,  zinc,  copper  and  silver  prevail  among  the  metals;  anti- 
mony and  arsenic  begin  to  appear.  Iron  deposits  are  also  found 
but  do  not  attain  the  importance  of  those  of  Fenno-Scandia. 
In  this  age  were  formed  the  tin  deposits  of  England,  France, 
Saxony,  and  Spain  all  connected  with  persilicic  intrusives. 

The  majority  of  the  deposits  of  the  Iberian  Peninsula  (includ- 
ing those  of  Rio  Tinto  and  the  Mesa  Central1),  of  the  Central 
Plateau  of  France  and  of  Central  Germany,  belong  to  this  age. 

The  Ural  Mountains  are  also  regarded  as  a  Hercynian  range 
truncated  by  erosion  and  at  least  many  of  its  deposits  of  iron, 
copper  and  gold  are  considered  as  induced  by  Hercynian  in- 
trusives. The  deposits  are  generally  of  a  deep-seated  type. 

1  The  quicksilver  deposit  of  Almaden  should  probably  be  included  among 
these. 


912  MINERAL  DEPOSITS 

Permo-Triassic  Epochs. — Between  the  Hercynian  ranges 
extended  desert  plains  and  saline  lakes  or  bays.  The  salt  deposits 
of  central  Germany,  the  sedimentary  copper  ores  of  the  Mans- 
field basin,  the  copper  ores  disseminated  in  sandstone  in  Germany 
and  Russia  belong  to  these  epochs. 

Jurassic  and  Cretaceous  Epochs. — Broad  transgressions  of 
the  seas  with  widespread  formation  of  oolitic  iron  ores  mark  the 
Jurassic  in  England  and  on  the  continent.  Glauconitic  sands 
and  chalk  accumulated  in  the  Cretaceous  seas.  The  igneous 
forces  were  quiescent.  Perhaps  we  do  not  err  in  attributing  to 
the  Jurassic  the  concentrations  of  lead-zinc  ores  in  the  Paleozoic 
of  Belgium  and  in  the  Trias  of  Silesia. 

Tertiary  Epochs. — Few  mineral  deposits  of  northern  Europe 
date  from  the  Tertiary  but  southern  Europe  was  the  scene 
of  great  activity  mainly  in  connection  with  the  igneous  out- 
bursts which  culminated  south  of  the  great  Alpine  arches  and 
overthrusts.  - 

1.  In  the  areas  occupied  by  the  Mediterranean  seas,  we  find 
widespread  sulphur  deposits  and  phosphates,  .both  in  the  sedi- 
mentary beds. 

2.  The  great  Alpine  intrusions  of  granitic  rocks  originated 
replacement  deposits  of  siderite  and  magnesite  in  the  Triassic 
and  Paleozoic  limestones  together  with  many  minor  metal  de- 
posits.    Among  the  latter  should  probably  be  placed  the  wide- 
spread lead-zinc  deposits  of  the  Alpine  Trias. 

3.  Farther  south,  in  Italy,  we  note  the  contact-metamorphic 
specularite  deposits  of  Elba  in  connection  with  intrusive  granite. 
The  magnetite  deposits  of  the  Banat,  likewise  of  contact-meta- 
morphic origin  also  belong  in  this  class. 

4.  The  Eocene  intrusion  of  greenstones  (peridotites,  gabbro, 
etc.)  in  the  Alpine  region  and  throughout  the  Mediterranean 
resulted  in  magmatic  copper  deposits,  zeolitic  copper  deposits 
(Monte  Catini),  magmatic  chromite  deposits  (Greece  and  Asia 
Minor)  and  contact-metamorphic  emery  deposits. 

5.  In  Hungary  and  Transylvania,  inside  of  the  Carpathian 
arches,  intrusions  and  effusions  of  andesite  and  dacite  took  place 
in  the  Miocene.     Accompanying  those  was  a  metallization  of 
gold  and  silver  very  similar  to  the  Cordilleran  types.     These 
veins  are  of  the  type  formed  near  the  surface. 

6.  The  last  metallization  of  cinnabar  accompanied  the  late 
Tertiary  (and  perhaps  the  present)  igneous  outbursts  in  Tuscany, 


METALLOGENETIC  EPOCHS  913 

Austria,  and  Serbia.     It  is  throughout  of  the  type  formed  near 
the  surface. 

Some  effects  of  the  Tertiary  metallization  is  noted  in  con- 
nection with  eruptives  in  Bohemia,  Saxony  and  the  Hartz  as 
well  as  in  the  Central  Plateau  of  France.  Lead,  silver,  nickel, 
cobalt,  antimony  are  the  principal  metals  supplied  in  these 
regions. 

ASIAi 

The  mineral  deposits  of  Asia  have  been  described  by  De 
Launay,  who  in  his  work  cited  below,  has  collected  a  vast  mass 
of  data  in  the  correlation  of  which  there  are  still  many  blanks. 

We  find  the  pre-Cambrian  deposits  represented  by  gold- 
bearing  veins  in  the  Jenisei  and  perhaps  the  Lena  districts 
in  Siberia,  and  in  Korea.  Again  we  meet  them  in  the  great 
Indian  platform,  formerly  connected  with  Africa.  Here  are 
deposits  of  gold-bearing  quartz  (Mysore),  of  iron,  of  manganese 
and  of  graphite. 

Deposits  of  Hercynian  or  late  Paleozoic  age  are  found  in 
many  ranges  in  central  Asia,  such  as  the  Ural,  the  Altai  and  the 
trans-Baikalian  mountains.  They  are  mostly  well-defined  veins- 
carrying  gold,  lead,  silver  and  other  metals.  To  the  same  age 
are  referred  the  tin  and  wolfram  deposits  of  Malaya,  Burma 
and  Annam,  many  of  which  are  accompanied  by  much  later 
placers.  Copper  deposits  in  sandstone  occur  in  northern 
Turkestan. 

The  deposits  post-dating  the  Himalayan  and  Alpine  systems 
are  apparently  rare.  A  few  of  them  occur  in  the  Caucasus. 
The  Tertiary  mineralization  accompanying  the  igneous  out- 
bursts bordering  the  Pacific  is,  however,  well  represented  in 
eastern  Asia.  We  find  gold-bearing  veins  of  the  Cordilleran 
type  in  Sumatra,  Celebes,  Borneo,  the  Philippines  and  Japan. 
In  the  latter  country  there  is  also  a  more  varied  mineralization 
of  the  same  date  including  veins  of  copper,  antimony  and  quick- 
silver. 

AFRICA 

In  the  main  Africa  consists  of  a  vast  platform  of  ancient  rocks. 

Arabia,  India  and  Madagascar  have  the  same  structure  and  were 

no   doubt   once   connected   with  the  African   continent.     The 

crystalline  rocks  are  overlain  by  sediments,  folded  in  places, 

M,.  De  Launay,  Richeasea  minerales  de  1'Asie,  Paris,  1911,  pp.  816. 


914  MINERAL  DEPOSITS 

of  pre-Cambrian  and  Paleozoic  age;  in  the  south  these  older 
rocks  are  covered  by  Karroo  beds  (Permo-Triassic) .  In  the 
north,  in  Algeria  and  Tunis,  there  are  Tertiary  folds  of  the  Alpine 
system  with  accompanying  igneous  rocks.  Here  we  find  a  weak 
metallization  with  deposits  of  lead,  zinc,  antimony  and  quick- 
silver. 

Tertiary  and  recent  eruptives  are  absent  from  the  main  part 
of  Africa  except  along  a  zone  from  Abyssinia  to  the  Great  Rift 
valley  where  volcanoes  rise  above  the  old  plateau. 

The  old  pre-Cambrian  land  mass  contains  abundant  gold 
deposits,  usually  of  the  high  temperature  type  as  in  Madagascar, 
in  Rhodesia  and  hi  Swaziland,  and  gold  is  likewise  found  in  wide 
distribution  in  the  old  conglomerates  of  pre-Cambrian  age  over- 
lying the  Swaziland  crystalline  rocks.  Most  famous  among 
these  conglomerates  are  those  of  the  Witwatersrand,  Transvaal, 
which  furnish  the  larger  part  of  the  world's  gold  production. 
Somewhat  later  are  perhaps  the  bed  veins  of  Pilgrims  Rest  and 
other  places  in  the  Transvaal. 

Other  deposits  of  deep-seated  origin  in  Rhodesia  and  Nama- 
qualand  carry  chromite  and  copper  ores  of  magmatic  origin. 

Tin  deposits  of  pegmatitic  or  deep-seated  vein  type,  with 
accompanying  placers  are  found  in  the  Transvaal  and  in  Nigeria. 

The  copper  deposits  at  Katanga,  Belgium  Congo,  are  econom- 
ically important  and  are  found  in  sandstone,  shale  and  limestone 
of  probably  Paleozoic  age.  Their  origin  and  time  of  formation 
are  in  doubt. 

The  latest  mineralization  in  South  Africa  appears  to  be  repre- 
sented by  the  diamond-bearing  peridotite  pipes  which  are  intru- 
sive in  Karroo  series.  They  are  probably  of  Cretaceous  age. 

AUSTRALASIA 

On  the  old  land  mass  of  Australia  no  ore  deposits  (except  gold 
placers)  appear  to  have  been  formed  since  the  beginning  of 
Mesozoic  time. 

The  pre-Cambrian  era  is  marked  by  a  wide-spread  metalliza- 
tion of  gold  in  western  Australia;  this  is  apparently  associated 
with  intrusive  granites.  In  New  South  Wales  we  find  the  excep- 
tional high  temperature  lead-zinc  deposit  of  Broken  Hill,  which 
probably  also  dates  from  pre-Cambrian  times. 

In  Victoria  and  adjacent  parts  of  New  South  Wales,  a  rich 
metallization  of  gold,  in  quartz  veins,  marked  the  close  of  the 


METALLOGENETIC  EPOCHS  915 

Silurian  and  also  followed  the  intrusion  of  granitic  rocks.  The 
copper-gold  ores  of  the  Great  Cobar,1  in  western  New  South  Wales 
are  probably  also  of  early  Paleozoic  age  but  the  deposits  are 
isolated  among  sediments  far  from  outcropping  igneous  rocks. 
Gold  quartz  veins  of  a  similar  age  appear  in  the  South  Island  of 
New  Zealand. 

South  and  West  of  the  New  England  region,2  in  New  South 
Wales,  numerous  intrusions  accompanied  by  vein  formation  took 
place  in  the  Devonian,  while  in  New  England  and  Queensland 
deposits  of  tin,  tungsten  and  bismuth  were  formed  during  and 
after  Permo-Carboniferous  granitic  irruptions. 

Rich  deposits  of  gold  and  silver  of  the  Cordilleran  type,  also 
deposits  of  quicksilver,  are  found  on  the  North  island  of  New 
Zealand.  They  occur  in  the  lavas  of  Tertiary  age  whieh  form 
a  part  of  the  igneous  girdle  of  the  Pacific. 

SOUTH  AMERICA 

In  South  America,  as  in  the  northern  part  of  the  continent,  we 
must  differentiate  the  eastern  part  in  which  strong  mountain 
building  forces  have  rested  since  the  close  of  the  Paleozoic  era 
from  the  western  margin,  which  is  marked  by  the  great  Cordil- 
leran Ranges,  in  which,  since  early  Mesozoic  tunes,  the  scenes 
have  been  set  for  a  tremendous  display  of  vulcanism  and  erogenic 
movements. 

In  the  eastern  part  iron  and  gold  are  the  principal  metals.  The 
former  occurs  in  Brazil  as  sedimentary  deposits  of  hematite  of 
great  extent  and  probably  "Algonkian"  age. 

A  little  later  than  these  and  connected  with  intrusive  pegma- 
tites are  the  gold-bearing  quartz  veins  of  Brazil,  mainly  in  Minas 
Geraes,  and  in  the  Guianas;  all  of  these  are  of  the  high  tempera- 
ture type. 

Along  the  Pacific  Coast  the  pre-Cambrian  is  largely  lacking 
and  practically  all  of  the  deposits  are  of  Tertiary  age.  A  few 
of  them,  in  Colombia  and  Chile,  are  of  the  type  formed  near  the 
surface  and  carry  gold  and  silver.  The  great  majority  are  of 
the  intermediate  type  and  have  suffered  considerable  erosion. 
Genetically  they  stand  in  connection  with  smaller  masses  of  intru- 

1  E.  C.  Andrews,  Report  on  the  Great  Cobar  copper  and  gold  field, 
Min.  Res.  17,  Geol.  Survey  New  South  Wales,  1913,  p.  188. 
*Idem. 


916  MINERAL  DEPOSITS 

sive  granite  or  diorite,  as  well  as  with  rhyolitic  and  andesitic 
porphyries.  The  principal  metals  are  copper  and  silver.  Great 
lead  and  zinc  deposits  are  not  common.  Gold  is  widely  spread 
but  even  the  greatest  of  the  gold  districts,  in  Colombia,  do  not 
compare  with  those  of  California  and  Australia. 

Tin,  with  silver,  occurs  in  a  belt  in  the  Cordillera  Real  in 
Bolivia.  Tungsten  deposits  occur  in  Peru  and  Bolivia,  quick- 
silver deposits  in  Peru. 

Among  the  many  districts  which  have  yielded  great  treasures 
of  silver  we  note  particularly  Cerro  de  Pasco  in  Peru,  and  Potosi, 
in  Bolivia;  at  both  places  supergene  enrichment  has  undoubtedly 
played  an  important  part.  A  widespread  mineralization  of 
copper  often  accompanied  by  tourmaline  is  found  in  the  central 
and  northern  parts  of  Chile. 

A  later  epoch  of  mineralization  verging  upon  the  Recent  has 
yielded  superficial  deposits  of  borax  and  nitrates  probably  derived 
from  volcanic  exhalations.  Gold  placers  of  Quaternary  age  have 
been  worked  at  many  places. 

CENTRAL  AMERICA 

In  Honduras,  Nicaragua,  San  Salvador  and  Costa  Rica — a 
region  in  which  volcanoes  and  effusive  rocks  abound — we  find 
many  representatives  of  gold-silver  veins  of  Tertiary  age  which 
were  formed  near  the  surface. 

THE  ANTILLES 

A  feeble  mineralization  of  copper  and  gold  is  observed  in  the 
greater  Antilles  and  hi  most  cases  it  accompanied  Cretaceous  or 
Eocene  intrusions  of  basic  rocks. 

NORTH  AMERICA 

In  describing  the  metallogenetic  epochs  of  North  America 
it  will  "be  convenient  to  separate  the  eastern  and  western  parts 
of  the  continent  for,  with  the  exception  of  their  earliest  history, 
they  have  little  in  common.  The  western  or  Cordilleran  part 
including  also  almost  the  whole  of  Mexico,  and  Alaska  contains 
'few  deposits  older  than  the  Cretaceous.  The  eastern  part 
contains  few  that  are  more  recent  than  that  age  and  most  of  them 
are  of  pre-Cambrian  age. 

In  the  eastern  part  we  distinguish  the  following  metallogenetic 
epochs: 


METALLOGENETIC  EPOCHS  917 

I.  The  Pre-Cambrian  Epochs. — The  pre-Cambrian  era  em- 
braces a  long  time  and  many  epochs  of  ore  formation.  Ages 
of  deep  weathering  and  sedimentation  alternated  with  times  of 
violent  igneous  action.  The  deposits  of  igneous  affiliations  are 
throughout  of  the  deep-seated,  high  temperature  types.  The 
metals  contained  are  iron,  copper,  nickel,  gold  and  silver.  Lead, 
zinc,  antimony  and  quicksilver  are  rare.  Naturally  the  deposits 
are  best  exposed  in  the  great  ice  polished  Canadian  shield  and 
adjacent  parts  of  the  United  States,  but  many  are  also  found 
along  the  Appalachian  ranges,  where  the  pre-Cambrian  may  be 
exposed. 

Most  ancient  among  the  pre-Cambrian  deposits  are  the  "iron 
formations"  contained  in  the  sedimentary  rocks  associated  with 
the  Keewatin  greenstones.  At  Vermilion,  Minnesota,  and  many 
other  places,  hematite  ores  have  developed  by  the  oxidation  of 
these  iron  formations. 

But  little  more  recent  are  the  many  magmatic  and  contact- 
metamorphic  deposits  connected  with  granitic  intrusions  in  the 
Grenville  series  of  sedimentary  rocks  in  the  Adirondacks  and  in 
Ontario.  Among  them  we  find  titanic  iron  ores,  magnetite, 
corundum,  molybdenite,  apatite,  phlogopite.  The  iron  ores 
occur  in  large  deposits.  The  great  zinc  and  manganese  deposits 
of  Franklin  Furnace,  New  Jersey,  should  perhaps  be  correlated 
with  these  concentrations. 

Iron  formations  were  repeatedly  laid  down  during  the 
Huronian  sedimentations  in  the  Lake  Superior  region,  but 
epochs  of  intrusions  of  granite  and  porphyry  intervened  and  in 
connection  with  one  of  these — 'the  Algoman  epoch1' — appears 
to  have  originated  the  majority  of  the  gold-quartz  veins  of 
Ontario  and  Quebec.  Though  they  are  found  in  many  districts 
they  have  been  most  successfully  worked  in  the  Porcupine 
region  of  central  Ontario.  Arsenopyrite  is  frequently  associated 
with  the  gold. 

The  Pre-Cambrian  epochs  close  with  the  great  Keweenawan 
intrusions  and  effusions  of  basic  rocks  like  gabbro,  norite,  diabase 
and  basalt.  These  magmas  were  rich  in  copper,  silver,  nickel, 
cobalt  and  arsenic  and  many  deposits  of  these  metals  were  formed 
during  this  epoch.  They  are  distributed  over  a  wide  area  hi  the 

1  W.  G.  Miller  and  C.  W.  Knight,  Metallogenetic  epochs  in  the  pre- 
Cambrian  of  Ontario,  Twenty-fourth  Ann.  Rept.,  Ontario  Bur.  Mines,  1915, 
pp.  243-248. 


918  MINERAL  DEPOSITS 

Lake  Superior  region1  and  include  the  zeolitic  type  of  native 
copper  deposits,  the  silver-cobalt  veins  of  Cobalt  and  the  mag- 
matic  nickel-copper  deposits  of  Sudbury. 

2.  Paleozoic  Sedimentary  Epochs. — During  the  long  period  of 
Paleozoic  sedimentation  in  the  Appalachian  region  at  least  three 
epochs   were    characterized   by   deposition    of   iron    ores.     We 
find  oolitic  hematite  of  Ordovician  age  in  Newfoundland,  and 
similar  oolites  in  the  widespread  Clinton  formation  in  the  Silu- 
rian, the  latter  reaching  their  maximum  development  in  Alabama. 
Less  important  siderites  were  deposited  as  "black  bands"  during 
the  Carboniferous.     The  phosphate  beds  of  Tennessee  also  belong 
here. 

3.  Paleozoic  Intrusives. — 1From  Nova  Scotia  to  Alabama  gra- 
nitic intrusions  took  place  during  the  early  Paleozoic  and  some  of 
them  even  extended  into  the  Carboniferous.     A  mineralization  of 
gold  quartz  veins  followed  these.     We  find  these  veins  in  Quebec, 
Nova  Scotia,  at  various  places  in  New  England  and  particularly 
in  the  southern  States  from  Maryland  to  Alabama.     The  age 
of  the  gold  deposits  in  the  latter  region  is  still  in  dispute.     Prob- 
ably the  majority  of  them  are  of  Cambrian  age.     Many  deposits 
of  pyrite  and  chalcopyrite  in  this  region  appear  to  date  from  the 
same  period. 

4.  Paleozoic   Epochs   of   Saline    Deposits.— Epochs   of   arid 
climates  with  accompanying  development  of  deposits  of  evapora- 
tion recurred  at  several  times  during  the  Paleozoic  era.     Thus 
there  are  deposits  of  gypsum  and  salt  in  the  Silurian  of  New 
York,  in  the  Carboniferous  of  Michigan  and  in  the  Permian  of 
Kansas  and  Texas. 

5.  Epochs  of  Triassic  Copper  Deposits. — The  important  period 
of  the  history  of  the  igneo-genetic  ores  in  the  eastern  part  of  the 
continent  closed  in  the  late  Paleozoic.     A  feeble  recurrence  of  ore 
formation  took  place  during  the  early  Mesozoic  when  the  Triassic 
traps  of  New  Jersey,  Connecticut  and  the  Bay  of  Fundy  were 
injected  as  sheets  or  overflowed  as  lava  streams.     Copper  ores 
of  the  zeolitic  type  and  some  contact-metamorphic   iron   and 
copper  ores  developed  in  places. 

6.  Cretaceous  and  Later  Periods  of  Lead  and  Zinc  Concen- 
tration.— Since  the  Triassic,  vulcanism  has  rested  and  in  the 
eastern  part  of  the  continent  metal  deposits  have  formed  only  by 

1  C.   R.  Van  Hise  and  C.  K.  Leith,  The  geology  of  the  Lake  Superior 
region,  Mon.  52,  U.  S.  Geol.  Survey,  1911,  p.  591. 


METALLOGENETIC  EPOCHS  919 

the  concentrating  power  of  flowing  surface  waters,  or  of  ground 
water  in  decaying  rocks,  or  of  ascending  waters  of  meteoric  origin. 

In  the  Mississippi  basin  and  in  the  Appalachian  valleys  the 
Paleozoic  beds  have  been  searched  by  saline  and  carbonate 
waters  containing  carbon  dioxide  and  hydrogen  sulphide.  This 
solution  and  deposition  has  resulted  in  the  many  lead  and  zinc 
deposits  which  we  find  in  the  limestones  of  that  region. 

7.  Tertiary  and  Recent  Periods  of  Rock  Decay. — For  a  long 
time,  in  fact  since  the  end  of  the  Paleozoic,  large  parts  of  this 
continent  have  been  a  land  area  and  its  rocks  have  been  exposed 
to  weathering  and  decay.  Such  periods  have  indeed  not  been 
absent  during  any  extended  time  intervals  and  in  the  iron  ores 
of  the  Lake  Superior  region  as  well  as  in  certain  manganese  ores 
of  Arkansas  we  find  indications  of  long  pre-Cambrian  and  shorter 
Paleozoic  epochs  of  weathering  and  oxidation. 

The  mild  climate  which  prevailed  in  the  southern  Appalachian 
region  during  the  Tertiary  favored  the  development  of  deposits 
caused  by  such  agencies.  The  ores  contain  limonite,  manganese 
dioxide,  phosphates  and  bauxite. 

In  the  Western  or  Cordilleran  part  of  the  Continent  we  distinguish 
the  following  metallogenetic  epochs: 

1.  The    Pre-Cambrian   Epochs. — The    western    part    of  the 
Cordilleran  region  is  lacking  in  great  exposures  of  the  pre-Cam- 
brian but  wherever  they  are  present  we  find  deposits  of  gold,  iron 
and  copper,  rarely  other  metals.     The  correlation  of  these  with 
the  eastern  epochs  is  often  difficult  but  on  the  whole  they  are 
connected   with  late  pre-Cambrian    (Algonkian)   intrusions  of 
granite  and  subsilicic  rocks.     As  may  be  expected,  the  deposits 
are  of  the  high  temperature  type  and  consist  mainly  of  veins  and 
lodes.     Among  them  may  be  mentioned  the  gold  lodes  of.  the 
Black  Hills  of  South  Dakota,  the  lenticular  gold-quartz  veins 
and  copper  deposits  of  Wyoming,  many  smaller  deposits  in  New 
Mexico,1  the  gold,  iron  and  copper  deposits  of  certain  parts  of 
southern  California  and  Arizona,  near  Yuma  and  Parker,  copper 
deposits  of  the  Jerome  district  in  Arizona  and  many  others.     As 
in  the  East,  lead,  zinc  and  antimony  are  rarely  found  in  these 
ores. 

2.  The  Early  Mesozoic  Epoch. — Throughout  the  Paleozoic  and 
the  larger  part  of  the  Mesozoic  the  great  interior  province,  now 

1  W.  Lindgren,  L.  C.  Graton  and  C.  H.  Gordon,  The  ore  deposits  of  New 
Mexico,  Prof.  Paper  68,  1910,  pp.  48-51. 


920  MINERAL  DEPOSITS 

occupied  by  the  Rocky  Mountains,  was  the  scene  of  almost 
uninterrupted  sedimentation.  The  phosphate  beds  of  Idaho 
date  from  this  period.  Not  so  along  the  Pacific  Coast;  for  here 
we  find  in  places  evidence  of  intrusions  and  lava  flows  dating 
back  to  the  early  Paleozoic,  and  during  the  Trias  and  early 
Jurassic  great  effusions  of  subsilicic  lavas  took  place.  Copper 
deposits  are  found  in  these  igneous  rocks  and  many  of  these  were 
doubtless  formed  shortly  after  this  igneous  activity.  Among 
them  we  may  probably  count  many  minor  deposits  in  California, 
the  Bonanza  deposit  in  the  Copper  River  district,  Alaska,  and 
the  ores  of  the  Prince  William's  Sound  in  the  same  territory. 
In  the  Jurassic  lavas  of  Vancouver  Island,  C.  H.  Clapp  has  found 
indication  of  a  mineralization  of  the  type  formed  near  the  surface.1 

3.  The  Late  Mesozoic  Epochs. — The  third  and  most  important 
ore  forming  period  followed  and  accompanied  the  great  batho- 
lithic  intrusions  of  the  Pacific  Coast  to  which  an  age  from  late 
Jurassic  to  early  Cretaceous  is  generally  assigned. 

These  intrusions  of  intermediate  quartz-monzonitic  or  grano- 
dioritic  character  took  place  upon  an  immense  scale  and  extended 
from  Alaska  to  Baja  California.  The  large  batholiths  of  the 
Sierra  Nevada  and  British  Columbia  are  the  most  striking 
features  but  innumerable  intrusions  of  smaller  volume  broke 
through  the  crust  along  both  sides  of  the  great  batholiths  and 
throughout  Oregon,  Idaho,  southern  California,  southern  Ariz- 
ona, western  Nevada,  Washington  and  Alaska.  One  of  the 
largest  of  these  is  exposed  in  the  great  granite  area  of  central 
Idaho.  But  throughout  this  revolution  and  during  the  birth  of 
mountain  ranges  on  the  coast  the  Cretaceous  was  being  quietly 
deposited  at  sea  level  all  over  the  eastern  Cordilleran  region 
from  Canada  to  southern  Mexico. 

An  epoch  of  intense  metallization  followed  these  intrusions, 
within  the  areas  indicated.  The  great  interior  masses  of  the 
batholiths  are  usually  free  from  deposits,  as  shown  in  the  High 
Sierra,  in  the  Clearwater  region,  and  in  British  Columbia.  But 
along  their  margins  mineral  deposits  formed  in  abundance,  as 
along  the  gold  belt  of  California,  and  along  the  two  contacts  of 
the  batholith  of  the  Canadian  and  Alaskan  coast  regions.  The 
latest  researches  by  the  Alaskan  division  of  the  United  States 
Geological  Survey  indicate  that  the  great  placer  fields  of  Alaska 
derived  their  gold  from  deposits  of  this  epoch.  Gold,  primarily, 

1  Econ.  Geol.,  vol.  10,  1915,  pp.  70-88. 


METALLOGENETIC  EPOCHS  921 

and  copper,  secondarily,  are  the  characteristic  metals.  Along  the 
Pacific  coast,  where  there  is  little  limestone  in  the  intruded  sedi- 
ments, lead  is  practically  absent,  but  in  the  interior,  as  in  Nevada 
and  Arizona,  where  the  intrusions  came  into  contact  with  Paleo- 
zoic limestone,  this  metal,  with  zinc,  begins  to  appear.  Silver  is 
everywhere  present,  but  scarcely  ever  important,  except  where 
lead  appears.  Arsenic  and  antimony  are  not  abundant,  mercury 
is  nowhere  present  in  commercial  quantities. 

4.  The  Early  Tertiary  Epoch.— As  if  exhausted,  the  igneous 
forces  appear  to  have  rested  until  the  close  of  the  Cretaceous  and 
then  broke  out  in  a  new  field,  along  the  eastern  margin  of  the 
Cordilleran  region,  at  that  time  largely  covered  by  a  plastic  man- 
tle of  Cretaceous  shales  and  sandstones,  several  thousand  feet  in 
thickness,  which  rested  on  great  accumulations  of  Paleozoic 
limestones. 

The  predominating  magmas  were  again  of  intermediate  char- 
acter, and  solidified  as  granular  or  porphyritic  rocks,  standing 
between  the  granites  and  the  diorites;  they  contrast  markedly 
with  the  potassic  and  acidic  magmas  of  pre-Cambrian  times. 
While  it  is  not  necessary  to  limit  strictly  this  igneous  activity 
to  a  certain  time,  there  is  little  doubt  that  most  of  it  took  place 
in  the  Eocene.  The  eruptions  mainly  took  the  form  of  in- 
trusions and  largely  that  of  laccoliths,  undoubtedly  because,  un- 
like the  conditions  of  the  shattered  rocks  of  the  Pacific  Coast,  they 
were  covered  by  this  heavy,  tough  and  still  yielding  mass  of 
Cretaceous  sediments.  We  find  an  enormous  number  of  these 
intrusions  at  various  horizons  between  the  Cambrian  and  the 
Cretaceous  or  as  dikes  or  stocks  that  break  through  the  under- 
lying pre-Cambrian.  They  are  not  comparable  in  extent  to  the 
great  batholiths  of  the  coast.  They  extend  from  British  Colum- 
bia, through  Montana,  Utah,  Nevada,  Colorado,  New  Mexico, 
eastern  Arizona,  and  attain  their  greatest  development  in 
eastern  Mexico.  For  reasons  already  indicated  many,  perhaps 
most,  of  these  intrusives  never  reached  the  surface.  Only  in  a 
few  cases,  as  in  Montana  and  in  Colorado,  near  Denver,  do  the 
strata  of  Laramie  or  Eocene  age  contain  volcanic  detritus. 

The  fourth  Cordilleran  epoch  of  metallization  followed  these 
intrusions;  contact-metamorphic  deposits  and  veins  were  formed 
in  abundance  around  their  margins.  The  characteristic  metals 
are  silver  and  gold  with  much  lead  and  zinc,  especially  where 
the  intrusions  cut  the  limestones.  Copper  and  iron  are  also 


922  MINERAL  DEPOSITS 

present  at  such  limestone  contacts.  Arsenic  and  antimony  are 
far  more  in  evidence  than  during  the  second  epoch,  but  mercury 
is  still  absent.  Rossland,  Butte,  Bingham,  Tintic,  and  Leadville 
are  representative  districts. 

5.  The  Late  Tertiary  Epoch. — Orqgenic  disturbances  followed 
the  intrusions;  the  whole   Cordilleran  region  was  lifted  high 
above  sea  level,  warped,  and  faulted.     These  disturbances  may 
have  facilitated  sub-aerial  eruptions;  at  any  rate  it  is  certain  that 
the  middle  and  close  of  the  Tertiary  witnessed  outflows  of  lavas 
upon  a  magnificent  scale. 

These  eruptions  spread  over  large  areas  of  the  western  part 
of  the  continent; less  pronounced  in  British  Columbia  and  Alaska, 
they  are  abundantly  represented  in  California,  Washington, 
Idaho,  Colorado,  Utah,  Nevada,  New  Mexico,  Arizona,  and 
attained  their  greatest  development  in  Mexico.  Andesites  and 
rhyolites  are  the  predominating  rocks.  In  some  places  the  flows 
attained  such  thickness  that  during  the  later  part  of  the  vol- 
canic epoch  intrusions  of  magmas  consolidated  in  them  with 
granular  structure. 

During  these  eruptions,  not  strictly  contemporaneous  through- 
out, a  fifth  metallization  took  place,  of  which  the  characteristic 
metals  are  gold  and  silver.  These  deposits  were  often  of  great 
richness  which  is  further  accentuated  by  secondary  processes; 
in  fact  most  of  the  "bonanzas"  belong  to  this  class.  Lead  and 
zinc  are  not  conspicuous  except  where  the  metallization  took 
place  in  limestone.  Copper  is  not  abundant.  Tellurium  and 
antimony  are  common.  Not  that  they  are  absent  in  older 
metallizations,  but  they,  especially  tellurium,  seem  particularly 
characteristic  of  this  epoch.  Quicksilver  occurs  in  two  belts,  in 
Nevada  and  Mexico.  The  Comstock,  Tonopah,  Cripple  Creek, 
Pachuca  and  El  Oro  are  representative  districts. 

Large  areas  of  volcanic  rocks  are  barren.  The  metal  deposits 
seem  to  have  formed  only  near  or  at  the  foci  of  igneous  activity, 
where  connection  could  be  established  with  underlying  magmas. 
The  most  recent  eruptions  were  mainly  basalts,  and  these,  except 
in  one  case,  do  not  seem  to  have  been  affected  by  metallization. 

6.  The  Post-Pliocene  Epoch.— The  youngest  metallogenetic 
province  is  that  of  the  Pacific  Coast  line.     It  is  of  very  late  age — 
Post-Pliocene  apparently,  and  is  characterized  by  mercury  ac- 
companied by  few  other  metals.     It  developed  in  the  Coast 
Ranges  of   California,  following  basalt  eruptions  and  contem- 


METALLOGENETIC  EPOCHS 


923 


poraneous  with  it  was  a  great  development  of  hot  springs.  In 
part  the  deposition  goes  on  at  the  present  time. 

Note  that  the  quicksilver  did  not  develop  simultaneously  with 
the  birth  of  the  Coast  Ranges;  these  are  much  older,  and  an 
active  circulation  of  atmospheric  water  was  undoubtedly  estab- 
lished long  before  the  quicksilver  deposits  were  formed. 

7.  Cretaceous  or  Later  Epochs  of  Copper  Concentration  in 
Sedimentary  Rocks. — In  addition  to  these  five  classes,  whose 
connections  with  igneous  rocks  are  indubitable,  the  disseminated 
copper  ores  of  the  southwest  should  find  mention.  They  occur 
in  sandstones,  shales,  or  conglomerates  ranging  from  the  Car- 
boniferous to  the  Cretaceous,  and,  in  most  cases,  chalcocite  is 
the  original  ore;  frequently  small  amounts  of  silver  are  present. 
New  Mexico,  Arizona,  Utah,  Colorado  and  Texas  offer  numer- 
ous examples  of  this  class.  While  their  origin  is  not  wholly 
clear,  many  observers  believe  that  they  represent  concentration 
by  ground  waters  of  small  amounts  of  copper  originally  derived 
from  the  pre-Cambrian  deposits  and  since  distributed  through 
late  sedimentary  beds.  In  similar  deposits  we  find  ores  of 
uranium,  vanadium  and  radium. 

Summing  up  we  have  then  in  the  Cordilleran  region: 


1.  Deposits  of  the  Pre- 

Cambrian  period: 

2.  Deposits  of  the  early 

Mesozoic  epoch: 

3.  Deposits   of   the   late 

Mesozoic  epoch: 


4.  Deposits  of  the  early 
Tertiary  epoch: 


Principal  metals 

Gold  and  copper. 

Copper. 

Gold. 


Gold,  silver,  copper, 
lead,  zinc. 


Gold,  silver. 
Quicksilver. 

Copper,  vanadium, 
uranium,  radium. 


5.  Deposits   of   the   late 

Tertiary  epoch: 

6.  Deposits  of  the  Post- 

Pliocene  epoch: 

7.  Cretaceous   or  later 

concentrations   in 
sedimentary  rocks. 

In  deposits  that  are  clearly  connected  with  igneous  rocks 
metallization  is  certainly  a  function  of  varying  pressure  and  tem- 


Principal  rocks  asso- 
ciated 

f  Granites. 

Diorites,  gabbro. 

Basalt,  diabase. 

Gabbro. 

Granodiorite. 

Quart  z-monzonite, 
gabbro. 

Granodiorite, 

Quartz-monzonite.  ^ 

Monzonite,  with  cor- 
responding por- 
phyritic  rocks.  ^ 

AndesrteTTatiteT^ 
[  Rhyolite. 

Basalt,  andesite. 

Sandstone,  shale, 
conglomerate. 


924  MINERAL  DEPOSITS 

perature;  these  factors  being  dependent  upon  depths  below  the 
surface  and  other  conditions;  metallization  is  also  dependent  on 
the  nature  of  the  rocks  in  which  deposition  takes  place.  Pri- 
marily, however,  it  is  probably  a  consequence  of  magmatic 
differentiation. 

It  is  well  established  that  magmas  of  different  types  contain 
different  associations  of  the  rare  metals.  For  instance,  tin  and 
tungsten  characterize  acidic  rocks,  while  nickel  and  cobalt  are 
found  chiefly  in  magmas  rich  in  ferro-magnesian  constituents. 
At  the  same  time  rocks  of  a  given  general  composition  may,  in 
different  localities,  vary  considerably  in  the  quantity  of  rarer 
metals  contained. 

Our  knowledge  of  the  content  of  rare  metals  in  igneous  rocks 
is  fragmentary,  but  it  is  at  least  often  the  case  that  ore  deposits 
formed  after  the  eruption  of  an  igneous  rock  will  contain  the 
rarer  metals  characteristic  of  it.  It  may  be  true  that  for  each 
differentiated  rock  type  there  are  corresponding  types  of  deposits, 
varying  with  the  conditions  of  deposition. 

As  periods  of  long  continued  differentiation  may  materially 
modify  the  composition  of  a  magma,  it  is  conceivable  that  this 
might  find  expression  in  a  progressive  change  in  the  character 
of  ore  deposits  successively  formed  during  these  periods.  The 
quicksilver  deposits  of  California,  for  instance,  may  be  the  ulti- 
mate result  of  such  long  continued  differentiation. 


As  we  look  back  over  the  wide  domain  of  mineral  deposits 
we  perceive  the  strong  tendency  to  concentration  of  common  or 
rare  elements,  by  magmatic  differentiation,  by  solution  or  by 
mechanical  transportation;  we  perceive  also  cycles  of  transform- 
ations, based  on  the  laws  of  stability  of  chemical  compounds. 
Even  when  deeply  buried  the  deposits  may  suffer  many  changes. 
Near  the  surface  they  are  constantly  subject  to  transmutations 
involving  both  concentration  and  dispersion.  A  few  stable 
compounds  are  formed  while  the  rest  are  of  the  elements  scat- 
tered by  mechanical  and  chemical  transportation.  Some  con- 
stituents are  carried  down  into  the  earth  by  the  underground 
circulation  of  water,  perhaps  to  form  new  deposits  in  other  rocks. 
Ultimately  erosion  sweeps  away  the  wreckage  of  the  old  deposits 
into  basins  of  sedimentation  where  the  elements  may  be  recon- 
centrated,  be  it  by  direct  precipitation  or  by  the  aid  of  living 


METALLOGENETIC  EPOCHS  925 

matter.  The  sediments  may  again  be  lifted  and  corrugated, 
again  destroyed  by  erosion  and  new  eras  of  concentration  begin. 
In  one  aspect  the  science  of  mineral  deposits  is  frankly 
utilitarian,  but  from  the  viewpoint  of  pure  knowledge  it  records 
the  principles  governing  the  cycles  of  concentration  of  the  ele- 
ments. It  traces  the  processes  by  which  the  primeval  gases  and 
magmas  have  become  differentiated  into  the  manifold  complexity 
of  the  earth's  crust. 

INDEX  TO  MINERAL  DEPOSITS  BY  ELEMENTS 
(Principal  Uses  in  Italics) 

NITROGEN.  Sodium,  potassium  and  calcium  nitrates  (fertilizers,  explo- 
sives'), saline  residues,  pp.  296-299.  Volcanic  efflorescence,  p.  297. 

FLUORINE.    Fluorite  (flux,  etc.).     Sedimentary  deposits,  in  fluo-apatite,  p. 

279.     All  vein  deposits,  pp.  524,  649,  657. 
Cryolite  (aluminum  ore  and  flux).     Pegmatite,  p.  774. 

CHLORINE.  Alkaline  chlorides  (soda,  hydrochloric  acid,  etc.).  Saline  de- 
posits, pp.  305-311. 

BROMINE.     Alkaline  bromides.     Saline  deposits,  pp.  290,  308,  311. 

IODINE.  Alkaline  iodides  and  calcium  iodate.  Saline  deposits  with  nitrate, 
p.  298. 

SULPHUR.     Native   (sulphuric  acid,   etc.).     Volcanic  emanations,  p.  382. 

Spring  deposits,  p.  382.     Alteration  of  gypsum,  pp.  382-387. 
Pyrite  and  pyrrhotite  (sulphuric  acid).     Sedimentary  deposits,  veins  and 
pyritic  replacement  deposits,  intermediate  or  high  temperature 
type,  pp.  387,  635.     Contact-metamorphic,  p.  751. 

SELENIUM.  Metallic  selenides  (electric  instruments,  coloring  matter). 
Gold-bearing  veins,  pp.  526-529.  Pyritic  deposits,  p.  892.  Vol- 
canic emanations,  p.  98. 

TELLURIUM.  Metallic  tellurides  (element  little  used).  Vein  deposits,  pp. 
521,  688.  Volcanic  emanations  with  sulphur,  p.  98. 

CARBON.     Diamond.     Placers,  p.  246.     Igneous  rocks,  pp.  786-789. 

Graphite.     (Refractory,   lubricant,   writing  material.)     Regional  meta- 
morphic.     Contact-metamorphic.     Igneous  rocks,  pp.  743-749. 

SILICON.  Quartz  (abrasive,  flux,  refractory  material,  etc.).  Detrital,  pp. 
207-208.  Sedimentary,  p.  251.  Spring  deposits,  p.  100.  Veins 
and  replacement  deposits,  general.  Pegmatite,  p.  766.  Igneous. 

BORON.     Sodium   and  calcium  borates   (various  industrial  uses).     Spring 

waters,  p.  61.     Saline  deposits,  pp.  299-305. 
Boracite.    Saline  deposits,  p.  302. 

Tourmaline,  axinite  (gem  material  in  part).     High  temperature  veins, 
pp.  659,  695.     Contact-metamorphic,  p.  741.     Pegmatite,  p.  775. 
Sassolite.     Volcanic  emanation,  p.  300. 

PHOSPHORUS.  Calcium  phosphate  (fertilizer).  Sedimentary,  pp.  275- 
286.  Residual,  p.  347.  Contact-metamorphic.  Igneous,  pp.  773, 
803. 


926  MINERAL  DEPOSITS 

Cerium-thorium  phosphate.     Monazile  (thorium  salts).     Placers,  p.  244. 

Pegmatite,  p.  772. 
Turquoise.     Aluminum  phosphate  (precious  stone).     Zone  of  oxidation, 

p.  276. 

ARSENIC.     Metallic  arsenides  and  sulpharsenides  (manufacture  of  AstOs, 
etc.).     Veins   of   all   zones,    particularly   intermediate   and   high 
temperature,  pp.  626,  649.     Contact-metamorphic,  p.  739.     Oxi- 
dation products,  p.  898. 
ANTIMONY.     Stibnite  and  sulph-antimonides   (metallic  ore).     Veins  and 

replacement  deposits  of  all  zones,  p.  501.    Oxidation,  p.  899. 
SILICATES.     Mica  (insulation  material,  refractory).     Pegmatite,  pp.  766- 

768. 
Asbestos   (refractory,  fireproof  fabrics,   etc.).     Hydration  of  magnesian 

silicates,  pp.  395-399. 
Talc  and  soapstone  (refractory,  powder,  etc.).     Regional  metamorphic, 

hydration  of  amphibole,  etc.,  pp.  393-395. 

Serpentine  (ornamental  stone).     Hydration  of  olivine,  etc.,  p.  389. 
Meerschaum.     Hydration  of  magnesian  minerals  and  dolomite,  p.  392. 
Pyrophyttite  (refractory).     Hydrous  alteration,  p.  395. 
Kaolin    including    halloysite   and    clay    (refractory,    ceramic,    building 

material).     Detrital,  pp.  208-210.     Residual,  pp.  325-328. 
Precious  stones.     Tourmaline,  topaz,  emerald,  aquamarine.    Pegmatites 
and  high  temperature  veins,  p.  775.     Peridote,  garnet.     Igneous, 
p.  789.    Garnet,  vesuvianite,  lapis  lazuli.     Contact-metamorphic. 
Cordierite,  sapphirine.     Regional  metamorphic.     Serpentine,  jade, 
nephrite  (semi-precious).     Hydration  and  regional  metamorphic. 
POTASSIUM.     Chloride  and  sulphate.     Brines   and   saline   residues,   pp. 

312-315. 

Alunite,  upper  vein  zones,  pp.  316,  479. 
Orthoclase,  leucite,  etc.     Pegmatite  and  igneous,  p.  316. 
Glauconite.     Sedimentary,  pp.  262-264,  316. 
SODIUM.     Chloride,  sulphate  and  carbonate.     Brines  and  saline  residues, 

pp.  305-311. 
LITHIUM.     Amblygonite,  spodumene,  etc.  (Lithium  salts,  precious  stones). 

Pegmatite  dikes,  p.  773. 
CALCIUM.     Calcite.    Sedimentary,  pp.  247-250.     Regional  metamorphic. 

All  vein  zones.     Contact-metamorphic. 
Gypsum  and  anhydrite.     Saline  residues,  pp.  293-295. 
BARIUM.     Barite.     Sedimentary,  p.  253.     Residual,  p.  345.     Concentra- 
tion from  surrounding   rocks,    pp.   376-380.      Witherite.     Veins, 
p.  376. 
STRONTIUM.     Celestite,    strontianite.     Saline     residues.     Concentration 

from  surrounding  rocks,  pp.  380-382. 

MAGNESIUM.  Magnesite  (refractory,  metallic  ore).  Sedimentary,  p.  350. 
Alteration  of  serpentine,  pp.  390-392.  Replacement  of  dolomite, 
p.  391. 

Chloride  and  sulphate  (industrial  uses,  metallic  ore).     Saline  residues, 
pp.  312-315. 


METALLOGENETIC  EPOCHS  927 

ALUMINUM.      Corundum    (abrasive,    refractory).      Contact-metamorphic, 

p   808.     Igneous,  pp.  805-808. 
Ruby  and  sapphire.     Placers,  p.  246.     Pegmatite,  p.  776.     Igneous 

p.  807. 

Bauxite  (refractory,  metallic  ore).     Residual,  pp.  350-356. 
Cryolite  (see  fluorine). 

IRON.  (Metallic  ores.)  Detrital,  p.  245.  Sedimentary,  pp  251-272. 
Residual,  pp.  329-338.  Pyritic  replacement  deposits  of  lower 
vein  zones,  pp.  635-644.  Contact-metamorphic,  pp.  726-730. 
Igneous,  pp.  799-805.  Metamorphosed  deposits,  pp.  824-828. 

MANGANESE.  (Metallic  ores,  various  uses.)  Sedimentary,  pp.  272-275 
Residual,  pp.  338-345.  Contact-metamorphic,  pp.  753,  758. 

CHROMIUM.  Chromite  (refractory,  metallic  ore,  chromium  salts).  Igneous, 
pp.  793-795.  Oxidation  products,  p.  895. 

NICKEL.  (Metallic  ores.)  Residual  silicates,  pp.  348-350.  Middle  vein 
zone,  pp.  625-631.  Igneous,  pp.  808-817.  Oxidation  products, 
p.  894. 

COBALT.  (Metallic  ore,  pigment.)  Residual,  p.  348.  Middle  vein  zone, 
pp.  625-631.  High  temperature  veins,  p.  703.  Oxidation  prod- 
ucts, p.  894. 

COPPER.  (Metallic  ores.)  Sedimentary,  pp.  413-415.  By  meteoric 
waters  in  sandstone,  pp.  400-407.  By  meteoric  waters  in  veins, 
pp.  415-418. 

Deposits  of  igneous  affiliations  as  follows:  Native  copper  in  lavas,  pp. 
425-443.  Upper  vein  zone,  p.  529.  Middle  vein  zone  and  related 
replacement  deposits,  pp.  634-648.  High  temperature  veins, 
pp.  695-701.  Contact-metamorphic,  pp.  730-737,  750-753.  Igne- 
ous, pp.  811-821.  Oxidation  products,  pp.  848-871. 

LEAD.     (Metallic  ores.)     By  meteoric  waters  in  sandstone,  pp.  400-402 

By  meteoric  waters  in  limestone,  pp.  444-464. 

Deposits  of  igneous  affiliations  as  follows:  Upper  vein  zone,  pp.  534-539. 
Middle  vein  zone  and  related  replacement  deposits,  pp.  590-620. 
High  temperature  veins,  pp.  701-703.  Contact-metamorphic, 
pp.  737-739.  Oxidation  products,  pp.  874-878. 

ZINC.     (Metallic  ores.)     Residual  deposits,  p.  346.     By  meteoric  waters  in 

limestone,  pp.  444-464. 

Deposits  of  igneous  affiliations  as  follows:  Upper  vein  zone,  pp.  529- 
[539.  Middle  vein  zone  and  related  replacement  deposits,  pp. 
590-620.  High  temperature  veins,  pp.  701-703.  Contact-meta- 
morphic, pp.  737,  753,  828.  Oxidation  products,  pp.  871-874. 

CADMIU M.     (Metallic  ore.)     In  almost  all  zinc  deposits,  p.  648. 

GOLD.     (Metallic  ores.)     Placers,  pp.  211-236.     Conglomerates  of  South 

Africa,  pp.  236-242. 

Deposits  of  igneous  affiliations  as  follows:  Upper  vein  zone,  pp.  504-545. 
Middle  vein  zone,  pp.  564-590,  620.  High  temperature  veins, 
pp.  674-698.  Contact-metamorphic,  pp.  639-641.  Pegmatite, 
Igneous,  p.  776.  Oxidation  products,  pp.  878-883. 

SILVER.  (Metallic  ores.)  By  meteoric  waters  in  limestone,  pp.  449,  461. 
By  meteoric  waters  in  sandstone,  pp.  403,  405. 


928  MINERAL  DEPOSITS 

Deposits  of  igneous  affiliation  as  follows:  Upper  vein  zone,  pp.  516-521, 
533-539.  Middle  vein  zone,  pp.  590-620,  622-631.  High  tempera- 
ture veins,  pp.  701-703.  Oxidation  products,  pp.  883-891. 

PLATINUM.  (Metallic  ores.)  Placers,  p.  742.  Igneous,  pp.  790-792. 
Oxidation  products,  p.  891. 

IRIDIUM,  PALLADIUM,  RHODIUM  AND  OSMIUM.  (Metallic  ores), 
pp.  790-792. 

MERCURY.  (Metallic  ores.)  Hot  spring  deposits,  p.  499.  Upper  vein 
zone,  pp.  487-501.  Middle  vein  zone,  p.  577.  High  temperature 
veins,  p.  692.  Oxidation  products,  p.  892. 

TIN.  (Metallic  ores.)  Placers,  p.  743.  High  temperature  veins,  pp.  657 
673.  Contact-metamorphic,  p.  741.  Pegmatite,  p.  768.  Igneous, 
p.  768.  Oxidation  products,  p.  895. 

BISMUTH.  Native,  sulphide  and  sulpho  salts  (metallic  ores).  Veins  and 
replacement  deposits  of  all  zones,  pp.  544,  699.  Pegmatite,  p. 
777.  Oxidation  products,  p.  897. 

MOLYBDENUM.  Molybdenite  (metallic  ore).  Veins  and  replacement 
deposits  of  all  zones,  particularly  in  high  temperature  veins,  p.  700. 
Pegmatite,  pp.  777-779.  Oxidation  products,  p.  897. 

TITANIUM.  Ilmenite  and  rutile  (metallic  ore,  etc.).  Placers,  p.  245. 
High  temperature  veins,  p.  700.  Contact-metamorphic,  p.  742. 
Pegmatite,  p.  769.  Igneous,  pp.  795-799. 

ZIRCONIUM.  Zircon  (refractory,  metallic  ore).  Placers,  p.  772.  Pegma- 
tite, p.  772. 

VANADIUM.  (Metallic  ores.)  By  meteoric  waters  in  sandstone,  pp.  407- 
412.  In  veins,  pp.  573,  620,  691.  Oxidation  products,  p.  896. 

URANIUM  AND  RADIUM.  By  meteoric  waters  in  sandstone,  pp.  407- 
412  Middle  vein  zone,  p.  626.  Pegmatite,  p.  770. 

TUNGSTEN.  (Metallic  ores,  etc.).  Middle  vein  zone,  p.  620.  High  tem- 
perature veins,  p.  673.  Contact-metamorphic.  p.  742.  Pegma- 
tite, p.  770.  Oxidation  products,  p.  896. 

CERIUM,  THORIUM,  ETC.  Monazite,  etc.  Placers,  p.  244.  Pegmatite, 
pp.  770-773. 

COLUMBIUM,  TANTALUM,  ETC.     Pegmatite,  p.  770. 


INDEX 


Aar,  Switzerland,  zeolitic  veins,  crystalliza- 
tion, 633 

Abe  Lincoln  mine,  New  Mexico,  water 
condition,  40 

Abilena  well,  Kansas,  analysis  of  water,  55 

Abitibi,    Ontario,    gold-quartz    veins,     676 

Abosanobori    mine,    Japan,    sulphur,    382 

Abosso,  West  Africa,  gold  conglomerate,  242 

Absorption  ratio,  30 

Acid  sulphate  waters  in  igneous  rocks,  57 

Acid,  sulphuric,  387 

Acmite,  765 

Actinolite,  738,  714,  739,  751,  753,  821 

Adalbert  vein,  Przibram,  Bohemia,  section, 
600 

Adirondack  graphite  deposits,  745,  magne- 
tite deposits,  803 

Admiralty  Island,  Alaska,  gold,  683 

Adsorption,  26 

Adularia,  group  of  ore  deposits,  77;  468. 
514,  631,  363,  428,  437,  468,  470-472,  475, 
479,  483,  508,  528,  620,  716 

Aegirine,  765 

Africa,  metallogenetic  epochs,  913 

Ajo,    Arizona,    725;    section    of    ore,    851 

Akmolinsk    district,    copper    deposits,    401 

Alabama  Coon  Mine,  Joplin,  Missouri, 
analysis  of  water  from,  905 

Alabandite,  895 

Alaska  gold-quartz  veins,  682;  stibnite,  503 

Alaska-Tread  well  gold  mine,  Alaska,  section, 
684,  694 

Alaskite,  682 

Albite,  development,  67;  in  schists,  74; 
group  of  ore  deposits,  77;  dikes,  570,  716, 
730,  738,  755,  764.  818,  827,  421,  428, 
430,  549,  564,  571,  580,  624 

Alderley  Edge,  England,  copper  ores,  401 

Algse,  as  precipitants  in  spring  waters,  99 

Algeria,  phosphate,  278;  senarmontite,  503 

Alibert  mines,  Russia,  graphite,  747 

Alkali,  288 

Alkalinity,  how  measured,  65 

Allanite,  770,  771. 

Alleghany,  California,  gold  masses,  226, 
gold  replacement,  574 

Alleghany  Springs,  Virginia,  composition 
of  water,  56 

Allophanite,  326 

Alma,  Colorado,  deposits,  617 

Almaden,  Spain,  quicksilver,  491,  489; 
cinnabar,  494 


Almandite,  749,  790 

Almeria,  Spain,  alunite,  317 

Alpine  veins,  631 

Alsace,  potassium  salt  beds,  316 

Alta  vein,  Helena  Montana,  701 

Altar,  Sonora,  Mexico,  stibnite,  503;  gold,  11 

Altenberg,  Saxony,  granite,  greisen  analyses, 

662;  cassiterite,  669 
Alteration,   of  intrusive    rock    in    contact 

metamorphism,     713;     of     sedimentary 

rocks,  663;  types  of,  478;  of  wall  rocks 

adjoining    gold-quartz     veins,     550;     of 

serpentine,   Cuba  iron  region,   336,   337 
Alum    well,    Versailles,    Missouri,    analysis 

of  water,  56 
Aluminum,  71,  350;  minerals,  350;  in  igneous 

rocks,  6;  tenor  of,  16;  in  waters  of  pyritic 

shale,  56,  59;  sulphate,  64;  salts   of,   98 

(see  also  minerals  by  name) 
Alundum,  808 
Alunite,  57,  316,  479,  493,  842,  539,  352,  499, 

541 

Amblygonite,  277,  774,  764 
American  Graphite  Co.,  mine,  745 
American   Nettie   mine,   Ouray,    Colorado, 

ores  as  barriers,  193 

American  River,  California,  gravel  bars,  219 
Amherst  County,  Virginia,  rutile  deposits, 
o769 

Ammeberg,  Sweden,  zinc  ores,  828 
Ammonium,  salts,  98 
Amount,  of  metal,  of  ore,  produced  in  the 

United  States,  17 
Amphibole,  asbestos,  395;  decomposing,  43; 

effect  of  vein  forming  solutions  on,  77; 

in  granular  rocks,  88,  737,  758,  765,  799, 

800,  813,  819,  393,  430 
Amphibolite,  analyses,  686,  687,  693,  553; 
-   of  deeper  zone,  75 
Amygdaloid,    definition,    137;    copper-dash 

beds,  435;  water  of,  48 
Amygdules,  definition,  137 
Analcite,  427,  363,  428,  437,  442,  625 
Analyses,   of  water,  interpretation,   64;  of 

igneous,  average,  6  (see  also  by  subject 

and  geographically) 

Anamorphism,  definition,  72;  zone  of,  74,  76 
Andalusite,  653,  712 
Andes,  silver  in  volcanic  ash,  14 
Andesine,    replaced    by    tourmaline,    177; 

sericite,  calcite,  178,  811,  430,  797 
Andesitq,  analyses,  481,  484;  copper  in,  9 


929 


930 


INDEX 


Andradite,  053,  714,  712,  716,  719-721, 
726,  730,  738,  735,  740,  751 

Andreasberg,  Harz  Mountains,  silver  depos- 
its, 623;  zeolites,  428 

Angels  Camp,  Calaveras  County,  Cali- 
fornia, 551,  569,  571,  694 

Anglesite,  874,  875,  611,  703 

Anhydrite,  291.  293,  310 

Ankerite,  529,  571,  572,  580,  712 

Annaberg,  Saxony,  cobalt  veins,  602,  625 

Annabergite,  894 

Anna  Lee,  Cripple  Creek,  Colorado,  ore 
chimney,  186 

Anorthite,  of  deeper  zone,  75,  712 

Anthophyllite,  396,  423 

Anticline,  118 

Antilles,  metallogenetic  epochs,  916 

Antimony,  114;  in  spring  water,  96;  solu- 
bility, 899;  minerals,  501  (see  also 
minerals  by  name) 

Antiochia,  Asia  Minor,  chromite,  794 

Antioquia,  Columbia,  springs,  52 

Anvil  Creek,  Alaska,  gravels,  225 

Apatite,  275,  773;  77,  276,  363,  486,  657, 
700,  714,  715,  729,  738,  745,  748,  749, 
755,  764-769,  773,  788,  798-801 

Apophyllite,  427,  430,  437,  439,  472,  493, 
625,  714,  788 

Appalachian  region,  hematites,  330;  gold, 
674;  manganese,  342;  zinc,  346;  gold- 
quartz  veins  of  southern,  674 

Aquamarine,  775,  767 

Aragonite,  247;  in  spring  deposits,  100,  383 

Arendal,  Norway,  ilmenite,  796 

Arfvedsonite,  765 

Argentine,  Colorado,  deposits,  617 

Argentite,  467,  883,  884;  deposits,  475; 
veins,  516, 520,  624,  626 

Argonaut  vein,  Amador  county,  California, 
567 

Arid  regions,  ground  water,  32,  40 

Arkansas,  zinc,  457;  bauxite,  354 

Arqueros,  Chile,  zeolitic  silver  veins,  428, 
623,  625 

Arrowhead  Spring,  California,  character  of 
water,  55;  source  of  salinity,  90 

Arsenic,  minerals,  649;  deposits,  649;  solu- 
bility, 898;  colloidal  origin,  28;  in  water, 
45,  49,  53,  96,  60,  101,  110,  114;  in  ochre- 
ous  deposits,  99  (see  also  minerals  by 
name) 

Arsenide,  vein  with  greywacke  inclusions, 
629;  in  pegmatites,  776 

Arsenobiamite,  897 

Arsenolite,  898 

Arsenopyrite,  649,  898;  gold  deposit,   739, 
552,   574,   586.   711,   718,   719,   738-741, 
755,  764,  769 
Artesian,  water,  34;  wells,  34,  60;  analysis 

of  water,  62;  basins,  33 
Asbestos,  amphibole,  395;  chrysotile,  396; 
uses,  398 


Asbestos  Hill,  Quebec,  398 

Asbolite,  348,  350,  894 

Asia,  metallogenetic  epochs,  913 

Aspin,  Colorado,  605,  611,  section  of  lime- 
stone, 176 

Assay  valuations,  table,  20;  assay  walls,  158 

Associated  gold  mine,  Kalgoorlie,  Aus- 
tralia, 691 

Atacama  desert,  Chile,  nitrate,  297 

Atacamite,  849 

Atlin,  British  Columbia,  hydromagnesite, 
390 

Atolia,  Kern  County,  California,  tungsten, 
621 

Auburn,  California,  vein  system,  567 

Augite,  742 

Aurichalcite,  87l" 

Aurora,  Missouri,  plan  of  workings,  456,  451 

Austin,  Texas,  celestite,  381 

Australia,  eastern,  map,  579;  gold,  11 

Australia,  Western,  geological  map,  689; 
gold-telluride,  688;  analyses  of  amphi- 
bolites,  693;  mangano-tantalite,  770 

Australasia,     metallogenetic     epochs,     914 

Auvergne,  France,  carbon  dioxide,  in  water, 
95 

Avala,  Servia,  quicksilver,  491,  492 

Average  analysis  of  igneous  rocks,  6 

Avery  Island,  Louisiana,  salt,  310 

Awaruite,  793 

Awavatz,  mountains,  California,  celestite, 
381 

Axinite,  623,  624,  657,  663,  712,  716,  740, 
741,755,623,624 

Ayrshire  mine,  Lomagundi,  Mashonaland, 
Africa,  auriferous  gneiss,  13 

Aztec  Spring,  New  Mexico,  analysis  of 
water,  44 

Azurite,  crystals  replacing  kaolin,  841, 
849,  401,  733 

Baddeleyite,  772 

Balaklala     copper     mine,    Shasta    County, 

California,  637 
Ballarat,  Victoria,  Australia,  buried  placers, 

224;  nuggets,  226;  indicators,   191;  gold 

region,  578,  582 

Baltimore,  Maryland,  chromite,  794 
Banat  province,  Hungary,  magnetite,  726; 

iron,  705 

Banded  structure  of  veins,  164,  534,  537 
Banka,  greisen,  section,  659;  tin,  671 
Bannock,  Montana,  gold  deposits,  740 
Banos  del  Toro,  Chile,  borate  deposition, 

300 

Baraboo  district,  Wisconsin,  ores,  373 
Baringer  Hill,  Texas,  rare  earths,  771 
Barite,  in  calcite,  71;  group  of  ore  deposits, 

77;    occurrence    and    origin,    376;    253; 

residual,   345;   replacing  limestone,    178; 

363,   383,   385,  402,   403,   468,  47C,.  475, 

489,   492,    529,    598,611,    620,    623,    625 


INDEX 


931 


Barium,     minerals,    376;    solubility,    377; 

in  spring  water,  45,  47,  49,  56,  62,  96, 
106,  110;  deposits  in  United  States,  378; 

foreign  deposits,  379;  uses  and  produc- 
tion,  380    (see  also   minerals  by   name) 
Barkevikite,  765 
Barriers,  impermeable,  effect  on  ore  bodies, 

191 

Barstow,  California,  strontianite,  381 
Bartlett  Springs,  California,  composition  of 

water,  46 
Basalt,    with    manganese,   43;    stability   in 

deeper  zone,  75;  occurrence,  113;  showing 

blowholes,  138 

Base-metal,  deposits,  474;  veins,  529 
Bassick    deposits,    Silver    Cliff,    Colorado, 

153,  529 

Batagol,  Siberia,  graphite,  747 
Batesville,      Arkansas,      manganese,      340; 

section,  340 

Batopilas,     Mexico,    argentite    veins,    521 
Baux,  France,  analysis  of  bauxite,  355 
Bauxite,    350,    775;    analysis,    355;    origin, 

352;  occurrence,  354;  uses  and  production, 

355 
Bay  of  Fundy,  Nova  Scotia,  native  copper, 

425 

Beaches,  221 
Bear    Creek,    Colorado,    vanadium-bearing 

sandstone,  410 

Beaver  County,  Utah,  sulphur  deposits,  382 
Beaver  Lake,  Mining  District,  Utah,  anda- 

lusite,  653 

Beaver  Valley,  Utah,  analysis  of  water,  58, 59 
Bed  veins,  154,  157 
Bedford  Alum  Spring,  Virginia,  composition 

of  water,  57 

Bellefountain    mine,    Nevada    City,    Cali- 
fornia, altered  granodiorite  552;  grano- 

diorite,  analysis,  553 
Belle  Isle,  salt  well,  310 
Belowda  Beacon,  Cornwall,  vein,  section; 

663 
Bendigo,   Victoria,   section  of  saddle  reef, 

141,  142;  578,  580;  spur  reef,  581;  trough 

reef,  581;  gold-bearing  quartz,  189;  mine 

water,  902 

Bennet  mine,  Virginia,  barite  deposit,  sec- 
tion, 379 
Beresowsk,  Ural  Mountains,  ladder  veins, 

139 

Bergen  Hill,  New  Jersey,  ores,  475 
Berggiesshubel,  Saxony,  contact  metamor- 

phism,  718;   magnetite,   726,   cassiterite, 

741 

Berlin  mine,  Nevada,  faulted  vein,  134 
Berners  Bay  mining  district,  Alaska,  683 
Bersbo,  Sweden,  copper,  820;  pyritic,  636 
Bertha  mines,  Virginia,  zinc,  section,  346 
Beryl,  775,  764,  767,  769 
Beuthen,  Silesia,  zinc-bearing  beds,  section, 

449 


Bilbao,  Spain,  iron  ores,  334 

Bilin  springs,  sodium  carbonate,  62 

Billiton,  tin,  671 

Bindheimite,  899 

Bingham,  Utah,  732;  alteration  of  rocks, 
557;  analyses,  558;  copper  deposits,  736, 
866;  cupriferous  monzonits,  3;  galena 
876;  section  of  ore  body,  867 

Biotite,  decomposition,  43;  development  at 
high  temperature,  67;  of  deeper  zone,  75; 
group  of  ore  deposits,  77;  auriferous 
biotite  gneiss,  14,  597,  653,  675,  703,  712, 
716,  749,  752,  765,  788,  767,  797,  799,  811; 
819-821 

Bird  Reef,  South  Africa,  series,  239 

Birmingham,  Alabama,  district,  sedimen- 
tary rocks,  265;  section,  266 

Bisbee,  Arizona,  copper,  725,  733;  depth  of 
oxidation,  830 

Bismite,  897 

Bismuth,  98,  699,  626,  657,  769,  629,  777; 
minerals,  897;  solubility,  897  (see  also 
minerals  by  name) 

Bismuthinite,  897,  544,  606,  626,  657,  690- 
698,  716,  764 

Bismutite,  897 

Bissell,  California,  magnesite,  390 

Bitterns,  317 

Biwabik,  iron-bearing  formation,  Mesabi 
district.  Minnesota,  section,  367 

Black  band,  258 

Black  Hills,  South  Dakota,  gold,  679;  mica. 
797;  ore  deposits,  sections,  587;  pegma- 
titic,  768;  tin  veins,  670;  gold  ores  as 
barriers,  193;  Pine  Ridge,  section,  34 

Black  Mountains,  gold,  514 

Black  reef,  series,  South  Africa,  238 

Black  sand,  California,  platinum,  790,  245 

Black  sea  mud,  253;  sulphur  content,    385 

Blanket  veins,  Rico,  Colorado,  191 

Blatt,  flaws,  134 

Bleiberg,  Carinthia,  zinc,  449 

Blue  Canyon,  California,  albite  dikes,   571 

Blue     Mountains,     Oregon,     oxidation     of 
gold  deposits,   882;   wall  rocks  of  veins. 
555;  copper  and  tourmaline,  697 
Bodenmais,    Bavaria,    pyritic    deposits, 
636,  819;  cadmium,  648 

Bog  iron  ore,  255 

Bog  manganese  ore,  273 

Bogoslowsk,  Russia,  iron,  705 

Boise  Basin,  Idaho,  773 

Boise  hot  springs,  Idaho,  sodium  carbonate 
waters,  62 

Bonanza  mine,  Alaska,  copper,  416;  chal- 
cocite,  246,  416 

Bonanza  mine,  Sonora,  California,  573 

Bonanza  Valley,  Yukon,  gravel  deposits, 
228 

Bonanzas,  473 

Bonneterre,  Missouri,  barite,  378 

Book  structure  of  veins,  165 


932 


INDEX 


Bor,  Servia,  enargite,  635 

Boracite,  300,  302 

Borate  deposits,  California  and  Nevada, 
map,  301;  marine,  300;  Tibet  and  Chili, 
300;  of  Tertiary  lake  beds,  303 

Borates,  299;  Volcanic  exhalations,  60 

Borax,  300 

Borax  Lake,  Lake  County,  California,  borax, 
300 

Borax  marshes,  302 

Boric  acid,  300  . 

Bornite,  837;  in  igneous  rocks,  817;  oxida- 
tion, 849,  851,  856;  in  quartz  veins,  634, 
711,  716,  735,  736,  739,  755,  764,  401 

Boron,  minerals,  300;  origin,  production, 
use,  304  (see  also  minerals  by  name) 
in  water,  45,  47;  in  volcanic  springs,  91, 
96;  associated  with  magmatic  emanations, 
79;  in  springs,  49,  51,  53,  60,  63 

Bort,  787 

Boulangerite,  874,  600 

Boulder,  Montana,  hot  springs,  105;  zeolites, 
428;  batholith  veins,  701 

Boulder  County,  Colorado,  deposits,  617; 
tungsten,  620;  wolframite  veins,  673; 
fluorite,  650 

Boundary  district,  British  Columbia,  igne- 
ous metamorphic  deposits,  750,  718 

Bourbon-l'Archambault  springs,  110 

Bourbonne-les-Bains,  France,  zeolites,  428 

Bournonite,  848,  874 

Brad,  Transylvania,  gold-quartz  reins,  505; 
section,  506 

Braden,  Chile,  copper-tourmaline,  696 

Branchville,  Connecticut,  pegmatitic  quartz, 
763 

Brandenburg,  Kentucky,  lithographic  stone, 
248 

Braunite,  339,  895,  344 

Brazil,  manganese,  344;  gold,  680 

Brazilite,  772 

Brazos  River,  Texas,  sulphur  deposits,  387 

Breckenbridge,  Colorado,  deposits,  559,  617, 
618;  galena  on  sphalerite,  876;  rooks, 
analyses,  5CO 

Breece  and  Wheeler  mine,  California,  227 

Breithauptite,  624,  629 

Bremen  mine,  Silver  City,  New  Mexico, 
ore-shoots,  192 

Brewsterite,  625 

Bridal  Chamber,  Lake  Valley,  New  Mexico, 
silver,  887 

Brinton,  Virginia,  arsenic,  649 

Briseis  mine,  Tasmania,  244 

Bristol,  England,  celestite,  381 

British  Guiana,  copper  in  rocks,  9;  lead  and 
zino,  10 

Brochantite,  deposition,  842,  849 

Brodbo,  Sweden,  rare  earths,  771 

Broken  Hill,  New  South  Wales,  stromeyerite, 
836,  884;  lead  and  zinc  deposits,  701 

Bromide,  New  Nexico,  gold  veins,  678 


Bromine,  308;  production,  311 ;  in  salt  brines, 

318;   in   spring   water,   47,   49,    96,    110 
Bromyrite,  884 
Brooklyn  mine,  Phoenix,  British  Columbia, 

section,  751 

Brussa,  Asia  Minor,  chromite,  794 
Buckingham   Township,   graphite,   section, 

747,  748 
Bullah  Delah,  New  South  Wales,  alunite, 

317 

Bull-Domingo  deposits,  Silver  Cliff,  Colo- 
rado, 529 
Bullfrog     district,      Nevada,     gold,     515; 

alunite,  493 

Bullion  district,  Nevada,  sketch  map,  705 
Bully  Hill  mine,  Shasta  County,  California, 

copper,  637 

Bunches,  definition,  183 
Bunker    Hill    and    Sullivan    mine,    Coeur 

d'Alene,    Idaho,  ore    zone  section,   596; 

mine  water  analysis,  902 
Bunker  Hill  vein,   Amador  County,   Cali- 
fornia, 570 

Bunker  Hill  vein,  Coeur  d'Alene,  Idaho,  596 
Bunny  mine,  St.  Austell,  Cornwall,  England, 

668 

Burma,  rubies,  776 
Burra-Burra   mine,    Ducktown,   Tennessee, 

mine  water  analysis,  907 
Burro  Mountains,  New  Mexico,  chalcocite, 

868 
Butte,    Montana,    copper    635,    858,    862; 

depth  of  oxidation,  830;  outcrops,  832; 
•water  conditions,  39;  sections,  155,  156; 

copper    in    granite,    8;    cadmium,    894; 

arsenic,   899;   mine   water   analysis,   905 
Byrcn  Hot  Springs,  California,  character  of 

water,  49 

Cable  mine,  Montana,  altaite,  741;  gold, 
739;  magnetite,  727 

Cache  la  .Poudre  River,  Colorado,  analysis 
of  water,  44 

Cactus  mine,  Utah,  697 

Cadmium,  minerals,  6*8,  894;  in  spring 
water,  56,  96  (see  also  minerals  by  name) 

Calamine,  346,  448,  871 

Calaveras  County,  California,  copper,  417 

Calaverite,  524,  878,  538,  691,  692 

Calcasieu  Parish,  Louisiana,  salt  dome  sec- 
tion, 309;  sulphur  386 

Calcio-carnotite,  408 

Calcio-volborthite,  409 

Calcite,  origin,  247;  replacing  andesine,  178 

Calcite,  continued,  363,  383,  437,  458,  468- 
471.  489,  508,  514,  563,  598,  703,  710,  712, 
716,  718,  730-742,  745,  751,  753,  755, 
759,  768,  773,  788 

Calcium,  carbonate  deposits,  99,  375;  re- 
placed by  zinc  carbonate,  26;  carbonate 
waters  in  igneous  rocks,  43;  in  sedimen- 
tary rocks,  45;  sulphate,  64,  49,  59; 


INDEX 


chloride  in  spring  water,  47,  49,  64,  96; 

in  springs,  53,  60,  110;  leaching  of,  73; 

development  in  schist,  74 
Caledonia  mine,  New  Zealand,  473,  508 
Caliche,  Chile,  analysis,  298;  origin,  248 
Calico,  California,  silver,  188 
Calico   Mountains,   California,   colemanite, 

303 
California,  borate  deposits,  map,  301 ;  mining 

districts     map,     566;     quicksilver,     495; 

type  of  gold  veins,  564 
California  Hill,   Terlingua,   Texas,  section, 

498 
Callaway  mine,  Ducktown,  Tennessee,  mine 

water  analyses,  907 
Callie  mine,  Virginia,  section,  333 
Calomel,  893,  487 

Calumet  and  Hecla  mine,  Michigan,  mine 
•    water   analysis,   440;    copper,   434;   con- 
glomerate, 434,  441 
Cameron's  Bath,   Roturoa  geyser  district, 

New  Zealand,  water  analysis,  59 
Camp  Bird  lode,  Ouray  County,  Colorado, 

536;  supergene  gold,  883 
Campiglia    Marittima,    Tuscany,    contact- 

metarn orphic  deposits,  741 
Camp  Seco,  California,  copper,  417 
Canadian,    asbestos    deposits,    398;    mica, 

768;  corundum,  807 
Cananea,    Mexico,    copper,    725,  732,  736; 

mine  water  analysis,  906 
Cannstatt,  springs,  96 
Cape  Colony,  Africa,  asbestos,  398;  nickel, 

817 
Capote  mine,  Cananea,  Mexico,  mine  water 

analyses,  906 
Carbon,  786,     solubility,  789;     dioxide     in 

rocks,  source,  92;  dioxide  in  water,  50, 

59,  64,   76,   90,   92;  dioxide  in   volcanic 

gases,  98 
Carbonaceous    material,    precipitation    by, 

191 

Carbonardo,  787 
Carbonate,  rocks,  relation  to  ore  deposits, 

250;  waters,  47,  59,  902 
Carbonates,  solubility,  840 
Carbonization,  73,  76 
Carlsbad,  Bohemia,  analysis  of  water,  62, 

64,    107;   spring   deposits,    10O;   juvenile 

origin,  89;  minerals,  97;  relation  to  Joach- 

imsthal,  111 
Carnallite,  312,  392 

Cam  Brea  lode,  Cornwall,  England,  668 
Carnotite,  408 
Carolina  phosphates.  282 
Carson  Hill,  California,  nuggets,  226;  gold, 

573 
.Cartersville;   Georgia,   ocher,  347;  bauxite, 

354;  barite,  378 

Casa  Grande,  Arizona,  copper,  725 
Cashin  mine,  Colorado,  copper,  404 
Cass  County,  Texas,  section,  329 


Cassiterite,   minerals,  768;  solubility,  896; 
contact-metamorphio  deposits,  741;  pla 
cers,  243;  veins,  657;  sources,  666,  669. 
670,  712,  764,  768-772 
Castlemaine,  saddle  reef,  579 
Catoctin,  type  of  copper  ores,  443 
Cauca,  Colombia,  chloride  springs,  52 
Cave  earth,  definition,  139 
Caves,  formation  of,  138 
Cebolla,  Colorado,  titanium,  742 
Celestite,  380,  383 
Cement,  Portland  and  natural,  249 
Cementation,  definition,  70;  zone  of,  72 
Central  America,  metallogenetic  epochs,  916 
Central     City,     Colorado,    deposits,     559; 

map  showing  veins,  151 
Central  Eureka  mine,   Mother  Lode  belt, 

California,  569  • 

Centrosphere,  definition,  72 
Cerargyrite,  884,  622,  703 
Cerium,  minerals,  770  (see  also  by  name) 
Cerro  de  Pasco,  Peru,  copper  veins,  635 
Cerussite,  346,  402,  716,  611,  703 
Cervantite,  501,  899 
Ceylon,  graphite,  744,  747 
Chabazite,  427,  430,    493,   625;  in   spring 

deposits,  104 
Chaguarama   Valley,    Venezuela,   cinnabar, 

500 

Chalcanthite,  849 

Chalcedony,    group    of    ore    deposits,    77 
Chalcocite,  836,  837;  blankets,  188;  zones, 
853;  oxidation,  849,  857,  858;  The  South- 
western   deposits,    867;    replacing    plant 
remain^,   400,   403;   colloidal  origin,   28, 
634,    703,    716,    718,  733,  736,  753,  817, 
Chalcopyrite,  812,  856,  67;  oxidation,  849; 
of  quartz  veins,  634;  deposits,  730,  363, 
528,   634,    704,    711,   714,   716,  717,  719, 
728-740,  751,  753,  755,  764,  771 
Chalk,  248 
Chalmersite,  848 
Chambered  veins,  157 
Chamosite,  262,  268 
Chafiareillo,  Chile,  silver,  884,  891 
Chandler  mine,  Minnesota,  section,  369 
Charters  Towers,  Queensland,  gold,  583 
Chemical  changes  produced  by  weathering, 

324 

Cherokee,  California,  diamonds,  787 
Cherts,  251;  radiolarian,  495;  section,  454, 

370 

Chester,  Massachusetts,  corundum,  807 
Chewelah,  Washington,  magnesite,  391 
Chile,  gold  in  pitchstone,   11;  nitrate,  de- 
posits of,  297 

Chiltern  Valley,  gravels,  224 
Chimneys,  definition,   183;  Cripple  Creek, 

186 

China,  stibnite,  502 
Chino.  New  Mexico,  copper,  860 
Chloanthite,  894,  626,  629 


934 


INDEX 


Chloride  Flat,  Silver  City,  New  Mexico, 
oxidized  silver,  191 

Chloride  springs,  Germany,  49 

Chloride  waters,  900;  in  rocks.  47,  51 

Chlorine,  associated  with  magmatic  emana- 
tions, 79,  97;  in  rocks,  91,  96 

Chlorite,  363,  394,  421,  430  437,  441,  443, 
564,  580,  623,  714,  738,  753,  758,  759, 
788,  800,  806,  818,  821 

Chondrodite,  712,  741,  759 

Christmas  Island,  guano,  278 

Chromite,  793;  price  of,  16;  diamondiferous, 
789 

Chromium,  895;  with  vanadium  ores,  409; 
in  spring  water,  97 

Chrysocolla,  842,  849 

Chrysotile,  396 

Chuquicamatat  Chile,  copper,  635,  870; 
arsenic,  899 

Cinnabar,  495,  892;  deposition,  113;  oxida- 
tion, 487-492,  450;  in  spring  water, 
63;  deposits,  474 

Circulation  of  deeper  waters,  37 

Clarksburg,  West  Virginia,  Goff  Farm, 
deepest  bore  hole  in  the  world,  81 

Classification,  of,  faults,  124;  fluviatile  and 
marine  placers,  221;  mineral  deposits, 
195,  205,  200;  phosphate  deposits,  277; 
residual  iron  ores,  330;  metalliferous 
deposits,  474;  pyritic  replacement  de- 
posits, 636 

Clausthal,  Germany,  character  of  spring  in 
mine,  106;  lead-silver  veins,  598 

Clay  County,  Alabama,  graphite,  746,  749 

Clay,  definition,  208;  aluminum  •  content, 
350;  residual,  325-328;  iron-stone,  258; 
origin,  327;  porosity,  30;  detrital  deposits, 
208;  tensile  strength,  327;  gouge,  150  (see 
also  varieties  by  name) 

Clear  Creek,  Colorado,  .deposits,  617,  618 

Cleveland  Hills,  England,  carbonate  ore, 
260 

Clifton,  Arizona,  azurite,  841;  springs,  51; 
chalcocite  zone,  852,  858,  868;  deposits, 
699,  719;  section  of  ore,  731;  copper, 
732,  860;  cupriferous  pyrite  veins,  699; 
section  of  ore  bodies,  724 ;  faults  abundant, 
142 

Climax,  Summit  County,  Colorado,  molyb- 
denite, 778 

Clinton  ores,  264,  267;  section  of  New  York, 
271;  tenor,  14 

Closed  fault,  definition,  123 

Coast  Range,  California,  quicksilver,  113, 
489 

Cobalt  district,  Ontario,  geological  section, 
628;  ore  analysis,  630;  quicksilver,  487; 
silver  veins,  627;  arsenic,  649 

Cobalt,  minerals,  895;  solubility,  895;  in 
spring  water,  45,  56,  96,  110;  in  ocherous 
deposits,  99;  nickel  veins,  625;  tourmaline 
veins,  703  (see  also  minerals  by  name) 


Cobaltite,  895,  629 

Cobar,  New  South  Wales,  copper,  698 

Cody,  Wyoming,  sulphur,  382 

Coeur  d'Alene  district,  Idaho,  lead-zinc 
deposits,  876;  lead-silver  deposits,  595; 
veins,  159;  stibnite,  503;  replacements, 
177,  190,  194,  347 

Colemanite,  300,  303,  305 

Colloids,  27;  in  filling  and  replacement,  181 

Colombia,  South  America,  platinum,  242; 
springs,  52 

Colorado  mining  regions,  map,  531 

Coloradoite,  487,  691 

Colorados,  outcrop,  159,  832 

Columbia  River,  gold,  220;  magnetite,  245 

Columbite,  770 

Comb  structure,  164 

Combination  mine,  Goldfield,  Nevada,  sec- 
tion, 543 

Commern,  Prussia,  lead,  402 

Composite  veins  or  lodes,  151 

Comstock  lode,  Nevada,  analysis  mine 
waters,  904;  composite  veins  or  lodes, 
151;  gold-silver,  518;  hot  waters,  112; 
condition  of  veins,  157;  section,  519; 
bonanzas,  473 

Concentration,  of  gold,  220;  of  minerals, 
78;  processes,  247,  215;  in  molten  mag- 
mas, 780;  within  rocks,  200 

Concepcion  del  Oro,  Mexico,  contact  phe- 
nomena, 713,  725 

Concretions,  definition,  162;  knotty  galena, 
402;  barite,  345;  cassiterite,  672 

Conglomerates,  gold-bearing,  occurrence, 
212;  South  Africa,  236;  copper-bearing, 
434 

Congress  Spring,  Saratoga,  New  York, 
analysis  of  water,  51 

Conjugated  system  of  fractures,  144 

Connarite,  348 

Connate  water,  35,  86 

Conneautsville,  Pennsylvania,  well,  analy- 
sis of  water,  51 

Conrad  vein,  Ophir,  California,  schist 
analysis,  553 

Consolidation  temperature,  762 

Contact-metamorphic  deposits,  706,  699, 
719;  depth  of  formation,  723;  physical 
conditions  at  contact,  722;  types,  725; 
succession  of  minerals,  718;  volume  rela- 
tions, 720;  mode  of  transfer,  721;  form 
and  texture  of  ores,  710 

Contraction  joints  in  sediments,  140; 
in  igneous  rock,  139 

Contrexeville  springs,  1 10 

Conversion  of  water  analyses,  65;  conversion 
tables,  21 

Copiatite  847 

Copper,  Maine,  molybdenite,  778 

Copper,  in  basic  lavas,  425,  443;  in  rocks, 
8;  gold  deposits,  698;  shales,  Mansfeld, 
Germany,  413;  in  veins  allied  to  contact- 


INDEX 


935 


inetaiuorphic  deposits,  699;  deposits  in 
sandstone,  399;  Lake  Superior  region,  431 ; 
of  Monte  Catini,  Italy,  442;  oxidation, 
848;  minerals,  848;  solubility,  849; 
Shasta  County,  California,  637;  with 
zeolites,  425;  tourmaline  deposits,  695; 
sedimentary  ores,  399, -407;  tenor,  14; 
sulphide  veins  in  basic  lavas,  415;  in 
intrusive  basic  rocks,  417;  veins,  634;  oc- 
currences, 402,  406;  titanium  and  molyb- 
denum vein,  700;  igneous  metasomatic 
deposits,  750;  amygdaloids,  43.5;  veins, 
Lake  Superior  region,  436;  mine  waters, 
440;  rock  alteration,  441;  mining  and 
smelting,  441;  in  magma  tic  emanations, 
79;  salts  of,  98;  in  spring  water,  45,  56, 
96,  97,  110,  112  (see  also  minerals  by 
(name)  in  basic  lavas  (Catoctin  type) 
443;  Calumet  conglomerate,  434,  717 

Copper  Mountain,  Prince  of  Wales,  Island, 
Alaska,  copper  deposits,  736 

Coquimbite,  847 

Coral  Springs,  Yellowstone  National  Park, 
analysis  of  water,  53 

Cordierite,  development,  67;  of  deeper  zone, 
75.  820,  879 

Cordilleran  region,  gold-quartz  veins,   576 

Cornubianite,  663 

Cornwall,  England,  radium,  412;  copper- 
tourmaline  veins,  695;  geologic  map,  667; 
metasomatic  processes,  660;  cassiterite 
veins,  666;  mine- water  conditions,  39, 
111;  veins,  190;  cassiterite,  896;  China 
clays,  328 

Cornwall,  Pennsylvania,  magnetite,  723, 
730;  rneta  morphia  in,  713 

Coro-Coro-,  Bolivia,  copper,  406 

Coromandel,  gold,  508 

Cortez,  Nevada,  section,  605 

Cortland  Series,  sulphide  ores,  806 

Corundum,  805,  705,  712,  724,  797 

Corundum  Hill,  Macon  County,  Georgia, 
corundum,  806 

Cote  Carline,  Louisiana,  salt,  310 

Cove  Creek,  Beaver  Valley,  Utah,  analysis 
of  water,  59 

Cove  Creek  mine,  Utah,  sulphur,  382 

Covellite,  857,  635 

Cowee  Valley,  North  Carolina,  rubies, 
776 

Cracker    Creek,    Oregon,    gold    veins,    882 

Cranberry  district,  North  Carolina,  mag- 
netite, 759 

Crawford  Mountains,  Utah,  phosphate 
analysis,  286 

Creede  district,  Colorado,  fault  fissure,  142; 
gases,  98;  supergene  gold,  883 

Creighton  mine,  Sudbury,  Ontario,  nickel, 
815;  section,  816 

Cresson  Spring,  Pennsylvania,  character  of 
water,  45 

Creswick  district,  Victoria,  placers,  218 


Crimora   mine,    Virginia,   manganese,   341; 

analysis  of  ore,  312 
Cripple  Creek  district,  Colorado,  depth  of 

oxidation,  830;   fissure  veins,    142;  gold 

deposits,  522;  gold  ores,  473;  gold-telluride 

lodes,  882;  original  surface,  476;  section, 

522;  sheeted  zone,  152;  vein  system,  143; 

water     conditions,     38;     gypsum,     104; 

wolframite,     673;     ore     chimney,     186; 

intersection  of  veins,  194 ;  gases,  98 
Crocidolite,  396 
Crocoite,  874,  895 

Croesus  vein,  Hailey,  Idaho,  diorite,  analy- 
sis, 555 

Cross  veins,  157 
Crust  of  earth,  composition,  5;  zones,  32; 

amount  of  water,  40 
Crustification,  163 
Cryolite,  350,  774 
Crystal  mine,  Port  Snettisham,  Alaska,  gold 

crystals,  881 
Crystalline  minerals,  26;  rocks,  composition 

of  waters  in,  44 
Crystallization  in  minerals,  relative  power 

of,  169;  of  magmas,  782;  process  of,  26; 

Riecke's    law,    75;    force    of,    146,    165 
Crystalloids,  26 

Cuba    residual    iron    ores,    334;    map,    335 
Cumberland,  Rhode  Island,  iron  deposits, 

795;   ilmenite,    799 
Cummingtonite,  653,  679-681,  696 
Cupriferous,    monzonite,    Bingham,    Utah, 

3;  shale,  average  analysis,  414 
Cuprite,  deposition,  842,  703,  734 
Cuprite,  Nevada,  sulphur  deposits,  382 
Cupro-scheelite,  696 
Cuylerville,  New  York,  salt  mine,  308 
Cuyuna  Range,  iron,  366 
Cyanite,  of  deeper  zone,  75,  712,  788 

Dacite,  Goldfield,  Nevada,  photomicro- 
graph, 544 

Daghardy  chromite  mine,  Brussa,  Asia 
Minor,  794 

Dahlonega,  Georgia,  gold  veins,  674; 
mining  system,  214 

Danburite,  738,  742 

Danby,  California,  salt,  308 

Dannemora,  Sweden,  758 

Dartmoor,  Devonshire,  England,  contact- 
metamorphic  deposits,  741 

Darwin,  California,  lead,  737 

Datolite,  427,  428,  430,  437,  439,  442,  765 

Daubree's  experiments  on  glass  plates,  141 

Dead  Sea,  analysis  of  water,  289 

Death  Valley,  borax,  302 

Decatur  County,  Georgia,  fuller's  earth,  210 

Deccan  diamond  mines,  India,  787 

Decomposition  of  minerals,  322 

Deep-seated  deposits,  78,  163 

Deer  Park  district,  Washington,  wolframite 
veins,  673 


936 


INDEX 


Deformed  pyritic  deposits,  823 

Dehydration,  291 

De  Lamar,  Idaho,  lamellar  quartz,  471; 
section  of  ore,  471;  spring  deposits,  102; 
vein  system,  gold,  513 

Delamar  mine,  Nevada,  plan,  588,  589 

Delessite,  437 

Deloro  mine,  Ontario,  arsenopyrite  gold 
deposits,  586 

Denver,  Colorado,  artesian  wells,  60 

Deposits,  igneous-metamorphic  not  dis- 
tinctly related  to  contacts,  750;  vein  and 
replacement  deposits  formed  at  high  tem- 
peratures and  in  genetic  connection  with 
intrusive  rocks,  651;  formed  by  evapora- 
tion of  surface  waters,  287;  by  mechanical 
processes,  207;  by  igneous  metamorphism, 
704;  of  igneous  origin,  texture,  161;  of 
native  copper  with  zeolites,  425;  produced 
by  chemical  processes,  247;  resulting 
from  regional  metamorphism,  421;  delta 
deposits,  analyses  of,  252;  related  to 
igneous  activity,  77  (see  also  by  subject) 

Depth  of  formation  of  mineral  deposits, 
137,  723;  of  oxidation,  830 

Derbyshire,  England,  fluorite,  650 

Derivation  of  minerals,  78 

Desoloizite,  411,  412 

Desilication,  73,  352 

Desmine,  427 

Detrital,  definition,  200;  deposits,  200 

Detroit  mine,  Arizona,  733 

Deviations,  122 

Devil's  Inkpot,  Yellowstone  National  Park, 
analysis  of  water,  57,  59 

Devil's  Lake,  analysis  of  water,  289 

Diabase,  analysis,  553;  copper  in,  8 

Diamond  placers,  246 

Diamonds,  786,  789 

Diaphorite,  600 

Diaspore,  350,  529,  539 

Diatomaceous  earth,  251 

Dietzeite,  298 

Differentiation  in  magmas,  784;  products, 
204 

Dikes,  pegmatite,  760;  albite,  570;  ilmenite, 
797;  sapphire  bearing,  806 

Dillon,  Montana,  graphite,  746 

Diopside,  in  limestone,  75,  597,  703,  704, 
710-719,  729-738,  740,  788,  827 

Diorite,  analyses,  687;  nickel  and  cobalt  in, 
8;  porphyry,  analyses,  560 

Dip  fault,  definition,  127 

Dip  of  ore  body,  148 

Dip-shift,  definition,  128;  slip,  definition, 
127;  slip  faults,  definition,  132;  throw, 
definition,  130 

Discharge  zone  of  water,  32 

Disintegration,  73,  320 

Dislocation,  use  of  term,  125 

Displacement,  use  of  term,  125 

Dissemination,  definition,  70 


Djebel  Hamimat,  Algeria,  stibnite,  503 

Dochida  River,  Transbaikal,  copper,  425 

Dognazka,  Hungary,  magnetite,  726 

Dclcoath  lode,  Cornwall,  England,  668; 
cassiterite,  742 

Dolcoath  mine,  Elkhorn,  Montana,  gold,  740 

Dolomite,  section  of,  174;  lead  and  zinc  con- 
tent, 10;  replaced  by  galena,  179;  cal- 
cium carbonate  waters  in,  45;  group  of  ore 
deposits,  77;  waters  of,  90,  93,  249,  363, 
444,  468,  489,  563,  712,  755,  757,  759 

Dolores  mine,  San  Luis  Potosi,  Mexico, 
contact  metamorphism,  713,  725 

Domes,  salt,  origin,  310 

Donetz  basin,  Russia,  quicksilver,  492 

Dos  Estrellas  lode,  511 

Douglas  Island,  mining  district,  Alaska, 
153,  683 

Downthrow,  in  faulting,  124 

Drag  of  ore,  122 

Dravo-Doyle  Company,  Pennsylvania,  coal 
mine  water  analysis,  903 

Dredging  process  of  working  gold,  236 

Drusy  structure,  163 

"Dry  ores,"  analysis,  825 

Ducktown,  Tennessee,  analysis  of  mine 
waters,  907;  copper,  655,  751;  pyritic 
deposits,  819;  pyrite,  388;  chalcocite 
zone,  853 

Dunderland,  Norway,  iron  ores,  827 

Durango,  Colorado,  ores,  538;  quadrangle, 
538 

Durango  lead  mine,  Mapimi,  Mexico,  depth 
of  oxidation,  830 

Durdenite,  878 

Durham,  England,  fluorite,  650 

Dutch  Flat,  California,  gold,  227 

Dyscrasite,  487,  884,  629 

Eagle  River  mining  district,  Alaska,  683 

Earth's  crust,  composition,  5;  definition, 
2;  zones,  32 

East  Union,  Maine,  olivine,  section,  812 

Eclogite,  deeper  zone,  75 

Economic  geology,  definition,  1;  scope,  1; 
notes  on  gold,  235 

Ederveen,  Netherlands,  spring  water, 
analysis,  254 

Eger,  hot  springs,  62 

Egersund,  Norway,  ilmenite,  796 

Eglestonite,  893 

Eifel,  on  Rhine,  carbon  dioxide  waters,  95 

Eisenerz,  Styria,  siderite,  650 

Eiserner  Hut,  outcrop,  159,  832 

Ekersund,  Norway,  ilmenite,  797 

Elba,  cassiterite,  664;  hematite,  728;  min- 
erals, 762 

Eldorado  County,  California,  placer  gold, 
234 

Electrum,  467 

Elements,  distribution,  5;  relative  abun- 
dance, 6 


INDEX 


937 


Elizabethtown,  '  New     York,     titaniferous 

iron  ores,  797 
Elk  City,  Idaho,  gold  227 
Elkhorn,  Montana,  deposits,  605,  740,  179 
Elkhorn  mine,  Idaho,  galena,  246 
Elkton  mine,  Cripple  Creek,  Colorado,  522 
El  Oro  mine,  Mexico,  veins,  157;  gold,  510, 

512 

El  Paso,  Texas,  tin  vein,  670 
El    Paso    mine,    Cripple   Creek,  Colorado, 

522;  mine  water  analysis.  905 
Ely,    Nevada,   copper,    15,  865,  732;  palla- 
dium, 892;  protore,  833;  plan  and  section, 

866;   contact   metamorphism,   716,   719; 

volume  relations,  721;  mine  water,  906 
Ely,  Vermont,  water  conditions,  39 
Embolite,  884,  703 
Emerald,  775 
Emery,  805 

Emery  County,  Utah,  uranite,  409 
Emmonsite,  878 
Empire  mine,  section,  717. 
Ems,  springs,  61,  110;  juvenile  origin,  89; 

minerals,  97 
Enargite,  634,  635,  649,  898;  photomicro-^ 

graph,  836 
Encampment  district,  Wyoming,  copper,  9, 

418 
Engels  mine,   Plumas  County,   California, 

bornite,  817 
Enrichment   of   mineral   deposits,    204;   of 

veins,  423;  sulphide,  473,  845 
Enstatite,  389,  393,  759,  788,  773,  806 
Enterprise  vein,  Boulder  County,  Colorado, 

620 

Eolian  deposits,  215 
Epidote,  437,  441,  443,  617,  624,  653,  682, 

699,   704,   712-720,    728,   730,   733,  737, 

740,  742.  751,  756,  759,  800,   810,  818, 

822,    827;    with   gold,    13;    development, 

67,  74,  75 
Epidotization,  713 
Epigenetic  deposits, '  texture,    163;   mineral 

deposits,  147,  148 
Epsomite,  834 
Equal  volume,  law  of,  70 
Equilibrium,  movable,  23 
Erzgebirge,     Bohemia     Hot     Springs,     62; 

greisen,  663 
Erythrite,  895 
Esch,  minette  measures,  261 
Eski-shehr,  Asia  Minor,  meerschaum,  392 
Esmeralda  County,  Nevada,  sulphur,  382 
Etta    mine,    Black    Hills,    South    Dakota, 

columbite,   770;   lithium,   774;   minerals, 

774;  spodumene,  162,  763;  tin,  769 
Euboea,  Greece,  magnesite,  391 
Eureka,  Nevada,  deposits,  605 
Eureka-Idaho,     ore-shoot,     Grass     Valley, 

California,  184 

Europe,  metallogenetic  epochs,  911 
Evergreen  mine,  Colorado,  bornite,  817 


Eutectic  texture,  161 

Euxenite,  770 

Evaporation  producing  precipitation,  24 

Expansion  joints,  140 

Fachingen,  spring,  61 

Faeroer,  Scotland,  copper,  425 

Fahlbands,  definition,  422 

Fahlun,     Sweden,    pyritic    deposits,     636; 

copper  deposits,  820 
Fairbanks,  Alaska,  placers,  223 
Fairplay,  Colorado,  deposits,  617 
Fairport  Harbor,  Ohio,  bittern,  317 
Famatinite,  899,  544,  635 
Fault,    definition,    123;   breccia,    124;    dip, 

124;  line,   123;  space,   123;,  strike,   124; 

surface,  123;  measurement  of  movement, 

123;  closed,  123;  open,  123 
Faulting,    115,    120;    complexity    of,    135 
Faults,    classified,    131,    133;    in    stratified 

rocks,  126;  mineralization  of.  134 
Fayalite,  738 
Faywood,  New  Mexico,  sodium  carbonate 

springs,  62 
Feather  River    dredging    ground,    section, 

223 
Federal  Lead  Company,   Missouri,  section 

of  mine,  460 
Federal  Loan  mine,  spring,  43;  analysis  of 

water,  44 

Feldspar,  in  deeper  zone,  76;  effect  of  vein- 
forming    solutions,    77;     yielding    salts, 

91;  gold  in,  11;  replaced  by  quartz  and 

sericite,   26;   occurrences  and  uses,   660, 

752,  753,  765,  766,  771,  821 
Feltrick-New  North  Pool  section,  Cornwall, 

Ferberite,  673    . 

Fergusonite,  770,  771 

Ferromagnesian  minerals,  copper  content,  9 

Fierro,  New  Mexico,  magnetite,  728 

Fiji,  copper  in  andesite,  9 

Filled     deposits,     primary     texture,     163; 

secondary  texture,  166;  r&le  of  colloids, 

181 

Finbo,  Sweden,  rare  earths,  771 
Fissure  veins,  134;  formed  by  water,  114 
Fissures  produced  by  torsional  stress,  141 
Fissuring  and  filling,  mode  of,  656 
Flamboyant  structure  of  crystals,  165 
Flaws,  definition,  134,  135 
"Flaxseed"  ore,  267 
Florence  mine,  replacement,  179 
Florida    phosphates,     284;     analysis,    286 
Flow  of  underground  water,  29 
Flowage  zone,  earth's  crust,  72 
Fluoresceih,  use  to  show  flow  of  water,  32 
Fluorine,  in  spring  deposits,  45,  60,  110,  97, 

103,    105    (see   also   minerals   by   name) 

with  magmatic  emanations,  79 
Fluorite,    replacing    limestone,    103,    105, 

179;  in  spring    deposits,    103;    deposits, 


938 


INDEX 


649;  group  of  ore  deposits,  77,  468,  475, 
492,  514,  523,  524,  529,  623-625,  633,  657, 
671,  712,  714,  716,  738,  741,  742,  764, 
765,  769,  771 

Folding,  115 

Foot  wall,  definition,  124 

Forsterite,  712,  715 

Fractional  crystallization,  786 

Fracture  zone  of  earth's  crust,  72,  76 

Fractures,  influence  on  water  circulation, 
35;  producing  cavities  in  rocks,  139 

France,  stibnite,  502;  bauxite  ores,  355; 
phosphate,  278 

Franklin  Furnace,   New  Jersey,  lead,  874; 

graphite,  744;  zinc  deposits,  654,  753; 
manganese,  343 

Franklinite,  753,  871 

Franklin  mine,  Georgia,  674 

Franklin  Mountains,  Texas,  tin  veins,  670 

Free  water,  30;  in  earth's  crust,  40 

Freiberg,  Saxony,  159;  silver-lead  deposits, 
475;  replacements,  190;  vein  intersection, 
194;  ore-shoots,  193;  cassiterite,  664, 
895;  proustite,  899;  mineral  deposits, 
111;  pyritic-galena  quartz  veins,  601 

Freihung,  Palatinate,  copper  ores,  401 

Fresnillo,  Mexico,  argentite  veins,  521 

Frostburg  vein,  Georgetown,  Colorado, 
section,  619 

Frozen,  definition,  151 

Fuchsite,  895 

Fuller's  earth,  209 

Fumarolic  gases,  98 

Furnace  Creek  region,  California,  cole- 
manite,  303 

Gabbro,  cobalt  in,  8 

Gadolinite,  770,  771 

Gadsen  County,  Florida,  fuller's  earth,  210 

Gaffney,  South  Carolina,  pegmatitic  tin, 
243,  768 

Gafsa,  Tunis,  284;  analysis,  286 

Gahnite,  754,  871 

Galena,  590,  703,  711,  714,  716,  717,  719, 
735,  737,  738,  741,  755,  765,  874,  876; 
ore  mineral,  4;  replacement  veinlets,  171, 
172;  replacing  dolomite,  179;  quartzite, 
177;  ore,  346;  oxidation,  842;  siderite 
veins,  595;  in  limestone,  69;  concre- 
tionary, 402 

Galicia,  potassium  salts,  316 

Gallatin  County,  Montana,  corundum,  808 

Gallium,  648 

Gallup,  New  Mexico,  monocline,  115 

Gangue,  definition,  4;  minerals,  103 

Garnet,  363,  421,  430,  597,  624,  675,  678, 
682,  701,  703,  704,  710,  713-720,  728- 
742,  751-753,  756,  758,  759,  772,  788, 
789,  797,  804,  807,  819-821,  827,  section, 
719;  deposits,  749,  77;  development,  67; 
in  limestone,  75;  of  deeper  zone,  76 

Garnetization,  713,  717,  719 


Garnierite,  348 

Garrison  mine,  Cortez,  Nevada,  section,  605 

Gases  in  rocks,  origin,  92;  in  spring  water, 
113 

Gash  veins,  definition,  138,  154 

Gathering  zone  of  water,  definition,  32 

Gel,  definition,  27,  257 

Gellivare,  Kiruna,  Sweden,  iron  mines,  802 

Gem  minerals,  775 

Genthite,  348 

Geocronite,  874;  section  showing  replace- 
ment of  galena,  172 

Geologic  body,  definition,  2 

Georgetown  Canyon,  Idaho,  section  phos- 
phate bed,  116 

Georgetown,  Colorado,  deposits,  559;  sec- 
tions, 619;  supergene  silver  deposits,  889 

Geothermal  gradients,  81 

German  potassium  salts,  312 

Germanium,  648,  672  (see  also  minerals 
by  name) 

Geyser  mine,  Colorado,  analysis  of  water, 
44;  sodium  carbonate  waters,  62;  springs, 
112;  sinter,'  101 

Geyser  regions,  composition  of  waters,  53,  57 
"Gibbsite,  350,  354,  539 

Gila  Bend,  Arizona,  celestite,  381 

Gila  River,  New  Mexico,  bauxite,  352; 
meerschaum,  393 

Gilpin  County,  Colorado,  deposits,  617, 
879;  radium,  412;  polished  ore,  563;  gold- 
bearing  veins,  620 

Glauconite,  260,  262,  316 

Glaucophane  rocks,  of  deeper  zone,  75 

Glenwood  Hot  Springs,  Colorado,  springs, 
51;  sodium  chloride,  100 

Globe,  Arizona,  chrysotile,  397;  copper 
deposits,  419,  868;  oxidation,  830 

Gogebic  Iron  Range,  365 

Gothite,  257,  363 

Gold,  715,  739,  741;  conversion  tables,  21; 
divisibility,  219;  fineness  and  relation 
of  placer  to  vein  gold,  229;  in  ashes  of 
trees,  233;  in  pegmatite,  11,  776;  in  sea 
water,  12;  in  sinter,  102;  in  rocks,  10; 
placer,  cost  of  working,  235;  origin,  212; 
production,  235;  relation  to  bed  rock,  229; 
relation  to  primary  deposits,  234;  re- 
deposition,  881;  size  and  mineral  associa- 
tion in  placers,  226;  solution  and  pre- 
cipitation, 232;  standard  of  prices,  16; 
ore-shoots,  184;  native,  467;  in  eluvial 
deposits,  213;  in  eolian  placers,  216;  in 
stream  deposits,  216;  in  kaolin,  485; 
formation  of  nuggets,  217;  in  quartz,  thin 
section,  574;  in  quartz,  583;  in  andesite, 
504;  in  rhyolite,  512;  flake  and  flour, 
220;  pay  streak  run  of,  231;  gold-copper 
deposits,  698;  oxidation,  of  ores,  878; 
solubility  of  ores,  881;  tenor  of  ores,  15; 
gold  placers,  211;  replacing  arsenopyrite, 
574;  section,  577;  alunite  deposits,  539; 


INDEX 


939 


conglomerates,  212,  236;  replacement 
deposits  in  limestone,  586,  in  porphyry, 
589,  in  quartzite,  588;  arsenopyrite  type 
of  contact  metamorphic  deposit,  739; 
selenide  veins  and  deposits,  475,  526; 
telluride  veins  and  deposits,  475,  521, 
688,  740;  quartz  veins,  deep  seated;  674; 
types  of  gold  quartz  veins,  564;  argentite 
quartz  veins,  516;  in  magmatic  emana- 
tions, 79;  in  spring  water,  97;  in  hot 
ascending  water,  114;  in  quartz,  189; 
alteration  of  adjoining  wall  rock  of  gold- 
quartz  vein  550;  (see  also  geographical 
entries)  (see  also  minerals  by  name) 

Goldcircle,  Nevada,  gold,  514 

Gold  Creek  mining  district,  Alaska,  683; 
analyses,  687 

Golden  Cycle  mine,  Cripple  Creek,  Colorado, 
522;  depth  of  oxidation,  830 

"Golden  Mile,"  Australia,  690 

Goldfield  district,  Nevada,  540;  original 
surface,  473,  477;  alunite,  316;  mine 
water  analysis,  906 

Goldfieldite.  544 

Goldroad  mine,  Arizona,  470 

Goroblagodat,  Ural  Mountains,  magnetite, 
803 

Gora  Magnitnaja,  Ural  Mountains,  726 

Goslarite,  444,  871;  834 

Gossan,  158,  832 

Gouge,  definition,  124 

Gouverneur,  New  York,  394 

Gowganda,  Ontario,  cobalt-silver  veins,  627 

Gradients,  geothermal,  81 

Granby,  British  Columbia,  pyritic  deposits, 
636 

Granby  Company,  British  Columbia,  ore 
deposits,  751 

Grand-Grille  spring,  Vichy,  France,  62 

Grandview,  Arizona,  copper,  404 

Granite,  analyses,  662;  copper  in,  8;  gold 
in,  11;  porosity,  30;  openings  in;  31; 
potassium  in,  316;  stability  of,  67;  waters 
of,  88,  91 

Granite  district,   Oregon,  gold    veins,    882 

Granite-Bimetallic  mine,  Montana,  gold 
and  silver,  590;  super  gone  gold,  883; 
supergene  silver,  888 

Granodiorite,  analysis,  553;  copper  in,  9 

Granulite,  826;  of  deeper  zone,  75 

Graphite,  705,  711;  properties,  origin,  oc- 
currences, 743;  artificial,  749,  753 

Grass  Valley,  California,  conjugated  systems 
of  fractures,  144;  diabase,  analysis,  553; 
water  condition,  38;  ore-shoots,  184; 
gold  veins,  568;  gold-quartz  veins,  164 

Gravel  plains,  221 

Gravels,  tertiary,  225;  porosity,  30 

Gravitative  adjustment  in  magmatic  diff- 
erentiation, 785 

Great  Boulder  gold  mine,  Kalgoorlie,  West- 
ern Australia,  991  902 


Great  Eastern  mine,  California,  section,  497 

Great  Fingall  mine,  Western  Australia,  gold, 
690 

Great  Geyser,  Iceland,  analysis  of  water 
53 

Great  Gossan  lead,  Virginia,  388 

Great  Northern  mine,  Canyon,  Oregon,  sec- 
tion of  ore,  577 

Great  Salt  Lake,  analysis  of  water,  289; 
precipitations,  247 

Great  Works  mine,  Cornwall,  England,  669 

Greeley,  Colorado,  composition  of  waters,  60 

Greenalite,  262,  362,  370 

Green  Mountain  mine,  Butte,  Montana, 
water  analysis,  905 

Green  River,  Wyoming,  sodium  carbonate, 
296 

Greensand,  260,  262;  marls,  316 

Greenwood,  British  Columbia,  copper,  750 . 

Greisen,  175,  660;  analyses,  662;  composi- 
tion, 663;  pipes,  769 

Grenockite,  648,  894 

Grenville  district,  Quebec,  graphite  vein, 
section,  748;  magnetite,  804;  magnesite, 
391 

Greywacke,  inclusions  in  arsenide  vein,  629 

Griqualand,  Africa,  asbestos,  398 

Grossularite,  712,  714,  730,  738,  791 

Ground-water  level,  29;  not  stationary,  32 

Guadalcazar,     Mexico,     quicksilver,     492; 


Guano,  277,  278 

Guanajuato,  Mexico,  zeolites,  428,  472; 
silver  ores,  473;  argentite  veins,  52C; 
silver  deposits  with  zeolites,  623 

Guiana,  eluvial  gold,  215 

Gulch  and  creek  gravels,  221 

Gulf  of  Karaboghaz,  Caspian  Sea,  illustra- 
tion of  bar  theory,  293 

Gympie,    Queensland,  ore-shoots,  184;  190 

Gypsite,  294 

Gypsum,  290,  293,  295,  310,  363,  383, 
430,  492,  499,  503;  occurrence,  104; 
origin  of  sulphur  deposits  in,  383;  dis- 
tribution of  beds,  294;  formation  of,  24; 
in  Red  Beds,  48;  in  sedimentary  rocks, 
53,  76;  crystals,  696 

Hachita,  New  Mexico,  water  conditions,  40 

Hade,  definition,  124;  of  ore  body,  149 

Haile  mine,  South  Carolina,  675,  879 

Hailey,  Idaho,  analyses,  555;  silver-lead 
veins,  592 

Haliburton  County,  Ontario,  corundum,  807 

Halloysite,  209,  326,  354 

Halogens,  in  magmatic  emanations,  79 

Halotrichite,  540 

Halsbrttcker  Spat  vein,  Freiberg,  Germany, 
159 

Hamman  Meskoutine,  Algeria,  sodium  car- 
bonate springs,  100,  102 

Hammock  structure  of  veins,  152 


940 


INDEX 


Hanging  wall,  definition,  124 

Hanover,  New  Mexico,  ore,  725 

Hargraves,  New  South  Wales,  gold,  583 

Harmony  channel,  Nevada  City,  gravel,  228 

Harmotone,  623,  625 

Harney  Peak,   Black  Hills,  South  Dakota, 

tin,  769 

"Hartsalz,"  314 

Hartville,  Wyoming,  iron  deposits,  374 
Harz  Mountains,  mineral  veins,  142 
Hastings  County,  Ontario,  corundum,  807; 

arsenic,  649 
Hauerite,  895 
Hauraki  Peninsula,  New  Zealand,  gold, 

507;  propylitization,  480;  cinnabar,  500 
Hausrnannite,  895 
Hawaiian  lavas,  copper  in,  9 
Heave,  definition,  129 
Hecla  mjne,  Coeur  d'  Alene,  Idaho,  595 
Hedenbergite,    653,    712,    714,    716,    726, 

728,  731,  738 
Hedley,    British   Columbia,   contact   meta- 

morphism,  719,  739 
Helena,  Montana,  sapphires,  246,  807 
Helvite,  765 
Hematite,  residual,  329,  803;  Appalachian 

brown    hematites,    330;    Brazilian    ores, 

268;     of    Lake     Superior     region,     357; 

oolitic,  260,  264,  268;  mud,  272;  ocher, 

346,^703,  712,  714,  717,  728,  751,  795 
Heptaborate,  305 
Heptahydrate,  291 

Hercules  mine,  Coeur  d'  Alene,  Idaho,  595 
Hercynite,  807 

Hereroland,  South  Africa,  deposits,  700 
Hermann  shaft,  Sulphur  Bank,  California, 

analysis  of  water,  62 
Heroult,  California,  magnetite,  728 
Hessite,  691,  883 

Heulandite,  430  . 

Hiddenite,  775 
Hidden     Treasure     vein,      Neal     district, 

Idaho,  576 

Highland  Boy  mine,  Bingham,  Utah,  636 
High-temperature  deposits,  classes  of,  655; 

replacements,    174;    minerals,  651;  defi- 
nition, 77 
Hill    End,    Bathurst,    New    South    Wales, 

gold,  226,  583 
Hill    Grove,    New   England   district,    New 

South  Wales,  gold,  583 
Hillsborough,  New  Brunswick,  manganese, 

273 

Hittero,  Norway,  rare  earths,  771 
Hogback  mine,  Alabama,  674 
Hokkaido,  Japan,  sulphur,  382 
Holgol  gold  mine,  Korea,  contact  metamor- 

phism,      718;  contact-metamorphic  ore, 

section,  731 

HoUinger  mine,  Ontario,  gold,  677 
Holocrystalline,  definition,  171 
Homburg,  springs  of  50,  96 


Homebush  placer,  Victoria,  232 

Homer  mine,  Colorado,  deflection  of  vein, 

121 
Homestake  lode,  Black  Hills,  South  Dakota, 

section  of  ore,  679;  mine  waters  of,  908 
Homilite,  764 

Hoole  canyon,  Yukon,  awaruite,  793 
Hopewell,  New  Mexico,  gold  veins,  678 
Horizontal  fault,  definition,  132 
Hornblende,  715,  729,  742,  752,  759,  773, 

797,  801,   811,  820,  827;  with  gold,   13; 

development,  67;  of  deeper  zone,  75,  76; 

group  of  ore  deposits,  77 
Hornfels,  definition,  708 
Horn  Silver  mine,  Frisco,  Utah,  wurtzite, 

873 

Horse,  definition,  124 
Horseshoe  gold  mine,  Kalgoorlie,  Western 

Australia,  691 

Horsetail  structure,  definition,  157 
Hot  Springs,  Arkansas,  water  analysis,  46 
Hot  springs,  producing  quicksilver  deposits, 

499;  prigin,  37;  of  sedimentary  rocks,  90 
Houghton,    Michigan,    amygdaloid    basalt, 

435;   copper-bearing   beds,   section,   432; 

mine  waters,  48 

Huancavelica  district,  Peru,  quicksilver,  490 
Hubnerite,  536,  673 

Huitzuco,     Mexico,     livingstonite,     493 
Hulsite,  741 
Humble  salt  well,  310 

Humboldt  County,   Nevada,   sulphur,  382 
Humite,  715 
Hunter  Hot   Springs,    Montana,   character 

of  water,  104;  zeolites,  428 
Hydrargillite,  350 

Hydrated  silicates  of  nickel,  deposits,  348 
Hydration,  33,  73,  76 
Hydraulic  limestone,  249 
Hydrogen  sulphide,   in   rocks,   source,    92; 

in  water,  56,  58,  64,  77,   90;   in  volcanic 

gases,  98 

Hydromagnesite,  390 
Hydrostatic  level,  definition,  29 
Hydrozincite,  444,  871 
Hypidiomorphic  mixture,  163 
Hypogene,  definition,  833 

Ibex    mine,  Leadville,   Colorado,  615,  617 

Idaho,  lead,  silver,  15;  gold-quartz  veins, 
analysis  of  wall  rocks,  555 

Idaho  Basin,  Idaho,  monazite  placer,  245; 
gravels,  235 

Idaho  City,  Idaho,  gold,  227 

Idaho-Maryland  vein,  Grass  Valley,  Cali- 
fornia, 572 

Idaho  Springs,  Colorado,  sodium  carbonate 
waters,  62;  barium  in  springs,  106 

Idria,   Austria,   quicksilver,  490,   492,  495; 

Igneous  activity,  influence  on  mineral 
deposits,  77 

Igneous  emanations[97,^203 


INDEX 


941 


Igneous  metaaomatic  deposits,  750 

Igneous  rocks,  average  composition,  5; 
acid  sulphate  waters  in,  57;  calcium  car- 
bonate waters  in,  43;  chloride  waters  in, 
51;  copper  in,  8,  10;  gold  in,  10,  12;  salts 
from,  91;  silver  in,  10,  12;  sodium  car- 
bonate waters  in,  60;  metamorphism  of, 
68;  corundum  in,  806 

Illinois  lead  and  zinc  district,  458 

Ilmenite,  712.  742,  764,  765,  768-773,  788, 
795,  799;  of  deeper  zone,  75;  dikes,  797; 
ore,  analysis  of,  798 

Ilsemannite,  409,  897 

Ilvaite,  712,  718,  726,  728,  730,  731,  738 

Imperial  mine,  Utah,  contact  metamor- 
phism, 719 

Impermeable  barriers,  effect  on  ore-shoots, 
191 

Impregnation,  definition,  70 

Imsbach,     German    Palatinate    ores,    475 

Independence  mine,  Cripple  Creek,  Colo- 
rado, 523;  section  of,  524,  525 

India,  manganese,  344;  potassium  salts,  316; 
mica,  768 

Indicator,  definition,  157;  Ballarat, 
Victoria,  191 

Infiltration  from  ocean,  47 

Injected  pyritic  deposits,  818 

Insizwa,  Cape  Colony,  sulphide  deposits, 
817;  section  of  olivine  norite,  818 

Inspiration  mine,  Globe,  Arizona,  copper, 
869 

Intermediate  temperatures,  replacements; 
at,  175 

Intermediate  zone,  rocks  of,  74 

Interpretation  of  water  analyses,  64 

Intersections,  effects  on  ore-shoots,  194 

Interstate-Callahan  mine,  Cceur  d'Alene, 
Idaho,  595,  597 

Inverell,  New  South  Wales,  greisen,  analysis, 
662 

Iodine,  in  spring  water,  47;  in  Byron  Hot 
Springs  water,  49;  in  spring  waters, 
60,  96,  110;  source  of  supply,  299; 
iodate,  298 

lodyrite,  884,  703 

Iowa  lead  and  zinc  district,  458 

Iowa  Hill,  California,  weathering  of  pebbles, 
329 

Iridium,  648,  790 

Iridosmine,  790 

Irkutsk,  Siberia,  graphite,  747 

Iron    Blossom    mine,    Tintic,    Utah,    609 

Iron,  occurrence,  792;  in  igneous  rocks,  6; 
in  spring  waters,  45,  114;  salts  of,  96; 
in  meteoric  waters,  96;  sulphate,  64; 
ranges,  364 

Iron  Hill,  Leadville,  Colorado,  section,  615 

Iron   hydroxid,    centrifugal   tendency,   329 

Iron  Mountain,  Iron  Springs,  Utah,  plan,  729 

Iron  Mountain,  Missouri,  hematite,  803 
section,  245  ., 


Iron  Mountain,  Wyoming,  ilmenite,  796; 
ore  analysis,  798 

Iron  Mountain  copper  mine,  Shasta  County, 
California,  837 

Iron  ores,  bog,  255;  metamorphosed,  824; 
residual,  329-337;  classification,  330; 
sedimentary,  254;  origin,  270;  tenor  of, 
14;  with  phosphorus,  3;  Lake  Superior 
region,  362;  concretionary,  259;  oolitic, 
260,  264,  268;  siderite  type,  257,  650; 
Clinton  ores,  264;  "flaxseed,"  "soft," 
"hard"  ores,  267;  cohere,  346;  laterite, 
351;  hematite,  357,  363;  titanic  ore,  795; 
magnetite  deposits,  728,  800;  meta- 
morphic,  824;  "dry  ores,"  825;  oxidation, 
846;  solubility,  846;  analyses,  Cuban  ore, 
336;  "valley"  and  "mountain"  ores, 
331;  igneous  metasomatic  deposits,  755 

Iron  Springs,  Utah,  laccolith,  depth,  723; 
magnetite,  728;  volume  relations,  721 

Ironstone  outcrop,  159,  832 

Ithaca,  New  York,  salt  beds,  307 

Ivahoe  gold  mine,  Kalgoorlie,  Western 
Australia,  691 

Ivigtut,  Greenland,  cryolite,  774 


Jagerfontein,  diamond  district,  Orange 
Colony,  787 

Jamesonite,  501,  703,  711,  738,  874,  899 

January  mine,  Goldfield,  Nevada,  plan  and 
section,  542 

Japan,  stibnite,  503 

Jarbidge  district,  Nevada,  gold,  514;  sec- 
tion of  lamellar  quartz,  and  adularia, 
471,  472 

Jarilla,  New  Mexico,  chalcopyrite,  725 

Jarosite,  847 

Jasper,  371,  372 

Jasperoids,  550,  606;  definition,  175,  176 

Joachimsthal,  Bohemia  Springs,  111;  ura- 
nium ores,  626;  radium,  412 

Johannesburg,  South  Africa,  gold-bearing 
conglomerate,  239 

Joints,  139;  water  in,  37 

Joplin  district,  Missouri,  analysis  of  well 
water,  46;  section,  452;  zinc  deposits, 
451,  873,  877/453;  mine  waters,  9C4; 
calcite,  166;  cadmium,  648,  894 

Jordan  alum  springs,  Virginia,  composition 
of  water,  56 

Josephinite,  793 

Judith  Mountains,  Montana,  replacements, 
179 

Jumbo  gold  mine,  Hart,  California,  513 

Juvenile,  waters,  87 

Kainite,  312,  314 

Kalgoorlie  gold   mine,   Western   Australia, 

689;  salt  waters,  902;  mineral  composition 

of  ores,  694 
Kamloopo,  gold  in  syenite,  11 


942 


INDEX 


Kanowna,  crystallized  gold,  233 

Kansas,  salt  deposits,  307,  308 

Kaolin,  485,  703,  745;  analyses,  326;  auri- 
ferous, 882;  development,  485;  origin,  327; 
in  feldspars,  69;  group  of  ore  deposits, 
77 

Kaolinite,  209,  326,  486,  541  ^ 

Kaolinization,  Transylvania,  504 

Karangahak'e,  New  Zealand,  gold,  508 

Karkaralinsk  district,  copper,  401 

Karst  topography,  Missouri,  452 

Kasaan  peninsula,  Alaska,  copper,  737 

Katahdin,  Maine,  bog  iron  ore,  257 

Katamorphism,  definition,  72 

Katanga,  Belgian  Kongo,  copper,  408 

Kearns  Keith  mine,  Park  City,  Utah,  sec- 
tion, 608 

Keltz  mine,  Tuolumne  County,  California, 
section  of  auriferous  quartz,  574 

Kennedy  mine,  Mother  Lode  belt,  Cali- 
fornia, 568 

Kerroesite,  900 

Kern  County,  California,  stibnite,  503 

Ketchikan,  Alaska,  copper,  736;  barite,  379 

Keweenaw  copper  deposits,  8,  432;  map,  433 

Khirgiz  steppes,  Russia,  copper,  401 

Kieserite,  291,  312 

Kidneys,  definition,  183 

Kilauea  Crater,  Hawaii,  analysis  of  gases,  88 

Kimberley,  South  Africa,  diamond  field,  787; 
geological  series,  239 

Kimberlite,  788 

Kingman,  Aiizona,  pyritic  galena,  603 

King's  mineral  spring,  Dallas,  Indiana, 
analysis  of  water,  56 

Kingston,  Ontario,  molybdenite,  773 

Kirkland  Lake,  Ontario,  673 

Kiruna,  Sweden,  magnetite,  800 

Kissingen,  springs,  50 

Klackberg,  Sweden,  758 

Klondike,  gravels,  225;  placer  deposits,  226 

Knebelite,  738 

Knox  County,  Maine,  peridotite,  811 

Kochbrunnen,  Wiesbaden,  Germany,  analy- 
sis of  water,  51 

Kokomo,  Colorado,  deposits,  617 

Kolar,  Mysore,  India,  gold  fields,  189,  688 

Kongsberg,  Norway,  fahlbands,  422;  quick- 
silver, 487;  silver  mines,  623;  veins,  191; 
zeolites,  428 

Korarfvet,  Sweden,  rare  earths,  771 

Kostainik,  Servia,  stibnite,  503 

Kran  mine,  Persberg,  Sweden,  sections,  757 

Krennerite,  878 

Kreutznach  springs,  50,  111 

Kristiania,  Norway,  contact-metamorphic 
deposits,  715,  723;  copper,  425;  magnetite, 
705;  mineralizers,  762;  type  of  ore  deposits, 
706 

Krohnkite,  849 

Kunzite,  775 

Kupferschiefer,  Mansfeld,  Germany,  413 


Kutais,  Trans-Caucasia,  manganese,  274 
Kyschtim,  Ural  Mountains,  deposits,  636: 
section  of  ore,  851 

Labradorite,  439,  813 

Ladak,  Kashmer,  British  India,  borax;  300 

Ladder  veins,  139;  definition,  154 

La  Ferrie're-aux-Etangs,  France,  iron  ore, 
269 

Lahontan  Basin  salt  deposits,  317 

La  Junta,  Colorado,  water  analysis,  60,  62 

Lake  City  district,  Colorado,  531,  538; 
galena,  876;  gold,  881 

Lake  of  the  Woods,  Ontario,  gold,  676 

Lake  Sanford,  New  York,  titaniferous  iron 
ores,  796 

Lake  Superior  region,  copper,  431;  para- 
genesis  of  copper  minerals,  437;  iron  ores, 
370;  tenor  of  ores,  14;  hematite,  357; 
manganese,  343 ;  geologic  map,  359 

Lake  Valley,  New  Mexico,  deposits,  605,  887 

Lake  View  Consolidated  gold  mine,  Kal- 
goorlie,  Australia,  691 

Lamalou  springs,  110 

Lands  End,  Cornwall,  England,  Lamorna 
granite,  analysis,  662,  greisen,  analysis, 
662 

Langbanshyttan,  Sweden,  manganese,  758; 
lead,  874 

La  Paz,  Bolivia,  tin,  672 

La  Plata  quadrangle,  Colorado,  531,  538 

Laramie  sandstone,  artesian  wells  in,  60 

Larder  Lake,  Ontario,  gold,  676 

La  Rose  vein,  Cobalt,  Ontario-,  627 

La  Sal  valley,  Colorado,  vanadium,  410 

Las  Condes,  Chile,  veins,  696 

La  Sirena  mine,  Zimapan,  Mexico,  deposits, 
738 

Last  Chance  lode,  Bingham,  Utah,  altera- 
tion of  rocks,  557;  analysis  of  rocks,  558 

Las  Vegas  spring,  New  Mexico,  sodium 
carbonate,  62 

La  Tolfa,  alunite,  317 

Laterite,  Cuban,  iron  rich,  335,  351 

Laumontite,  427,  430,  437,  439,  442,  472, 
473,  483 

Laurel  Creek  mine,  Rabun  County,  Georgia, 
corundum,  806 

Lautarite,  298 

Lavas,  copper  in,  8,  415,  425 

Lavenite,  765 

Lead,  minerals,  874;  solubility,  874;  occur- 
rence in  rocks,  10;  silver-zinc  deposits, 
701;  in  limestone,  47;  in  oxidized  zone, 
875;  supergene  sulphides,  876;  Missouri 
analysis,  461;  tenor  of  ores,  15;  lead- 
silver  "veins,  598,  701;  in  spring  water, 
56,  96,  110,  114;  in  magmatic  emanations, 
79,  salts  of,  98;  deposits  in  sedimentary 
rocks,  47,  399,  i44:  sulphide  enrichment  in 
deposits,  876;  deposits  of  contact- 
metamorphic  type,  737;  Mississippi 


INDEX 


943 


Valley  deposits,  450,  458,  876  (see  also 
minerals  by  name) 

Leadville,  Colorado,  deposits,  558,  605; 
sections,  613,  615,  616;  manganese,  342; 
water  conditions,  39;  analysis  of  quartzose 
porphyries,  10 

Leadville-Boulder  County,  belt  of  deposits, 
558,  617 

LeChatelier's  law  of  pressure,  23 

Lenox,  Massachusetts,  overturned  anticline, 
119 

Lenticular  veins,  154 

Lepidolite,  657,  659,  774 

Lepidomelane,  765 

Lea  Challanches,  France,  fahlbands,  424 

Leucite,  98;  potassium  in,  316 

Leucophane,  765 

Leucopyrite,  740 

Level  of  discharge,  relation  to  circulation, 
142 

Liberty  Bell  mine,  Telluride  district,  Colo- 
rado, 533 

Lila  C.  mine,  Inyo  County,  California, 
borates,  303 

Lime,  249 

Limestone,  definition,  247;  origin,  247; 
replacement  of,  122,  176,  178,  179;  ore, 
shoots  in,  192;  mineralization,  75,  250; 
porosity,  30;  calcium  carbonate  waters, 
45 

Limits  of  stability,  67 

Limonite.  96,  255,  329,  703,  733,  734;  col- 
loidal origin,  28,  257;  deposits,  99;  oolitic, 
260;  oolitic  in  swamps  and  lakes,  255; 
ocher,  346 

Lignite,  as  source  of  carbon  dioxide,  93 

Linarite,  874 

Lincoln,  California,  granodiorite,  analysis, 
553 

Linked  veins,  152 

Linnaeite,  895 

Lithium,  minerals,  773;  in  water,  45;  in 
spring  waters,  97,  111  (see  also  minerals 
by  name) 

Lithographic  stone,  248 

Lithosphere,  definition,  72 

Livingstone  Reef  Series,  South  Africa,  239 

Livingstonite,  493 

Livonia,  New  York,  salt,  308 

Llano  region,  Texas,  quartz-magnetite 
ores,  827 

Lode  formation,  defined,  691 

Lodes  or  composite  veins,  151 

Lofoten  Island,  iron  ore,  827 

Lohitsch,  Styria,  spring  deposits,  107 

Lollingite,  649,  624,  755,  765 

Lomagundi,  Mashonaland,  Africa,  aurif- 
erous gneiss,  13 

London  shaft,  Silverton,  Colorado,  section 
of  vein,  535 

Longfellow,  Arizona,  ore  body,  733 

Long  Island,  ground-water  table,  33 


Longitudinal  fault,  definition,  127 

Lorraine,  minettes,  261 

Los  Pilares;  Nacozari,  Sonora,  Mexico, 
copper,  860 

Lost  Packer  vein,  Idaho,  634 

Lost  River,  Seward  Peninsula,  Alaska,  con- 
tact-metamorphic  deposits,  741 

Lot-et-Garonne,  France,  phosphates,  281 

Louisa  County,  Virginia,  pyrite,  388 

Louisiana,  salt  deposits,  309;  sulphur,  386 

Low  temperature,  replacement  at,  175 

Luderitz  Bay,  German  West  Africa,  dia- 
monds, 246 

Ludwigite,  712,  741 

Luxembourg,  minettes,  261 

Luxeuil,  France,  zeolites,  104,  428 

Luxullianite,  659 

Luzonite,  635 

Lyon  Mountain,  New  York,  magnetite,  804 

Madagascar,  gold,  14;  graphite,  747 

McClelland  well,  Cass  County,  Missouri, 
analysis  of  water,  62 

Madame  Berry,  Victoria,  richest  drift  mine, 
236 

Madrid,  New  Mexico,  graphite,  746 

Magdalena,  New  Mexico,  lead  and  zinc, 
725,  737 

Magmas,  constitution,  780;  crystallization 
of,  782;  minerals,  786;  differentiation, 
784;  concentration,  780;  mineral  deposits; 
780;  exhalations,  89;  emanations,  79; 
water  of,  60,  86 

Magnesium,  in  spring  waters,  47,  74,  91, 
110;  leaching  of,  73;  production  and  use, 
392;  minerals,  76;  deposits,  389;  dolomite, 
249;  serpentine,  389;  occurrences,  391;  in 
igneous  rocks,  6  (see  also  minerals  by 
name);  chloride,  49,  53,  62;  carbonate,  64 

Magnesite,  76,  250,  348,  390 

Magnetite,  67,  74,  703,  704,  710,  712,714- 
719,  726-728,  733-737,  739,  741,  751,753, 
758,  764,  768,  799,  800,  811,  812;  with 
gold,  14;  placers,  245;  contact-metamor- 
phic  deposits,  726;  tenor  of,  14;  quartz 
ores,  827 

Main  Reef  series,  South  Africa,  239 

Makes,  definition,  142 

Malachite,  401,  703,  733,  849;  ore  mineral,  4 

Malacolite,  423,  773 

Malaga,  Spain,  niccolite,  794 

Malay  Peninsula,  tin,  671;  struverite,  770 

Mallardite,  834 

Mammilary  texture,  163 

Mammoth,  Final  County,  Arizona,  mine 
.  water,  902 

Mammoth  mine,  Shasta  County,  California, 
copper,  637 

Mammoth  Cave,  Kentucky,  139 

Mammoth  Hot  Springs,  Yellowstone  Na- 
tional Park,  tufa,  100 

Mancayan,  Philippine  Islands,  enargite,  635 


944 


INDEX 


Manganese  Blue  mine,  Arizona,  733 

Manganese,  bog  ore,  273;  residual,  338-345; 
origin,  344;  sedimentary  ore,  272;  price 
of,  16;  in  lacustrine  and  marine  beds, 
273;  nodules,  274;  igneous  metasomatic 
deposits,  753;  primary  sources,  339; 
solubility,  895;  production  and  uses,  345; 
minerals,  338,  895;  tenor  of  ore,  16;  in 
spring  waters,  45,  114;  in  meteoric  waters, 
96;  salts  of,  98  (see  also  minerals  by 
name) 

Manganite,  339 

Manganosiderite,  594,  895 

Mangano-tantalite,  770 

Mansfeld,  Germany,  copper,  413 

Marcasite,  363,  388,  449,  458,  474,  544; 
oxidation,  848;  in  spring  water,  113, 

Marienbad,  sulphuric  acid  in  spring  waters, 
62 

Mariposite,  409,    571,    573,    677,    794,    895 

Marquette  Range,  iron  deposits,   364 

Martha    Lode,    Waihi,    New  Zealand,  508 

Mass  copper  mine,  Michigan,  water  analy- 
sis, 901 

Massa  Marittima,  Tuscany,  copper,  699; 
mine  waters,  111 

Mary  mine,  Ducktown,  Tennessee,  section, 
752 

Mary  Creek  iron  mine,  Virginia,  section, 
332 

Marysboro,  Victoria,  pay  streak,  231 

Marysvale,  Utah,  tiemannite,  onofrite,  493; 
alunite,  317 

Marysville,  Montana,  batholith,  depth, 
723;  contact  metamorphism,  715 

Massicot,  874 

Mayari  iron  district,  Cuba,  334;  dia- 
gram of  serpentine  alteration,  337 

May  Day  mine,  Tintic,  Utah,  872 

Meadow  Lake,  California,  copper-tourma- 
line veins,  695,  697;  section  replacement 
veinlet,  177 

Mechernich,  Prussia,  lead,  402 

Meerschaum,  392 

Meggen,  Germany,  pyritic  deposit,  253 

Meissen,  Saxony,  kaolin,  327 

Melanocerite,  765 

Melanterite,  847 

Melones,  California  deposits,  551 

Mendenhall  glacier,  analysis  of  amphibolite, 
686 

Mendota  vein,  faulting,  120 

Menominee  Range,  iron  deposits,  364 

Mercur  district,  Utah,  ore  deposits,  586; 
value  of,  3; 

Mercury  (see  quicksilver) 

Mesabi  Range,  iron  deposits,  366,  368; 
analysis  of  surface  water,  901 

Mesothorium,  771 

Metacinnabarite,  487,  893,  894 

Metacolloids,  definition,  28 

Metacryst,  definition,  171 


Metahewettite,  408 

Metalliferous  deposits  formed  by  ascending 
thermal  waters,  at  intermediate  depths 
and  in  genetic  connection  with  intrusive 
rocks,  546;  near  surface  and  in  genetic 
connection  with  igneous  rocks,  465 

Metals,  price  of,  16;  relative  abundance, 
6;  traces  in  rocks,  8;  United  States  pro- 
duction, 17;  measurement  of,  17 

Metallogenetic,  epochs,  regions,  provinces, 
denned,  909;  main  epochs,  910;  Europe, 
911;  Asia,  913;  Africa,  913;  Australasia, 
914;  The  Antilles,  916;  South,  Central 
and  North  America,  915,  916 

Metamorphism,  definition,  68;  contact, 
69,  707;  of  mineral  deposits,  204;  regional, 
202;  in  production  of  ore  deposits,  421; 
successive  epochs,  714;  zones,  72;  piezo, 
724  (see  also  deposits) 

Metamorphosed,  iron  ores,  824;  pyritic 
deposits,  823;  zinc  ores,  828;  deposits, 
processes  involved,  822 

Metasomatic,  magnetite  deposits,  755; 
processes,  168,  478,  549,  653,  659;  in 
kaolin  development,  processes,  485; 
rocks,  California  gold-quartz  analyses, 
553;  texture  of  rocks,  171 

Metasomatism,  contact,  715;  definition, 
26,  69;  in  mineral  deposits,  168;  deposits 
not  distinctly  related  to  contacts,  750 

Metasome,  definition,  69,  171 

"  Metastibnite, "  100 

Metcalf,  Arizona,  ore  body,  733 

Meteoric  waters,  86 

Meteorites,  diamondiferous,  789 

Mexican  mine,  Goldenville,  Nova  Scotia, 
quartz  vein,  584 

Mexico,  argentite  veins,  520;  tin,  670 

Mey  Spring,  Haute  Savoie,  character  of 
water,  49 

Miami,  Arizona,  copper,  858,  869 

Miargyrite,  625,  884 

Miask,  Urals,  ilmenite,  796 

Mica,  766;  in  schists,  68;  in  granular  rocks, 
88 

Michigan,  salt  production,  307;  brines,  48; 
copper,  431;  water  conditions,  copper 
mines,  40;  analyses  of  mine  waters,  901; 
section  of  basin,  307 

Microcline,  764;  of  deeper  zone,  75 

Micro-organisms,  as  precipitants  in  spring 
water,  99;  sulphur  bacteria,  384 

Microperthite,  716;  of  deeper  zone,  75 

Middle  Park  Springs,  Colorado,  sodium 
carbonate  waters,  62 

Milan  mine,  New  Hamsphire,  823-;  deforma- 
tion of  vein,  824 

Mill  City,  Nevada,  scheelite,  742 

Millerite,  493,  625,  894 

Mimetite,  874,  898 

Mina  Rica  vein,  Ophir,  California,  amphi- 
bolite analysis,  553 


INDEX 


945 


Minas  Geraes,  Brazil,  diamond  fields,  787; 
gold  deposits,  680;  hematites,  268;  man- 
ganese, 344 

Minasragra,  Peru,  vanadium,  411 

Mine  Hill,  Franklin  Furnace,  New  Jersey, 
section  of  ore  body,  754 

Mine  La  Motte,  Missouri,  lead  and  zinc 
ores,  460 

Minerals,  crystalline,  26;  colloid,  27;  crys- 
tallization power,  169;  decomposition, 
322;  formation  of,  22;  derivation,  78; 
high-temperature,  651;  in  gold  placers, 
227;  formed  by  magmatic  concentra- 
tions, 780;  of  pegmatite  dikes,  760; 
relation  to  igneous  activity  and  to  meta- 
morphic  zones,  76,  77;  relation  to  mineral 
springs,  109;  deposits  of,  resulting  from 
rock  decay  and  weathering,  319;  texture 
of  deposits,  161;  deposits  genetically  con- 
nected with  igneous  rocks,  203;  veins, 
60;  rare,770 

Mineral  springs,  relation  to  mineral  deposits, 
109 

Mineralization  of  faults,  134;  successive 
phases.  469 

Mineralizers,  651,  760,  783 

"Minettes,"  110,  261 

Mineville,  New  York,  magnetite,  803 

Mine  waters,  38,  48,  111,  440,  904;  analyses, 
440,  441,  901,  902,  904,  905,  906,  907; 
of  Lake  Superior  region,  440;  chloride,  900; 
carbonate,  902;  sulphate,  59,  903 

Minerogenetic  provinces  and  regions,  de- 
fined, 909 

Mining  districts,  California  and  Nevada 
map,  566;  eastern  Australia,  579 

Minium,  874 

Minnie  Moore  mine,  Wood  River  district, 
Idaho,  591 

Mirabilite,  291,  296 

Mississippi  River,  salts,  42;  analysis  of 
delta  muds,  252 

Mississippi  Valley,  lead  and  zinc  deposits, 
450,  877;  genesis,  461 

Missouri,  barite,  378;  composition  of  waters, 
61;  lead  deposits  of  southeastern  section, 
15,  459;  mine  waters,  902;  zinc,  450 

Mittel-Sohland,  Saxony,  sulphide  ores,  812 

Mizpah,  gold,  516 

Moa  iron  district,  Cuba,  334;  analyses  of 
ore  and  serpentine,  336 

Moccasin  Creek,  California,  albite  dikes, 
570 

Moccasin  district,  Montana,  ore  deposits, 
587 

Modoc  mine,  Organ  Mountains,  New 
Mexico,  silver,  590 

Modum  parish,  Norway,  fahlbands,  423 

Mohave,  California,  gold,  11 

Mohawk  mine,  Goldfield,  Nevada,  analysis 
of  ore,  545 

Molly  Gibson  mine,  Aspen,  Colorado,  612 


Molokai,  Hawaiian  Islands,  hematite  mud, 

272;  hematite,  338 
Molybdenite,  423,  620,  698,  700,  704,  711, 

716,  730,  733,  735,  740,  764,  765,  768,  769 

771,  777,  808,  897;  price  of,  16 
Molybdenum,     minerals,     897;    solubility, 

897;  in  magmatic  emanations,  79;  copper 

veins,  700   (see  also  minerals  by  name) 
Monazite,  764,  770-773;  placers,  244;  pro- 
duction of,  245 
Monheimite,  872 
Monocline,  origin,  115 
Mono,   Inyo  County,  California,  scheelite, 

742 

Mono  Lake,  California,  water  analysis,  289 
Monte  Catini,  Itayl,  copper,  442 
Monteponi,  Italy,  cinnabar,  488;  zinc,  450 
Montezuma,  Colorado,  deposits,  617 
Montpelier,     Idaho,     phosphate     deposits, 

282,  283 

Montreal  mine,  Michigan,  section,  364 
Montroydite,  487,  893 
Monumental  mine,   California,   573;    gold, 

229 

Monzonite,  analyses,  558;  copper  in,  8 
Moravia,  graphite,  748 
Moravicza,  Hungary,  magnetite,  726 
Morenci  district,  Arizona,  861 ;  contact  meta- 

morphism,    720;    chalcocite    zone,    853; 

sulphide  zone,  861 

Moresnet  district,  Belgium,  zinc,  448 
Morning  mine,  Coeur  d'Alene,  Idaho,  595, 

597 
Morning  Star  Dyke,  Victoria,  ladder  vein, 

140 

Morococha,  Peru,  copper  veins,  635 
Morro    Velho    mine,    Brazil,    section,    681; 

veins,  159;  ore  body,  190 
Mother  Lode,  California,  Mariposa  County, 

California,  section,  567;  vein  system,  145; 

mines,  production,  572;  water  conditions, 

38;  veins,  159;  placer  gold,  234 
Mottramite,  401 

Mottram  St.  Andrews,  copper,  401 
Mount  Baldy  district,  Utah,  ore,  470 
Mount     Bischoff,     Tasmania,     cassiterite- 

bearing  dikes,  660;  placers,  243;  tin,  663, 

671 

Mount  Bohemia,  Michigan,  copper,  417 
Mount   Lyell,    Tasmania,   pyritic   deposits, 

636,  639,  862;  ore  analysis,  639 
Mt.  Margaret  gold  field,  Western  Australia, 

690 
Mount    Morgan,     Queensland,    gold    and 

copper,   861,    879;   supergene   gold,    883 
Mountain  iron  ores,  331 
Movement  of  water,  examples,  37 
Mud,  analyses  of,  252;  pyrite  in,  253 
Murchison  gold    field,   Western  Australia, 

690 
Murfreesboro,     Pike     County,     Arkansas, 

diamonds,  787 


946 


INDEX 


Muscovite,  323,  421,  675,  712,  742,  745,  752, 
763,  764-767;  development,  67,  in  schists, 
74;  of  deeper  zone,  75 

Muso,  Colombia,  emeralds,  775 

Mustard  gold,  878 

Nacimiento,  New  Mexico,  copper,  404 

Nacozari,  Sonora,  Mexico,  base  metal 
veins,  529 

Naeverhaugen,  Norway,  iron  827 

Nagyagite,  675 

Nankat,  Turkestan,  copper,  401 

Nantahala  Valley,  North  Carolina,  talc,  394 

Napa  Consolidated  mine,  California,  sec- 
tion, 496 

Napa  Soda  spring,  character  of  water,  63 

National,  Nevada,  gold,  515;  stibnite,  502 

Native  elements  (see  by  name) 

Native  elements  in  pegmatites,  773 

Natrochalcite,  849 

Natrolite,  427,  428,  430,  625 

Natron,  291   (see  also  minerals  by  name) 

Nauheim,  springs  of,  50,  93 

Navassa  Island,  guano;  279 

Navajo  Reservation,  Arizona,  pyrope  and 
peridot,  790 

Naxos,  Greek  Archipelago,  emery,  805,  808 

Necks,  definition,  183 

Nederland,  Colorado,  tungsten,  620 

Needle  Mountains,  Colorado,  quadrangle, 
531,  538 

Neihart,  Montana,  zinc,  873 

Nelson  County,  Viginia,  rutile,  769 

Nephelite,  765 

Nernst's  law  of  solution,  25 

Neu  Hoffnung  vein,  Freiberg,  Germany, 
ore-shoots,  193 

Nevada  mining  districts,  map,  56d 

Nevada  quicksilver  deposits,  490;  borate 
deposits,  301 

Nevada  City,  California,  geological  section, 
568;  gold,  227:  granodiorite,  analysis, 
553;  ore-shoots,  184;  water  conditions, 
38;  ribbon  structure,  166,  167;  Pittsburgh 
vein,  571;  gold-quartz  vein,  184 

Nevada  County,  California,  placer  gold,  234 

New  Caledonia,  349,  794 

New  Almaden,  California,  quicksilver,  496 

New  Almaden  Vichy,  character  of  water, 
63 

New  Brancepeth,  England,   witherite,  377 

New  Chum,  reef  line,  580 

New  England,  New  South  Wales,  pegmatite 
minerals,  769;  bismuth,  777;  granite, 
analysis,  662:  greisen,  662 

Newfoundland,  mine  water  conditions,  39 

New  Guinea,  cppper,  425 

New  Haven,  Connecticut,  wells,  35 

New  Idria,  California,  quicksilver,  492, 
493,  497 

New  Mexico,  contact-m  etamorphic 
deposits,  732 


New  North  Pool-Feltrick,  Cornwall,  Eng- 
land section,  669 

Newport  Mine,  Gogebic  district,  Michigan, 
analysis  of  water,  901;  iron,  363;  section, 
365 

New  South  Wales,  gold  districts,  582: 
artesian  waters  of,  60;  diamonds,  789 

New  Zealand,  gases  of  gold  deposits,  98 

Niccolite,  794,  649,  894,  624,  629 

Nickel,  792;  residual  silicate  deposits,  348- 
350;  Sudbury,  Ontario;  813;  minerals, 
solubility,  894;  tenor  of,  16;  in  spring 
water,  45,  56,  96,  110,  112;  in  ocherous 
deposits,  99;  silicates,  analyses,  349 
(see  also  minerals  by  name) 

Nickel  Plate  gold  Mine,  British  Columbia, 
metamorphism,  713,  739 

Nikitowka,  Russia,  quicksilver,  498 

Nikolai  greenstone,  Alaska,  416 

Nitrate  deposits,  Chile,  297 

Nitric  acid,  in  spring  water,  96 

Nitrogen,  volcanic  gases,  98 

Nome,  Alaska,  gold  beaches,  222,  226 

Nomenclature  of  faults,  123;  of  ore  oxida- 
tion, 833 

Nontronite,  329,  745 

Norberg,  Sweden,  758 

Nordmark,  Sweden,  758 

Normal  Faults,  definition,  132;  shift,  defini- 
tion, 128 

North  America  metallogenetic  epochs,  916 

Northampton,  Massachusetts,  wells,  35 

North  Star  mine.  Grass  Valley,  California, 
569,  160;  diabase,  analysis,  553;  auriferous 
'ore-shoots,  189;  vein,  569 

Norway,  iron  ores,  827 

Nottely   River,   North  Carolina,   talc,   394 

Novaculite,  Arkansas,  208 

Nova  Scotia,  gold-quartz  veins,  118,  584; 
chalcocite  nodules,  405 

Nuggets,    of    gold,    217;    weight    of,    226 

Oberstein  a.  d.  Nahe,  Germany,  copper,  425 
Oblique  fault,  definition,   127;  slip  faults, 

definition,  132 
Ocher,  96;  residual,  346 
Ocherous  deposits,  56,  99 
Octahedrite,  765 
Offset,  definition,  126 
Offset  of  a  stratum,  definition,  130 
Ojo   Caliente  Spring,   Taos,    New   Mexico, 

analysis  of  water,  62,  113;  metalliferous 

vein     origin,     113;    boron,    92;    sodium 

springs,  100 

O.  K.  mine,  Utah,  stereogram,  700 
Okufo  mine,  Japan,  contact  metamorphism, 

719 

"Old  Bed,"  Mineville,  New  York,  ore,  804 
Old  Faithful  Geyser,  Yellowstone  National 

Park,  analysis  of  water,  53 
Oldhamite,  809 
Oligoclase,    766,    827;    of  deeper   zone,   75 


INDEX 


947 


Olivine,  389,  393,  726,  727,  788,  790,  791, 
794,  795,  799,  812.  817.  rock,  analysis, 
812;  developed  at  high  temperatures,  67; 
of  deeper  zone,  75 

Olivenite,  898 

Oliver  shaft,  Arizona,  735 

Omaha  mine,  Grass  Valley,  California, 
section  of  auriferous  quartz,  574 

Onofrite,  892,  487,  493 

Ontario,  corundum,  807;  gold-quartz  veins, 
676;  silver  bearing  cobalt-nickel  veins,  626 

Onyx,  origin,  248 

Ookiep,  Cape  Colony,  Africa,  817 

Oolites,  definition,  163 

Oolitic,  hematite,  260,  264;  limonites,  260; 
silicate  ores,  marine,  260;  texture, 
163;  pyrolusite,  274 

Opal,  383,  391,  489,  493;  499,  origin,  28; 
group  of  ore  deposits,  77;  deposition, 
113 

Open  fault,  definition,  123 

Openings  in  rocks,  30;  origin,  137:  produced 
by  compressive  stress,  142;  produced  by 
folding  of  sedimentary  rocks,  141;  pro- 
duced by  shearing  stress  under  the.  in- 
fluence of  gravity,  142 

Ophir  mine,  Comstock  lode,  Nevada,  ore, 
520 

Ophir  mining  district,  Placer  County,  Cali- 
fornia, copper,  418;  replacements,  178; 
granodiorite  and  amphibolite,  analyses, 
553;  gold-quartz  veins,  191 

Oran,  Algeria,  zeolites,  428 

Oravicza,  Hungary,  magnetite,  726 

Ore,  definition,  4;  amount  produced,  17; 
deposition  by  saline  waters,  419;  deposits, 
250;  shoots,  182;  terminology  of  dimen- 
sions, 184;  in  limestone,  192;  precipita- 
tion by  silicates,  841;  tenor  of,  14;  in- 
troduced, 202 

Organ  district,  New  Mexico,  metamorphism, 
737;  veins,  590 

Oriskany  ores,  Virginia,  333 

Oro  La  Plata  mine,  Leadville,  Colorado 
section,  616 

Oroville,  California,  dredging  ground,  223, 
230 

Oroya-Brownhill  mine,  Kalgoorlie,  Western 
Australia,  gold,  691 

Orpiment,  898 

Orthite,  764 

Orthoclase,  437,  551,  660,  712,  714,  740, 
745,  764-766,  769,  819;  in  pegmatite; 
763;  with  gold,  13;  developed  at  high 
temperatures,  67;  changed  to  sericite, 
70;  of  deeper  zone,  75 

Orthotectic,  definition,  811 

Oruro,  Bolivia,  tin,  672 

Oscura  Range,  New  Mexico,  copper,  405 

Osmium,  790 

Otijiaongati.  South  Africa,  deposits,  700 

Ottrelite,  363 


Ouray  district,  Colorado,  53Q.  536;  replace- 
ments, 178;  ores  of  American  Nettie 
mine,  193 

Outcrops,  of  deposits,   831;  of  veins,   158 

Ouvarovite,  794 

Overthrust  faults,  118 

Overthrusts,  definition,  134 

Overturned,  anticlines,  119;  folds,  117 

Ovifak,  Greenland,  graphite,  744;  iron, 
792 

Owens  Lake,  California,  sodium  deposits, 
296;  potassium  chloride,  317 

Oxidation,  73,  473;  depth  of,  830;  examples, 
860;  of  metallic  ores,  829;  outcrops,  831; 
principles  of,  833;  of  iron  carbonates, 
329;  nomenclature,  833;  supergene  sul- 
phide zone,  835,  842;  of  metals,  846-899; 
mine  waters,  900 

Oxide  ores,  768 

Oxidized  and  residual  deposits,  texture  of, 
163;  silver  ores,  191,  188 

Oxyfluorides,  98 

Ozark  region,  lead  and  zinc,  877 

Pachuca,  Mexico,  argentite  veins,  520; 
silver  ores,  473 

Pacific   Congress,    character   of    water,    63 

Paigeite,  741 

Pala,  San  Diego  County,  California,  ambly- 
gonite,  774;  tourmaline,  775 

Palatinate,  quicksilver,  489 

Palen,  Montana,  gypsum,  294 

Palestine,  Texas,  salt  domes,  311 

Palladium,  681,  790,  891,  791 

Pandermite,  305 

Pandiomorphic  texture,  163 

Paradox  valley,  Colorado,  vanadium  ores, 
410 

Paragenesis,  cassiterite  veins,  658;  of  min- 
erals in  Lake  Superior  copper  ores,  437; 
in  pegmatites,  762;  silver  deposits,  625; 
of  metalliferous  deposits,  interior  types, 
562 

Paragonite,  560 

Parallel  displacement  faults,  124 

Parisite,  765 

Park  City,  Utah,  deposits,  605,  607;  forma- 
tion, phosphate  deposits,  282 

Pasco,  Peru,  vanadium,  411 

Paso  Robles  Hot  Springs,  character  of  water, 
63 

Passagem  lode,  Brazil,  gold,  681,  776; 
gold-tourmaline  veins,  695 

Passau,  Bavaria,  graphite,  748 

Patronite,  411,  897 

Paystreak,  gold  placers,  231 

Paystreaks,  development  of,  217 

Pearceite,  replacing  galena,  172,  173, 
649,  884,  898 

Peat,  as  source  carbon  dioxide,  93 

Pecos    River,    New    Mexico,    copper,    697 

Pectolite,  430 


948 


INDEX 


Peekskill,  Westchester  County,  New  York, 
emery,  806 

Peerless  mine,  Keystone,  South  Dakota, 
lithium,  774 

Pegmatite,  occurrence  and  general  character, 
763;  acidic  types,  764;  basic  types,  765; 
pipes,  765;  potassium  in,  316;  dikes 
with  juvenile  water,  88;  connected  with 
quartz  veins,  92 

Pelican  vein,  Georgetown,  Colorado,  sec- 
tion, 619 

Penokee-Gogebic  Range,  iron  deposits,  365 

Pentlandite,  811,  815,  894 

Peralillo  veins,  Chile,  696 

Pereta,  Tuscany,  stibnite,  503 

Peridot,  789 

Peridotite,  diamondiferous,  246 

Permian  salt  beds,  50;  Kansas,  section,  307 

Perowskite,  742,  788 

Pereberg  mines,  Sweden,  composition  of 
ore,  758;  map,  756 

Perseverance  gold  mine,  Kalgoorlie,  West- 
ern Australia,  691 

Persistent  minerals,  67 

Perthite,  766 

Peru,  quicksilver,  490 

Petalite,  774 

Peterboro  County,  Ontario,  corundum,  807 

Petite  Anse,  Louisiana,  salt,  310 

Petzite,  878,  691 

Pfal,    Bavarian    Forest,    quartz    vein,    159 

Pharmacosiderite,  898 

Philipsburg,  Montana,  metamorphism,  715; 
magnetite  ore,  section,  727 

Phillipsite,  428 

Phlogopite,  768,  773 

Phoenix,  British  Columbia,  copper,  750 

Pholerite,  209 

Phonolite,  potassium  in,  316;  Cripple  Creek, 
523 

Phosphate,  sedimentary  beds,  281;  rocks, 
275;  occurrence,  281;  origin,  278;  deposits, 
use,  277;  residual,  347;  "rock,"  285; 
"land  pebbles,"  284;  "river  pebbles," 
285 

Phosphates,  analyses,  286 

Phosphoric  acid,  in  spring  water,  96 

Phosphorus,  in  shells  of  animals,  279; 
in  rocks,  45;  minerals,  275;  in  spring 
water,  97  (see  also  minerals  by  name) 

Piedmontite,  339,  342 

Piezo-metamorphism,  724 

Pilbara  gold  fields,  Western  Australia,  692 

Pintadoite,  408 

Pipe  veins,  definition,  138 

Pipes,  definition,  183;  pegmatite,  765; 
"diamond,"  788;  pipe-like  deposit,  153; 
greisen,  769 

Pisolitic  texture,  163 

Pitch  of  a  fold,  118;  of  the  ore-shoot,  149 

Pitchblende,  412 

Pitching  ore  shoots,  18? 


Pitchstone,  gold  in,  1 1 

Pitkaranta,  Finland,  contact-metamorphic 
deposits,  741;  orbicular  structure,  711 

Pitsfield,  Victoria,  mines,  224;  placers,  232 

Pittsburgh  vein,  Nevada  City,  California, 
571 

Placer    County,     California,    copper,    417 

Placer,  deposits,  210;  copper,  426;  tin,  896; 
gold,  origin,  212;  relation  to  primary 
gold  deposits,  234 

Placers,  classification,  221 

Placerville,  California,  diamonds,  787; 
vanadium  deposits,  section,  410 

Plagioclase,  742,  769,  798,  799,  819;  with 
gold,  13;  development,  67;  of  deeper  zone, 
75 

Plantz  vein,  Ophir,  California,  granodiorite, 
analysis,  553 

Plasticity  of  clay,  327 

Platinum,  739,  740,  790,  891;  occurrence, 
7;  solubility,  892;  with  gold,  242;  value 
of,  243;  placers,  242;  sand  of  California, 
analysis,  790  (see  also  minerals  by  name) 

Plattnerite,  874 

Plombieres  springs,  104,  109;  zeolites,  428 

Plumbojarosite,  791,  874,  842 

Plunge  of  ore-body,  149 

Pneumatolytie,  term  discarded,  653 

Pneumotectic,  definition,  811 

Pockets,  definition,  183 

Podridos,  outcrops,  832 

Polyadelphite,  754 

Polybasite,  612,  884,  899,  625,  629 

Poly  erase,  771 

Poncha  Spring,  Colorado,  sodium  carbonate 
waters,  62 

Pontgiband,  lead-silver  mines,  springs  of, 
61 

Popocatepetl,  Mexico,  sulphur,  382 

Porcupine  district,  Ontario,  gold,  676 

Porosity,  definition,  30 

Porphyry,  analyses,  560,  642;  gold  ant  silver 
in,  11;  spring  water  in,  51 

Porterville,  California,  magnesite,  3S1 

Portland  mine,  Cripple  Creek,  Colorado, 
522;  vein  filling,  523;  water  conditions,  39 

Port  Snettisham  mining  district,  Alaska,  683 

Potassium,  in  brines,  317;  in  spring  water, 
47;  leaching  of,  73;  salts  of,  98,  312;  min- 
erals, 312,  in  rocks,  316  (see  also  min- 
erals by  name)  sources  of,  315,  318 

Potchefstrom  system,  238 

Potosi,  Bolivia,  tin  veins,  672 

Powellite,  897 

Pre-Cambrian  gold  veins,  Ontario,  676; 
of  Cordilleran  region,  678 

Precious  stones,  775,  789 

Precipitation,  of  copper,  849;  by  carbona- 
ceous material,  191;  causes  of,  24;  of 
ore  by  ammonium  humate,  256;  of  ores  by 
silicates,  841;  of  silver,  885;  in  oxidation 
of  metalh'c  ores,  841 


INDEX 


949 


Prehnite,  427,  436,  428,  430,  437,  439, 
442.  714 

Premier  diamond  mine,  Pretoria,  Transvaal, 
787,  789 

Pressure,  influence  of,  23,  654;  decrease  of, 
in  producing  ore-shoots,  188 

Pretoria  series,  South  Africa,  238 

Price  of  metals,  16 

Priceite,  305 

Primary,  ore-shoots,  473;  length  and  depth, 
187;  causes  of,  188;  form,  182 

Primary    texture    of    filled    deposits,    163 

Propylitization,  175,  479;  Transylvania,  504 

Prosser  mine,  Portland,  Oregon,  limonite 
ore,  257 

Protore,  definition,  833 

Proustite,  649,  883,  898,  899,  624,  629 

Providence  Hill,  California,  gold-coated 
magnetite,  233 

Przibram,  Bohemia,  lead-silver  veins,  599; 
water  conditions,  39;  cassiterite,  664 

Pseudomorphic  textures,  168 

Pseudomorphism,  69 

Pseudophenocryst,  definition,  171 

Psilomelane,  272,  338,  895;  colloidal  origin, 
28 

Pueblo,  Colorado,  well,  character  of  water,  54 

Puerto  Mexico,  salt  domes,  311 

Put  in  Bay,  Ohio,  celestite,  380 

Pyrargyrite,  883,  899,  626 

Pyrenees,  water  of  Triassic,  49 

Pyrite,  .363,  422,  541,  647,  704,  711,  713- 
719,  726,  728,  730,  733,  735-742,  745, 
797,  751,  753,  251,  856,  764,  771;  spring 
water,  113,  59;  oolitic,  269;  as  sulphur 
ore,  388;  replacing  calcite,  577;  section  of 
ore,  852;  in  rocks,  45;  in  schist,  74;  in 
spring  water,  91,  98;  replacing  shale,  121; 
replacing  quartzite,  177;  with  gold  and 
silver,  13;  replacing  feldspar,  26;  experi- 
ments with,  839;  oxidation,  847,  903,' 832; 
enargite  veins,  634;  galena-quartz  veins, 
601;  waters  of  shales,  96 

Pyritic  deposits,  injected,  818;  of  Mount 
Lyell,  Tasmania,  639;  of  Rammelsberg, 
Germany,  644;  of  Rio  Tinto,  Spain,  640, 
861;  replacement  type,  635 

Pyrolusite,  338,  895;  oolitic,  analyses,  274 

Pyromorphite,  874,  276,  401 

Pyrope,  789,  788 

Pyrophyllite,  395 

Pyroxene,  393,  423,  699,  717,  718,  737, 
740,  742,  745,  746,  753,  756,  759,  765, 
769,  791,  798-800,  804,  817,  group  of  ore 
deposits,  77;  decomposition,  43 

Pyrrhotite  685,  703,  704,  711,  716-719, 
730,  732,  737-741,  753,  764,  811,  812; 
oxidation,  848,  857,  894,  388,  98 

Quaquaversal,  118 

Quartz,  barrel  quartz,  118,  585;  conversion 
point,  763;  fluid  inclusion,  analysis,  633; 


with  gold,  13;  ribbon  structure  in,  166; 
replaced  by  galena,  171;  Alpine  type, 
adularia-zeolite  vein,  631;  detrital  de- 
posits, 207 ;  tetrahedrite-galena  veins,  590; 
fluorite  veins,  109;  group  of  ore  deposits, 
77;  lamellar,  471 

Quartzite,  68;  with  siderite,  69,  177 

Quartzose  porphyries,  Leadville,  Colorado, 
analysis,  10 

Quebec,  graphite,  748;  bog  iron  ore,   257 

Queensland  gold  districts,  582 

Quelites    spring,    character    of    water,    49 

Quemados,  outcrops,  832 

Queretaro,  Mexico,  stibnite,  502 

Quicksilver,  deposits,  cinnabar,  474;  dis- 
tribution, 489;  genesis,  498;  geological  fea- 
tures, 491;  relation  to  other  ore  deposits, 
501;  structure,  493;  metasomatic  ores  and 
occurrences,  487;  production  and  use, 
489;  minerals,  492;  tenor  of,  16;  in  spring 
water,  96,  103,  892;  secondary  sulphides, 
893  (see  also  minerals  by  name) 

Quincy,  Massachusetts,  pegmatite  pipes, 
765 

Quincy  mine,  Hancock,  Michigan,  analysis 
of  water,  51,  441,  901 

Rabbit   Hole,    Nevada,   sulphur,    382,   499 

Radium,  in  spring  water,  114;  in  carnotite, 
vanadium,  uraninite,  410,  412  (see  also 
minerals  by  name) 

Radjang-Lebong,  Sumatra,  gold  field,  526, 
528,  879 

Ragtown,  Nevada,  soda,  296 

Raibl,  Carinthia,  zinc,  450 

Raibl,  Silesia,  zinc,  450 

Rainy  Lake,  Ontario,  gold-quartz  veins,  676 

Rambler   mine,    Wyoming,    platinum,    791 

Rammelsberg,  Harz  Mountains,  Germany, 
copper,  636,  823;  pyritic  deposit,  644,  823 

Randolph  County,  Georgia,  bauxite  de- 
posits, 354 

Rare  earths,  770 

Rarer   elements    contained    in    waters,    96 

Raton,  New  Mexico,  graphite,  746 

Rawhide,  Nevada,  gold,  513 

Rawhide  mine,  California,  571 

Ray.  Arizona,  copper,  868 

Rea  vein,  section,  677 

Real    del     Monte,     argentite     veins,     520 

Realgar,  898,  98 

"Red  Beds"  copper  ores,  403;  waters  of, 
48 

Red  Cliff  district,  Colorado,  deposits,  618 

Red  Gulch,  Colorado,  chalcocite  nodules, 
403 

Redington  cinnabar  mine,  California,  sec- 
tion, 494,  496 

Red  Mountain,  Colorado,  altered  rocks,  486 

Red  Mountain,  New  Zealand,  awaruite,  793 

Redruth,j[Cornwall,  England,  warm  springs, 
lit 


950 


INDEX 


Reforma  mine,  Guerrero,  Mexico,  pyritic 
deposits,  862 

Rehoboth,  South  Africa,  deposits,  700 

Reichenstein,  Silesia,  gold,  740 

Relict  texture,  definition,  171 

Renfrew  County,  Ontario,  corundum,  807 

Replacement,  89;  and  filling,  161;  definition, 
26;  at  high-temperature,  174;  at  in- 
termediate temperature,  175;  at  low- 
temperature,  175;  mode  of,  169;  of  shale 
by  pyrite,  121;  structures,  173;  criteria  of, 
179;  role  of  colloids  in,  181;  plants  by 
limonite,  255;  veinlets,  171, 177 

Replacement  deposits,  formed  at  high  tem- 
perature and  pressure  and  in  genetic  con- 
nection with  intrusive  rocks,  651;  in 
limestone,  148;  gold-bearing  in  limestone, 
586;  in  porphyry  gold-bearing,  589; 
auriferous  in  quartzite,  588;  of  silver-lead 
in  limestone,  604;  pyritic,  635 

Republic  iron  mine,  Michigan,  analysis  of 
water,  901 

Republic,  Washington,  gold-selenide  veins, 
526;  laumontite,  472;  quartz  veins,  472; 
zeolites  in  silver  veins,  625;  gold  tellurides, 
879 

Residual  iron  ores,  329;  distribution  and 
stability,  336;  classification,  330 

Residual  manganese  ores,  338;  origin,  344 

Residual  sea  water,  441;  barite,  345; 
deposits,  texture,  163;  clay,  325,  327; 
ochers,  346;  phosphates,  347;  salines,  287; 
zinc  ore,  346 

Reverse  faults,  definition,  132 

Reynolds  Mountain,  Virginia,  manganese 
breccia  ore,  343 

Rhodesia,  Africa,  asbestos,  398 

Rhodium,  790 

Rhodochrosite,  274,  895,  339,  344,  363,  505, 
529,  536,  538,  755 

Rhodonite,  274,  339,  895,  529,  703 

Rhyolite,  waters  of,  53;  of  deeper  zone,  75; 
geyser  springs,  92 

Ribbon  structure,  166 

Rice  Lake,  Manitoba,  gold  veins,  678 

Rich  Hill  mine,  Virginia,  section  showing 
brown  ore  deposits,  331 

Rico  district,  Colorado,  531,  536;  banded 
ore,  537;  veins,  157;  blanket  veins,  191 

Riddles  mines,  Oregon,  nickel,  348 

Ridgeway  mine,  Silverton,  Colorado,  sec- 
tion of  ore,  535 

Riebeckite,  765 

Riecke's  law  of  crystallization,  75 

Rio  Tinto,  Spain,  pyritic  deposits,  254,  636, 
640,  861;  section,  641 

River  and  bar  gravels,  221 

Riverside,  California,  tin,  670 

Rockbridge  Alum  Springs,  Virginia,  analysis 
of  water,  56 

Rockrun,  Georgia,  bauxite,  354 

Rock  salt,  composition,  311 


Rocks,  gases  in,  92;  openings  in,  30,  137; 
origin,  137;  secular  decay,  213;  stability, 
66;  contents,  7,  8,  12;  saturated,  30 

Rodaito,  Chile,  silver  veins,  zeolites,  428, 
625 

Rohitch  Styria  spring  deposit,  390 

Romaine,  Quebec,  molybdenite,  777 

Rooiberg,  Transvaal,  tin,  671 

Rooiberg  district,  Transvaal,  tin,  671 

Roosevelt  tunnel,  Cripple  Creek,  Colorado, 
522 

Roros,  Norway,  pyritie  deposits,  636,  820 

Roscoelite,  409,  896,  573,  620,  691 

Rosenbusch  rule  of  magmatic  crystalliza- 
tion, 783 

Rosenbuschite,  765 

Rosita  Hills,  Colorado,  317 

Rossland,  British  Columbia,  697;  water 
conditions,  39 

Roswell,  New  Mexico,  wells,  character  of 
water,  54 

Rotary  faults,  134;  definition,  124 

Rothschonberger  Stolln,  Freiberg,  Saxony, 
analysis  of  water,  904 

Roturoa  geyser  district,  New  Zealand, 
analysis  of  water,  53,  59;  hot  springs,  102 

Round  Mountain,  Nevada,  gold,  514 

Routivare,  Sweden,  titanic  iron,  797 

Rubies,  776;  in  gravels,  246 

"Run  of  gold,"  231 

Ruthenium,  790 

Ruth  mine,  Ely,  Nevada,  composition  of 
mine  water,  906 

Rutile,  351,  700,  742,  745,  764,  765,  773, 
788,  797,  799;  deposits,  769 

Ryepatch  mine,  Unionville,  Nevada,  sec- 
tion, 606 

Saarbrucken,  black  band  ore,  259 
Saarlouis,     Lorraine,     lead,     copper,     402 
Sacramento     Hill,     Bisbee,     Arizona,     733 
Saddle    Mountain,    Arizona,    copper,    725 
Saddle  reefs,    Nova   Scotia,   584;  Victoria, 

578,  580 
St.  John  del  Rey  mine,  Brazil,  gold,  680; 

section,  681 
St.  Lawrence  mine,  Butte,  Montana,  mine 

water  analysis,  905 
St.    Lawrence    River,    Canada,    magnetite, 

245 
St.  Michaels  Mount,  Lands  End,  Cornwall, 

England,  greisen,  analysis,  662 
St.  Nectaire,  arsenic  in  spring,  101 
St.  Urbain,  Quebec,  titanic  iron,  795,  797 
Salem,  India,  magnesite,  391 
Saline,  deposits,  solution,  47;  residues,  287; 

waters,  288;  analyses,  289;  precipitation, 

290;  in  spring  water,  110 
Salinity,  how  measured,  65 
Sail  Mountain,  Georgia,  asbestos,  396 
Salt,  beds,  305,  307,  312;  deposits,  mode  of 

formation,  24,  292;  wells,  306;  domes,  309' 


INDEX 


951 


composition,  production,  uses,  311;  plains, 

308  (see  also  sodium  chloride) 
Salton,  California,  salt,  308 
Salts,  solubility,  24;    from    igneous  rocks, 

91;    from    sedimentary    rocks,    46,    90; 

normal  succession,  290;  in  sea  water,  5; 

in  volcanic  springs,  91 
Samarskite,  770,  764 
San  Diego  district,  California,  tourmaline, 

775 
Sands,   varieties,  227;  porosity,   30;  water 

in,  32 
Sandstone,  porosity,  30;  water  in,  54,  45; 

water  reservoir,  34 
San  Felipe,  Cuba,  iron,  334 
San  Francisco  district,  Utah,  garnetization, 

718 

Sanger  mine,  Oregon,  gold  vein,  882 
San  Juan  Capistrano,  character  of  water,  63 
San    Juan,    Department    Freirina,    Chile, 

cobalt-tourmaline  veins,  703 
San  Juan  region,   Colorado,  geology,  531 ; 

map,  531;  ore  deposits,  477,  529;  veins, 

159,  191 
San   Miguel   County,  New  Mexico,  copper, 

404 
San  Pedro,  New  Mexico,  chalcopyrite  ores, 

725;  contact-metamorphic  deposits,  732 
San  Rafael  lode,  511 
Santa  Barbara  County,  California,  diatom- 

aceous  earth,  251 
Santa   Cruz    County,    California,    beaches, 

226 

Santa  Eulalia  mine,  Mexico,  contact-meta- 
morphic lead  deposits,  738 
Santa  Fe  mine,  Chiapas,  Mexico,  gold,  725, 

739 
Santa    Maria,    Sonora,    Mexico,    graphite, 

746 
Santander,  Spain,  cinnabar,  488;  zinc,  450; 

cadmium,  648 

Santa  Rita,  New  Mexico,  copper,  868 
Santiago,  Cuba,  iron  ores,  334 
Sao  Paulo,  Brazil,  copper,  425 
Sapphires,  246,  807,  788,  805 
Sapphirine,  797 
Saratoga  Springs,  New  York,  character  of 

water,  48 

Saturated  Belt,  definition,  32 
Saturation  of  rocks,  30,  32 
Sauce,  Argentina,  wolframite,  673 
Saugus,  California,  colemanite,  303 
Savage  mine,  Nevada,  mine  water  analysis, 

904 
Saxony,  cassiterite,  658,  669;  argentiferous 

cobalt-nickel  veins,  625 
Scapolite,  712,  715-717,  741,  746,  748,  755, 

768,  773;  group  of  ore  deposits,  77,  709 
Schalen  blende,  874 
Scheelite,  621,  677,  742,  896,  712,  741 
Schefferite,  754 
Schemnitz,  Hungary,  deposits,  529 


Schist,  gold  in,  11;  amphibolite,  analysis, 
553;  origin,  68 

Schladming,  Styria,  fahlbands,  424 

Schlegelmilch,  South  Carolina,  quartz 
vein,  section,  153 

Schlieren,  793;  definition,  781 

Schmiedeberg,  Silesia,  magnetite,  726 

Schneeberg,  Saxony,  veins,  626 

Schorl,  661 

Schttrmann's  Series  and  law,  843 

Schwarzenberg,  Saxony,  contact-metamor- 
phism,  741 

Scorodite,  101,  898 

"Seam  diggings,"  152,  214;  definition,  570 

Sea  water,  gold  and  silver  in,  13;  boron  in, 
300;  salts  in,  5;  zinc,  in,  10;  in  rocks, 
30;  sulphur  in,  385 

Searles  Marsh,  California,  borax,  302; 
potassium,  317 

Secondary,  shoots,  187;  textures,  166 

Secular  decay  of  rocks,  213 

Sedalia,  Chaffee  County,  Colorado,  sulphide 
deposits,  813 

Sedimentary,  copper  ores,  406;  deposits, 
texture,  162;  iron  ores,  254,  269;  man- 
ganese ores,  272;  phosphate  beds,  275; 
sulphide  deposits,  251 

Sedimentary  rocks,  alteration  of,  68,  663; 
water  in,  33,  45,  47,  53,  59;  salts  from,  90; 
average  composition,  5 

Selangor  hot  springs,  103 

Selenides,  526 

Selenium,  642,  879,  892;  gold  selenide  de- 
posits, 475;  veins,  526;  in  spring  water, 
97;  volcanic  emanations,  98  (see  also 
minerals  by  name) 

Selukwe,  Rhodesia,  chromite,  794 

Senarmontite,  501,  503,  899 

Sepiolite,  392 

Sericite,  562,  563,  595,  638,  677;  in  feldspar, 
69;  development,  74;  group  of  ore  de- 
posits, 77;  replacing  andesine,  178 

Sericitization,  60,  479 

Serpentine,  389,  712,  7;  water  of,  62; 
development,  67,  74;  analysis,  336; 
alteration  of,  336,  337;  with  magnesia, 
43:  asbestos,  396;  as  building  stone,  390 

Seven  Devils,  Idaho,  contact-metamorphic 
deposits,  706 

Shale,  impermeable,  31 ;  replaced  by  pyrite 
121;  section,  Rico,  Colorado,  192 

Shannon  mine,  Clifton,  Arizona,  section, 
724 

Shannon  Mountain,  Arizona,  contact- 
metamorphic  deposits,  733 

"Shasta  County,  California,  copper,  418,  637 

Shaw  mine,  California,  albite  dikes,  571 

Shear  zone,  152;  definition,  124,  135 

Sheep  Creek  mining  district,  Alaska,   683 

Sheet  openings,  31 

Sheeted  zone,  135,  145,  152 

Shift,  definition,  126 


952 


INDEX 


Shoots  of  successive  mineralization,  187 
"Shot  ore,"  255 
Siberia,  graphite,  747 

Sicily,  sulphur,  384,  385;  celestite,  381 
Siderite,  363,  590,  594,  598,  660;  in  quart- 
zite,  69;  group  of  ore  deposits,  77;  de- 
posits, 650;  Jurassic,  259;  of  marine  and 
brackish-strata,  257;  hematite-chamosite, 
268 

Sierra  Famatine,  Argentina,  enargite,  635 
Sierra  Hachita  district,  New  Mexico,  copper, 

699 

Sierra  Mojada,  Mexico,  deposits,  605 
Sierra   Nevada,   buried  placers,   224;   gold 
quartz  veins,  565;  gold,   11;  water  con- 
ditions, 37;  platinum,  7 
Sieve  texture,  definition,  171 
Silesia,  zinc,  448;  section,  449;  cadmium,  648 
Silicate,  ores,  oolitic,  262;  minerals  in  lime- 
stone, 75 

Silicates,  as  ore  precipitants,  841;  attacked 

by  water,  60;  in  vein  forming  solutions, 

77;  replacing  limestone  or  dolomite,  93 

Silicification,  9f  limestone,  175,  176,  250 

Silicon,  in  spring  water,  47,  56,  58,  91,  99; 

in  mine  waters,   113,   59,   64,   71;  from 

aea  water,  247;  a  "gel,"  27;  from  plutonic 

intrusions,     75;     origin,     251;     "geyser 

waters,"  64;  (see  also  minerals  by  name) 

Sillimanite,   702,   804;   of  deeper  zone,   75 

Silt,  analysis,  252 

Silver,    622;   conversion   tables,   18   21;  in 
sea  water,  12;  native,  467,  435;  in  rocks, 
10,     12,     102;     argentite-gold     deposits, 
475;   argentite   veins,    520;   replacement 
veinlet,    631;    lead-zinc    deposits,    701;  ' 
lead  veins,  590;  minerals,  883;  precipita- 
tion, 885;  supergene  deposition,  886;  tenor 
of  ores,  15;  solubility,  884,    cobalt-nickel 
veins,   625,   626;   oxidation   of   deposits, 
884;  replacement  deposits,  604;  in  mag- 
matic  emanations,  79 ;  in  veins  related  to 
springs,  113;  (see  also  minerals  by  name) 
Silver  Bell,  Arizona,  copper,  725,  735 
Silver  City,  New  Mexico,  ore  shoots,  192 
Silver  Cliff,  springs,  112;  ore  deposits,  529 
Silver  Crown  lode,  Silverton,  Colorado,  150 
Silver  Islet,  Lake  Superior,  veins,  627 
Silver  Peak,  Nevada,  gold,  682,  776 
Silver  Plume,  Colorado,  Mendota  vein,  120 
Silver  Reef,  Utah,  silver,  405 
Silver  Wreath  Vein,  Willow  Creek,  Idaho, 

analysis  of  rock,  555 

Silverton,  Colorado,  cross  section,  of  vein 
structure,    167;   fissure  veins,    142,    159; 
deposits.  505,  531,  534 
Sindbad  valley,  Colorado,  vanadium,  410 
Sink-holes,  formation  of,  139 
Sinter,  deposits,  100 
Skaggs  Springs,  character  of  water,  63 
Skarn,  ores,  825;  rocks  defined,  716 
Skutterud,  Norway,  fahlbands,  423 


Slip,  definition,  125,  127 

Slates,  origin  of,  68 

Slocan  district,  British  Columbia,  veins, 
594,  648 

Smaltite,  649,  895,  898,  626,  624,  629 

Smectite,  209 

Smithsonite,  170,  346,  871,  611,  703 

Smuggler-Union  mine,  Telluride  district, 
Colorado,  533 

Smuggler  vein,  Telluride,  Colorado,  section, 
533 

Smyrna,  Asia  Minor,  corundum,  808; 
chromite,  794 

Snake  River,  Idaho,  gold,  220;  copper,  426 

Snarum,  Norway,  fahlbands,  423;  ilmenit'e, 
796 

Snowstorm   bed-vein,    Idaho,   section,    154 

Soapstone,  76,  393 

Sodalite,  91,  98,  765 

Soden,  springs,  50 

Sodium,  in  spring  waters,  47-64;  leaching 
of,  73;  salts  of,  98;  waters  in  rocks,  59, 
60;  chloride,  111,  305;  nitrate,  296;  sul- 
phate, 295;  in  water,  90,  96,  100,  107; 
(see  minerals  by  name) 

Soffioni,  Tuscany,  Italy,  98,  300 

Soil,  formation  of,  320 

Solenhofen,  Bavaria,  lithographic  limestone, 
248 

Solowioff  Mountain,  Ural  Mountains,  plati- 
num, 791 

Solubility  of,  iron,  846;  copper,  849;  zinc, 
872;lead,  874;  gold,  879;  silver,  884;  salts, 
24;  silver  salts,  885;  mercury,  892;  cad- 
nium,  894;  nickel,  894;  chromium,  895; 
manganese,  895;  tin,  895;  tungsten,  896; 
vanadium,  896;  molybdenum,  897;  bis- 
muth, 897;  arsenic,  898;  antimony,  899; 
barium,  377;  cobalt,  894;  sulphides,  838, 
884 

Solution,  22;  agency  in  concentration  of 
minerals,  79;  law  of,  25;  producing  cavi- 
ties in  rocks,  138;  of  gold,  232;  copper, 
849;  saline  deposits.  47 

Sombrerete,  Mexico,  argentite  veins,  521 

Sombrero  Island,  guano,  279 

Sonoma  County,  California,  geysers,  char- 
acter of  water,  57 

Sons  of  Gwalia  gold  mine,  Western  Aus- 
tralia, 690 

Soret's  principle,  784 

South  Africa,  geology,  237 

South  America,  metallogenetic  epochs.  915 

South  Lorrain,  Ontario,  cobalt-silver  veins, 
627 

South  Republic  mine,  Republic  Washington, 
zeolites,  625 

Southwestern  chalcocite  deposits,  867 

Spandite,  344 

Spain,  potassium  salts,  317 

Specularite,  362,  430,  704,  710,  726,  730, 
736,  737,  741,  753,  758,  768,  773,  825;  in 


INDEX 


953 


metamorphic  schists,  74;  group  of  ore 
deposits,  77 

Speerenberg,  Germany,  bore-hole  tempera- 
ture, 81.  306 

Sperrylite,  740,  808,  791,  891;  with  gold,  11 

Spessartite,  274,  339,  342,  344 

Sphalerite,  450,  704,  711,  714,  716-719, 
733-741,  753,  764,  765,  771,  Sweden,  828, 
871,  809,  873;  Joplin,  454 

Spherosiderite,  258 

Spindletop,  Texas,  salt,  310 

Spinels,  712,  797,  798,  806,  811,  813,  819, 
820;  developed  at  high  temperature,  67 

Spodumene,  763,  774,  775;  crystals  at 
Etta  mine,  162 

Springs,  production  of  35;  warm,  35;  cold, 
37;  composition  of,  43;  minerals  deposited, 
108;  deposits  at  surface,  99;  elements 
in  waters,  96 

Sprudel,  Carlsbad,  Bohemia,  analysis  of 
water,  62 

Stability  of  rocks,  66 

Stalactitic  texture,  163 

Stanley     Basin,     Idaho,     quicksilver,     488 

Stannite,  896 

Stassfurt,  Germany,  potassium  salts,  312 

Stassfurt-Egeln  anticline,  section,  314 

Static  zone  of  water,  33 

Staurolite,  712;  752,  of  deeper  zone,  75 

Steamboat  Springs,  Nevada,  granodiorite, 
9;  analysis  of  water,  51,  53,  63;  deposits, 
100 ;  boron,  92 ;  minerals  of ,  96 ;  hot  springs, 
113;  cinnabar,  499 

Stephanite,  884,  899,  624,  629 

Sterling,  New  Jersey,  magnetite,  799 

Sterling,  Scotland,  native  copper,  425 

Sterling  Hill,  Franklin  Furnace,  New  Jersey, 
zinc,  754 

Stibiconite,  501,  899 

Stibnite,  501,  899;  at  Steamboat  Springs, 
Nevada,  100;  deposits,  474 

Stilbite,  104,  472,  430,  483,  625 

Stockworks,  139,  152,  466 

Stolzite,  874 

Stone  ore-shoot,  Iron  Hill,  Leadville,  Colo- 
rado, section,  615 

Stratigraphic  throw,  definition,  130 

Stratton's  Independence  mine,  Cripple 
Creek,  Colorado,  524,  525 

Stream  deposits,  216 

Striberg,  Sweden,  analysis  of  ore,  825 

Striegau,  zeolites,  762 

Strike,  fault,  classes,  132;  definition,  126; 
of  tabular  ore-body,  148;  shift,  definition, 
128;  slip,  definition,  127;  slip  faults,  defi- 
nition, 132 

Stromeyerite,  836,  884,  639 

Strontianite,  380,  108 

Strontium,  380;  in  spring  water,  45,  47, 
56,  97,  96 

Structures  of  filled  deposits,  166 

Struverite,  770 


Styrian  magnesite  deposits,  391 

Success  mine,  Coeur  d'Alene,  Idaho,  597 

Sudbury,  Ontario,  nickel, 811, 813; platinum, 
791;pyrite,  636 

Sulitjelma,  Norway,  pyrite,  636,  820 

Sulphantimonides,  502 

Sulphate,  waters,  903;  acid,  57;  in  sedimen- 
tary rocks,  53 

Sulphates,  solubility,  840;  of  metals,  96 

Sulphide,  sedimentary  deposits,  251;  en- 
richment, 473,  845;  enrichment  in  silver 
deposits,  886;  enrichment  in  zinc  and 
lead  deposits,  873;  supergene,  842;  pene- 
tration, 819;  ores  of  igneous  origin,  808, 
ore  in  pegmatite,  776 

Sulphides,  dissemination  of,  422;  veins  in 
basic  lavas,  415;  veins  in  intrusive  basic 
rocks,  417;  solubility,  839;  unstable,  73 

Sulphur,  76,  98,  382,  717;  origin  of  deposits 
in  gypsum,  383;  in  spring  water,  113; 
production  and  uses,  387;  sulphuric 
acid,  387,  57,  62 

Sulphur  Bank,  California,  springs,  113,  499; 
analysis  of  water,  62,  63;  sulphur  deposits, 
382;  cinnabar,  499 

Sumatra,  gold,  528;  platinum,  739;  tin, 
671 

Superficial  or  secondary  ore  shoots,  187 

Supergene,  definition,  833;  textures  of  sul- 
phide zones,  836;  sulphides,  842;  sul- 
phide enrichment,  845;  iron,  846;  copper 
sulphides,  850;  copper  sulphides,  theory, 
854;  zinc,  871;  lead,  874;  gold,  878;  silver, 
883;  other  metals,  891;  mine  waters 
900 

Susanville,  Oregon,  cinnabar,  488 

Swaziland  schists,  238 

Swedish,  "dry  ores,"  825;  magnetite  de- 
posits, 755,  800,  758 

Switzerland,  mineral  veins,  631 

Syd  Varanger  ore,  Norway,  section,J526 

Syenite,  gold  in,  11 

Sylvanite,  878 

Sylvite,  313 

Syncline,  118 

Syngenetic  mineral  deposits,  147 

Taberg,    Sweden,    magmatic    ore   deposits 

758;  ilmenite,  798 
Table    Mountain,    Cape    oi    Good   Hope 

sandstone,  238 
Tables  for  conversion,  18-21 
Tacoma,  gold,  220  m      w 

Talc,  393;  temperature  developed, ^67  ^74, 

in  metamorphic  schists,  74;  intermediate 

zone,  74 

Talcville,  New  York,  talc,  394 
Tamarack  mine,  Michigan,  copper,  434 
Tamaya  mines,  Chile,  695 
Tangential  thrust,  115 
Tantalite,  770 
Tapanhoaiicanga,  Brazil,  214 


954 


INDEX 


Tarkwa,   West  Africa,  gold  conglomerates, 

242 

Tarnowitz,  Silesia,  zinc,  section,  449 
Tauern,  Austria,  gold  quartz  veins,  134 
Taunus,  spring,  61 

Tavoy  district,  Burma,  wolframite,  673 
Tecolote  district,  New  Mexico,  copper,  404 
Telemarkeu,  Norway,  "ladder  veins,"  139 
Telluride,     contact-metamorphic    gold    de- 
posits, 740;  gold  veins,  688;  of  gold,  878 
Telluride     district,     Colorado,     531,     533; 

section,  532 
Tellurite,  878,  883 
Tellurium,  98,  642;  gold  telluride  deposits, 

475     (see    also    minerals    by    name) 
Temescal  Mountains,  California,  tin  veins, 

670 
Temperature,  influence  of,  24;  decrease  of, 

in  producing  ore-shoots,  188;  in  deposition 

of  minerals,   654;  of  consolidation,  762; 

replacements  at  intermediate,  175;  at  low, 

175;   at  high,   174;   of  stability   of  salt, 

315;  underground,  80;  measurement,   84; 

of  salt  lakes,  315 

Teniente,  Chile,  copper  tourmaline,  696 
Tenmile  district,  Colorado,  deposits,  618 
Tennantite,  649,  883,  898,  538 
Tennessee  phosphates,  282;  lead  and  zinc 

ores,  459 
Tenor  of  ores,  14 
Tephroite,  342,  344,  754 
Teplitz,  barite  in  springs,  105;  hot  springs, 

62 
Terlingua  district,  Texas,   quicksilver,   490, 

497,  893;  section,  498 
Terlinguaite,  893 
Terminology  of  ore-shoots,  183 
Tertiary,    borate    beds,    California,    origin, 

303;   gravels,    Sierra   Nevada,   225;   lake 

beds,  303 
Tetradymite,  740 
Tetrahedrite,  528,  529,  538,  598,  625,  697, 

711,  883;  galena-siderite  veins,  590,  899, 


Texas,  manganese,  342;  salt,  310 

Textures  of,  epigenetic  deposits,  163; 
metasomatic  rocks,  171;  pegmatite  dikes, 
161;  residual  and  oxidized  deposits,  163; 
sedimentary  deposits,  162;  secondary 
deposits,  166;  deposits  of  igneous  origin, 
161;  mineral  deposits,  161;  definitions 
of  sieve  and  relict,  171;  oxidized  zone,  835; 
supergene  sulphide  zone,  836 

Thallium,  648 

Thames  district,  New  Zealand,  metasomatic 
processes,  480;  rock  analyses,  481,  482; 
gold,  507;  ore  shoots,  184 

Thaumasite,  430 

The  Dallas,  Oregon,  copper  in  basaltic 
lava,  8 

Thenardite,  291 

Thermal  springs,  source  of  water,  88 


Thermopolis,  Wyoming,  sulphur,  382 

Thetford,  Quebec,  asbestos  deposits,  398 

Thomsonite,  427 

Thorite,  770 

Thorium,  minerals,  770 

Thorn  Mountain  mine,  North  Carolina, 
section,  767 

Throw,  definition,  126;  perpendicular,  129 

Thuringia,  iron  ores,  264 

Thuringite,  262 

Ticonderoga,  New  York,  graphite,  744,  746 

Tiemannite,  892,  487,  493 

Tilly  Foster  mine,  New  York,  magnetite, 
759 

Tin,  699;  magmatic  emanation,  79;  in 
water,  96,  97,  98;  veins,  664;  deposits, 
243;  tenor  of,  16;  solubility,  896;  minerals, 
895  (see  also  minerals  by  name) 

Tintic,  Utah,  deposits,  605,  609,  610;  depth 
of  oxidation,  830;  water  conditions,  40; 
replacement  veinlets,  171,  172,  181; 
photomicrographs,  enargite,  836;  super- 
gene  gold.  881,  883;  zinc  ore,  873 

Tinton,  South  Dakota,  tin,  769 

Titanite,  714,  749,  753,  768,  773;  of  deeper 
zone,  75 

Titanium,  742;  in  bauxite,  351;  iron,  795; 
spring  water,  97;  copper  veins,  700 

Titanium  garnet,  742 

Tomboy  mine,  Telluride  district,  Colorado, 
533;  supergene  gold,  883 

Tonopah,  Nevada,  gold  tellurides,  879; 
faulted  vein,  135;  gold-silver  veins,  516; 
gold  ores,  473;  selenides,  526;  wolframite, 
673;  metasomatic  processes,  483;  analyses 
of  rocks,  484;  quartz  and  adularia,  470; 
mineralization,  476;  supergene  enrich- 
ment, 890;  silver-gold  veins,  depth  of 
oxidation,  830 

Topaz,  776,  709,  712,  714,  742,  764,  657, 
659,  660,  666;  group  of  ore  deposits,  77; 
in  rhyolite,  672 

Torsional  stress,  141 

Tourmaline,  177,  657,  659,  660,  663, 
675,  678,  685,  690,  695-697,  700,  712,  714, 
715,  741,  742,  764,  769,  775,  788,  363, 
477,  818;  in  feldspar,  69;  group  of  ore 
deposits,  77;  veins  701;  boron  minerals, 
92;  copper  deposits,  695;  cobalt  veins,  703 

Tourmalization,  663 

Transbaikalia,  copper,  425 

Transverse  fault,  definition,  127 

Travertine,  99 

Tread  well,  Alaska,  gold,  683;  tenor  of  ore,  16 

Tremolite,  75,  393,  704,  712,  717,  730, 
732,  734,  737,  741,  753 

Tres  Cruces,  Bolivia,  tin,  672 

Triassic  basalts,  New  Jersey,  Connecticut, 
zeolitization,  425 

Triphylite,  774 

Troilite,  809 

Trona,  291,  302 


INDEX 


955 


Troostite,  871  '. 

Tufa,  99 

Tuffs,  60  J 

Tulameen  River,  British  Columbia,  plati- 
num, 79! 

Tulare  County,  California,  magnesite,  391  ^ 

Tully,   New  York,  salt  wells,  sections,  306 

Tungsten,  deposits,  3,  620,  896;  price 
of,  16;  minerals,  -896;  solubility,  896 
(see  also  minerals  by  name) 

Tunis,  phosphate  beds,  278,  281 

Turgite,  257,  363 

Turquois,  276 

Turret,  Chaffee  County,  Colorado,  sulphide 
deposits,  813 

Tuscany,  Italy,  quicksilver,  490;  soffioni, 
300 

Twin    Buttes,    Arizona,    copper,    725 

Tyee,  Vancouver  Island,  636 

Tyrol,  salt  deposits,  309 ;  potassium  salts, 
316 

Ukiah    Vichy,     character    of    water,     63 

Ulexite,  300,  302,  305 

Underground    water,    chemical    works,  66, 

composition,  42;  flow,  29;  origin,  86 
Underlie  of  ore  body,  149 
Upper    Mississippi   Valley,   lead   and   zinc, 

section,  458 

Upthrow,  in  faulting,  124 
Ural    Mountains,    7,    790;    platinum,    242; 

gold,  11;  magnetite,  803 
Uralite,  421 
Uranium,   minerals,   408;   occurrence,   410; 

origin,   use,   411    (see  also    minerals    by 

name) 

Uraninite,  412,  626 
Utah  Hot  Springs,  analysis  of  water,  51; 

of  sedimentary  rocks,  90;  ouvarovite,  794 
Uvanite,  408 

Vaal  River,  South  Africa,  246;  diamonds,  787 

Vaalite,  788 

Vadose  water,  definition,  86,  89 

Valencianite,  468 

Valentinite,  899 

Valkyr  Mountains,  gold,  11 

Vallalta-Sagron,  quicksilver,  490 

Valley  iron  ores,  331 

Valuations,  3,  4;  assay,  20 

Values,  decrease  in  depth,  189 

Vanadinite,  407,  874,  896 

Vanadium,  Colorado,  deposits,  408 

Vanadium,   minerals,   408;  solubility,  896; 

deposits,  407,  411;  ores,  in  sandstone,  399, 

407,  410;  origin  and  use,  411;   (see  also 

minerals  by  name) 
Vancouver  Island,  contact-metamorphism, 

719 

Vegetable  remains,  in  spring  deposits,  102 
Veins,   deposited  by   waters  of  the  upper 

circulation,  418;  formed  at  high  tempera- 


ture and  pressure  and  in  genetic  connec- 
tion with  intrusive  rocks,  651;  in  relation 
to  country  rock,  157;  length  and  depth, 
159;  deflection  of,  121;  walls  of,  158; 
systems  of,  156;  copper  sulphide  in  basic 
lavas,  415;  spacial  relations  of,  149 

Veitsch,  Austria,  magnesite,  391 

Vekol,  Arizona,  copper,  725 

Velardena,  Mexico,  alteration  of  intrusive 
rock,  713;  contact-metamorphic  deposits, 
725 

Ventersdorp,  South  Africa,  volcanics,  238 

Vermilion  Range,  iron  deposits,  370 

Vertical,  faults,  definition,  132;  shifts,  defi- 
nition, 128 

Vesuvianite,  704,  712,  714,  715,  735,741,742, 
746 

Vesuvius,  copper,  9 

Vichy  Springs,  water  analysis,  61;  minerals, 
97 

Victoria,  Australia,  buried  placers,  223; 
gold,  578;  ladder  veins,  139;  nuggets,  226; 
grade,  231;  type  of  gold  quartz  veins,  564; 
gravels,  235 

Victoria  Reef,  Western  Australia,  analysis 
of  mine  water,  902 

Vieille  Montagne,  Belgium,  wurtzite,  874; 
zinc,  448 

Vigsnas,  Norway,  pyritic  deposits,  636,  820 

Vindicator  mine,  Cripple  Creek,  Colorado, 
522;  water  conditions,  38 

Virgilina  district,  Virginia  and  North  Caro- 
lina, bornite,  634 

Virginia,  barite,  379;  lead  and  zinc,  459; 
rutile,  769;  soapstone,  394 

Virginia  Hot  Springs,  analysis  of  water,  46 

Virginius  vein,  Ouray,  Colorado,  532 

Vivianite,  255,  276 

Vogelsgebirge,  springs  of,  61 

Volborthite,  407,  409 

Volcanic    ash,    copper  in,  9;  silver  in,   14 

Volcanic  gases,  95 

Volcanism,  influence  on  water  circulation, 
36 

Volume  relations,  in  metamorphism,  720; 
law  of  equal,  70 

Volume  of  gold  and  silver,  19 

Vulcan  Iron  mine,  Michigan,  analysis  of 
water,  901 

Wabana,  Newfoundland,  iron  ore,  268 
Wad,  272,  338,  895 
Wagoner,  Luther,  assays  for-gold,  12. 
Wagon  Wheel  'Gap,  Colorado,  metalliferous 

vein  origin,  113;  fluorine  in  spring  water, 

105,  650 

Wahnapitae,  Ontario,  gold,  676 
Waihi,  New  Zealand,  gold,  508;  cross  section, 

509;  metasomatic  processes,  480  ;selenides, 

526;  analyses,  482 
Wall     mine,     Virgilina,     North     Carolina. 

bornite  and  chalcocite,  837 


956 


INDEX 


Wall  rock,  character  of,  effect  on  ore-shoots, 
190 

Walkerville,  Montana,  copper,  8 

Wardner  lead  mines,  Coeur  d'Alene,  Idaho, 
597;  mine  waters,  902 

War  Eagle  mine,  Rossland,  British  Col- 
umbia, replacement  veinlet,  section,  697 

Warm  springs,  location,  36 

Warren,  Idaho,  quicksilver,  488 

Washington  Camp,  Arizona,  contact-meta- 
morphic  deposits,  725;  arsenic,  899 

Washington  County,  Missouri,  residual 
barite,  345 

Water,  connate,  35;  depth  in  crystalline 
rocks,  37;  deep  circulating,  202;  discharge 
zone,  32;  gathering  zone,  32;  in  crystalline 
rocks,  composition,  44;  iron  extraction 
by  surface,  254;  sulphate  type,  289; 
movement,  examples,  37;  oceanic  type, 
composition,  289;  residual  type,  composi- 
tion, 289;  static  zone,  33;  volcanic  type, 
composition,  289 ;  underground  (see  under- 
ground water)  in  sand  and  gravel,  32; 
analyses,  interpretation,  64;  circulation, 
influence  of  fractures,  35;  courses,  auri- 
ferous grade,  231;  level,  definition,  29; 
depth  of,  32,  40;  table,  definition,  29; 
amount  in  earth's  crust,  magmatic  emana- 
tions, 79 

Waterberg  system,  South  Africa,  238 

Wavellite,  276,  363 

Weathering,  319;  decomposition  of  minerals, 
322;  processes,  320;  residual,  201;  zones 
of,  73 

Webster,  Jackson  County,  North  Carolina, 
corundum,  806 

Weeks  Island,  Louisiana,  salt  deposits,  310 

Weights,  conversion  tables,  21 

Wellington  lode,  Breckenridge,  Colorado, 
559,  560 

West  Gore  mine,  Nova  Scotia,  stibnite, 
900 

West  Kootenai,  British  Columbia,  gold  in 
dike,  11 

West  Point,  California,  gold  veins.  568 

Westphalia,  black  band  ore,  259;  strontium, 
381;siderite,  650 

Westralia-Mt.  Morgan  gold  mine,  Western 
Australia,  690 

WTieal  Vor,  669 

Wheeling,  West  Virginia,  well,  34 

Whetstones,  origin,  207 

White  channels,  gravels  of,  235;  deposits, 
221,  225,  230,  231 

White  Horse,  Northwest  Territory,  contact- 
metamorphism,  718 

White  Knob,  Idaho,  garnetization,  713, 
717,  719 

White  River  region,  Alaska,  native  copper, 
426 

Wickes,  Montana,  manganese  ore,  273 

Wieliczka  salt  mine,  Galicia,  309 


Wiesbaden   springs,    96,    111,    50;   juvenile 

origin,  89 

Wildbad,  Wurttemberg,  waters,  96 
Wilkinson  County,  Georgia,  bauxite,  355 
Willemite,  753,  871,  716 
Willow  Creek,  Idaho,  rocks,  analyses,  555 
Windham  Bay,  mining  district,  Alaska,  683 
Wisconsin  lead  and  zinc  district,  458 
Witherite,  376 
Witwatersrand,  gold-bearing  conglomerates, 

237;    gold,    genesis,    238;    system,    238 
Wolcott,  New  York,  Clinton  ore,  271 
Wolframite,  712,  741,  660,   620,  770,  896, 

670;  in  veins,  88,  673 
Wollastonite,  791,  704,   712,  716-719,  737, 

739,  749;  in  limestone,  75 
Wood  River  district,  Idaho,  rocks,  analyses, 

556;  silver-lead  veins,  592;  type  of  veins, 

590 

Wood's    Creek,    Montana,    cassiterite,  896 
Wrangell,  Alaska,  barite,  379 
Wulfenite,  779,  897 
Wyoming,  Soda  Lakes,  296 
Wurtzite,  871,  873,  874 
Wyssokaja  Gora,  Ural  Mountains,  magne- 
tite, 726,  803 

Xanthosiderite,  257 

Yauli  district,  Peru,  quicksilver,  490 
Yellow    Pine    district,    Nevada,    platinum, 

791,  892 

Yellowstone  park,  sinters,  100 
Yogo    Gulch,    Montana,    sapphire-bearing 

dike,  806 

Ytterby,  Sweden,  rare  earths,  771 
Yttrialite,  770 
Yttrium,  770 
Yuba   River,   California,   Tertiary  section, 

226 
Yukon  district,  Alaska,  gold,  776 

Zacatecas,  argentite  veins,  521 

Zechstein,  413 

Zeehan  Tasmania,  tin,  672 

Zeolites,  472,  475,  712,  716,  765,  802,  818; 
deposition,  623 ;  with  quicksilver  deposits, 
493;  occurrence,  427;  veins,  631;  group  of 
ore  deposits,  77 

Zeolitic,  copper  ores,  425;  origin,  426; 
enrichments,  623;  replacement,  472 

Zeolitization,  202,  427,  425 

Zinc,  in  rocks,  3,  10;  solubility,  872;  meta- 
morphosed deposits,  828;  supergene 
shoots  of  ore,  872;  supergene  zinc  sul- 
phide, 873;  minerals,  871;  contact- 
metamorphic  type,  737;  lead  silver 
deposits,  701;  igneous  metasomatic  de- 
posits, 753;  minerals,  754;  in  sedimentary 
rock,  444;  residual  ore,  346;  tenor  of 
ores,  15  (see  also  minerals  by  name)  in 


INDEX  957 

spring    waters,    56,    96,    112,    magmatic  Zirconia,  Henderson  County,  North  Caro- 

ema  nations,  79  lina,  zircon,  772 

Zincite,  343,  754.871  Zoisite,   development,   712,   714,   753,421, 

Zinnwald,  Saxony,  vein,  section,  670;  joints,  67;  in  schists,  74 

139  Zuni    Mountains,    New    Mexico,    copper- 
Zircon,  244,  495,  772,  788  bearing  beds,  404 


6114      14 


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